CORNELL UNIVERSITY 
 
 •The 
 
 $iaw?x Betpnnartj IGtbrarg 
 
 FOUNDED BY 
 
 ROSWELL P- FLOWER 
 
 for the use of the 
 
 N. Y. State Veterinary College 
 
 1897 
 
 651 
 
QP 31.S64°192T niVerS,,yLibrary 
 
 iiNi manual of veterinar y physiology, 
 
 3 1924 001 030 679 
 
The original of this book is in 
 the Cornell University Library. 
 
 There are no known copyright restrictions in 
 the United States on the use of the text. 
 
 http://archive.org/details/cu31924001030679 
 
A MANUAL OF VETERINARY 
 PHYSIOLOGY 
 
A MANUAL 
 
 OF 
 
 Veterinary Physiology 
 
 BY 
 
 MAJOR-GENERAL SIR F. SMITH 
 
 K.C.M.G., C.B.' 
 
 FELLOW AND HON. ASSOCIATE OF THE ROYAL COLLEGE OF VETERINARY SURGEONS 
 FORMERLY DIRECTOR-GENERAL ARMY VETERINARY SERVICE 
 
 FIFTH EDITION 
 
 CHICAGO 
 
 ALEX. E G E R 
 
 1921 
 [All rights reserved] 
 
Ho.5^3 
 
 PRINTED IN GREAT BRITAIK 
 
 qP 
 
 31 
 c. I 
 
DEDICATED 
 TO THE MEMORY OF 
 
 SIR MICHAEL FOSTER, K.C.B., 
 
 M.A., M.D., LL.D., D.C.L., F.R.S., 
 
 LATE PROFESSOR OF PHYSIOLOGY IN THE 
 
 UNIVERSITY OF CAMBRIDGE 
 
PREFACE TO FIFTH EDITION 
 
 This book was due for revision in 1914, but the outbreak of War 
 and my recall to military service prevented its being taken in 
 hand; to meet the demand, two small editions were successively 
 reprinted. 
 
 As the War continued, it was decided either that the work 
 must remain out of print or the text be brought up to date by 
 another hand. Dr.T. H. Milroy, Professor of Physiology in the 
 Queen's University of Belfast, very kindly undertook the revision 
 of that portion of the book dealing with general physiology. My 
 thanks are due to him for his help rendered at great personal 
 inconvenience in the midst of his own exacting duties. The 
 first five chapters were in the press when my release from Military 
 Service, in the autumn of 1919, rendered it possible for me to 
 undertake revision on a broader basis than was originally 
 intended. Several chapters have accordingly been recast or 
 entirely rewritten. 
 
 I have been taken to task for calling this work a Veterinary 
 Physiology, but the title was not chosen without due considera- 
 tion. It is not a comparative physiology, but a work intended 
 for veterinary students and practitioners. Throughout its pages 
 an attempt has been made to show the bearing of physiology on 
 the practice of veterinary medicine, surgery, and hygiene.* 
 
 The chapter on Generation and Development is long, but 
 I make no apology for this; the subject is of extraordinary 
 interest to the veterinarian. The breeding of animals is not 
 hampered by social laws and that neglect- of heredity which in 
 human beings leads so largely to the multiplication of the unfit. 
 
 * It is the great regret of my life that with unrivalled facilities for 
 clinically studying two big groups of diseases affecting horses — namely, 
 those of the Digestive and Locomotor systems — I have been able to add so 
 little to the common stock of knowledge. Ignorance of the diseases of 
 animals will remain until those forces capable of overcoming pathological 
 secrets combine to attack the question. The clinician, pathologist, 
 physiologist, biochemist, biologist, and entomologist are all in a greater 
 •or less degree concerned ; in the future, laboratory team work must play 
 3. far larger part if progress is to be made. 
 
viii A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The section on breeding has been dealt with by Mr. J. B. Robert- 
 son, M.R.C.V.S., whose practical knowledge of the subject is- 
 balanced by his scientific interest. I regard myself as excep- 
 tionally fortunate to have secured his able co-operation. 
 
 We sometimes require to be reminded of the fact that in these 
 islands all the domestic animals have been brought to a state 6i 
 development unapproached by any other country in the world.* 
 The local conditions, so favourable to the improvement of breeds, 
 have fortunately been supplemented by a special gift of native 
 intelligence. 
 
 The all-important study of animal nutrition may be claimed 
 to have been very largely initiated in this country; it was at 
 Rothamsted, at the private expense of the late Sir John Lawes, 
 that researches, extending over twenty years, were carried out, in 
 conjunction with the late Sir Joseph Gilbert, into the economical 
 feeding of animals and the storage of food by the body.f None 
 of the physiological conclusions of these observers, though seventy 
 years old, have been cast aside as out of date. They established 
 the fundamental facts on which the scientific work of Animal 
 Nutrition Stations on the Continent of Europe and in the United 
 States of America has been so largely built. I am especially' 
 indebted to the work of Dr. H. P. Armsby, Director of the Insti- 
 tute of Animal Nutrition of the Pennsylvania State College,. 
 U.S.A., which I have freely drawn upon in endeavouring to 
 present a summary of the highly complex subject of the means 
 by which the system obtains its nourishment from food, and of 
 what it does with it subsequently. 
 
 In a review of the last edition there was a complaint that the 
 physiology of the cell was inadequately dealt with. I fear that I 
 am impenitent ; if more information is required, there are numer- 
 ous works from which it can be obtained. I need all the space 
 I can command for subjects of greater practical importance, 
 some of which are not dealt with elsewhere, and many of which 
 are so scattered through periodicals as to be unavailable, except- 
 ing to the research student. 
 
 For the same reason the chemical facts have been dealt with 
 
 * " Every breed of live-stock with which he was familiar, and which 
 it was the ambition of his countrymen to raise successfully, came originally 
 from Great Britain. It was one of the most remarkable things in human 
 history that practically all the great domestic animals on which man- 
 kind relied for sustenance and support had been brought to perfection in 
 Great Britain and distributed throughout the world. If there were noth- 
 ing else in British history to make it glorious, that alone was a matter for 
 unceasing pi ide."— Report of speech by H.E. the American Ambassador 
 on " British Stock," Morning Post, August 3, 1920. 
 
 t My thanks are due to Dr. E. J. Russell, F.R.S., the present Director 
 of the Rothamsted Experimental Station, for copies of the photographs- 
 of Sir John Lawes and Sir Joseph Gilbert. 
 
PREFACE TO FIFTH EDITION 
 
 Sir John Lawes, F.R.S. 
 
 great debt he owes 
 to the labours of the 
 late Professor G . 
 Colin, of the Veteri- 
 nary School of Paris, 
 as the father of 
 veterinary physio- 
 logy. Though his 
 work appeared more 
 than fifty years ago, 
 and vast strides have 
 been made since his 
 day, nothing can dis- 
 turb the accuracy 
 of his experimental 
 work. I am indebted 
 to Mr. A. H. Hunter, 
 M.R.C.V.S., of Le 
 Havre, for the 
 trouble he has taken 
 in securing me a copy 
 of a photograph of 
 this great man. 
 
 only at sufficient 
 length to elucidate 
 the text, while the 
 concluding chapter 
 on the chemistry of 
 the body, very 
 kindly contributed 
 by Professor Milroy, 
 is intended only as 
 an outline of a sub- 
 ject which has grown 
 far beyond the needs 
 of any other than a 
 student desirous of 
 a University degree, 
 for whom the neces- 
 sary textbooks are 
 available. 
 
 I made a special 
 point in the last 
 edition of drawing 
 the attention of the 
 English reader to the 
 
 Sir Joseph Gilbert, F.R.S. 
 
A MANUAL OF VETERINARY PHYSIOLOGY 
 
 I have been favoured with help from many members of the 
 profession whose names appear in the text, and to whom I am 
 indebted for new facts or observations which I no longer have an 
 opportunity of repeating or making. I would mention in this 
 connection the staff of the Army Veterinary School, Aldershot, 
 where, through the co-operation of Major-General L. J. Blen- 
 kinsop, C.B., D.S.O., Director-General of the Army Veterinary 
 Service, several observations were made for me; likewise Princi- 
 pal Bradley, M.D., D.Sc, and the members of the staff at the 
 Royal (Dick) Veterinary College, Edinburgh; and Captain 
 J. T. Edwards, B.Sc, of the Royal Veterinary College, London. 
 
 " Dr. A. S. Lea, 
 F.R.S., who so will- 
 ingly took over the 
 duties of critic and 
 proof-reader of the 
 previous editions, 
 has, to my great 
 regret, since died; 
 but 1 have been 
 happily fortunate in 
 securing the assist- 
 ance of Mrs. F. 
 Bullock, M.A., who, 
 besides relieving me 
 of the labours of 
 proof - reading and 
 indexing, has given 
 me the advantage of 
 her critical judg- 
 ment. For her valu- 
 able co-operation I 
 tender my most sin- 
 cere thanks. The 
 student is urged to 
 consult her index, as in a work of this kind it is impossible, 
 excepting through this channel, to bring together the many 
 scattered references bearing on the same subject. 
 
 My thanks are also due to those authors and publishers who 
 have been good enough to permit reproductions of illustrations 
 from their works, all of which arc acknowledged in the text. 
 
 Finally, I recognise that the time has arrived, or is fast approach- 
 ing, when my work must close and younger men take up the 
 subject. I observe with great regret that there appears a 
 disinclination in the young school to specialise in physiology, 
 in which case it will pass into the hands of others outside 
 
 Professor G. Colin. 
 
PREFACE TO FIFTH EDITION xi 
 
 the profession, who will be unable to draw the necessary practical 
 lessons, owing to their ignorance of clinical matters. Pure physio- 
 logy is one thing, but what the practitioner requires is physio- 
 logy in its application to disease, and it cannot be said that the 
 fringe of this question has yet been touched; it can only be done 
 by those whose training renders them familiar with the habits 
 and diseases of animals. Pathology has been happily defined 
 as 'physiology out of health,' and the chief reason why we study 
 the functions of the normal body is to be enabled to apply the 
 information thus obtained to the study of disease. 
 
 F. S. 
 
 December, 1920. 
 
CONTENTS 
 
 CHAPTER 
 
 I. The Blood - 
 
 II. The Heart ... . , 
 
 III. The Bloodvessels . 65 
 
 IV. Respiration ioi 
 V. Digestion 158 
 
 VI. The Liver and Pancreas - 246 
 
 VII. Absorption 272 
 
 VIII. The Endocrine Glands and Internal Secretions 300 
 
 IX. The Skin - - 308 
 
 X. The Urine - 325 
 
 XI. Nutrition - 357 
 
 XII. Animal Heat 402 
 
 XIII. The Muscular System - 422 
 
 XIV. The Nervous System - - 452 
 XV. The Senses - 566 
 
 XVI. Locomotor System - 631 
 
 XVII. The Foot - 700 
 
 XVIII. Generation and Development 737 
 
 XIX. Growth, Decay, and Death 811 
 
 XX. The Chemical Basis of the Body 822 
 
 Index to Authorities - - 837 
 
 Index of Subjects 840 
 
aio- 
 
 Inches 
 
 
 
 Cent/mefres 
 
 ° \- 
 
 = 
 
 J 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 to 
 
 
 
 
 
 
 
 = 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 |i- 
 
 
 = 
 
 
 
 
 = 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 5 
 
 
 
 
 
 
 
 
 
 
 
 
 ■ / h 
 
 
 
 
 
 
 
 
 
 
 
 == 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 90- 
 
A COMPARISON OF SOME BRITISH AND 
 METRIC UNITS 
 
 To convert degrees Fahrenheit into degrees Centigrade, subtract 32, 
 multiply by 5, and divide by 9. 
 
 To convert degrees Centigrade into degrees Fahrenheit, multiply by 9, 
 divide by 5, and add 32. 
 
 11 inch= 25-4 millimetres = 2-54 centimetres. 
 1 foot=304-8 ,, =30-48 
 
 1 yard - =91-44 
 
 1 mile= 1,609-3 metres = 1-609 kilometres. 
 
 i_,engm -, 1 micromillimetre = it ,Vti millimetre. 
 ! 1 millimetre = ,', inch. 
 
 1 centimetre = -? inch. 
 
 1 metre =1,000 millimctres = 39-37 inches. 
 
 1 kilometre =1,000 metres =0-62 miles= 3,281 feet. 
 
 To convert millimetres into inches, x 0-039. Converse, x 25-4. 
 To convert metres into yards, x i-og. Converse, x 0-914. 
 To convert inches into metres, x 0-0254. Converse, x 39-37. 
 
 1 grain=o-o648 gramme=64-8 milligrammes. 
 1 ounce (avoir.) = 28-35 grammes = 437-5 grains. 
 1 pound ,, =453-60 ,, = £ kilogramme (approx.). 
 
 Weight -\ 1 kilogramme =1,000 grammes = 2-2 pounds (avoir.). 
 1 gramme = 15-4323 grains. 
 
 1 milligramme = 1^ gramme = 0-01543 grain= ¥ 1 I grain 
 
 (nearly) . 
 
 To convert grammes into grains, x 15-432. Converse, x 0-0648. 
 
 To convert grammes into ounces (avoir.), x 0-03527. Converse, x 28-35. 
 
 To convert kilogrammes into pounds, X2-2. Converse, x 0-454. 
 
 ' 1 fluid ounce = 28-42 cubic centimetres= 28-42 grammes. 
 1 pint =568-34 ,, ,, =568-34 grammes. 
 
 1 gallon = 4-54 litres = 10 pounds (avoir.). 
 
 1 cubic inch =16-38 cubic centimetres. 
 
 foot =28-33 litres. 
 1 cubic centimetre = o-o6i cubic inch=i gramme of distilled 
 
 water at its greatest density = 17 minims. 
 1 litre= 1,000 cubic centimetres = 1-76 pints= 35-2 fluid ounces 
 
 = 61 cubic inches=l kilogramme. 
 1 cubic metre= 1,000 litres=35-3 cubic feet. 
 
 .To convert cubic centimetres into ounces fluid, x 0-0352. Converse, 
 x 28-42. 
 
 To convert cubic centimetres into cubic inches, xo-o6i. Converse, 
 x 16-38. 
 
 To convert litres into fluid ounces, x 35-2. Converse, x 0-0284. 
 
 To convert litres into pints, x 1-76. Converse, x 0-568. 
 
 To convert litres into gallons, xo-22. Converse, x 4-543. 
 
 To convert litres into cubic feet, x 0-03532. Converse, x 28-33. 
 
 Capacity 
 
 (1 foot-pound= 0-138 kilogramme-metre. 
 : 1:" 
 
 Work< I foot-ton =309-12 kilogramme-metres. 
 1 1 kilogramme-metre =7-25 foot-pounds. 
 
 1 Unit of Heat (British) = heat necessary to raise 1 pound of water through 
 
 i°F. 
 1 Small Calorie=heat necessary to raise 1 gramme of water through i° C. 
 1 Large Calorie = heat necessary to raise 1 kilogramme of water through 
 
 i°C. 
 Mechanical equivalent of heat-unit = 772 foot-pounds. 
 
 ,, calorie =424 gramme-metres. 
 
 „ kilo-calor:e= 424 kilogramme-metres. 
 
Frog 
 
 (on the flat) (sute view) Bir ' 1 
 
 Camel 
 
 Blood of •mammal 
 
 Frog's Corpuscle 
 after addition of water 
 
 ® 
 
 4& 
 
 Mammalian Mammalian 
 
 after addition of syrup after addition of salt 
 
 •- ®® f 
 
 - %>2I©9 
 - "M IF 
 
 1. Red blood-corpuscles. 
 
 •4 ^& i 
 
 : : ;:rc- 
 
 *# 
 
 /**\ 
 
 1 
 
 ^ 
 
 
 2. The colourless corpuscles of human blood, x 1000. a, eosinophile cells ; 
 6, finely granular oxyphile cells ; c, hyaline cells : d, lymphocyte ; 
 e, polymorphonuclear neutrophils cells (Kanthack and Hardy). The 
 magnification 13 much greater than in 1. 
 
 3. Cover-glass preparation of spinal cord of ox, x 250. 
 (Staintd with methylene blue). 
 Dendritic processes 
 
 Large 
 
 multipoi 
 ncrve-fi 
 
 Axis-cyl 
 
A MANUAL OF 
 VETERINARY PHYSIOLOGY 
 
 CHAPTER I 
 THE BLOOD 
 
 The special functions of the blood are to nourish all the tissues 
 of the body, and thus aid in their growth and repair; to furnish 
 material for the purpose of the body secretions; to supply the 
 organism with oxygen, without which life is impossible ; and finally 
 to convey from the tissues the products of their activity. To 
 enable all this to be effected the blood is constantly in circula- 
 tion, is rapidly renewed, is instantaneously purified in the lungs, 
 and, by means of certain channels, is placed directly in com- 
 munication with that nourishing fluid absorbed from the intes- 
 tines by which it is constantly repaired. 
 
 Physical Characters.— Blood is a red, opaque, rather viscous 
 fluid, the tint of which depends upon whether it is drawn from 
 an artery or a vein; in the former it is of a bright scarlet colour, 
 whilst in the latter it is of a purplish red. The colour is due to a 
 pigment called hemoglobin contained in the red corpuscles. 
 Whether the colour is scarlet, as in blood from an artery, or 
 purplish, as from a vein, depends on the amount of oxygen with 
 which the haemoglobin is combined. 
 
 ' The reaction of blood, as tested by glazed litmus paper, is 
 always distinctly alkaline. When, however, the true reaction 
 of the fluid is determined by modern methods it is found to be 
 only very slightly alkaline. The principle of the method em- 
 ployed for the accurate determination of the blood reaction is 
 based upon the fact that in all aqueous solutions the product of 
 the hydrogen-ion and the hydroxyl-ion concentrations is a 
 constant— namely, the ionisation constant of water usuall3 
 stated in the following form, the enclosures in brackets denoting 
 gramme-ionic concentrations per litre: 
 
 [H+]X[OH-]=K=io- I t (approximate). 
 
■a A MANUAL OF VETERINARY PHYSIOLOGY 
 
 In meratral solutions the [H+] and [OH"] are equal— namely, 
 iXMf-?. In acid solutions the [H+] is greater and in alkaline 
 solutionslessthanthe[OH-]. If, therefore, either the hydrogen- 
 ion or the hydroxyl-ion concentration can be determined, the 
 true reaction of the fluid stated in terms of [H+] or [OH~] can 
 be accurately expressed. 
 
 The modern method consists in the determination of the hydro- 
 gen-ion concentration of the blood, making use of the hydrogen 
 electrode, and the concentration values so arrived at are usually 
 stated in terms of the negative exponent with the symbol P H , 
 the minus sign being omitted. P H 7'5 is therefore another form 
 of IX 10 -7s, and would denote a slightly alkaline fluid such as 
 blood, while P H 6 would signify a slightly acid fluid such as urine. 
 A rise in the P H value signifies an increase in alkalinity. In 
 order to gain an idea of the actual concentration of the hydrogen- 
 ions in blood, it is advisable to state it directly rather than in 
 logarithmic form; thus P H 7 - 35, which may be regarded as an 
 average value for blood, equals 0-446x10-7, or 0-0000000446 
 gramme hydrogen per litre of blood. The reaction of blood is 
 mainly dependent upon the ratio of carbonic acid to bicarbonate, 
 
 H CO 
 
 — One of the most important properties of blood is the 
 
 NaHC0 3 
 
 maintenance of a fairly constant reaction, although the exchange 
 between the tissues and the blood must result in the entrance 
 and removal of acid and basic radicles. The mechanisms which 
 contribute to the regulation of the reaction are extremely im- 
 portant. These are in part intrinsic to the blood itself, the so- 
 called ' buffer salts,' of which by far the most important is sodium 
 bicarbonate. These buffer salts may be regarded as soaking up 
 the added H+ or OH~-ions.* In addition, the respiratory 
 mechanism and the kidneys regulate the reaction, the former 
 responding to a rise in the hydrogen-ion concentration by an 
 increased pulmonary ventilation, and so removal of the carbonic 
 acid, the latter by an excretion of a more acid urine. Recently 
 much valuable work has been done on the bicarbonate or alkali 
 reserve store of the blood, and the effect of various conditions 
 upon it, such as shock, haemorrhage, diabetic coma, etc. 
 
 Excepting in the herbivora, considerable amounts of acid may 
 be administered by the mouth or formed in the tissues without 
 affecting the reaction of the blood, the acid being neutral- 
 ised by the ammonia split off from proteins. Proteins may, in 
 
 * The expression ' buffer salts ' is intended to indicate, the fact that they 
 prevent the shock to the reaction of the blood on the addition of acid or 
 alkali. Bayliss thinks the term ought to be avoided, and points out that 
 it originally arose through a mistranslation of the word ' tampon ' employed 
 by the French writer who first dealt with the question. He used it to indi- 
 cate that the sodium bicarbonate soaked up the excess of acid or alkali. 
 
THE BLOOD 3 
 
 fact, act in a dual capacity, either as bases neutralising acidity, or 
 as acids neutralising alkalinity. 
 
 The carbon dioxide of the blood is in the main united with 
 bases, especially soda, and the total carbonic acid content of a 
 sample of blood is a measure of its capacity not only for carrying 
 carbonic acid, but also for maintaining the normal reaction of the 
 blood. In the herbivora the carbonic acid content is readily re- 
 duced under the influence of acids administered by the mouth or 
 formed in the body, and the protective mechanism is thus reduced. 
 
 The recently-drawn blood of the cat and dog has a peculiar 
 and decidedly disagreeable smell ; this is not observed in the 
 blood of the horse and ox. The taste of blood is saltish, due to 
 the amount of sodium chloride it contains. 
 
 The specific gravity varies in different animals: in the horse, 
 ox, and pig, 1-060 ; in the sheep, 1-050 to 1-058; in the dog, 1-050 
 (Colin). According to Hoppe-Seyler, the specific gravity of the 
 liquor sanguinis of the horse is 1-027 to 1-028, and the specific 
 gravity of the cells 1-105. This considerable difference between 
 the specific gravity of, the cells and the liquor sanguinis in the 
 horse accounts for the rapid manner in which the cells sink in 
 horse's blood when drawn from the body, producing during the 
 process of clotting the so-called ' buffy coat.' 
 
 The viscosity of the blood is an important determining factor 
 regulating the blood-pressure and velocity. Thus the substitu- 
 tion of simple saline solutions for whole blood, such as would 
 occur in intravenous injection after haemorrhage, results only in 
 a very temporary rise in blood-pressure, while the addition of a 
 colloid — for example, gum arabic — to the saline not only raises 
 the pressure immediately after injection, but enables the pressure 
 to be maintained at the higher level. The osmotic pressure of the 
 colloid is also of great value, as, owing to the fact that the capil- 
 lary walls are practically impermeable to colloid, the filtration 
 or loss of water from the vessels is checked by the counter- 
 osmotic effect. 
 
 The viscosity of blood is four or five times greater than that of 
 water. It is rather higher in the dog and cat than in man. 
 
 Composition of the Blood.— Blood consists of a fluid portion 
 or plasma and of the blood cells or corpuscles suspended therein. 
 
 Plasma, or liquor sanguinis, is a yellow-coloured, somewhat 
 viscid, fluid, containing in solution — 
 
 Protein. 
 Extractives. 
 Mineral matter. 
 Enzymes. 
 
 Unknown substances [i.e., immune bodies). 
 (See p. 10.) 
 
4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The corpuscles are — 
 
 Red corpuscles, or erythrocytes. 
 White corpuscles, or leucocytes. 
 Platelets. 
 
 , So far as the presence of the various constituents is concerned, 
 the blood of any one animal presents a very uniform character, 
 but their proportions in the blood of different animals are liable to 
 great variation. The composition of the blood is also affected 
 according to the source from which it is derived; the blood from 
 an artery is not exactly like that from a vein. 
 
 Plasma forms about 66 per cent, of the blood; and consists of 
 three proteins, which are held in solution- — viz., fibrinogen, para- 
 globulin [serum- globulin), and serum-albumin. The two former 
 belong to the globulin group of proteins ; the latter to the albu- 
 minous group, of which egg-albumin is typical. 
 
 The three proteins may be separated by the employment of 
 certain neutral salts. Serum-albumin is precipitated by satura- 
 tion with ammonium sulphate. Its solution in a neutral or acid 
 medium is thrown down by heat. The temperatures of coagula- 
 tion, 70 to 75 C. (158 to 167° F.), would appear to indicate that 
 there are two or three different proteins classed together as serum- 
 albumin. 
 
 This substance is found not only in blood-plasma, but in lymph. 
 It also forms a part of such secretions as milk. Its source is 
 believed to be the protein substances of the food which are 
 taken up from the intestinal canal, though there is no experi- 
 mental proof of the correctness of this view. Serum-albumin 
 obtained from the blood of the horse may readily be made to 
 crystallise. /£'- 
 
 Paraglobulin, or serum-globulin, is distinguished by being 
 precipitated by saturation with magnesium sulphate, or half 
 saturation with ammonium sulphate. In neutral or faintly acid 
 solutions it is coagulated by heating to 75 C. (167 F.), and there 
 is reason to think from the behaviour under analysis of the para- 
 globulin obtained from serum that this substance is probably a 
 mixture of two or three allied proteins. It is supposed that the 
 source of paraglobulin is twofold — first the protein substances 
 of the food, and secondly the disintegration of the white cells 
 of the blood ; but in both cases proof is wanting. 
 
 Bayliss draws attention to the fact that the serum proteins 
 of the blood are remarkable for being chemically very inert 
 substances. They do not serve for the nutrition of the tissues, 
 but their chief function, apart from the important share they 
 take in the process of clotting, appears to be to give a viscosity 
 
THE BLOOD 5 
 
 to the blood, so that this shall not lose water to the tissues too 
 rapidly.* 
 
 Fibrinogen constitutes but a small proportion of the total 
 protein of plasma. It is precipitated from solution by half 
 saturation with sodium chloride. It belongs to the group of 
 proteins known as globulins, and is a substance of remarkable 
 interest, for on its conversion from a fluid to a solid condition 
 depends the phenomenon of the coagulation of the blood. Like 
 the other proteins in plasma, it coagulates on heating, but 
 at a much lower temperature, for fibrinogen is coagulated at 
 56° to 6o° C. (133 to 140 F.j. Blood so treated is no longer 
 capable of clotting, owing to its fibrinogen being coagulated. 
 The source of fibrinogen in the body is unknown ; it is supposed 
 to be connected with the destruction of leucocytes, and there is 
 also some evidence of the liver being concerned in its formation. 
 
 Perhaps the nearest approach to pure plasma is the fluid found 
 in the pericardial and abdominal cavities. That which is effused 
 into the chest during, an attack of pleurisy is plasma to start with, 
 but if it has undergone coagulation and fibrin (false membranes) 
 has been formed, then the resulting fluid is serum. 
 
 In the following table is shown the composition of the blood 
 plasma of different animals.- The figures are expressed as 
 grammes in 100 c.c. of blood. It will be observed that in the 
 horse the globulins exceed the albumins, while in the dog and 
 pig the reverse holds good. The poverty of dog's blood in total 
 proteins, as compared with that of .the horse and pig, is also 
 striking. 
 
 
 Total 
 Proteins. 
 
 Serum- 
 Albumin. 
 
 Para- 
 globulin. 
 
 1 
 
 Fibrino- 
 gen. 
 
 Dog - 
 
 Sheep - - - - 
 
 Horse - 
 
 Pig 
 
 603 
 
 7' 29 
 8-04 
 805 
 
 3' 17 
 383 
 
 2' 80 
 4-42 
 
 2 - 26 
 
 3'°° 
 4' 79 
 2-98 
 
 o'6o 
 o - 46 
 
 o'45 
 0-65. 
 
 So long as the blood is in circulation or prevented from clotting, 
 its fluid portion is termed * plasma ' ; but if blood be allowed to 
 coagulate, in course of time it separates into a solid clot and a 
 liquid portion, and this liquid is no longer known as plasma, but 
 as serum. Serum is therefore plasma which is modified as the 
 result of coagulation, and as this latter process is attended by 
 the production of fibrin, it may be said that serum is plasma 
 minus the fibrin-forming elements. 
 
 Serum does not contain fibrinogen, for the reason that the latter 
 * British Medical Journal, June 29, 191 8. 
 
6 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 has been used up in the process of clotting, but it possesses, in 
 addition to serum-albumin and paraglobulin, a body known as 
 fibrino- globulin, believed to be split off from fibrinogen in the 
 act of clotting, and nucleo-protein, supposed to be derived from 
 the fibrin ferment; some observers doubt the existence of this 
 latter substance in the serum. The total protein content of 
 serum is 8 or 9 per cent., and thus resembles the protein content 
 of plasma; the extractives and salts are the same in both fluids. 
 In the following table a comparison is made between the 
 proteins of plasma and serum : 
 
 Plasma. Serum. 
 
 Serum-albumin. Serum-albumin. 
 
 Paraglobulin. Paraglobulin. 
 
 Fibrinogen. Fibrino-globulin. 
 
 Nucleo-protein. 
 
 Corpuscles. — Blood examined under the microscope is found 
 to contain an enormous number of bodies termed ' corpuscles ' 
 floating in the liquor sanguinis. These corpuscles are of two 
 kinds, red and white; the former are the more numerous, the 
 latter are the larger. 
 
 The Red Corpuscles constitute 33 per cent., or one-third of the 
 total blood. Viewed under the microscope, they are found to 
 
 Fig. 1. — Diagram showing Relative Size of Red Corpuscles of 
 Various Animals (Stewart). 
 
 be biconcave discs, circular in shape, and possessing no nucleus 
 (Plate I.) ; they are soft, flexible, elastic bodies, capable of having 
 their shape readily altered by pressure, and this enables them to 
 pass along the finest capillaries. The colour of a single corpuscle 
 is yellow, but when heaped together they appear red, and thus 
 give the colour to the blood. 
 
 In all mammals, excepting the camel tribe, the red cells are 
 circular and biconcave; in the Camelidse they are elliptical and 
 biconvex. In all vertebrates below mammals they are bi- 
 convex, oval, and nucleated (Plate I.). The corpuscles vary in 
 size in different animals, being smallest in the deer tribe and 
 largest in the elephant, the relative sizes being shown in Fig. 1. 
 
THE BLOOD 7 
 
 When a drop of blood is shed, the red cells at first move quite 
 freely each over the other. In a short time they tend apparently 
 to become sticky, and when this state is reached they have a ten- 
 dency to lie in long rows, with their flat surfaces in close contact, 
 their appearance resembling that of a pile of pennies. This con- 
 dition is not marked in horse's blood. 
 
 A red blood-cell is composed of a spongy stroma, holding in its 
 meshes the red colouring matter. The stroma or framework of 
 the corpuscle consists of nucleo-albumin, lecithin, cholesterin, 
 and salts; the red colouring matter, haemoglobin, forms no less 
 than 32 per cent, of the total solid matter of the living corpuscle. 
 
 Great discussion has taken place as to whether the corpuscle is 
 a perforated mass of protoplasm containing no covering, or 
 whether it possesses a cell-wall, as its microscopical appearance 
 indicates. At present the general feeling is that there is no 
 cell- wall, but that there is a condensation "of the cell-substance 
 at the periphery, while within the spongy substance of the 
 interior is lodged the haemoglobin, probably in an amorphous 
 condition, certainly not in a crystalline state, and perhaps not 
 in solution. It is further supposed that the large amount of 
 lecithin and cholesterin present in the stroma determines the 
 nature and degree of the permeability of the external layer of the 
 cell. For example, water, alcohol, ether, and a solution of urea 
 can pass in, but not neutral salts. 
 
 The number of corpuscles in the blood is determined approxi- 
 mately by the method either of Gowers or Malassez. The 
 principle on which these methods are based is the same— a known 
 quantity of blood is diluted with a known bulk of artificial serum 
 and thoroughly mixed; of this a small drop is placed in a count- 
 ing-chamber of known capacity, which is ruled into squares, and 
 examined under the microscope. The blood-cells occupying the 
 squares are counted, as may readily be done, and the mean of 
 them taken. In the horse the mean number of red blood- 
 corpuscles per .cubic millimetre is 7,212,506, and in the ox 
 5,073,000. Taking the amount of blood in the horse as 29 litres 
 (50 pints, or 66 pounds), this gives 204,113,750,000,000 as the 
 approximate number of red cells in the body (Ellenberger).* It 
 is evident that a loss of water from the blood means a larger 
 relative proportion of red cells present, while an excess of water, 
 by diluting the blood, would show a loss of red cells; thus the 
 number of the red cells is increased by sweating, by the excretion 
 of water from the bowels and kidneys, and by starvation, while 
 it is diminished by copious draughts of water, also by pregnancy. 
 But apart from these conditions, it is undoubted that an actual 
 increase or decrease in the number of red cells may occur, this 
 * ' Physio logie der Haussaugethiere.' 
 
8 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 numerical variation being especially marked in some diseases. 
 The shape of the red cell is affected by the amount of fluid in the 
 plasma; if the latter be artificially concentrated, water diffuses 
 from the corpuscle to the plasma, and in consequence the cor- 
 puscle itself shrinks and becomes wrinkled (Plate I.). If the 
 plasma be diluted, the red cells swell. A 0-9 per cent, solution of 
 sodium chloride causes the corpuscles neither to shrink nor swell; 
 this strength is known as ' physiological salt solution ' (an isotonic 
 solution), and may be employed for the purpose of transfusion. 
 
 Each red cell offers a certain absorbing surface for oxygen, 
 which, if calculated on the total number of corpuscles, is some- 
 thing enormous, being equal for the horse to a square having a 
 side of 164 metres (180 yards). The opacity of blood is due to 
 the red cells reflecting light as the result of their peculiar shape; 
 if the cells be destroyed, the blood becomes transparent or, as 
 it is termed, ' laky,' of which more will presently be said. 
 
 The greater part of the red cell consists, as already stated, of 
 haemoglobin, a substance possessing a remarkable affinity for 
 oxygen; this it obtains at the lungs, and leaves behind in the 
 tissues. The haemoglobin of the red cells, therefore, exists in 
 two states, one in which it is charged with oxygen called ' oxy- 
 haemoglobin,' and the other in which it has lost its oxygen and 
 is known as ' reduced haemoglobin,' or simply haemoglobin. The 
 process of oxidation in the lungs and reduction in the tissues is 
 constantly occurring at every cycle of the circulation, with the 
 ultimate result that the red blood-disc, the life of which is prob- 
 ably only a matter of a few days, gets worn out and dies. In this 
 condition it is cast off from the system, being got rid of through 
 the medium of the liver. In addition to the blood-stream, other 
 seats of destruction are suggested — viz., the spleen, bone- 
 marrow, and lymph-glands — but no definite statement can at 
 present be made. When the red cells die, their haemoglobin 
 is set free, and decomposed into an iron-free residue from which, 
 probably, all the pigments of the body are formed, certainly those 
 of the bile. 
 
 The seat of formation of the red cells is in the red marrow of 
 bones, where they are formed from certain nucleated colourless 
 cells; there are several varieties of blood-forming cells (erythro- 
 blasts) in the red marrow, and it is not definitely settled which 
 of these furnish the red blood-cells. All other seats of formation 
 are doubtful. In the embryo the future red cells for a certain 
 period are nucleated and contain no haemoglobin, but. these are 
 gradually replaced by non-nucleated, haemoglobin-holding cor- 
 puscles before birth. It is interesting to observe that both in 
 the embryo and in the adult the red cells are derived from a 
 nucleated precursor. 
 
THE BLOOD y 
 
 , By the time the corpuscle takes its place in the blood as a cell 
 which has lost its nucleus it is on the downward path. This and 
 other considerations have caused its probable life in the blood- 
 stream to be estimated at only a few days. 
 
 Haemolysis. — It has been pointed out that in a normal con- 
 dition the haemoglobin is contained wholly within the red cells, 
 and that there is no passage of colouring matter from the cells to 
 the fluid in which they are carried. Anything which kills the 
 red cell, or, if we adopt' the cell-wall view, anything which breaks 
 down the envelope, allows the haemoglobin to escape. 
 
 The cells may be destroyed by alternately freezing and thawing 
 the blood, or by the passage through it of electric shocks, or by 
 the addition of certain agents such as chloroform, ether, bile salts, 
 tannic or boric acids, etc. The haemoglobin becomes liberated 
 from the broken-up cells, and stains the naturaUy yellow plasma 
 a red colour. The blood under these circumstances, as we have 
 seen, is no longer opaque, but transparent, and the term ' laky ' 
 well describes its appearance. The entire process is described as 
 ' haemolysis.' 
 
 Most of the above causes of haemolysis act as protoplasmic 
 poisons; they kill the cell, and as the osmotic pressure of the 
 plasma is slightly less than that of the corpuscular contents, the 
 haemoglobin diffuses out. Poisons such as ether and chloroform 
 are probably haemolytic owing to their chemical effect in dissolv- 
 ing the cholesterin and lecithin of the corpuscles. Other poisons 
 mayunite with these substances and rendef them soluble, by which 
 means haemoglobin escapes from the cell. Snake venom and the 
 poison of bees and of certain spiders produce haemolysis, so also 
 some pathological toxins, of which the most noteworthy in the 
 horse is that producing so-called azoturia. The most remarkable 
 example of haemolysis is that produced by adding to, or injecting 
 into, the blood of one animal the serum of an animal of a different 
 species . This leads in certain cases to destruction of the red cells, 
 and a chain of results of the highest practical importance . If, for 
 example, the fresh serum of the blood of the dog be added to the 
 washed red corpuscles of the rabbit, the latter are destroyed and 
 the colouring matter liberated. If the Serum of the dog be 
 previously heated to 55 C. (132" F.), it may be added in any 
 quantity to the washed blood-cells of the rabbit, without pro- 
 ducing any effect on them; evidently something has been 
 destroyed by the process of raising the temperature. This some- 
 thing can be restored by adding to the heated dog's serum some 
 serum which is not haemolytic for rabbit's cells (say rabbit serum). 
 The effect now is to render the dog's serum once more haemolytic. 
 
 So far the phenomenon is physiological. The serum of any animal, 
 however, can be rendered haemolytic for the corpuscles of another 
 
io A MANUAL OF VETERINARY PHYSIOLOGY 
 
 species by injecting the first with the red corpuscles of the second. 
 If, for example, the corpuscles of species A, say an ox, be injected 
 into species B, say a goat, the serum of the goat becomes actively 
 haemolytic for the corpuscles of the ox, and will cause hsemoglobm- 
 uria and death when inoculated to the latter animal. The goat's 
 serum will also haemolyse the corpuscles of the ox in vitro. If it be 
 heated to 55° C, however, it loses its haemolytic action, which can be 
 restored to it in the way mentioned above. The explanation is 
 that there are two substances in the haemolytic serum concerned in 
 the production of haemolysis. One is relatively unstable, and is 
 destroyed by a temperature of 55° C. This is the destroying agent, 
 which is known as the complement. The other substance is spoken 
 of as the immune body, the antibody, the amboceptor, or substance 
 sensibilisatrice. It is not destroyed by a temperature below 65 ° C. 
 When a hemolytic serum is heated to 55° C. for half an hour, its 
 haemolytic action is lost, because the complement has been destroyed. 
 It can be restored by adding fresh, though non-haemolytic, serum 
 from another animal — a guinea-pig, for example — because the 
 complement is not specific, and is the same in all animals. If a 
 haemolytic serum be heated before injection into the animal, it still 
 causes haemolysis, because the complement is supplied by the 
 animal's serum. The immune body, on the other hand, is specific 
 for the corpuscles of the species against which it has been prepared, 
 and for those of no other. These phenomena are closely related 
 to the reaction of the body towards bacteria, and are concerned in 
 the production of antitoxins. 
 
 Agglutination is the process by which the red blood-corpuscles 
 are collected together in clumps, under the influence of an agent 
 in the blood known as an agglutinin. Agglutination frequently 
 precedes haemolysis, but it is independent of it, for if the comple- 
 ment of a haemolytic serum be destroyed by heating to 55 ° C, 
 the agglutinating substance remains, being relatively unaffected 
 by heat. 
 
 If an animal, say a rabbit, be injected with the blood-cells of 
 the dog, the serum of the rabbit, which normally has no effect on 
 the blood-cells of this animal, becomes powerfully haemolytic for 
 dog's corpuscles, and, further, it agglutinates for the corpuscles of 
 the dog. An agglutinin has, therefore, been experimentally p v 
 duced in the serum of the rabbit. The phenomenon of agglutina- 
 tion is employed in bacteriology as an important aid to diagnosis. 
 
 Precipitins. — If the serum of one animal be injected into an 
 animal of a different species, it is found that the serum of the 
 receiver is capable of causing a precipitate in the normal serum 
 of the donor; for example, if a rabbit be injected with the serum 
 of a dog, the rabbit serum will in course of time produce a pre- 
 cipitate if added to dog's serum, but not if added to the serum of 
 any other animal. Such substances are known as precipitins, 
 and are employed for the purpose of identifying different bloods 
 and for other purposes. They may, for example, be used for the 
 determination of the flesh of different animals — horse-flesh, for 
 
THE BLOOD 
 
 instance, when sold as beef; for if a rabbit be injected with an 
 extract of horse-flesh, its serum will produce a precipitate with 
 extracts of horse-flesh, but not of ox-flesh. 
 
 Blood Platelets. — These may be seen in the circulating blood, 
 but more easily in blood which has been shed — certain small 
 colourless cells one-quarter the size of a red corpuscle, and usually 
 of a round or oval shape. In shed blood they agglutinate and 
 rapidly disintegrate, but under suitable conditions they may be 
 kept alive, when they exhibit amoeboid movements. At one 
 time they were regarded as disintegration products of the red cells, 
 but this view is no longer held, and it is probable they are dis- 
 tinct cellular elements. Of their function little or nothing is 
 known, but that they play an important part in blood-clotting 
 is undoubted. 
 
 Haemoglobin is the red colouring matter of the blood, and is 
 remarkable for being one of the most complex substances in 
 organic chemistry. It contains the elements C, H, 0, N, S, and 
 Fe. The molecule of haemoglobin is probably the largest of any 
 known substance which is capable of being crystallised. If, as 
 is most usually assumed, its molecule contains one atom of iron, 
 then, on this assumption, and from a knowledge of its percentage 
 composition, the molecular formula for the haemoglobin of dog's 
 blood may be represented as C 758 H 1203 N 196 S 2 FeO 218 , which is 
 some 16,000 times that of a molecule of hydrogen. It has been 
 supposed that the size of the haemoglobin molecule is connected 
 with the heavy atom of iron which it has to carry. The function 
 of the iron is closely connected with the power the pigment has of 
 combining with oxygen. This compound of oxygen-haemoglobin 
 constitutes a chemical system peculiar to itself, for there is no 
 other like it known. 
 
 Haemoglobin is a protein, but is distinguished from the majority 
 of the other members of its class by the comparative ease with 
 which it may be obtained in a crystalline form; while, on the 
 oilier hand, its behaviour in a dialyser is not that of a crystalloid, 
 but that of a colloid. . 
 
 Haemoglobin, under the influence of heat, acids, or alkalies, is 
 broken up in the presence of oxygen into a simple protein globin, 
 and a pigment, hamatin. It is therefore a compound of a pro- 
 tein body with haematin ; the protein portion of the molecule repre- 
 sents the greater part, the pigment being only 4 per cent, of the 
 total In the protein portion is found aU the sulphur, and in the 
 pigment all the iron of the molecule. The pigment haematin 
 exists in the living blood as haemoglobin, the great difference 
 between these two substances being that haematin forms a 
 stable, while haemoglobin forms a feeble combination with 
 oxygen. 
 
12 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The union of oxygen with haemoglobin gives a true chemical 
 compound, i gramme (15$ grains) of haemoglobin uniting with 
 1-34 c.c. (0-08 cub. in.)of the gas, and forming oxyhaemoglobin. 
 This is not a stable compound ; it readily gives off its oxygen either 
 in the presence of oxidisable substances, or in an atmosphere free 
 from oxygen, and by so doing becomes reduced haemoglobin. 
 Other conditions which cause haemoglobin to readily part with its 
 oxygen are a relatively high temperature, such as that of the 
 blood, and the presence of salts and acids, especially carbon 
 dioxide. It is oxyhsemoglobin which gives the bright colour to 
 arterial blood, and it is the presence of partly reduced haemoglobin 
 which gives the darker tint to venous blood. The change in 
 colour which venous blood undergoes on exposure to the air is 
 due to the absorption of oxygen by haemoglobin. By a similar 
 absorption the blood is charged with oxygen in the lungs, brought 
 back to the heart, and distributed all over the body to the tissues, 
 where it gives up the bulk of its oxygen, and as partially reduced 
 haemoglobin is brought back by the veins to the heart for dis- 
 tribution to the lungs, where it renews its oxidised condition. 
 Excepting in the latest stages of asphyxia, haemoglobin is never 
 completely reduced in the body. 
 
 Oxygen is not the only gas with which haemoglobin is capable 
 of forming a chemical compound. Carbon monoxide unites with 
 it more readily than oxygen, forming the definite compound 
 carbonic oxide haemoglobin. In great contrast to oxyhemo- 
 globin, this is a remarkably stable compound, the carbon mon- 
 oxide holding so tenaciously to haemoglobin that oxygen cannot 
 displace it. This explains the highly poisonous nature of this 
 gas. The spectrum of CO haemoglobin closely resembles that of 
 oxyhsemoglobin (see Fig. 2). A still more poisonous compound 
 is nitric oxide haemoglobin, since this gas is attached to the 
 haemoglobin even more tenaciously than carbon monoxide. 
 
 Haemoglobin also forms an important compound with carbon 
 dioxide. This compound may be formed even when the haemo- 
 globin is already nearly saturated with oxygen ; the explanation 
 which has been offered is that, while the oxygen is united to the 
 pigment portion of the molecule, the carbon dioxide is united to 
 the protein portion. Buckmaster has shown that haemoglobin, 
 can unite with large quantities of carbon dioxide and give it off 
 again at a lower pressure. 
 
 When examined spectroscopically, oxy- and reduced haemo- 
 globin produce quite distinctive spectra, by which they may be 
 readily recognised. To state the matter broadly, oxyhsemoglobin 
 gives two well-marked dark absorption bands or shadows in the 
 green portion of the spectrum, one band being wide, the other 
 narrow, while reduced haemoglobin gives one wide single band in 
 
THE BLOOD 
 
 13 
 
 nearly the same position (Fig. 2). The change from oxy- 
 hemoglobin to reduced haemoglobin may readily be brought 
 about in spectroscopic examination by the addition of an alkaline 
 solution of ferrous tartrate (Stokes's fluid) to the blood. 
 
 Crystals of haemoglobin, when seen in bulk, are of a dark red or 
 bluish-red colour ; they are extremely soluble in water, the solu- 
 tion being dichroic — viz., green by reflected and bluish-red by 
 transmitted light. The blood of the horse, cat, dog, and guinea- 
 
 Oxyhamoglobin 
 
 Reduced haemo- 
 globin 
 
 IlilHIfl Carbonic oxide 
 11 ' ' hemoglobin 
 
 Methsemoglobia (in 
 acid solution) 
 
 Acid haematin (in 
 ethereal solution) 
 
 Alkaline haematin 
 
 Haemochromogen 
 
 Hsematoporphyrin 
 (in acid solution) 
 
 Haemato porphyrin 
 (in alkaline solu- 
 tion) 
 
 b c d £ * F 
 
 Fig. 2.— Table of Spectra or H*mom,sbuj and its Derivatives 
 (Stewart). 
 
 pig readily yields crystals of oxyhemoglobin; that of the ox, 
 sheep, and pig crystallises with difficulty The crystals are 
 generally rhombic prisms or needles, but the form differs accord- 
 ing to the animal (Fig. 3) ■ Reduced hemoglobin can be crystal- 
 lised only with great difficulty, and in an atmosphere free from 
 
 ° X The n total amount of hemoglobin in a horse's body is about 
 4 kilogrammes (8-8 pounds), and the amount of iron contained 
 in this is about 17 grammes (257 grams). This calculation is 
 
M 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 based on the assumption that the amount of blood in the body 
 
 is 29 litres (50 pints). 
 
 In dried red blood-cells haemoglobin exists in the proportion of 
 
 90 to 94 per cent., in the corpuscle under normal conditions it 
 
 represents 32 per cent, of 
 its weight ; in the total 
 blood of the horse it forms 
 13-15 per cent., in the ox 
 9-96 per cent., in the sheep 
 10-34 per cent., in the pig 
 12-7 per cent., and in the 
 dog 9-77 per cent. (Ellen- 
 berger) . * The younger the 
 animal the less haemo- 
 globin ; males have more 
 than females, and castrated 
 animals more than entires 
 (G. Muller).t 
 
 The functions of haemo- 
 globin cannot be studied 
 until the subject of respi- 
 ration is dealt with. 
 
 Methaemoglobin is a 
 derivative of haemoglobin, 
 and may be produced by 
 allowing blood to be ex- 
 posed to the air until it 
 becomes brown in colour; 
 or it may be prepared by 
 the action of ferricyanide 
 of potassium or nitrite of 
 
 u, b, From man; c, from cat; d, from guinea- amyl, etc., On OXyhaemo- 
 pig; e, from hamster; /.from squirrel (Frey). globin. This Substance 
 
 ' contains the same amount 
 of oxygen as haemoglobin, but it will not part with it, excepting 
 in the presence of reducing agents ; for respiratory purposes it is 
 therefore useless. It is not a normal constituent of the blood, 
 but may be found in the urine whenever a sudden breaking- 
 down of red corpuscles occurs, as, for example, in the so-called 
 azoturia of the horse. Its spectrum is seen in Fig. 2. 
 
 Hsematin, as one of the decomposition products of haemoglobin, 
 has been previously referred to. It wiU be remembered that it 
 is obtained by decomposing haemoglobin by boiling, or the 
 addition of alkalies, acids, or acid salts; in either case the haemo- 
 globin splits up into a substance containing the iron, known as 
 * ' Physiologic der Haussaugethiere.' \ Ibid. 
 
 Fig. 3. — Oxyhemoglobin Crystals. 
 
THE BLOOD 15 
 
 haematin, and a protein substance or substances termed globin. 
 Haematin in the dry state strongly resembles iodine in appear- 
 ance ; it has a metallic lustre, a blue-black colour, is not crystal- 
 lisable, and yields, when pulverised, a dark brown powder which 
 contains8-82 per cent, of iron. It is a remarkably stable substance 
 and the colouring matter presents a distinctive spectrum both 
 in an acid and in an alkaline solution (see Fig. 2) . Alkaline solu- 
 tions of haematin can take up and give off oxygen as does haemo- 
 globin. When haematin is treated with glacial acetic acid and 
 common salt, it yields haemin or haematin hydrochloride, which, 
 when examined microscopically, is found to consist of prismatic 
 crystals, dark or nearly black in colour (see Fig. 4). Haemin 
 crystals may be readily produced by warming the dried blood 
 with a drop of glacial acetic acid on a slide ; this fact is used'as a 
 microscopical test. 
 
 ■ni 
 
 Fig. 4.— Crystals of H^min Fig. 5.— Crystals of 
 
 (Frey). H^kmatoidin (Stewart). 
 
 When reduced haemoglobin is decomposed'by acids or alkalies, 
 oxygen being carefully excluded, it yields haemochromogen, a 
 substance presenting a definite spectrum; this test is employed 
 in the detection of old blood-stains (see Fig. 2). Haemato- 
 porphyrin is obtained by the action of strong sulphuric acid on 
 haematin, which thereby loses its iron; hsematoporphynn is 
 really haematin from which the iron has been removed; it is iso- 
 meric with bilirubin. The spectrum of this substance in acid 
 and alkaline solutions may be seen in Fig. 2. 
 
 Hydrobilirubinis obtained by the action of reducing agents on 
 haematin; it very closely resembles urobilin, a pigment found in 
 
 Haematoidin (Fig. 5) is found in old blood-clots and in the 
 ovary ■ it is a crystaUine iron-free product derived from haematin, 
 and gives the same reaction with nitrous acid as bile pigment-- 
 viz a play of colours. Haematoidin is, in fact, chemicaUy 
 identical with bilirubin, and the name is now of interest merely 
 as indicating the close genetic relationship of the pigments of bile 
 
16 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 to the colouring matter of blood. Notwithstanding this close 
 relationship, it has not as yet been found possible to convert 
 hsematin into bilirubin. The nearest approach to bilirubin is 
 iron-free haematin (haematoporphyrin) . Again, both haematin 
 and bilirubin may be made to yield an identical product (hydro- 
 bilirubin) ; this product closely resembles urobilin, a pigment found 
 in the urine, and urobilin beyond all doubt is derived from bili- 
 rubin in the digestive canal, under the influence of putrefactive 
 organisms. 
 
 White Corpuscles, or Leucocytes.— There are certain cor- 
 puscles found in the blood, lymph, connective tissue, and in 
 pathological products such as pus, which possess a great family 
 resemblance. In the blood they are known as ' white corpuscles,' 
 and it seems quite certain that between the blood and the tissues 
 a free interchange of corpuscular elements occurs ; this process is 
 
 Fig. 6. — Amceboid Movement (Stewart). 
 A, B, C, D, Successive changes in the form of an amoeba. 
 
 exaggerated under pathological conditions, as in the case of in- 
 flammation and suppuration. 
 
 One characteristic of these cells is their power of spontaneous 
 movement — ' amoeboid,' as it is termed. In Fig. 6 successive 
 changes in the form of an amoeba are shown, and very similar 
 changes occur in these cells. These changes in shape assist 
 materially in the passage of the corpuscle through the walls of 
 the vessels into the tissues. In consequence of their migratory 
 habits these cells are frequently referred to as wandering cells. 
 The white cells of the blood are so called in contradistinction to 
 the coloured or red cells, compared with which they exist in the 
 proportion of i to 300-700, depending on the source of the blood. 
 The white cells at the present time are more frequently referred 
 to as blood-leucocytes. They are not all of one kind; there are 
 microscopical differences in their structure which has enabled 
 a classification to be made, and though this is by no means final, 
 particularly in view of the limited knowledge possessed regarding 
 
THE BLOOD 17 
 
 these cells, yet it is useful as a means of identification and descrip- 
 tion. Ehrlich regards the white corpuscles as divisible into two 
 main groups— lymphocytes and leucocytes. The former are dis- 
 tinguished by the cell-body being free from granules, while amoe- 
 boid movement may be absent. The opposite condition exists in 
 leucocytes. 
 
 Two varieties of Lymphocytes are found in the blood — 
 small and large. The former, the size of a red cell, is represented 
 by about one-quarter of all the white corpuscles ; the latter, two 
 or three times larger in size, do not represent more than 1 per 
 cent, of the whole number of white corpuscles. 
 
 Three varieties of Leucocytes are described — polynuclear, or 
 polymorphonuclear, which represent the bulk of the white blood- 
 cells, with a sub-group known as eosinophiles ; uninuclear occur- 
 ring to the extent of from 2 to 10 per cent, of the white cells; 
 and mast-cells, which represent less than 1 per cent. Not only 
 the nature of the nucleus, but also the reaction which the cell 
 granules give with dyes, enables these groups to be distin- 
 guished from one another. The granules of the uninuclear cell 
 readily stain, with neutral dyes; the polynuclear ■ cells stain 
 with neutral or acid dyes, while the mast-cells" demand basic 
 dyes. Some leucocytes and lymphocytes may be seen in 
 Plate I., 2. 
 
 The origin of the lymphocytes and leucocytes has been 
 variously attributed to lymphatic tissue and bone-marrow, but 
 very little is at present known of the subject. There is a free 
 communication between the lymphatic system and the blood via 
 the thoracic duct, and though a mixture of material gains access 
 to the blood by this route, there is no doubt that much of it is 
 pure lymph. 
 
 The white corpuscles contain about 10 per cent, of solids. The 
 cell protoplasm consists of proteins belonging to the globulin 
 and nucleo-protein groups, while the nucleus consists of nuclein, 
 which is remarkable as being a very stable substance, and also 
 as containing phosphorus. The effect of nucleo-protein when 
 injected into the circulation will be dealt with at p. 23. 
 
 The white corpuscles, as well as the red, are constantly being 
 used up and as constantly replaced. They also possess, as we 
 have seen, the power of passing through the walls of the vessels 
 into the surrounding tissues, from which they are removed by 
 the lymph-channels, and so find their way back to the blood. 
 No doubt many corpuscles leave the blood, for whose destruction 
 we are unable to account, but it is suggested that by their death 
 they influence the composition of the blood-plasma, as in this 
 fluid their component parts must become dissolved after their 
 death. 
 
18 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 During the life of the white corpuscle great activity prevails; 
 it is constantly giving up and taking in material which must 
 affect the composition of the plasma. It is known that the 
 white cell possesses the power of digesting certain substances, 
 both solid and liquid. The researches of Metschnikoff have 
 paved the way towards a better understanding of the probable 
 manner in which protection against certain diseases is obtained. 
 He has shown that the white cells take up the bacteria into their 
 interior and digest them, a process termed phagocytosis ; it is 
 really a fight between bacteria and leucocytes. The difference 
 in the resisting power to disease possessed by ' fit ' over ' unfit ' 
 animals, and the greater protection afforded by maturity as 
 compared with youth, are facts which may be directly connected 
 with the question of phagocytosis. 
 
 The polynuclear cell above described appears to be the leuco- 
 cyte best adapted to ingesting bacteria, but all are capable of 
 doing good work, and the thoroughness with which it is done 
 depends upon the composition of the plasma. It appears 
 essential that this should contain a substance which acts upon 
 the bacteria, and renders them an easy prey to the leucocyte. 
 This substance, known as an opsonin, may be conveyed to the 
 plasma by the leucocytes, and it may also be artificially increased 
 by the injection into the body of suitable bacteria or of products 
 obtained from them. 
 
 It is also probable, as distinct from the doctrine of phago- 
 cytosis, that the white cells of the blood may be closely con- 
 cerned in the production of certain protective substances which 
 destroy bacteria, bacteriolysins, the existence of which helps to 
 explain the theory of immunity. 
 
 Coagulation. — We are now brought to a consideration of the 
 subject of blood-clotting, a process by which the naturally fluid 
 blood becomes converted into a solid. 
 
 If blood be drawn from the body and left at rest, it will be 
 found within a few minutes to have undergone the process of 
 clotting. The fluid first becomes a jelly and then a firm clot or 
 crassariientum, taking a complete cast of the vessel in which 
 it is placed, and so firm in consistence that it may be inverted 
 without any blood being lost. In a short time the clot begins 
 to contract, and by so doing squeezes out a fluid known as serum 
 (Fig- 7)- This gradually accumulates, and as it becomes abun- 
 dant the clot sinks. The blood of the horse is remarkable for 
 the slow rate at which coagulation occurs, and the red cells, 
 being specifically heavier than the plasma, have time to fall in 
 the fluid before the process is completed. The result of this is 
 that the upper solid layer is considerably decolourised, forming 
 the so-called buffy coat, which, though natural to the blood of 
 
THE BLOOD 
 
 19 
 
 the horse, is indicative in other animals of the presence of an 
 inflammatory process in the system. 
 
 We have here closely followed the account given by human 
 physiologists of the coagulation of the blood in the horse, but 
 the appearance described is by no means invariable. Coagulation 
 in this animal is often complete in 
 less than five minutes, when, of 
 course, no buffy coat forms, and we 
 are inclined to believe that rapid 
 coagulation and non-buffy coat are 
 the rule rather than the exception; 
 we have repeatedly observed in the 
 case of the horse the formation of 
 the clot to be so rapid as to seem 
 almost instantaneous. One fact in 
 connection with horse's blood is 
 undoubted, and that is that coagu- 
 lation is more easily slowed or pre- 
 I vented by cold and by neutral salts 
 than it is in the blood of any other 
 warm-blooded animal. May it not 
 be that some confusion has thus arisen, and we have come to 
 regard this abnormally easy slowing of clotting by cold and 
 salts as if it were markedly a characteristic of horse's blood, as 
 it clots naturally ? 
 
 According to Nasse, the average time occupied in coagulation 
 is as follows : 
 
 Fig. 7. — Diagram of Clot 
 with Buffy Coat (Stewart). 
 v, Lower portion of clot with 
 red corpuscles; w, white cor- 
 puscles in upper layer of clot ; 
 c, cupped upper surface of 
 clot ; s, serum. 
 
 Pig 
 
 Sheep 
 
 Dog 
 
 Ox 
 
 Horse 
 
 J to 1 J minutes. 
 
 i „ ii „ 
 1 „ 3 
 5 .. J 3 
 5 -. 13 
 
 In our experience the extreme time mentioned for the horse is 
 exceptionally long. 
 
 If the clot be examined microscopically, it is found to consist 
 of fine fibrils, entangled in which are the blood- corpuscles; if the 
 fibrin produced be washed completely free from blood, its appear- 
 ance is well described by its name. 
 
 If the blood, instead of being allowed to clot spontaneously, 
 be whipped with a rod or bunch of twigs, or, as we say, ' de-' 
 fibrinated,' the fibrin separates rapidly and coats the rod, 
 while no coagulation in the remaining fluid can occur. The 
 power of spontaneous clotting lies, then, in the production of 
 fibrin. 
 
 These changes may be graphically represented thus : 
 
20 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 On Clotting. 
 
 Blood. 
 
 ,,, / Serum. 
 
 Plasma. | Fibrin . 
 
 (IS*. }<**■ 
 
 Corpuscles. 
 
 I Blood platelets 
 
 When Whipped. 
 
 f^sma. {g^ , 
 
 Blood. { fRed. iDefibrinated 
 
 Corpuscles. \ WMte. f blood. 
 
 1 I Blood platelets.; 
 
 Fibrin is a yellowish-white, stringy-looking, bulky mass. Its 
 bulky appearance would lead to the belief that it exists in blood 
 in large quantities; it is found, however, to be by weight rela- 
 tively small (0-2 to 0-4 per cent.). 
 
 The cause of coagulation has kept physiologists busy for 
 many years, and even at the present time the matter has by no 
 means been settled. The theory most generally accepted is that 
 of Hammarsten — viz., that clotting is due to the conversion of a 
 fluid fibrinogen into a solid fibrin, under the influence of a sub- 
 stance spoken of as fibrin ferment, which is described below. 
 
 If blood be prevented from coagulating, plasma can be obtained, 
 and this plasma, depending upon the agents used in its pro- 
 duction, will teach the main facts of coagulation. If it be 
 obtained by cooling the blood, then the plasma will clot spon- 
 taneously if we allow the temperature to rise; if the plasma be 
 obtained by previously mixing the blood with a definite amount 
 of magnesium Sulphate or common salt, clotting will occur on 
 diluting it. If it be obtained by acting on blood with oxalates, 
 then clotting can be brought about by the addition of a lime salt. 
 The clot formed by the plasma coagulating is precisely the same 
 as that formed by the blood coagulating ; it is, of course, colourless. 
 
 If the above plasmas be acted upon by adding common salt to 
 half saturation, a precipitate of fibrinogenoccurs; this is a protein 
 belonging to the globulin group, and has previously been alluded 
 to. If this precipitate be redissolved by diluting the fluid, and 
 allowed to stand, it clots spontaneously. If a solution of pure 
 fibrinogen be prepared, it does not clot spontaneously, but it may 
 be made to do so by the addition of a drop of serum or the 
 washings of a blood-clot. 
 
 The substance which brings about coagulation of the blood 
 is contained in the plasma ; it is not found in the serum, as shown 
 by the fact that the latter is incapable of spontaneous coagula- 
 tion. The difference between plasma and serum, as we have 
 
THE BLOOD 
 
 already seen, is that the former contains fibrinogen and the v 
 
 latter does not. 
 
 Nor is clotting a function of the red cells, for lymph is capable 
 
 of clotting, and there are no red cells in it; and, further, there is , , 
 
 an abundance of red cells in defibrinated blood, yet clotting is 
 
 impossible. 
 
 We have previously learnt that in the plasma both fibrinogen 
 
 and serum-albumin are present; the proof that serum-albumin 
 
 takes no share in the process of clotting is that, though serum 
 contains an abundance of serum-albumin, yet spontaneous coagu- 
 lation is impossible, nor can it be made to clot by the addition of 
 such bodies as lead to clotting in a fibrinogen solution. 
 
 Everything points to the fluid fibrinogen of the plasma being 
 converted into the solid substance fibrin, and in whatever other 
 particulars physiologists differ regarding the question of coagula- 
 tion, all are agreed on this fundamental fact . In the observations 
 above described it has been shown that a solution of pure fibrino- 
 gen does not clot spontaneously, but that it at once coagulates 
 on the addition of a drop of serum or washed blood-clot. 
 
 From this fact we learn that the conversion of fluid fibrinogen 
 into solid fibrin cannot occur without the agency of another 
 substance . The fibrinogen will not coagulate of itself ; it requires 
 to be rendered active by something contained in a drop of serum 
 or a washed blood-clot. This substance is generally spoken of | 
 as fibrin ferment; its present-day name is thrombin. The term \ 
 ' ferment ' was employed since in some respects the substance 
 resembled the class of bodies known as ' ferments,' inasmuch 
 as a very small amount appeared to be capable of acting on 
 an indefinite amount of fibrinogen; it further resembled a fer- 
 ment in its action, being closely dependent on temperature. 
 It is now, however, more generally believed that a small amount 
 of the ferment will not act upon an indefinite amount of fibrino- 
 gen, the amount of fibrin formed being proportional to the amount 
 
 of ferment present. ,,,.,. , x 
 
 It was stated above that a drop of serum added to a solution 
 of fibrinogen at once causes clotting. Evidently, then, blood- 
 serum contains in abundance the substance known as ' fibrin 
 ferment ' or ' thrombin.' If the serum be boiled or heated to 
 6s° C (150 F ) a drop of it added to a solution of fibrinogen 
 does not cause clotting; but this function can be restored by the 
 addition to it of an alkali. This observation should largely, if 
 not entirely, exclude thrombin from being classed as a ferment. 
 A drop of stale serum will not produce clotting with fibrinogen. 
 The reason why stale serum refuses to act is unknown. 
 
 Thrombin may readily be prepared by extracting a dried blood- 
 clot with water. The addition of a few drops of this extract to 
 
22 a MANUAL OF VETERINARY PHYSIOLOGY 
 
 hydrocele or pericardial fluid at once causes coagulation, for these 
 fluids contain fibrinogen, but no ferment. Thrombin is a colloid, 
 and its resistance to putrefaction suggests it is not a protein 
 substance. According to some observers, thrombin is present in 
 the circulating blood, but if blood be received with special precau- 
 tions directly from an artery -into a large bulk of alcohol, the clot, 
 when dried and extracted as above, yields no thrombin. It 
 seems reasonable to infer that as the blood does not coagulate 
 in the vessels during life, there is no thrombin present. Neverthe- 
 less, if solutions of thrombin are injected into the circulation they 
 do not produ ce intravascular clotting with the certainty that might 
 be expected, and it has been suggested that in such cases the 
 thrombin has been destroyed in the liver, but the evidence of this 
 is not convincing. We shall look at the question again. 
 
 If thrombin does not exist as such in the blood-stream, to what 
 is its origin to be attributed ? Blood received direct from the 
 vessels into a solution of oxalate of soda will remain uncoagu- 
 lated indefinitely, but the addition of calcium chloride at once 
 causes clotting. This result was originally explained by saying 
 that the lime in the blood was thrown out of solution by the 
 oxalate/ and that the addition of calcium caused coagulation. 
 But an 'oxalate solution of fibrinogen is coagulated by an oxalate 
 serum containing thrombin. Evidently, therefore, in the first 
 of these two experiments thrombin was absent, and in some way 
 or other was produced by the addition of calcium chloride. It 
 is now supposed that the calcium is not concerned in the action 
 of the thrombin on fibrinogen, but that its function is to assist in 
 the production of thrombin from some antecedent substance. 
 To this substance the name pro-thrombin or thrombogen has 
 been given. According to some views, this substance exists in 
 living blood, while others regard it as a product of the breaking 
 down of the platelets and leucocytes of the blood after the latter 
 has been shed. 
 
 If both thrombin and pro-thrombin are formed after the blood 
 is shed, what is it that stimulates the production of pro-throm- 
 bin ? One view which finds many supporters is that the pro- 
 thrombin is liberated from the disintegrating blood-cells by the 
 action of a kinase,* thrombokinase, which, together with 
 calcium salts, converts thrombogen into thrombin. 
 
 Another view is that the circulating plasma contains throm- 
 bogen, but not thrombokinase, the latter being liberated from the 
 leucocytes and blood-platelets after shedding. In the presence 
 of calcium, thrombokinase converts thrombogen into thrombin. 
 
 Not only is thrombokinase present in the above cellular 
 elements of the blood, but it exists in all the tissues of the body. 
 * Kinases are activating substances. 
 
THE BLOOD 
 
 23 
 
 The practical importance in the case of a wound of having 
 thrombokinase at hand to act on the thrombogen, and so cause 
 coagulation by the production of thrombin, is very evident, for 
 the cessation of haemorrhage is due to the formation of fibrin 
 in the mouths of the vessels. 
 
 There is a substance known as ' nucleo-protein,' readily 
 obtained from tissue-cells, thymus, kidney, lymphatic glands, 
 and other organs, which is very closely identified with thrombin. 
 If nucleo-protein be prepared, and a large dose injected into the 
 veins of an animal, intravascular clotting and death at once occur. 
 On the other hand, if small doses be injected at intervals, no such 
 effect is produced — in fact, the blood is rendered uncoagulable. 
 To this phenomenon the term ' negative phase ' 
 has been applied, in contradistinction to the posi- 
 tive phase in which clotting at once occurs. The 
 explanation which has been offered of the nega- 
 tive phase is that an antibody is produced which 
 neutralises the thrombokinase. But it has also 
 been held that two different substances are 
 obtainable from the above tissue-cells — one, 
 leuconuclein, which accelerates, and the other, 
 histon, which retards coagulation; whichever 
 effect is observed is said to depend upon the 
 relative amount of each present. 
 
 The view that an antibody exists in living 
 blood which prevents coagulation in the vessels 
 is urged by some. Such an antithrombin has 
 not at present been extracted from the inner 
 wall of the bloodvessels, but the theory offers 
 an explanation, not only of why blood remains 
 fluid during life, but also of the fact that coagu- 
 lation of the blood in the vessels after death is 
 a slow process. It also offers a reasonable 
 explanation of a very old experiment on the horse, in which 
 the jugular vein, after being occluded by ligatures and excised, 
 maintains the enclosed blood in a fluid condition for one or two 
 days, so long as it is left in contact with the wall of the vessel; 
 clotting, nevertheless, at once occurs on removal. Fig. 8 shows 
 diagrammatically this so-called ' living test-tube ' experiment, 
 the explanation of which was for years attributed to the in- 
 fluence exerted in some way or other by the normal endothelium 
 of the vessel on the contained blood. 
 
 Intravascular clotting may occur under pathological conditions, 
 as when the inner coat of the bloodvessel is injured, and the cells 
 then act as a foreign body. This may be seen in a ligatured 
 vessel, while pathologically the horse provides a classical example 
 
 Fig. 8. — Vein 
 of a Horse 
 tied B E- 
 tween two 
 Ligatures. 
 
 1, Plasma ; 2, 
 white corpus- 
 cles ; 3, red 
 corpuscles. 
 
24 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 in the injury occurring to the iliac arteries, which leads to 
 thrombosis. 
 
 Circumstances influencing Coagulation. — Clotting in shed 
 blood may be retarded or hastened by certain conditions. The 
 blood of a horse received into a vessel so constructed as to expose 
 it to a freezing temperature may be kept fluid for an indefinite 
 period, though coagulation will at once occur when the tem- 
 perature is allowed to rise. The probable explanation of the 
 phenomenon is that the low temperature keeps the corpuscles 
 from disintegrating. 
 ; Clotting is delayed by the addition to the blood of certain 
 neutral salts of the alkalies and alkaline earths ; the best salt to 
 employ for the purpose is magnesium sulphate ; The plasma 
 
 -— so obtained is spoken of as salted plasma, and is largely used in 
 physiological experiments on the blood. It is not known hqw 
 neutral salts prevent coagulation; it may be by keeping the 
 corpuscles intact, or by inhibiting the conversion of pro-thrombin 
 to thrombin. The addition of dilute acetic acid or the passage 
 of a current of carbonic acid gas through blood prevents coagula- 
 tion by precipitating fibrinogen. The addition to blood of a 
 weak solution of potassium or sodium oxalate prevents clotting by 
 combining with the lime in the blood. Without the presence of j 
 calcium, thrombogen cannot, as we have seen, become converted' 
 into thrombin; if, however, a soluble calcium salt be added,' 
 the power of clotting is restored. Sodium fluoride has much the 
 same effect on blood as sodium oxalate; it combines with the 
 lime ; but the power of clotting is not restored by merely adding 
 a soluble calcium salt. Some tissue extract must also be in) 
 eluded, for the fluoride has interfered with the formation of tbi 
 thrombokinase. 
 
 The action of certain organic substances in retarding blood- 
 clotting is very remarkable. If peptone be injected into the 
 blood of a dog, such blood will not clot; it can be shown that 
 this is not directly due to the action of the peptone, for the 
 
 I latter may be added to blood without inhibiting coagulation. 
 
 : Peptone introduced into the circulation causes the secretion in 
 the liver of a substance which prevents blood-clotting, an anti- 
 
 i thrombin. Leech extract contains a similar substance. This 
 
 I antithrombin — designated hirudin — is secreted by the salivary 
 glands of the leech. When an extract of these is injected into 
 the circulation, the blood loses its power of clotting, and, unlike 
 peptone, leech extract, when added to blood drawn from the body, 
 prevents coagulation. The bleeding following leech-bites and 
 the fluid condition of the blood in the body of the leech are due 
 to the action of this antithrombin. 
 Clotting of blood is retarded if the fluid as shed is received into 
 
THE BLOOD 25 
 
 a vessel the wall of which is thinly coated with oil. The shape 
 of the collecting vessel has also an influence over coagulation, 
 clotting being much slower in a deep, smooth vessel than in a 
 rough, shallow one. By increasing the foreign surface to which 
 the blood is exposed clotting is hastened. This is the explanation 
 of the effect of washing a bleeding wound, while the application 
 of compresses influences in some way or other the formation of 
 thrombin. 
 
 The Extractives of the blood are fats, cholesterin, lecithin, 
 creatine, urea, hippuric acid, uric acid, and grape-sugar, all in 
 small and varying quantities. The amount of fat in the blood \ 
 during, digestion is 0-4 to 0-6 per cent.; in dogs fed on a fatty 
 diet it may reach 1-25 per cent., and may give the serum a milky 
 appearance. There is twice as much fat in the Serum of recently- 
 fed horses as in the serum of those kept starving. Other ex- 
 tractives such as soaps are found to the extent of 0-05 to o-i per 
 cent.; urea, 0-02 to 0-04 per cent.; sugar, o-i to 0-15 per cent. 
 The corpuscles contain neither sugar nor fat, and possess a larger 
 amount of cholesterin and lecithin than the plasma. 
 
 The characteristic Difference between Arterial and Venous 1 
 Blood is that the former contains more oxygen and less carbonic \ 
 acid than the latter, though there is always, in fully arterialised I 
 blood, about twice as much carbon dioxide as there is oxygen \ 
 (see p. 123). Arterial blood also contains more water, fibrinogen, j 
 extractives, salts, and sugar, fewer blood-corpuscles, and less urea ; . 
 its temperature is, on the average, i° C. lower. The dark colour 
 of venous blood is not due to the greater amount of C0 2 it con- 
 tains, but to the diminution of oxygen in the red blood-cells. The 
 alteration in colour effected by the addition of reagents and gases 
 to blood is probably due partly to alterations in the shape of the 
 corpuscles themselves, which become more concave on the 
 addition of oxygen and less concave on its removal, but mainly 
 to the fact that oxyhaemoglobin is brighter in colour than haemo- 
 globin. 
 
 The Salts of the blood are divided between the plasma and 
 the corpuscles. The distribution of these is not the same in all 
 animals; in the horse and pig, for example, sodium exists only 
 in the plasma, there being none in the corpuscles, while in both 
 animals the potassium in the corpuscles is very high; in the ox 
 and dog both corpuscles and plasma contain sodium. Sodium 
 chloride is the most abundant salt of the blood, potassium chloride 
 and sodium carbonate come next, and lastly phosphates of cal- 
 cium, magnesium, and sodium. The chief inorganic substance 
 of the ceUs is potassium phosphate. The following table from 
 Bunge bears on the question of the salts of the blood in different 
 animals : 
 
26 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 
 1,000 grammes of corpuscles contain — 
 
 1,000 grammes of serum contain — 
 
 Horse - 
 Ox - 
 Pig - 
 
 K. 
 
 4-920 
 0-747 
 5'543 
 
 Na. 
 
 
 
 2-093 
 
 
 
 CI. 
 
 1-930 
 i'°35 
 i'5°4 
 
 K. 
 
 0-27 
 
 0-254 
 
 0-273 
 
 Na. 
 4' 43 
 4 - 35i 
 4-272 
 
 a. 
 
 3' 75° 
 3-7I7 
 3- 61 1 
 
 Water free from salts is destructive to protoplasm ; no doubt, 
 therefore, one important function*of the salts in the blood is to 
 maintain the vitality of the tissues by preserving the osmotic 
 equilibrium. Sodium chloride is here especially valuable, and its 
 extensive presence in blood (60 per cent, to 90 per cent, of the 
 total amount of ash) corresponds to its importance. As the blood 
 is simply the carrier of the salts, and the only channel by which 
 the tissues can obtain them, it by no means follows that all the 
 mineral matter found in it is essential to its own repair and 
 constitution. 
 
 The Temperature of the Blood in the different domestic 
 animals varies from 37-8° C. to 40-54° C. (100° to 105° F.), the 
 warmest blood in the body being found in the hepatic veins. 
 
 The Quantity of Blood in the Body cannot be determined by 
 mere direct bleeding alone. After all the blood is drained off, 
 the vessels require to be washed out, and the quantity of blood 
 in the water estimated by the colour present ; the body has then 
 to be minced and macerated, and the quantity of blood in the 
 resulting fluid estimated by the colour test, comparison being 
 made with a standard solution of blood. 
 
 By Haldane and Lorrain Smith's carbon monoxide process the 
 amount of blood in the living animal may be calculated. The 
 essential steps in this process are to estimate first colorimetrically 
 the percentage of haemoglobin in the blood, and then the extent 
 to which this is saturated by breathing a measured volume of 
 carbon monoxide. In this way the total capacity of the blood 
 for carbon monoxide may be ascertained, and the carbon mon- 
 oxide capacity being the same as the oxygen capacity, the 
 volume of the blood may be readily calculated. 
 
 Sussdorf* puts the proportion which the weight of the blood 
 bears to the body weight as follows : 
 
 Horse 
 
 Ox 
 
 Sheep 
 
 Pig 
 
 Dog 
 
 -j^ = 6-6 per cent, of the body weight. 
 
 A =771 
 
 A =4-6 ! „ 
 
 i\ to ts=5"5 to 9 - i per cent, of the body weight. 
 
 Ellenberger's ' Physiologie der Haussaugethiere.' 
 
THE BLOOD 27 
 
 The same observer gives the amount of blood in the body of 
 the horse at 29 litres (66 pounds, or nearly 50 pints). 
 
 The Distribution of Blood in the Body (Fig. 9) is believed to 
 be as follows : 
 
 About one-fourth in the heart, lungs, large vessels, and veins. 
 » ,, liver. 
 
 „ ,, skeletal muscles. 
 
 „ ,, other organs. 
 
 It is probable that in the horse the liver would contain less 
 than one-fourth the bulk of blood, while the skeletal muscles 
 would contain more. Under certain conditions the abdominal 
 veins are capable of containing the whole of the blood in the 
 body. When an organ is active it receives more blood than when | 
 in a state of rest ; this increase has been variously estimated at ! 
 from 30 to 50 per cent. 
 
 Liver 29-5 
 
 A-fu-scZes 2 9-2 
 
 GreatVessehHeartMungs 2 27 
 ' Bows S-Z 
 
 hut stmt stcgenitcil orqans S-Z 
 Skin zi 
 
 Kidneys 16 
 
 Nbrve Centres L 2 
 Sp-lee-n. ,' . OZ^ 
 
 p IG „ Diagram to illustrate the Distribution of the j Blood in the 
 
 Various Organs of a Rabbit, after Ranke's Measurements (Stewart). 
 The numbers are percentages of the total blood. 
 
 Regeneration of the Blood after Haemorrhage. — Regenera- 
 tion of the fluid portions of the blood is extremely rapid, experi- 
 ments showing that after slight haemorrhage the normal volume 
 is regained within a few hours, and after severe hemorrhage in 
 from twenty-four to forty-eight hours. This is supported by 
 clinical observation ; in the days of severe bleedings venesection 
 to the extent of producing syncope was frequent, yet in a very 
 short time the volume was restored. In these cases it is the 
 plasma which is rapidly replaced. The red cells and haemoglobin 
 take longer to prepare, probably several days, perhaps even two 
 or three weeks. In the dog exact observation shows that a 
 haemorrhage of from 2 to 3 per cent, of the body weight is readily 
 recovered from, while a loss of 4-5 per cent., which represents half 
 the blood in the body, is generally fatal. Percivall tells us* that 
 in the horse he occasionally drew 3 gallons of blood, which may 
 be taken as half the amount in the body, apparently without 
 fatal consequences. 
 
 * ' Hippopathology, ' vol. i., p. 95. 
 
28 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Transfusion of warm normal or physiological saline solution 
 (NaCl 0-9 per cent.) is capable of keeping the heart beating even 
 after severe haemorrhage, showing that the immediate essential 
 factor is the restoration of bulk to the empty vascular system. 
 In order that the blood-pressure should be kept up after saline 
 transfusion, the addition of gum to the fluid is necessary (Bayliss). 
 Transfusion of blood is not without danger, for, as has been 
 shown in dealing with haemolysis, the serum of one animal may 
 be toxic to another. 
 
 The Gases of the Blood are more conveniently dealt with in 
 the chapter on Respiration (see p. 123). 
 
 Composition of the Blood.— Reviewing the various analyses 
 which have been published of the blood of animals, the following 
 represents the average composition of the fluid : 
 
 The Plasma. 
 
 Water - 9° parts per cent. 
 
 Proteins 8 or 9 parts. 
 
 Fats ° -1 part. 
 
 Extractives 0-4 ,, 
 
 Salts o-8 „ 
 
 The Corpuscles. 
 These represent from one-third to half the weight of the blood, 
 and consist of — 
 
 Water 64 parts per cent. 
 
 Solids 35 ,, consisting of 32 per cent, haemo- 
 
 globin, o-i per cent, proteins. 
 Salts 1 part. 
 
 Taking the blood as a whole, the following represents approxi- 
 mately its composition in every 100 parts : 
 
 Water 81 parts. 
 
 rHasmoglobin 13 parts. 
 „ .. , Proteins 4 ,, 
 
 Sollds J 9 » \ Salts I „ 
 
 lExtractives o-6 ,, 
 
 The Blood in Disease. — The blood plays two distinct parts in 
 disease; it is a carrier and distributer of infection to the body-cells, 
 and, further, it may itself undergo profound pathological change. 
 
 All the specific infective diseases of animals are spread through the 
 body by means of the blood-stream. It is true that the initial source 
 of entry may be an allied passage — the lymph-stream — but it is by 
 means of the blood that the final and complete invasion of the body 
 is effected. Nor does this observation apply to specific diseases only; 
 if we take two such opposite conditions as anthrax and poisoning by 
 arsenic, it is the blood in each case which is responsible for the dis- 
 tribution of the infecting agent. 
 
 The blood- tissue itself may be the seat of disease; micro-organisms 
 may live and multiply in the plasma, and infect the whole body as in 
 anthrax. Some of the organisms may be so small as to be ultra- 
 
THE BLOOD 29 
 
 microscopic, and it should be remarked that some of the most 
 acute and fatal infectious diseases of animals are caused by organisms 
 of this class — for example, rinderpest, foot and mouth disease, 
 rabies, and African ' horse sickness.' And, in spite of the fact 
 that these microbes have not been seen, their existence is undoubted, 
 the best evidence of which is that some of them are sufficiently large 
 to be caught in the pores of a filter, leaving the nitrate sterile. Other 
 organisms attack the blood-cells, either from without or within — for 
 example, the important group of Trypanosomes, the malaria para- 
 site, the organism of Texas fever, and suchlike. In these cases the 
 product of red-cell destruction may show itself by the discoloured 
 urine, and is evident in the tissues — for instance, the liver and spleen. 
 
 Compared with the red corpuscles, the white are seldom affected 
 with disease, but there are certain pathological conditions associated / 
 with a great increase in their number {leucocytosis), and others in 
 which the white cells are reduced in number (leucoptznia). 
 
 There are other conditions affecting the blood — for instance, 
 Purpura — which cannot be attributed to parasitic agency. In this 
 disease, either from defects in the blood or vessel-wall, haemorrhage 
 takes place into the tissues. No organ appears to be able to escape, 
 though probably the subcutaneous and muscular tissues are the 
 most frequent seat of the haemorrhage. 
 
 Quite as strange and obscure is the dietetic disease of equines 
 known as hemoglobinuria, in which the animal in the middle of work 
 suddenly falls paralysed; the urine becomes coffee-coloured and 
 loaded with methsemoglobin, in consequence of the destruction of 
 the red cells. What the destructive agent is, is at present unknown, 
 but it is probably one of the poisonous products of protein disintegra- 
 tion, which will be found dealt with in the chapter on Digestion. 
 
 Blood-letting in the treatment of disease was at one time so uni- 
 versal that it came to be regarded as the ' sheet-anchor ' of life, and 
 animals were regularly bled in order to keep them in health. ' Blood- 
 letting ' was killed by abuse ; it is now a question whether the pendu- 
 lum has not travelled too far in the other direction, and the employ- 
 ment of a physiological means in the treatment of disease been too 
 long neglected. Towards the end of the eighteenth century a full 
 blood-letting for the horse was from 4 to 5 pints. During the first 
 half of the nineteenth century 8 pints were considered a moderate 
 bleeding. Under pressure of acute disease 3 gallons were drawn, and 
 Percivall tells us he had heard of 4 gallons being taken (see p. 27). 
 
 This is the abuse we allude to as having caused the fashion to 
 change. Such heavy blood-lettings must have been responsible 
 for considerable mortality. Percivall describes the ' impression on 
 the system,' which was considered a certain indication if blood- 
 letting was to prove beneficial. As the pulse began to sink, the 
 horse became very uneasy, jerking the head up and down, moving 
 backwards until finding support for the hind-quarters ; respirations 
 increased, deep sighing followed, the body rocking from side to side, 
 in danger of falling headlong; shivering; and, after the operation, 
 sweating. A second and even third bleeding was employed. 
 
 The efficacy of bleeding healthy animals to improve nutrition was 
 fully accepted, and Percivall declares his opinion that, if persisted 
 in, it became necessary for preserving health. An increased: disposi- 
 tion to fatten was observed in young animals submitted to moderate 
 bleeding, and farmers employed this knowledge regularly in fatten- 
 ing their young stock. 
 
CHAPTER II 
 
 THE HEART 
 
 The blood in the body has to be kept in constant motion, so that 
 the tissues which depend upon it for their oxygen may be con- 
 tinuously supplied, and also that the blood when rendered 
 impure by the. changes in the tissues may be rapidly and effectu- 
 ally conveyed to those organs where its purification is carried 
 out. 
 
 The heart is the organ which pumps the blood over the body, 
 not only distributing it to the tissues, but forcing it on from 
 these back to the heart again, to be prepared for redistribution. 
 It may be described as a hollow muscle divided into two com- 
 partments, usually known as right and left, but in quadrupeds 
 really anterior and posterior, each compartment being divided 
 into an upper half or auricle and a lower or ventricle^ Opening 
 into the auricles are large veins which convey the blood back to 
 the heart, while from the ventricles other vessels, arteries, take 
 their origin for the conveyance of blood from the heart. The 
 auricles and ventricles are separated by a valvular arrangement, 
 and the two sides of the heart are separated by a muscular 
 partition (Fig. 10). 
 
 Thus far the general arrangement of both right and left sides is 
 much the same, each having to receive and then to get rid of a 
 certain quantity of blood received by it. But the blood sent into 
 the right side of the heart is very different from that received 
 by the left, and with this difference we must for a moment deal. 
 The whole of the impure or venous blood in the body is brought 
 into the right side of the heart for the purpose of being distributed 
 to the lungs, where it is purified ; this arterial or purified blood 
 is brought back from the lungs into the left heart for distribution 
 to the body. The passage of the impure or venous blood from 
 the right side of the heart through the lungs to the left side is 
 known as the Pulmonic circulation, that of the blood, thus 
 purified, through the body back to the right side of the heart is 
 called the Systemic circulation (Fig. n). * 
 
 3° 
 
THE HEART 
 
 3 1 
 
 Mention has been made of valves in the cavities of the heart ; 
 they are found on both sides separating auricle from ventricle, 
 and are known as the right auriculo-ventricular or tricuspid 
 valve, and the left auriculo-ventricular or mitral valve. In addi- 
 tion to these, valves are found in the vessels arising from the ven- 
 tricles — viz., in the pulmonary artery and the aorta ; these valves, 
 pulmonary and aortic, are known as the semilunar valves. No 
 valves are found guarding the entrance of the vessels (veins) into 
 the auricles. In order to understand the function of these 
 valves, which play such an 
 important part in the phy- 
 siology of the heart, it is 
 necessary that we should 
 briefly detail the course 
 which the blood takes from 
 the time it enters the right 
 auricle until it completes 
 the round of the circulation 
 and finds itself at this auricle 
 again. 
 
 Course of the Circula- 
 tion. — The venous blood 
 from the whole of the body 
 flows into the right auricle 
 by means of the anterior 
 and posterior venae cavae; 
 it then passes through the 
 tricuspid valve into the right 
 ventricle ; from the right 
 ventricle it travels to the 
 lungs by means of the pul- 
 monary artery, where, hav- 
 ing been exposed to the 
 action of the air and become 
 greatly changed in its gase- 
 ous composition, it returns 
 
 to the heart as pure blood by means of the pulmonary veins, 
 pouring itself into the left auricle. The blood now passes through 
 the auriculo-ventricular opening into the left ventricle, and 
 thence into the aorta to be pumped all over the body, being 
 distributed by means of the arteries and capillaries; it is then 
 collected by the veins, and eventually brought back to the 
 heart to undergo afresh its distribution to the lungs and body 
 
 (Fig. n). 
 
 The use of the valves is to allow of and to insure the trans- 
 ference of blood from auricles to ventricles, and from the ventricles 
 
 Fig. 10. — Diagram of the Circulation 
 thkough the heart. 
 
 i and 2, The ven<E cavas; 3, right auricle; 
 4, right ventricle ; 5, pulmonary 
 artery; 6, 6, pulmonary veins ; 7, left 
 auricle ; 8, left ventricle ; 9, aorta 
 dividing into anterior or posterior. 
 The arrows represent the direction 
 taken by the blood-stream. 
 
32 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 to the aorta and pulmonary artery without any chance of re- 
 gurgitation. This they do in virtue of the fact that they are 
 so constructed and arranged as to open only in that direction 
 towards which the blood has to be sent. 
 
 Position of the Heart. — The heart occupies a position in the 
 middle line of the chest, and is suspended from the spine by 
 means of its arterial trunks. These are its only means of sup- 
 port. Some assistance may be afforded by its connection with the 
 root of the lungs ; but in thinking of the heart as a pump we 
 must remember that all the movements it executes in its constant 
 work are carried out as the organ literally hangs from the spine. 
 It rests on nothing; the apex is clear of the sternum; the peri- 
 cardium keeps it in its place, but is no mechanical aid to keeping 
 it in position. In the dog the pericardium obtains attachment 
 to the diaphragm, but not in the horse and ruminants. Further, 
 
 Fig. ii. — Diagram of the Circulation of the Blood. 
 
 i, The heart; a, anterior; 3, posterior aorta; 4, anterior vena cava; 5, pulmonary 
 artery; 6, pulmonary veins; 7, mesenteric arteries; 8, intestinal capillaries; 
 9, portal vein; 10, the liver, the veins from which open into (12) the posterior 
 vena cava; n, the circulation through the hind extremities; 13, the circula- 
 tion through the kidney. 
 
 in the dog and cat the heart rests on the upper face of the ster- 
 num, whereas in the horse it does not touch the sternum. Fig. 12, 
 or better still Fig. 45, gives an accurate notion of the position 
 of the heart. It will be observed that the organ is tilted for- 
 ward, the base lying in front of the apex. 
 
 The base of the heart is uppermost, and the organ in the horse 
 occupies a position opposite to the third, fourth, fifth, and 
 sixth ribs. It is between the fifth and sixth ribs, just above 
 their sternal insertion, that the impulse of the heart can be felt. 
 On its right face is the right lung, and on its left, part of the left 
 lung, the big triangular notch in which exposes the left ventricle 
 and enables it to make its impulse felt against the chest-wall. 
 The anterior part of the heart is formed by the right auricle and 
 ventricle, the posterior by the left auricle and ventricle. The 
 pulmonary artery runs along the left face, and the posterior 
 vena cava lies on the right face. 
 
THE HEART 
 
 33 
 
 Heart Muscle. — The heart muscle in structure is considered to 
 come midway between skeletal and involuntary muscle. Until 
 recently its fibres were described as short, branching, anastomos- 
 ing, andpossessing no sarcolemma. There isgood reason to believe 
 that this account is no longer correct, and that the fibres, instead 
 of being short, form a continuous sheet, fibrils passing from fibre 
 to fibre, and so constituting an anastomosis in every direction. 
 There is evidence also that a sarcolemma is present, though it is 
 delicate in structure. The fibrils, or sarcostyles, of which the 
 fibres are composed, are both longitudinally and transversely 
 striated. 
 
 Fibres, described as the 
 fibres of Purkinje, are 
 found beneath the endo- 
 cardium of the horse, ox, 
 and sheep. They give a 
 greyish appearance to the 
 part, and on microscopi- 
 cal examination are found 
 to consist of large poly- 
 hedral, clear cells, con- 
 taining granular sub- 
 stance, and one or more 
 nuclei. These fibres are 
 connected with a peculiar 
 band of muscle which 
 forms a connection be- 
 tween the auricle and 
 ventricle. This band or 
 bundle is known as the 
 Auriculo-ventricular or 
 A.V. Bundle, and is the 
 discovery of Kent and 
 His. Originally it was 
 
 held that the muscular structure of the auricle and ventricle was 
 distinct, the chambers being separated by a fibrous ring; but in 
 the hearts of the ox and sheep this bundle of muscle connecting 
 the auricles with the ventricles can easily be traced by its paleness.. 
 It is found in all mammalian hearts. In the mammalian heart 
 there are two small masses of so-called ' nodal ' tissue, inter- 
 mediate between muscle and nerve in structure— namely, the 
 sinu-auricular and the auriculo-ventricular nodes (Keith and 
 Flack). The former is situated at the junction of the superior 
 vena cava and right auricle, and the latter at the base of the mter- 
 auricular septum on the right side below and to the right of the 
 coronary sinus. These nodes are connected with one another 
 
 Fig. 12. — Right Auricle and Ventricle 
 of Calf, to showAuriculo-Ventricular 
 Band (after Keith). 
 
 i, Auriculo-ventricular (A.V.) node; 2, main 
 auriculo-ventricular bundle; 3, right septal 
 division of the bundle; 4, moderator band. 
 
34 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 through the interauricular septum, and the S.A. node, which may 
 be regarded as the pace-maker of the heart, is in intimate con- 
 nection with the cardiac nerves. From the A.V. node, Fig. 12 
 (1) is given off the main bundle (2) ; this divides into right and 
 left divisions, one running to the right ventricle, the other to the 
 left. The right heart bundle (3) runs downwards into the ven- 
 tricular septum, and is distributed to the musculi papillares and 
 moderator bands. In the left heart the bundle reaches the left 
 ventricle from the right auricle ; it then runs down the ventricular 
 septum, and, like its fellow in the opposite side of the heart, is 
 distributed to the musculi papillares and moderator bands. 
 
 The muscular walls of the auricle and ventricle are con- 
 structed of layers of red fibres, varying greatly in thickness and of 
 extremely complex disposition, especially around the ventricular 
 cavities. The ventricular walls are thicker than those of the 
 auricles, and the left side of both cavities is better developed than 
 the right. In certain portions of the right auricle the wall is so 
 thin as to be semi-transparent, and appears, in fact, to consist of 
 little else than the two layers of serous membrane which cover 
 and line the heart. The ventricular walls are of unequal thick- 
 ness; at the apex of the heart they are reduced to a few fibres 
 of muscular tissue one-eighth of an inch in thickness. Chauveau, 
 in fact, says that at this point there is nothing more than two 
 layers of serous membrane — viz., that lining and that covering 
 the heart. 
 
 The varying thickness of the walls of the heart is due to the 
 complex arrangement of the various layers of fibres ; these may, 
 broadly speaking, be divided into two main groups — an internal, 
 belonging to each auricle and each ventricle separately, and an 
 external, belonging to both auricles and both ventricles. Ex- 
 cluding the A.V. bundle just described, the fibres of the auricles 
 are confined to the auricles, and the fibres of the ventricles to the 
 ventricles ; the advantage gained by this arrangement is obviously 
 connected with the independent contraction of auricles and 
 ventricles. 
 
 The muscular layer peculiar to each ventricle is very complex, 
 but, generally speaking, may be described as a scroll of fibres of 
 several layers running obliquely around each ventricle, and 
 where the scrolls meet forming the ventricular septum. The 
 scrolls. are attached above to the auriculo- ventricular ring, but 
 are left open below. If we can imagine these scrolls separated 
 from the other muscle of the heart, they would present the appear- 
 ance of a pair of hollow cones. Covering this internal layer is an 
 external, belonging in this case to both ventricles; it takes its 
 origin from the auriculo-ventricular ring, and describes a spiral 
 course in descending from base to apex of the heart, where the 
 
THE HEART 35 
 
 fibres form what Henle described as a vortex (Fig. 13) ; they 
 then pass upwards through the opening in the scrolls of the inner 
 layer, and so gain the interior of the ventricles. Some of the 
 fibres pass to the columnce carnece, others to the musculipafiillares, 
 while the majority gain insertion into the auriculo- ventricular 
 ring from which they had their origin. As pointed out by 
 Chauveau, whose account of the arrangement of the fibres in 
 the heart of the horse we have mainly followed, this layer forms 
 between its origin and insertion figure of eight loops, the smallest 
 loop being at the apex of the heart, where at its centre it leaves 
 a very small space, through which a probe may be passed into 
 the ventricle without piercing anything but the external and 
 internal layer of serous membrane. 
 
 The fibres of the auricle are much simpler in their arrangement. 
 Those peculiar to each cavity are disposed in several fasciculi, 
 some circular, especially those 
 surrounding the mouths of the 
 vessels, others, in the general 
 body of the auricle, in inter- 
 woven loops. The septum is 
 formed where the two sets 
 proper to each auricle meet. 
 The fibres common to both 
 auricles are generally arranged 
 transversely. 
 
 It may be added that anat- 
 omists are not agreed as to FiG I3 ._ ApEX 0F Heart _ showing 
 
 the arrangement of the fibres Vortex Arrangement of Fibres 
 
 of the heart muscle, and this (after Krause). 
 
 may to some extent be due 
 
 to the fact that the system is not the same in all animals. 
 
 The arrangement just outlined has, however, this advantage, 
 that it certainly appears to provide for the squeezing and wring- 
 ing movement to which the ventricular contents are exposed, 
 the shortening of the heart wall from base to apex, and the con- 
 traction of the musculi papillares at the moment the valves 
 close. Far simpler is the disposition of the muscular fibres of 
 the auricles; physiologically these cavities have been regarded 
 as the dilated extremity of the vessels entering the heart, and 
 their function as more that of a well than of a pump. Neverthe- 
 less, the existence of a network of muscular pillars in the auricles, 
 especially the dense bands in the left, warn us to be careful not 
 to regard these cavities in the horse as mere passive channels 
 of the circulation. Keith dismisses the reservoir theory of the 
 auricles as being opposed to their structure => n d describes them 
 as loading pumps for the ventricles. 
 
36 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 !&£ 
 
 Keith* further points out that the muscular walls of the heart 
 must have some fulcrum from which to act. The auricles are fixed 
 by means of the pericardium to the lungs and walls of the thora- 
 cic cavity, and a point from which they can act is thus obtained. 
 When the ventricles contract, the base of the aorta, where it arises 
 from the left ventricle, becomes a fulcrum. Nevertheless, we know 
 that in some animals the heart will continue to contract rhythmi- 
 cally for some time after removal from the body, even when the 
 above-mentioned fulcra are detached. There are certain fibrous 
 
 rings found in the heart 
 where the valves are 
 situated to which these 
 and the muscular fibres 
 obtain a firm attachment. 
 The ring surrounding the 
 aortic opening in the ox 
 has in its substance one 
 or more pieces of bony 
 tissue, and this is also 
 common in the horse. 
 
 The cavitiesof the heart 
 are lined by the endo- 
 cardium, which is re- 
 flected over the valves; 
 this membrane in the 
 left auricle of the horse 
 is of a peculiar grey 
 
 11. — Left Ventricle of Horse exposed Colour, due to the fibres 
 to show Mitral Valve. f Purkinie 
 
 Valves of theHeart. — 
 The auricula- ventricular 
 valves are made up of 
 fibrous membrane, in 
 which a small proportion of muscular fibre is found close to 
 the attached border. The mitral or bicuspid valve in the horse 
 consists of one large distinct segment, and several smaller ones 
 united to form a second; the tricuspid consists of three segments, 
 one, much larger than the others, being placed opposite to that 
 portion of the ventricle which leads to the pulmonary artery. 
 
 The free edges of all the valves are held in position by large and 
 
 small tendinous cords (chorda: tendi tees) composed of fibrous 
 
 tissue, which are inserted into musculi papillares found on the 
 
 internal surface of the ventricle ; the cords from one papilla do 
 
 not all pass to one segment of the valve, but to all (Fig. 14). 
 
 -Vrthur Keith 
 
 Fig. 
 
 Portion of valve; 2, columns camecz, on the 
 upper surface of which are found the musculi 
 papillares, to which the chorda tendinecs are 
 attached. 
 
 * ' The Functional Anatomy of the Heart 
 M.D., F.R.S., British Medical Journal, March 
 
 Professor 
 , 1918. 
 
THE HEART 
 
 37 
 
 The function of the papilla is to restrain the valves from being 
 forced too far into the auricle during the contraction of the ven- 
 tricle, and this they accomplish by gradually shortening as the 
 walls of the ventricle approximate, and thus exerting traction on 
 the cords (Fig. 15) . This shortening is brought about by the layer 
 of muscular fibres which at the apex passes from, the external 
 face of the heart into the interior of the ventricles, and thence 
 to the fleshy columns and papillary muscles (p. 35) . Other bands 
 pass from one side of the ventricle to the opposite wall; they are 
 called moderator bands, and their function is to protect the ven- 
 tricular wall from undue distension. 
 
 - aur. 
 
 --vent 
 
 Fig. 15. — Diagram to illustrate the Action of the Valves of the Heart 
 
 (Huxley). 
 
 In A the auricle is contracting, ventricle dilated, mitral valve open, semilunar 
 valves closed. In B the auricle is dilated, ventricle contracting, mitral valve 
 closed, semilunar valves open. Aur., auricle; vent., ventricle; v., v., vein; 
 a., aorta; m., mitral valve; s., semilunar valve. Note the manner in which 
 the papillas have shortened in B, in order to compensate for the approxima- 
 tion of the ventricular walls to the surfaces of the mitral valve. 
 
 The valvular flaps meet in the most perfect apposition when 
 the ventricles contract ; their edges are inverted, and the sides of 
 the valves curl in and lie so close to their fellows that nothing 
 can escape upwards into the auricles (Fig. 16). This may be 
 readily demonstrated in the dead heart if one ties the aorta 
 and pulmonary veins, and introduces into the left auricle a tube 
 which admits a powerful jet of water; the left side of the heart 
 distends and hardens, and at last water forces its way out of 
 the hole in the auricle through which the tube is inserted. If 
 the auricle be now opened, the ventricle is found cut off from 
 view by a tense, membranous, parachute-like dome, convex 
 
38 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 towards the auricle, which is the mitral valve in position ; not a 
 drop of water will escape from the ventricle, though the heart be 
 turned upside down, and it requires some little force to depress 
 the valve. 
 
 During the filling of the ventricle the auriculo-ventricular 
 valves are coming into position; the blood is under them, and 
 the final systole of the auricle, by raising the pressure in the 
 ventricles, forces these valves into their place, and bulges them 
 upwards towards the auricular cavity. We have seen that even 
 in the dead heart their fit is so perfect that they render the 
 ventricle water-tight. But in the living heart the walls of the 
 ventricles are approximating as they contract on their contents, 
 and though the internal diameter is being reduced in every 
 
 direction, this does not disturb 
 the accuracy with which the 
 valves apply themselves to each 
 other. The papillary muscles 
 compensate for the approxima- 
 tion of the ventricular walls 
 by constantly shortening, and 
 through the chorda tendv<rce 
 maintain the segments of the 
 valves in apposition and prevent 
 further encroachment on the 
 auricle. 
 
 The semilunar or sigmoid valves, 
 which guard the entrances of the 
 aorta and pulmonary artery, are 
 composed of fibrous tissue, and 
 possess at the centre of each 
 segment a small hard body, the corpus Arantii, which is particu- 
 larly well marked in the aortic valves. It is generally sup- 
 posed that these shot-like bodies complete the central sealing 
 when the valves are closed, but this view gives too little attention 
 to the fact that the valves not only meet at their free border but 
 overlap. Chauveau states that, with his finger in the pulmonary 
 artery of the horse, he has tried to hold back one flap in order 
 to keep the opening patent, but the two remaining segments t. 
 applied themselves so closely to his finger that the orifice was | 
 closed That these bodies are additional aids to the closing of 
 the valves completely tight is undoubted, but the overlapping 
 of the valves is the more important factor. 
 
 When the sigmoid valves are not in action, they still stand out 
 in the blood-stream, and do not lie against the wall of the vessels 
 as was at one time supposed, nor do those in the aorta cover the 
 opening •. of the coronary arteries. It is probable that the valves ' 
 
 Fig. 16. — Tricuspid Valve of the 
 Horse in Closed Position seen 
 from the Auricle. 
 
 Note the cracks in the surface, which 
 show where the margins of the 
 valves meet, and fold in against 
 each other like the lips of a 
 toothless mouth. 
 
\. u ' ,-\ THE HEART 39 
 
 are enabled to stand out in the blood-stream through the action 
 of vortex currents, and while thus waiting for their turn in the 
 heart's cycle they form a triangular orifice with curved sides. 
 
 It is generally believed that both the aortic and pulmonary 
 valves are closed by the regurgitation of the blood; but it has 
 been pointed out that the blood, as it is leaving both ventricles, is 
 streaming through orifices which at that time are mere chinks, 
 owing to the pads of heart muscle which take their origin from 
 all sides of the mouths of the vessels. Vortices are thus created 
 in the space between the arterial root and the edge of the valves. 
 These vortices tend to press the edges of the valves together, 
 and the valves consequently close the moment the blood actually 
 ceases to stream through the narrow crevice. In this way there 
 is no regurgitation, as the valves are closed before the recoil of 
 the aorta. If this explanation be correct, the second sound of 
 the heart must be considered due to the sudden tension, and not 
 to the closure, of the aortic valves at the time of the aortic recoil. 
 
 It is of the utmost importance to bear in mind that the force 
 of the aortic reflex is not expended wholly on the valves, but 
 largely on the muscular tissue of the ventricle, which here, as the 
 result of the orifice contracting, forms a large circular pad. In 
 order to permit this strain to fall on the heart wall itself, the 
 diameter of the aorta is much greater than the opening out of the 
 ventricle. 
 
 Movements of the Heart. — If the exposed mammalian heart 
 be watched at work, a great deal may be learnt of its action. It 
 will be observed that both auricles contract together and both 
 ventricles together ; further, that certain changes in shape occur. 
 The contraction either of auricle or of ventricle is spoken of as its 
 systole, while the subsequent relaxation is described as its dia- 
 stole. The contraction of the ventricles is succeeded by a pause, 
 during which the heart is in a state of relaxation. 
 
 A Cardiac Cycle is the term used to describe the changes which 
 occur in the heart during the time which elapses between one 
 contraction or relaxation of the auricle, and the one which 
 immediately succeeds it. 
 
 We may take the moment when the blood is entering the 
 auricles from the venae cavse and pulmonary veins as the most 
 convenient point to start from. This flow is brought about by 
 the pressure of blood in the veins, which, though low, is yet higher 
 than that in the auricles. The influence of gravity is also a 
 great aid to the filling of the auricles, for, speaking broadly, ex- 
 cepting in the limbs, the greatest number of the veins in the body 
 of quadrupeds is above the heart. The emptying of the anterioi 
 and posterior venae cavse is largely assisted by gravity ; even in 
 the latter there is a downward incline in the vessel from the last 
 
40 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 valves met with in the iliac veins to the right auricle. Besides 
 the influence of gravity there is the contraction of the abdominal 
 wall and consequent pressure on the viscera to be considered, 
 so that at each expiration the blood is spurted along the posterior 
 vena cava towards the auricle. Finally, there is an aspiration 
 in the veins produced by the auricle, and caused by a 
 relaxation of its walls after the previous contraction. There 
 is also an aspiration in the thorax, the result of inspiration, 
 which produces a negative pressure in the veins leading to the 
 heart, and so draws blood towards the heart from the veins lying 
 outside the thorax. 
 
 By a combination of these means the auricles are filled with 
 blood, and a wave of contraction which first appears at the 
 vessels leading into them passes over the chambers ; the auricular 
 appendage becomes pale, the auriculo-ventricular groove is 
 drawn upwards, and the auricles, by a sudden sharp and brief 
 contraction, empty their contents into the ventricles. At this 
 moment there is a backward positive wave produced in the 
 anterior vena cava, which shows itself by a pulsation in the 
 jugular veins at the root of the neck, well seen in the horse. 
 
 The effect of the auricular contraction is to complete the filling 
 of the ventricles and to close the valves by raising the pressure in 
 those cavities. During the whole time the valves are open the 
 ventricle is filling ; what the systole of the auricle effects is the 
 final filling of the ventricle. The auriculo-ventricular valves, 
 which during the filling have been gradually coming into position, 
 are now, under the sudden increase of intraventricular pressure, 
 rapidly closed, and bulge into the auricle, their closure con- 
 tributing to the production of the first sound of the heart. 
 The valves are prevented from going too far by the chorda 
 tendinece, which are acted upon by the gradually contracting 
 musculi papillares, as previously explained. At this moment 
 the blood imprisoned in the ventricles is shut off from the 
 auricles by the closed auriculo-ventricular valves, and shut off 
 from the aorta and pulmonary artery by the closed semilunar 
 valves. The ventricles now contract and raise the pressure 
 within their cavities in order to force open the semilunar valves, 
 for until the pressure within the ventricle exceeds that in the 
 aorta and pulmonary artery, these two avenues of escape are 
 closed. While the intraventricular pressure is being increased 
 by the walls contracting tighter and tighter on the imprisoned 
 blood, the heart is changing in shape; it is becoming more 
 globular, its walls are growing tenser, it is shortening from base 
 to apex,' and in its writhing, screwing efforts to overcome the 
 pressure in the aorta and pulmonary artery it is twisting slightly 
 from left to right, and from before backwards, rotating on its 
 
THE HEART 41 
 
 vertical axis, and bringing more of the left ventricle against the 
 sides of the chest. The intraventricular pressure is now suffi- 
 cient to cause the aortic and pulmonary valves to yield, and blood 
 rushes into these vessels under the systole of the ventricles. The 
 aorta and pulmonary artery fill with blood, elongate and curve; 
 the heart, rotating so long as the ventricles are contracting, makes 
 its impulse against the chest wall, and empties its contents into 
 the arteries. The auriculo-ventricular groove now moves down- 
 ward towards the apex of the heart, the ventricular walls relax 
 and elongate, the pressure within them falls, and the blood from 
 the over-full arteries is prevented from regurgitating into the 
 ventricles by the semilunar valves coming into position. The 
 closure of these valves creates the second sound of the heart. 
 The ventricles are at this stage momentarily isolated, the auriculo- 
 ventricular valves are closed, so also are the semilunar. This is 
 the second time in the cardiac cycle that the ventricles have 
 been shown to be cut off from the other part of the heart. We 
 shall examine the question in greater detail presently. The 
 great arteries now contract and shorten, the heart rotates back- 
 wards to the left, the auriculo-ventricular valves open, the 
 auricles and ventricles, neither contracting nor dilating, assume 
 a passive condition during a period known as the pause, the blood 
 flows into the auricles, and from the auricles into the ventricles ; 
 the auricles now contract, and the whole process is repeated. 
 
 We have thus the contraction of the auricles, the contraction 
 of the ventricles, and the pause. The time that each of these 
 occupies has not been determined with accuracy; the results 
 obtained by Chauveau and Marey in the horse show that the 
 auricular Systole is brief, the ventricular systole twice as long, 
 and the pause equal in length to the ventricular systole ; but the 
 time values as given by them would cause the horse to have a 
 pulse-rate of 60, which is abnormal. From 36 to 40 beats per 
 minute is the normal rate, and this gives a period of 1-5 seconds 
 for a complete cycle of the heart. 
 
 There is a well-marked interval between the contraction of the 
 auricles and that of the ventricles, during which not only is the 
 ventricle getting up pressure, but the papillary muscles are 
 contracting to prevent the valves being pressed up further into 
 the auricles. Chauveau draws especial attention in the horse to 
 this interval, which has been named the intersystolic period. It 
 must not, of course, be confused with the pause of the heart which 
 follows ventricular systole. The auricles have a longer period of 
 rest than the ventricles, but, as we shall see later, they are not 
 entirely idle between each systole. One important point may 
 here be conveniently stated, that no matter how fast the heart 
 is beating, the frequency depends, not on the duration of 
 
42 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the ventricular systole, but on the length of the subsequent 
 pause. 
 
 Summary of Events occurring during a Cardiac Cycle. — 
 Dividing the events into three periods, and starting with the con- 
 traction of the auricles, the following is a summary of the changes 
 occurring in the heart : 
 
 First Period. — The contraction of the auricles completes the 
 filling of the ventricles. 
 
 Second Period. — -The auriculo- ventricular valves are closed, the 
 ventricles contract, the aortic and pulmonary valves open, blood 
 is pumped into the aorta and pulmonary artery, the impulse of 
 the heart is made against the wall of the chest, the first sound is 
 produced, the auricles fill with blood, and the whole is followed 
 by a short pause. 
 
 Third Period. — The aortic and pulmonary valves close, the 
 second sound of the heart is produced, followed by a long pause, 
 during which diastole of both auricles and ventricles occurs, the 
 auriculo-ventricular valves open, and blood flows into all the 
 chambers. 
 
 The impulse of the heart, to which we have previously referred 
 as being felt externally between the fifth and sixth ribs, is not 
 given by the apex, but by the lower half of the left ventricle. 
 There is no such thing as an apex-beat ; the apex practically does 
 not move as long as the heart is retained within the pericardium, 
 but if the latter be opened, the apex is tilted forward with each 
 contraction. The Use of the Pericardium is to prevent over- 
 distension of the heart. 
 
 Cardiac Sounds. — There are four causes of sound in the heart, 
 but as they work in pairs only two sounds are heard. We have 
 previously indicated where these occur in the heart's cycle. The 
 first sound is a long, booming one, and is made up of two causes— 
 the muscle sound of the contracting ventricle and the closure of 
 the auriculo-ventricular valves. The proof that the valves are 
 not wholly responsible for the first sound is that the bloodless, 
 beating heart still gives out a sound during ventricular systole. 
 
 The cause of the second sound has never been questioned ; it is 
 due to the closure of the aortic and pulmonary valves; it may 
 be abolished by hooking these back, and re-established by releas- 
 ing them. Under pathological conditions when the aortic valve 
 is destroyed a murmur takes the place of the normal heart sound. 
 When an animal is bled to death the second sound disappears 
 before the first — in fact, it is abolished immediately the amount 
 of blood propelled into the aorta is insufficient to distend this 
 vessel properly. 
 
 As the first sound of the heart is heard from the beginning 
 and during the major part of the systole of the ventricle, it is 
 
THE HEART 43' 
 
 termed the ' systolic ' ; while the second sound, occurring at the 
 beginning of diastole, is termed the ' diastolic' On auscultation 
 the two sounds are heard with unequal intensity at different 
 parts of the cardiac area. They are heard better on the left side 
 than on the right, not because the heart is nearer to that side 
 than to the other, but for the reason that there is a larger gap 
 in the left lung, which exposes the heart and allows its impulse 
 to be felt against the chest wall. The two sounds are very 
 accurately represented by the words lub diip. 
 
 Intracardiac Pressure. — Most important additions to the 
 physiology of the heart have been made by studying the pres- 
 sure existing in its chambers. The pressure exerted upon the 
 blood by the heart varies from moment to moment ; the pump 
 is for ever being charged and discharged, and both these pro- 
 cesses depend upon the condition of internal pressure existing 
 at the time. An examination of this internal pressure not only 
 throws light upon the circulation, but also furnishes a better 
 understanding of the mechanism of the heart itself. 
 
 It is interesting to note that the first experimental work done 
 in this connection was by a French veterinary surgeon, Chauveau, 
 in conjunction with a phy- 
 sicist, Marey. Their work, 
 for beauty, originality, and 
 exactitude, has only re- 
 cently been exceeded. Ob- 
 servations were made on 
 the horse by means of an 
 instrument known as the 
 
 cardiac sound, a diagram FlG i7 ,_Diagram of Cardiac Sound. 
 
 of which may be seen in A _, ,, , . , „ . , 
 
 _. t • j -ui A, Elastic ampulla for auricle ; V, for 
 
 Fig. 17. It IS a double ventricle; T, tubes connected with re- 
 
 tube, having at its ex- cording tambours. 
 
 tremity two elastic balls 
 
 separated so that when the apparatus is introduced into the 
 heart, on the right side through the jugular vein, and on the left 
 through the carotid artery, in each case one ball lies in the auricle 
 and one in the ventricle. The air in the apparatus is compressed 
 when the heart's cavities contract, and the compression moves a 
 lever placed in connection with a recording surface. The intro- 
 duction of the apparatus causes no pain, and as there are no 
 sensory nerves in the lining membranes of the bloodvessels or 
 heart, its presence gives rise to no inconvenience. Chauveau, in 
 one of his memoirs, states that the pulse-rate was not disturbed, 
 and the introduction of the instrument did not cause the animal 
 to cease feeding. 
 A tracing taken by means of the apparatus just described is 
 
44 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 seen in Fig. 18. In it may be Seen curves obtained simultaneously 
 from the auricle and ventricle ; the vertical dotted lines indicate 
 coincident periods in both chambers. Taking the auricular 
 curve, there is a sharp, sudden rise, indicating auricular systole, 
 followed by a sudden fall in pressure, and the contents of the 
 chamber are discharged. This is succeeded by two minor rises 
 and falls in pressure before the pause in the heart's cycle D is 
 reached. The curve of intraventricular pressure shows a slight 
 and temporary rise at the moment the auricle reaches its maxi- 
 mum of pressure, and immediately afterwards a sharp, sudden 
 rise in the intraventricular pressure starts. The pressure is main- 
 tained for a short time when once it has reached its maximum, 
 
 Fig. 18. — Curves of Endocardiac Pressure taken with Cardiac Sounds. 
 
 Aur., Auricular curve; Vent., ventricular curve; AS, period of auricular systole, 
 including relaxation; VS, of ventricular systole, including relaxation; 
 D, pause. 
 
 and the curve is in consequence flattened; this flattening is 
 called the systolic plateau, and is followed by an abrupt fall the 
 moment the pressure within the ventricles is sufficiently low 
 to allow the aortic and pulmonary valves to close. The fall in 
 pressure which represents the end of the ventricular systole 
 is followed by the pause D, and this is once more succeeded 
 by a contraction of the auricles. 
 
 All that we have attempted to do in the above is to focus 
 attention on the fact that certain positive and negative waves of 
 pressure are constantly occurring in all chambers of the heart. 
 Their shape on a recording surface depends upon the nature 
 of the apparatus employed ; their significance remains, and must 
 now engage our attention. 
 
THE HEjRT 45 
 
 There are three ' standard movements ' in the heart to which 
 other cardiac events may be referred in point of time; in the 
 measurement and interpretation of pulse tracings in clinical 
 work these are important landmarks. One is the closure of the 
 semilunar valves of the aorta and pulmonary artery, another is the 
 opening of the auriculo- ventricular valves. The first is known 
 for brevity as the ' S.C. period,' the second is known as the 
 ' A.O. period.' 
 
 In Fig. 18 the S.C. period occurs near the end of the ventricular 
 plateau, just when the pressure in the ventricle becomes less than 
 that in the aorta and pulmonary artery; the A.O. period occurs 
 at the bottom of the down-stroke. Between these two points, 
 brief as the period is, there exists within the heart rather a pecu- 
 liar condition : the semilunar valves are closed, but the pressure 
 in the ventricles is still too high to admit of the auriculo- ventricu- 
 lar valves opening, so that for this brief period no blood is entering 
 the ventricles, which are screened off from all parts of the circu- 
 latory system. This period • is known as the postsphygmic. 
 Meanwhile the ventricular walls are relaxing, and as the cavity 
 of the ventricle expands the pressure falls sufficiently to allow 
 the auriculo- ventricular valves to open, and blood pours in from 
 the auricle. 
 
 This is now the period of heart pause, during which both 
 auricles and 'ventricles are filling simultaneously; it is succeeded 
 by the contraction of the auricles, by which the ventricles are still 
 further distended, and under the steadily increasing pressure in 
 the ventricles the auriculo-ventricular valves are closed. This 
 point occurs in Fig. 18 shortly after the beginning of the up- 
 stroke on the ventricular curve. The closure of the auriculo- 
 ventricular valves is referred to as the third standard movement. 
 The ventricles are now full of blood, shut off from the auricles 
 by the auriculo-ventricular valves, and shut off from the general 
 circulation by the closed aortic and pulmonary valves. This 
 period, which is extremely brief, is known as the presphygmic, 
 the period of rising pressure, or, as Stewart graphically puts it, 
 the period during which the ventricles are ' getting up steam.' 
 No blood can leave the ventricles until the pressure in their 
 cavities rises above the aortic and pulmonary pressures. With 
 the opening of the semilunar valves the blood is leaving the. 
 heart, and it continues to leave it during the period shown on 
 the tracing as the systolic plateau. 
 
 If Fig. 18 be again referred to, and the curve given by the 
 auricles examined, three well-marked waves will be seen; the 
 first and largest, known as 'A,' we have previously referred to 
 as corresponding to the contraction of the auricle; the second 
 positive wave, known as ' C,' occurs during the rising pressure in 
 
46 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the ventricles, and is probably due to the bulging of the auriculo- 
 ventricular valves into the interior of the auricle. Chauveau, 
 having experimented with a finger in the contracting auricle of 
 the horse, says this upward bulging does occur. The cause of the 
 third wave, known as 'V,' is not yet agreed upon. The interest 
 of these auricular curves is mainly clinical ; similar waves may be 
 observed in veins near the heart, and the venous pulse thus 
 observed may be employed to indicate irregularities in auricular 
 contraction. We shall see presently that the contraction of the 
 heart is initiated in the right auricle, so that a clinical examina- 
 tion of the behaviour of this cavity through the medium of the 
 jugular pulse is a method of diagnosis of the utmost importance. 
 In modern investigations of heart irregularities the most valuable 
 information is being afforded by the A to C interval, or the time 
 distance between the waves representing auricular and ventricular 
 systole. In Fig. 19 may be seen a venous pulse-tracing from the 
 
 Fig. 19. — Simultaneous Record of Jugular Pulse, Ventricular Con- 
 traction, Auricular Contraction, and Carotid Pulse, in the Dog 
 
 (CUSHNV AND GrOSH). 
 
 a, c, v, The three elevations of the jugular pulse. (Time-trace, 
 fifths of a second.) 
 
 dog in relation to auricular and ventricular contraction. The 
 information thus obtained regarding the condition of the auricle 
 may be compared with that obtained by the cardiograph and 
 from the pulse- tracing in relation to the condition of the ventricle. 
 Intracardiac Pressures. — The positive and negative pressure., 
 in the heart in large dogs have been measuied with the following 
 results : 
 
 Right auricle: maximum positive, 20 mm. (J inch) of mercury; 
 
 minimum negative, - 10 mm. (| inch). 
 Left ventricle: maximum positive, 230 to 240 mm. (9I to 
 
 gj inches); minimum negative, -30 to 50 mm. (if to 
 
 2 inches). 
 Right ventricle: maximum positive, 70 mm. (2J inches)- 
 
 minimum negative, -25 mm. (1 inch). 
 
THE HEART 
 
 47 
 
 In the horse the maximum pressure in the left ventricle has 
 been found to be from 178 to 318 mm. of mercury (7J to i2§ 
 inches), or a column of blood 2-4 to 4-3 metres (9 to 14 feet) in 
 height. In the right ventricle of the same animal the maximum 
 pressure was 34 mm. (i| inches), equal to a column of blood 
 0-46 metre (i£ feet) high. 
 
 Starling* points out that, provided the blood-pressure in the 
 heart does not exceed the above physiological limits, and the in- 
 flow of blood remains constant, it is immaterial to the heart what 
 blood-pressure it has to contend with. The more the blood-pres- 
 sure rises, the greater the quantity of blood which passes through 
 the coronary arteries, and consequently the better blood-supply 
 to the heart muscle. 
 
 The negative pressure within the heart has not been satis- 
 factorily explained. According to one view, the heart behaves 
 like the bulb of a flexible syringe, which is discharged by pressure 
 and filled by the elastic dilatation of its own walls ; but there are 
 objections to this view, inasmuch as no-elastic dilatation can be 
 demonstrated by experiment, nor have any of^the other views 
 put forward been supported by conclusive experimental evidence. 
 The explanation that the dilatation was due to the aspiratory 
 effect of the air-tight thorax may be negatived by the fact 
 that the negative pressure may be still recorded with the thorax 
 open. In fact, the explanation of cardiac dilatation has yet to 
 be found. 
 
 The Cardiac Impulse has been studied by means of an instru- 
 ment termed a cardiograph, which transmits the impulse of the 
 heart on the chest wall to a recording apparatus. Curves so 
 obtained are often difficult to interpret; they are the graphic 
 record, not of one event, but of a series of events, the chief of 
 which are variations in ventricular pressure and changes in 
 volume. In Fig. 20 is a cardiogram which shows a small eleva- 
 tion corresponding to auricular contraction, followed by a large 
 rise due to ventricular systole, with a sudden and then pro- 
 longed drop indicating relaxation of the ventricles. In the horse 
 the impulse of the heart occurs on the cartilages of the fifth and 
 sixth ribs, close to the articulation with the rib, the centre of the 
 shock being the fifth intercostal space (see Fig. 45, p. 107). 
 
 The Capacity of the Heart may be ascertained by enclosing 
 it in a chamber termed a cardiometer, and measuring the change of 
 volume during systole and diastole. 
 
 Observations so conducted show that the ventricle does not 
 
 empty itself iat each systole ; as much as one-third of the blood 
 
 may be left in it. Colin, many years ago, showed this for the 
 
 horse, and stated that not more [than two-thirds or three-fourths 
 
 * ' The Law of the Heart,' E. H. Starling, M.D., F.R.S., 1918- 
 
48 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 of the ventricular charge was expelled. The quantity of blood 
 with which the heart is capable of dealing cannot be ascertained 
 by measuring the capacity of the chambers. Munk gives the 
 capacity of the horse's ventricle at i litre (176 pints), equivalent, 
 roughly, to 1 kilogramme (2-25 pounds) blood, and states that 
 each ventricle contains one-thirtieth of the blood in the body, 
 so that when both contract one-fifteenth of the total blood is 
 ejected. Both ventricles deliver the same amount of blood, for 
 there is as much entering the heart from the pulmonary veins as 
 from the venae cava?. 
 
 Work of the Heart. — This may be calculated if we know the 
 amount of blood being discharged from the heart at each Stroke, 
 and the pressure against which it is propelled. The amount 
 
 pumped out at each systole 
 of the ventricle is liable to 
 great variation ; at least, 
 such are the results of ex- 
 periments on the dog, in 
 which animal it has been 
 shown that the contraction 
 volume of the left ventricle 
 diminishes as the size of 
 
 _ the animal increases. It 
 
 „ , is obvious that the right 
 
 Fig. 20. — Cardiogram taken with Mareys . • 1 j 1 , 
 
 Cardiograph (Stewart). ventricle does less work 
 
 A, Auricular systole ; V, ventricular systole; %*? th \ left ' f ° r treason 
 D, diastole. The arrow shows the direc- tnat « has to pump the 
 tion in which the tracing is to be read. same volume of blood 
 
 against a much smaller 
 peripheral resistance; it has been said, indeed, that the right 
 heart does one-quarter the work of the left. 
 
 If we take the amount of blood pumped at each stroke into 
 the aorta of the horse as about 1 kilogramme (2-25 pounds) in 
 weight, and the pressure under which it is forced upwards as 
 equivalent to a column of blood 3-048 metres (10 feet) in height, 
 then the work of the left ventricle at each stroke is equal to 
 10 kilogrammes (22-5 pounds) raised 0-304 metre (1 foot) high, 
 or for twenty- four hours, allowing the work of the right heart to 
 be one-fourth that of the left, 212,275-86 kilogramme-metres 
 (1,539,000 foot pounds). This amounts to about one-thirtieth 
 of a horse-power per diem; Munk places it at one-thirty-sixth of 
 a horse-power. If the amount of blood expelled by the left 
 ventricle at each stroke be equal to 1 kilogramme, then in a state 
 of repose the entire blood in the body of a horse passes through 
 the heart in about thirty beats, or in forty-five seconds. Munk 
 says that in the horse the entire blood passes through the heart 
 
 a 
 
THE HEART 49 
 
 in fifty seconds, in"the ox in forty seconds, and in the dog in 
 twenty seconds. Since the amount of work performed by the 
 heart is increased during exercise, the above calculations only 
 hold for a horse in a state of repose. During work a larger quan- 
 tity of blood passes through the heart and a high blood-pressure 
 occurs under the influence of the muscular walls. This high 
 pressure is necessary in order to force open the aortic valves and 
 insure that there shall be a sufficient flow of blood through the 
 parts supplied even by the finest vessels. Starling* points out 
 that the heart muscle does its work with the greatest judgment 
 and adjusts its energy to the mechanical conditions present. If 
 the muscle receives a larger quantity of blood, the heart tem- 
 porarily dilates and the muscle fibres lengthen ; this lengthening 
 increases the energy of the heart muscle, and with a large 
 quantity of blood passing into the coronary arteries the muscle 
 substance is kept well supplied with oxygen, the tone of' the 
 heart is restored, and the organ returns to its normal size, 
 although doing more work. A diseased or fatigued heart does 
 not recover, but may remain > dilated during the whole work 
 period, and eventually under stress become permanently en- 
 larged. 
 
 Coronary Circulation.— The vascular system which supplies 
 the heart substance is lodged in grooves in its wall, and much dis>- 
 cussion has arisen, not only as to the moment at which the 
 arteries receive their supply, but also as to the effect on the 
 coronary bloodvessels of the contraction of the surrounding 
 heart muscle. It is now generally admitted that the coronary 
 arteries receive their blood during the ventricular systole, and 
 not, as was originally thought, during the closure of the aortic 
 valves. The latter, as we have seen, do not cover the openings 
 of the coronary arteries. The question of the effect of the heart's 
 contraction on the vessels lodged in its walls is far more difficult 
 of solution. If the squeezing of the heart will assist the passage 
 of blood in one direction it must retard it in another, for the 
 veins and arteries lie side by side, the blood flowing in opposite 
 directions; the effect on the thick-walled arteries is, however, 
 less than on the thin-walled veins. When the ventricle begins 
 to contract, it can be shown that the pressure and velocity in the 
 coronary arteries is increased; but as the contraction proceeds, 
 and the muscle is, as it were, being wrung, the coronary vessels 
 are clamped, and the blood in the arteries is driven back on the 
 aorta, while that in the veins is forced onwards to the right auricle. 
 At diastole the coronary arteries at once refill, and, as we have 
 seen above, a further charge is pumped in at the beginning of 
 contraction. Other experiments appear to show that with each 
 
 * ' The Law of the Heart,' E. H. Starling, M.D.. F.E.S., 1918. 
 
 4 
 
50 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 systole of the heart the coronary system is emptied towards the 
 venous side, and at each diastole it is filled. 
 
 The effect of occluding the coronary arteries is of the utmost 
 practical interest. If all the arteries be clamped the heart at once 
 stops ; but if the observation be limited to one vessel only, that 
 portion of the ventricle supplied by it ceases to beat. The arrest 
 of the ventricle is a curious condition, giving rise either at once 
 or soon after to the phenomenon known as fibrillar contraction, 
 in which the surface of the heart presents vibrating, twitching, 
 disorderly movements, to which the term delirium cordis has aptly 
 been applied. It is as if each fibre of the heart were irregularly 
 contracting on its own account, independently of its neighbours. 
 Fibrillation of the auricles may also be experimentally pro- 
 duced, but the auricles, unlike the ventricles, appear to possess a 
 capacity for returning to co-ordinate contraction. Consider- 
 able attention is now being paid clinically to fibrillation of 
 the auricles, which Lewis* finds is the commonest persistent 
 irregularity exhibited by the human heart, constituting approxi- 
 mately 50 per cent, of all such cases. He has also observed it 
 in the horse, and it is quite likely that it may turn out to be 
 a relatively frequent condition. Though fibrillation of the 
 ventricles means immediate death, fibrillation of the auricles 
 does not. 
 
 The Cause of the [Heart-Beat. — It seems incredible that the 
 use of the heart should have remained unknown until the early 
 part of the seventeenth century. Even now some of the chief 
 features in its working are obscure, and concerning one of them, 
 the cause of the heart-beat, much difference of opinion exists. 
 
 An ordinary skeletal muscle is under the control of the nervous 
 system by which its movements are carried out, but the hollow 
 heart muscle, whose never-ceasing action is maintained for years 
 with perfect regularity, is known to beat independently of any 
 nervous supply. The evidence of this is conclusive ; the hearts 
 both of the frog and of mammalia are capable under similar con- 
 ditions of contracting rhythmically for hours, even for days, 
 when entirely removed from the body, and therefore when no 
 longer in connection with the nervous system. The discovery 
 of nervous bodies called ' ganglia ' in the substance of the heart 
 wall at once appeared to afford a solution of the vexed problem 
 of why the heart was capable of spontaneous movements, but 
 it was shown that the embryonic heart was capable of spontane- 
 ous movement before any sign of ganglia appeared in its walls. 
 The position in which the inquiry stands to-day is practically 
 represented by the above; physiologists are not agreed as to 
 
 * T. Lewis, M.D., D.Sc, ' Auricular Fibrillation,' Heart, vol. i\, 
 No. 4, 1910. 
 
THE HEART 51 
 
 whether the heart muscle, independently of its ganglia, sets up 
 its own movements, or whether these are initiated by the nervous 
 elements embedded in its walls. The former is called the myo- 
 genic, the latter the neurogenic, theory. Both these views must 
 be briefly examined. 
 
 The Neurogenic Theory. — In the walls of the frog's heart, and 
 those of a few other cold-blooded animals, intrinsic nerve ganglia 
 have been discovered, and from this it has been argued that some 
 such arrangement exists in the hearts of mammalia. This, how- 
 ever, has never been demonstrated beyond doubt. Three 
 intrinsic ganglia, known after their discoverers as Remak's, 
 Bidder's, and von Bezold's, are situated mainly in the venous 
 end of the heart — viz., in the auricles, at the junction of auricle 
 and ventricle, and in the interauricular septum. Ganglia have 
 been described as occurring in the ventricles, but at present no 
 conclusive evidence has been brought forward to prove this. 
 
 The neurogenic theory requires that in these ganglia, situated, 
 it will be observed, at that end of the heart which initiates the 
 c ontraction, impulses are originated which pass out to the neigh- 
 bouring muscular tissue, and give rise to a regular sequence of 
 events. But there is no evidence in vertebrates that the heart 
 possesses an intrinsic motor nervous system charged with the 
 spontaneous production of rhythmical contractions. The whole 
 of the nerves in a strip of heart wall may be cut without the 
 tissue losing its property of spontaneous contraction. 
 
 The Myogenic Theory demands that the heart muscle, indepen- 
 dently of any nervous supply, shall possess the power of con- 
 tracting automatically and rhythmically, and the experiment last 
 named lends considerable support to this theory. The contraction 
 of the heart from base to apex is provided for by the A.V. bundle 
 of conducting muscular tissue described at p. 33, which links 
 up auricle and ventricle, while the base of the heart is provided 
 with muscle, more pronouncedly automatic than that found in 
 the ventricles, which insures the normal sequence of events 
 from base to apex. 
 
 Within the right auricle, below and to the right of the coronary 
 sinus, lies the auriculo-ventricular node (Fig. 12), the com- 
 mencing portion of the A.V. bundle; if this connection be cut 
 or compressed, disturbance of conduction follows known as 
 heart-block. 
 
 In a work of this kind it is not necessary to enter deeply into 
 vexed questions such as the one we are here considering, but 
 two more points in favour of the muscular theory of contraction 
 may be briefly mentioned. 
 
 The normal direction of the wave of contraction of the heart 
 muscle from auricle to ventricle may, undej; certain conditions, 
 
52 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 be reversed — viz., from ventricle to auricle. For example, if a 
 ligature be passed around the heart of the frog between the 
 sinus and the auricle (Stannius's experiment), the auricle and 
 ventricle cease to beat; if now the ventricle be stimulated to 
 contract, the auricle follows. On the theory of muscle con- 
 duction this experiment can be explained, but not on that of 
 nerve conduction. 
 
 The balance of evidence is in favour of Gaskell's myogenic 
 theory as the conducting medium of automatic action, but this 
 still leaves the question of the nature of heart automaticity 
 untouched. 
 
 Heart Automaticity. — A heart, even one which has been 
 apparently dead for some time, may be revived by placing it in 
 an atmosphere of oxygen and transfusing through its vessels a 
 solution containing sodium chloride, calcium chloride, potassium 
 chloride, sodium bicarbonate, and grape-sugar. It was Ringer 
 who was the first to show that a fluid of a definite strength con- 
 taining sodium, potassium, and calcium chlorides would keep 
 the frog's heart beating for days ; but it is only recently that the 
 mammalian heart, and one even that has been apparently dead 
 some days, has been shown to be capable of resuscitation. 
 Not only are sodium, potassium, and calcium chlorides required 
 to be of a definite strength, but practically no substitute is effi- 
 cient. The dextrose and sodium bicarbonate are added to in- 
 crease the efficacy of the work, but they are not' essential ; on a 
 diet of inorganic salts the heart is capable of beating rhyth- 
 mically for days. 
 
 We have at p. 8 referred to the action of a 0-9 per cent, solu- 
 tion of sodium chloride as a physiological solution. It is sodium 
 chloride which, in a strength of 0-5 to o-6 per cent., is mainly 
 responsible for the normal osmotic pressure of the blood, and 
 though alone it is not capable of maintaining the heart-beat,, 
 nor of furnishing the tissues with all the inorganic material they 
 require, nevertheless it is more effective in these respects than 
 any other single salt. The presence of calcium, though the 
 amount in blood is only small, appears to be necessary to con- 
 traction; at any rate, the addition of calcium to the 'fed' 
 heart, or its application to the isolated strip, not only brings on 
 contractions, but increases the length of time during which they 
 are continued. The physiological effect of potassium appears 
 to be connected with relaxation rather than with contraction. 
 
 Facts such as the above suggest that the heart is capable of 
 generating in its own substance a stimulus to contraction, and 
 the hypothesis of an inner stimulus, though it takes us no nearer 
 to a conception of the nature of the stimulus, assists the imagina- . 
 tion. It is not considered that the inner stimulus is represented 
 
THE HEART 53 
 
 by the inorganic salts, but rather that in the presence of these 
 it is capable of doing its work. 
 
 The Physiological Properties of Cardiac Muscle differ from 
 those of skeletal muscle. The stimulation of skeletal muscle 
 with a weak current causes a weak contraction, with a strong 
 current a more powerful contraction. But the heart muscle 
 behaves differently; the amount of contraction it exhibits under 
 stimulation is the full amount which it is at the time capable 
 of exhibiting, no matter whether the current be a weak or a 
 strong one, the ' all or none ' effect. This remarkable difference 
 has been explained by the fact that in skeletal muscle the fibres 
 are isolated from one another, while in heart muscle they form a 
 continuous sheet, and the excitation travels from fibre to fibre. 
 
 Another difference distinguishing heart from skeletal muscle 
 is the reaction to repeated electrical stimulation. If a series of 
 rapid induction shocks be passed into skeletal muscle, it is thrown 
 into a condition known as tetanus ; but heart muscle cannot be 
 tetanised, for the reason that electrical stimuli applied to it 
 during the period of its contraction produce no effect whatever. 
 It is only during diastole that a response to stimulation is ob- 
 tained. 
 
 The non-irritable period of the heart muscle during its systole 
 is named the refractory period, and the response to stimula- 
 tion obtained during diastole is described as an extra contrac- 
 tion. Having executed this contraction out of its regular 
 sequence, the heart makes the succeeding pause longer than 
 usual, by which means it picks up its rhythm as though it had 
 never been disturbed. The pause is known as a compensating 
 pause. 
 
 The heart muscle stands alone in being non-irritable during 
 its period of actual contraction ; neither skeletal nor plain muscle 
 exhibits this phenomenon. The refractory period is in all proba- 
 bility connected with the internal metabolic processes concerned 
 in the building up of the heart's contractile material, and it 
 appears at present impossible to separate these from the funda- 
 mental processes of rhythmicity. 
 
 The direction of a contraction through the mammalian heart 
 is from auricles to ventricles via the auriculo- ventricular bundle ; 
 the rate of conduction through this bundle is slower than that 
 through the' ordinary heart muscle, which explains the slight 
 pause between the contraction of the auricles and that of the 
 ventricles. The rhythm set by the auricles is under normal 
 conditions taken up by the ventricles; but experimentally it 
 can be shown that interference with the A.V. bundle may lead 
 to the rhythm of the ventricles being slower and independent of 
 . that of the auricles. The precise results obtained depend upon 
 
54 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 whether there is a partial or complete block between the auricles 
 and ventricles. The condition thus experimentally produced has 
 its clinical counterpart in irregularities due to heart-block. 
 
 One word more may be said in connection with the transmis- 
 sion of a contraction through the heart. Chauveau has shown 
 that the two auricles in the horse do not contract precisely 
 together; there is a slight delay in the contraction of the left, 
 and this is explained by saying that the transmission of the 
 contraction from right auricle to left auricle takes time. 
 
 Nervous Mechanism of the Heart. — Up to this point the 
 question of the rhythmical contraction of the heart has been con- 
 sidered ; we have now to take up another and distinct question- 
 viz., the influence of the nervous system in regulating the activity 
 of the heart. The heart receives nerve supplies from two 
 portions of the central nervous system: one is concerned with- 
 the transmission of impulses which slow or stop the heart, hence 
 called inhibitory ; the other conveys to the heart impulses which 
 stimulate or augment the activity, and are in consequence 
 known as augmentor or accelerator nerves. From the vagus 
 are derived the inhibitory effects, and from the sympathetic the 
 augmentor. It is obvious that these nerves are antagonistic, 
 the one endeavouring to slow the heart, the other pressing it on. 
 The balance between these two opposite conditions results in 
 the normal rate of heart-beat. 
 
 Not only have these two nerves opposite functions, but they 
 are also structurally different, the vagus being a medullated, the 
 sympathetic a non-medullated nerve. 
 
 An immense amount of work has been done in endeavouring 
 to elucidate the physiological effects on the heart of the vagus 
 and sympathetic, and the results have not always been concor- 
 dant. This may be due to the fact that the frog has usually 
 furnished the experimental material. The physiology of this 
 creature's heart is known more completely than that of any other 
 animal's, but what is known cannot always be held true of the 
 mammal, owing to anatomical differences in the arrangements 
 of the nerves. 
 
 In the frog there is a vago-sympathetic nerve in the neck, in 
 which the fibres of both nerves are anatomically mixed, though 
 functionally distinct ; the mixing up does not occur in the chest, 
 as in the mammal, but close under the skull. When this vago- 
 sympathetic is stimulated, either inhibitory or augmentor effects 
 may be obtained, according to which one of the set of nerve- 
 fibres in the mixed nerve happens to be more efficiently stimu- 
 lated. The mixing of the fibres takes place in the ganglion of the 
 vagus nerve, which lies just outside the skull. If the intracranial 
 fibres of the vagus are stimulated, the effect on the heart is 
 
THE HEART 
 
 r\ Jugular ganglion. 
 
 55 
 
 G.Tr.Vg. Gan- 
 glion on the 
 trunk of the GTtVq 
 vagus ; the black 
 line through it 
 is the internal 
 branch of the 
 spinal accessory. 
 
 Vg. The vagus \[n c jl 
 
 nc., nc. The 
 cardiac branches^ 
 of the vagus con-5i 
 veying inhibit- 7J 
 ory fibres to 
 heart. 
 
 . nc, nc. Car- 
 diac branches of 
 the sympathetic 
 conveying aug- 
 inentor fibres to 
 the heart. 
 
 , GTh.+ and 
 G.Th. 5 Fourth 
 and fifth thora- 
 cic ganglia on 
 sympathetic 
 chain. 
 
 r y Right vagus. 
 
 , o k Inhibitory fibres from 
 
 v F spinal accessory enter- 
 ing the vagus. 
 
 C. S y. Th e cervical lym - 
 pa.helic nerve. 
 
 G.C. Inferior 
 g ui^licn. 
 
 cervical 
 
 A.:;>. Subclavian artery, 
 
 Ait V. Annulus of Vieits- 
 
 G.St. Ganglion stellai'im. 
 
 D.I I , D.III, The inferior 
 D. If roots of the second and 
 ttiird .spinal nerves, passing 
 by means of r.c. the ramus 
 communicans to the gang- 
 lion stellatum. These are 
 the augmentor fibres, pass- 
 ing both by the annulus and 
 n jti inferior cervical ganglion to 
 1 li] the heart by nc. nc. 
 
 D1V . 
 
 The dotted line in certain 
 thorncic nerves, D.I., D.IV., 
 and D.V., indicates that they 
 may contribute augmentor 
 
 ^-^i fibres to the sympathetic. 
 
 Fig. 21. — Diagrammatic Representation of the Cardiac Inhibitory and 
 
 Augmentor Fibres in the Dog (Foster). 
 The upper portion of the figure shows the inhibitory, the lower the augmentor. 
 
 fibres. 
 
56 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Fig 
 
 purely inhibitory, and if the fibres of the sympathetic are stimu- 
 lated just before they enter the ganglion, the effect is entirely 
 
 augmentor. In the mammal 
 the vagus and sympathetic 
 are distinct, even if, as in 
 some animals, they run in 
 the same sheath, while ac- 
 celerator fibres do not join 
 the sympathetic until it 
 enters the thorax. 
 
 In the mammal the vagus 
 arises from the medulla ; 
 the inhibitory fibres it re- 
 ceives are derived from the 
 spinal accessory and join the 
 vagus within the skull ; the 
 cardiac branches of this 
 nerve are given off from it 
 in the thorax. The sym- 
 pathetic supply to the heart 
 comes out of the spinal cord 
 certainly at the second and 
 third dorsal nerves, probably 
 at others, and by means 
 of the rami communicantes 
 passes to the stellate gan- 
 glion, thence to the inferior 
 
 cervical ganglion on the cervical sympathetic trunk, and from 
 
 this ganglion to the heart The above arrangement applies 
 
 especially to the dog, and 
 
 is shown diagrammatically 
 
 in Fig. 21 ; it is not quite 
 
 the same for all mammals. 
 
 The rich plexus of nerves 
 
 on the surface of the horse's 
 
 heart may be seen in 
 
 Fig. 22, and that of the 
 
 calf in Fig. 23. 
 Function of the Vagus. 
 
 — If the vagus in the neck 
 
 be cut and its peripheral 
 
 end stimulated, the rate of 
 
 the heart-beat is slowed 
 
 and the force of the beat 
 
 diminished. If stronger stimulation be applied, the heart stops 
 
 in a condition of dilatation, and becomes swollen with blood. 
 
 -Nerves on the Surface of the 
 Horse's Heart (Pettigrew). 
 
 Nerve descending to the auricle; b, c, 
 coronary vessels; e, d t vessels in an- 
 terior ventricular furrow. 
 
 Fig. 23. — Nerves on theTSurface of the 
 Left Ventricle of the Calf (Pettigrew). 
 
 The nerves take a spiral direction, like the 
 muscle fibres; c, apex of ventricle. 
 
THE HEART 57 
 
 Slowing of the heart-rate is the most prominent effect of vagus 
 stimulation, and this in the mammal is more apparent in its 
 effect on the auricles than on the ventricles. The strength of 
 the ventricular contraction may continue undiminished at the 
 time the auricles are suffering from inhibition, and should the 
 stimulation be sufficiently strong to cause the auricles to cease 
 contracting, the ventricles for a brief time are inhibited, and 
 then beat again independently. In other words, in the mammal 
 the vagus is essentially an auricular nerve. The phenomenon 
 of inhibition does not show itself immediately on the application 
 of the stimulus ; there is at least one contraction before the heart 
 slows or ceases to beat. This delay is known as the latent 
 period (Fig. 24). In some animals, such as the cat, even the 
 
 Fig. 24. — Tracing showing Influence of the Arrest of the Heart on 
 Blood-Pressure, due to Stimulating the Vagus (Foster). 
 
 x, Marks on the signal line when the current is thrown into, and y shut off from 
 the vagus. The time-marker below marks seconds, a corresponds in point 
 of time with x ; the heart does not at once cease to beat. The first beat, 
 b, occurs a short time after shutting off the current. The notches in the 
 tracing are due to the beats of the heart. 
 
 stronger stimulation of the vagus only slows the heart; it does 
 not stop it; whereas in the dog relatively weak stimulation may 
 bring the heart to a standstill. Fig. 25 shows the effect on 
 blood-pressure of weak and stronger stimulation of the vagus. 
 
 Inhibition does not last long; it may, of course, cause death, 
 but as a rule it is overcome and the heart starts again. 
 Moderate stimulation of the vagus sufficient to slow the rate 
 may be tolerated for some time. In the frog the same escape 
 from inhibition may be seen. The heart subsequently makes 
 up for lost time by working at a greater rate or strengthening 
 its contractions, until it regains its usual rate of working. When 
 an inhibited heart behaves in this way, its action is spoken of 
 
58 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 as secondary augmentation, and the phenomena, though best seen 
 in the frog, may also, though in a less marked degree, be seen 
 in the mammal. 
 
 Reflex inhibition of the heart may arise through a sensory 
 surface, as in painful impressions, or in a blow on the abdomen, 
 and in other ways. Afferent impulses — that is, impulses con- 
 veyed to a nerve centre from without — must pass through the 
 centre from which the inhibitory fibres of the heart arise, and 
 though such a centre has not been defined with exactitude, the 
 existence of a cardio-inhibitory centre in the medulla is un- 
 doubted. From this centre impulses issue which in a normal 
 
 Fig. 25. — Blood-Pressure Tracings : Rabbit (Stewart). 
 
 Vagus stimulated at 1 ; stimulation stronger in B than in A (Hiirthle's 
 manometer). 
 
 state throughout the whole life of the animal are passing down 
 the vagus, keeping a constant watch and control over the rate 
 of the heart, and the proof that such a view is not a fanciful one 
 is shown by the result of cutting off these impulses by dividing 
 both vagi, when the frequency of the heart-beats is greatly in- 
 creased. This constant action of a nerve centre is known as tonic 
 activity, and this in all probability is the outcome of impulses 
 flowing into it through sensory nerves ; a mechanism so arranged 
 produces what is known as a reflex tonus. 
 
 -■ The Action of Poisons on- the Heart has been appealed to in 
 order not only to decide their pharmacological effect, but also as a 
 physiological means of research in connection with its nervous 
 
THE HEART 59 
 
 mechanism. Atropine causes the heart-rate to become quick- 
 ened, on account of the inhibitory effect of the vagus being sus- 
 pended. If the vagus under these conditions be stimulated, no 
 inhibition follows, and it is argued that this is due to a paralysis 
 of the nerve terminations of the inhibitory fibres in the heart 
 muscle, much the same as that of the motor end-plates in muscles 
 under the action of curare. Muscarine, a poison obtained from 
 certain mushrooms, causes the heart to slow down, and finally 
 to stop. Pilocarpine has much the same effect, and it is assumed 
 that both these alkaloids stimulate the inhibitory fibres. This 
 view is strengthened by the fact that atropine abolishes the in- 
 hibitory effect produced by muscarine and pilocarpine, and it is 
 considered that this result is due to the paralysis of the terminals 
 of the inhibitory fibres in the heart muscle. 
 
 Function of the Sympathetic Nerve. — The course of the 
 accelerator fibres derived from the sympathetic has been pre- 
 viously described. Their distribution in the heart is mainly to 
 the ventricles. The vagus fibres, it willbe remembered, are 
 principally distributed to the auricles. If the sympathetic trunk 
 in the neck of the mammal be stimulated, no effect follows, for, as 
 Fig. 21 shows, there are no accelerator fibres above the inferior 
 cervical ganglion. If certain branches issuing from this ganglion 
 be stimulated, the heart-beats are increased in frequency, and 
 sometimes, but not always, in force; in other cases they are 
 increased in force and not in frequency. There is reason to 
 think these differences are explained by the probability that the 
 accelerator fibres consist of two sets, one increasing the fre- 
 quency of the heart-beat and the other increasing the force. 
 On stimulation, accelerator effects can be obtained from either 
 the right or left ganglion, but the augmentor effect is best ob- 
 tained from the left side. 
 
 The accelerator fibres, like the inhibitory, are in a state of con- 
 stant or tonic activity, as evidenced by the fact that, if they be 
 divided, the heart-rate is thereby decreased. It has been assumed 
 that this constant activity is carried on by a centre situated in 
 the medulla, and that to it pass afferent impulses of a reflex 
 nature which either increase or decrease its activity. No better 
 example of a reflex stimulation of the accelerator centre could be 
 witnessed than the fright of a nervous horse startled ; the heart 
 may almost be heard thumping against the chest wall. 
 
 No such accelerator centre has actually been located in the 
 medulla, but the probability of its existence is considerable. 
 On the other hand, it is not a physiological necessity; accelera- 
 tion might be brought about by inhibition of the cardio-inhibitory 
 centre, and experiment has shown the possibility of the effect 
 being produced through this channel. 
 
60 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The Nature of Inhibition.— The discovery of the inhibitory 
 action of the vagus was a great addition to physiological know- 
 ledge; it settled the important point that such an effect could 
 be produced through the nervous system, and has been the 
 means of adding considerably to our physiological knowledge, 
 of other organs besides the heart. The actual means by which 
 inhibition in the heart is brought about is still a matter of 
 speculation; the view most generally accepted is that pro- 
 pounded by Gaskell. He regards the vagus as the protecting 
 nerve of the heart, and reasoning from the observed fact that 
 after its stimulation and consequent inhibition there is on recovery 
 an improvement in the rate or force of the heart-beats, he con- 
 cludes that during inhibition there is a building up — anabolism— 
 of the muscular tissue which results in the improved condition 
 of the heart. Such changes are of an opposite character to 
 those occurring during contraction, which are of a katdbolic or 
 tissue-destroying nature, by which complex substances are 
 converted into simpler ones, with the production of heat and 
 energy. The latter changes are regarded as brought about by 
 the sympathetic system. During inhibition the heart is being 
 repaired, during contraction its substance is being used up, and 
 Gaskell believes that all muscular tissue is similarly provided 
 with anabolic and katabolic nerves. 
 
 The Depressor Nerve. — The nervous mechanisms considered 
 upto this point are concerned in bringing about some modified 
 action of the heart, under the guiding influence of a nerve centre 
 in the medulla. We have now to consider the case where a 
 nerve running from the heart to the medulla is engaged in a 
 regulative action which, unlike that of the vagus or sympathetic, 
 is not a direct action on the heart itself, but is brought to bear 
 indirectly on the heart mainly through the instrumentality of the 
 vascular (arterial) system. This nerve is the depressor. It is a 
 branch of the vagus distributed to the heart — some say to the 
 walls of the aorta — and from there runs up the neck as a separate, 
 branch in the horse, cat, and rabbit, but in other animals is 
 contained in the trunk of the vagus. It joins the superior 
 laryngeal nerve, and finally reaches a centre in the medulla 
 which regulates the movements of the bloodvessels of the body, 
 known as the vasomotor centre. The impulses which pass along 
 it are afferent — viz., they pass to the central nervous system 
 and not out of it. The heart in this way is placed directly in 
 communication with the centre which presides over the vasculai 
 system, a centre by whose varying activities the arteries of the 
 body are made smaller (constricted) or larger (dilated), according 
 to the needs of the system. If the heart is labouring and its 
 muscular structure becoming weakened, impulses pass up the 
 
THE HEART 61 
 
 depressor to the vasomotor centre, resulting in impulses being 
 sent out which cause the abdominal arteries to dilate and hold 
 more blood. By this means the peripheral resistance is dimi- 
 nished, the blood-pressure falls, and the heart is eased, since it 
 now has less work to do in ejecting its contents. 
 
 If the depressor nerve be divided, no effect follows ; if the end 
 in contact with the heart be stimulated, there is no result; but 
 if the central or upper end be stimulated, the blood-pressure falls 
 (see Fig. 26). 
 
 By some the depressor nerve has been described as the sensory 
 nerve of the heart, as they hold that there are signs of pain when 
 it is stimulated in an animal not under' an anaesthetic. But 
 
 Fig: 26.— Blood-Pressure Tracing: Rabbit (Mercury Manometer) 
 
 (Stewart). 
 
 Central end of depressor stimulated at i; stimulation stopped at 2. 
 
 Time-trace, seconds. 
 
 this view is not generally accepted, and as a matter of fact {he 
 heart may be handled, pinched, pricked, or otherwise injured, 
 without provoking the least sign of pain on the part of the animal. 
 Colin's experiments in this direction on horses appear quite con- 
 clusive. Not only is it considered that the external surface is in- 
 sensible to pain, but the internal surface also; for, as previously 
 noted, the experimental introduction of foreign bodies into the 
 cavities of the heart appears to produce no pain. Under patho- 
 logical conditions the results are otherwise; foreign bodies, so 
 common in the heart of the cow, cause great suffering, therefore 
 there must be sensory nerves, though normally their excitability 
 
 is probably low. 
 
 It is supposed that in the distending aorta the sensory nerve 
 
6* A MANUAL OF VETERINARY PHYSIOLOGY 
 
 endings are stimulated, and that the impulses so obtained are 
 conveyed to the medulla by the depressor, and increase the 
 tonic activity of the cardio-inhibitory centre. It can certainly 
 be shown experimentally that every contraction of the heart 
 sends a series of impulses up this nerve, which give rise not 
 only to a reflex dilatation of the splanchnic arterioles, but also 
 to a reflex cardiac inhibition. 
 
 We have now seen how complex are the influences exercised 
 by the nervous system on the rhythm, force, and tone of the heart 
 muscle, the moderating influence in these directions exercised 
 by the vagus, and the stimulating functions of the sympathetic. 
 The factors determining which influence at the moment is best 
 suited to the existing conditions are to be found in the peri- 
 phery, and not in the central nervous system; from the periphery 
 ■ — viz., skin, muscles, viscera, bloodvessels, and heart — impulses 
 are transmitted to the brain which are obeyed by the cardio- 
 inhibitory or accelerator centres in the medulla. 
 
 The Chemical Stimulus. — One further feature still remains 
 for consideration, and that is the influence of certain ' internal 
 secretions ' on increasing or diminishing the function of the heart 
 muscle. It is now known that the adrenal capsules and pituitary 
 body are capable of furnishing extracts which, when introduced 
 into the circulation, exercise a powerful effect on the heart. 
 An extract of the medulla of the adrenal capsules increases the 
 rhythm, force, and tone of cardiac muscle. An extract of pitui- 
 tary body contains two substances ; one increases the tone, but 
 does not affect the rate of rhythm of the muscle, while the second 
 diminishes the force of the heart-beat. It may well be that these 
 ductless glands pour into the circulation their various secretions 
 in the proportions suited to the most efficient working of the 
 heart, and by their action on the nerve-centres maintain them 
 in effective working order.* The action of these internal secre- 
 tions on the movements of the bloodvessels will be referred to 
 later. 
 
 Electrical Changes in Heart Muscle (see Chapter XIII.). 
 
 Pathological. 
 
 Disease of the heart in the lower animals is uncommon. It might 
 have been thought that horses would be particularly exposed to this 
 class of trouble, bearing in mind the enormous strain placed on the 
 heart during labour, and the utter want of consideration shown by 
 the vast majority of those who ride and drive them. But it is not so. 
 The hearts of horses exposed to the greatest strain seldom show any 
 pathological change; probably the most uncommon lesions found 
 
 * See ' The Factors which make for an Efficient Circulation,' by 
 Professor Sir E. A. Sharpey Schafer, F.R.S., British Medical Journal, 
 October 29, 1910, 
 
THE HEART 63 
 
 on post-mortem examination are those affecting the heart. The 
 heart may dilate under strain, but such dilatation when accompanied 
 by hypertrophy is compensated, and no indication of trouble exists 
 during life. It is evidence of the gross strain to which horses are 
 exposed that ruptures of the heart are by no means uncommon. It 
 is strange they are not more frequent. They probably would be but 
 for the saving reason that degenerations of the heart substance are 
 rare. When the heart ruptures, it gives way in the auricle, where 
 the wall is thinnest — so thin, indeed, that in certain parts of the 
 auricle daylight may easily be seen through the tissue. It is gener- 
 ally the right and not the left auricle which suffers, showing how 
 great is the resistance offered by the pulmonary vessels as the result 
 of engorgement due to severe work. 
 
 Valvular disease is not unknown, but so rare that probably there 
 is no practitioner with a large experience in the examination of horses 
 for soundness who ever thinks of examining the heart ! On the 
 other hand, irregularities in the heart's action are very common; 
 frequently purely functional in character, unassociated with organic 
 change, they do not interfere with the usefulness of the animal. A 
 horse condemned for heart disease on the strength of an intermittent 
 pulse may remain a living reproach to the practitioner. 
 
 The views on heart disease in man have within the last few years 
 been undergoing profound modification through the work of Mac- 
 kenzie,* Lewis,! and other observers. It is too much to expect 
 that the complete revolution in doctrine which these new ideas 
 necessitate will be at once brought about; the process must be gradual, 
 but when accomplished it will be found that very little modification 
 from the views now put forward will be necessary. Briefly, the 
 modern theory of the cajuses of heart failure looks to the heart muscle 
 itself as the prime seat of disorder; indeed, the coarser and more 
 obvious conditions, such as murmurs and irregularities, may not 
 even be pathological, or, if pathological, are of secondary interest 
 to the all-important inefficiency existing in the heart muscle. Mac- 
 kenzie has presented a statement of the modern views of heart 
 trouble in man, of which the following is a brief epitome : 
 
 The popular conception of heart failure is associated with valvular 
 trouble, the thickened and shrunken valve producing incompetence, 
 with leakage from the ventricle, distension of the auricle, stasis in 
 the lungs, back pressure into the veins, with consequent dropsy. 
 Functional murmurs may exist in a perfectly healthy heart under 
 physiological dilatation; a tricuspid regurgitation may be regarded 
 as a safety-valve function, as it is in certain diving animals; and a 
 mitral regurgitation may exist for fifty years without crippling the 
 heart in its work. Irregularities of the heart may not only be 
 normal, but may even, within certain limits) be considered as evidence 
 of the integrity of the heart muscl«. 
 
 These unorthodox views from one who has confined his attention 
 to the study of heart trouble for many years must cause even the 
 most sceptical to pause. According to Mackenzie, the essential causes 
 of heart trouble in man are consequent on auricular fibrillation, a con- 
 dition which alters the whole aspect of the mechanism of heart failure. 
 To the phenomenon of fibrillation attention has been drawn at 
 p. 50. The muscle fibres, instead of contracting in a normal and 
 
 * ' Heart Failure,' James Mackenzie, M.D., LL.D., British Medical 
 
 Journal, April 8, ign.-P- 793- „, T „„ ^ ^ c -c-dc 
 
 f ' Clinical Disorders of the Heart Beat,' T. Lewis, M.D., D.Sc, F.R.S. 
 
64 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 orderly manner, contract irregularly, rapidly, and independently, 
 so that the chambers not only cease to contract as a whole, but actually 
 stand still, while all their fibres are in incessant movement. When 
 this condition occurs in the ventricles, sudden death ensues; when 
 it occurs in the auricles, death does not follow, for fibrillation cannot 
 pass along the bundle connecting the auricles with the ventricles. 
 The degree of heart failure which follows auricular fibrillation depends 
 upon the extent to which changes have occurred in the structure of 
 the heart muscle of both auricle and ventricle. 
 
 In severe inflammatory chest invasions of the horse, the heart, 
 but especially its sac, may become acutely affected. There are few 
 attacks of severe pleurisy in the horse which are not associated with 
 pericarditis, followed not only by great thickening Of the heart sac, 
 but by more or less extensive effusion into it. The heart then 
 becomes enveloped in a water jacket, and this greatly adds to the 
 gravity of the case. In the above acute cases the heart muscle 
 suffers, and haemorrhages into it are common and widespread. 
 
 In the dog the heart's action is normally intermittent. 
 
 Foreign bodies in the heart of cattle, especially cows, are well 
 known, and give rise to a peculiar train of symptoms. Vegetations 
 on the valves of both the dog and pig are recognised in connection 
 with certain infectious diseases. 
 
CHAPTER III 
 
 THE BLOODVESSELS 
 
 The use of the bloodvessels is to distribute the blood over the 
 body, to bring it into contact with the tissues, and to return it to 
 the heart . To accomplish this work there are arteries, capillaries, 
 and veins. 
 
 The systemic Arteries arise from one common trunk, the 
 aorta, which by the process of dividing and subdividing like 
 the branches of a tree forms the arterial system. This system, 
 measured by its total cross section, is very much larger than 
 the parent trunk; in fact, its sectional area, and hence its cubic 
 capacity, has been estimated as several hundred times greater. 
 
 The large arteries differ somewhat in construction from the 
 small ones. The microscope shows that while the large vessels 
 are principally elastic, the small ones are mainly muscular. This 
 does not preclude the small vessels from exhibiting the elasticity 
 possessed by the large ones, for muscular tissue is itself highly 
 elastic. 
 
 This elastic property of arteries is an essential feature in their 
 construction; it admits of a vessel stretching both in width 
 and length, and at the same time insures its recovery to its 
 original dimensions after the stretching force ceases to act. 
 When we remember the intermittent force exercised by the left 
 ventricle on the arteries, we have no difficulty in understanding 
 the necessity for this elastic property. The arteries are always 
 full; for example, in the horse during rest, every contraction of 
 the left ventricle throws into them i litre (176 pints) of blood, 
 which must be accommodated, and this is provided for by the 
 distension of their walls. For every litre of blood entering the 
 aorta, an equal amount must pass out at the periphery, and the 
 reduction in the diameter of the vessels brought about by the 
 exit of this fluid is due to the elastic recoil of the arterial wall. 
 A further use of the elastic arterial wall will be studied when the 
 flow of fluid through tubes is described. 
 
 .' Another essential feature possessed by arteries is their power 
 of contractility. Just as the larger arteries are principally 
 
 6 5 5 
 
66 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 elastic, so the smaller ones are principally contractile. This 
 contractility or power of reducing their diameter is produced by 
 the muscular coat previously mentioned. Though the smaller 
 vessels possess this muscular coat, it by no means follows that 
 they are always fully contracted; in fact, special nerves exist 
 for the purpose of supplying the needful impulses to the muscular 
 tissue which controls or regulates the variations in the diameter 
 of the vessels. In this way the contraction or relaxation of the 
 muscular artery depends upon the set of nerves brought into 
 operation; this movement of the smaller muscular vessels is a tap 
 which regulates the blood-supply to any given part of the body. 
 
 Capillaries.— The minute arteries terminate in the capillaries, 
 and the diameter of the capillary bed thus formed is calculated to 
 be 800 times the diameter of the aorta. It is in these vessels that 
 the interchange between the blood on the one hand and the tissues 
 on the other takes place, and this is rendered easy by the fact 
 that the wall of the capillary consists simply of a very thin mem- 
 brane composed of cells known as endothelial plates. The capil- 
 lary is capable of expanding and contracting, and so of accommo- 
 dating more or less blood as the case may require; this is brought 
 about by the elastic nature of the membrane composing the capil- 
 lary wall (for there are no nerves supplied to them), under the 
 influence of the varying internal fluid pressure. 
 
 The size of the capillaries varies; in places such as the lungs 
 they are relatively large, in other parts such as the skin they aTe 
 very small. Their size depends upon the amount of blood 
 which has to pass through them ; in consequence they are larger 
 during active exercise than during rest. If thtey be observed 
 microscopically in the living animal, the capillaries are seen 
 as a network enclosing small islands of tissue. These are the 
 areas where the interchange between the blood and tissues occurs. 
 They represent the important part of the circulation, in spite 
 of the fact that the longest capillary does not exceed a length of 
 0-5 mm. (^V inch), and that the blood is not contained within 
 it for more than one second. 
 
 The Veins receive their blood from the capillaries. They are 
 thinner walled than the arteries, and their walls collapse when 
 they are empty. Though some degree of variation exists in their 
 structure, yet, speaking generally, they contain less elastic 
 and muscular material than arteries, and more fibrous tissue. 
 In certain veins., such as the vense cavae and those of the pregnant 
 uterus, there is a considerable development of muscular tissue 
 in the walls. 
 
 The venous system is larger than the arterial, and its capacity 
 is regarded as being two or three times greater. The abdominal 
 veins are capable of holding the whole blood of the body, as may 
 
THE BLOODVESSELS 67 
 
 be seen, for instance, at post-mortem examinations. The veins 
 as they pass from the capillaries towards the heart become 
 reduced in number and increased in size, and they terminate 
 in the right auricle of the heart by means of two trunks, the 
 united areas of which greatly exceed that of the aorta. 
 
 In the veins valves are found. These are well marked in the 
 veins of the head, neck, and extremities. The valves look 
 towards the heart, and supply 'a simple and essential means of 
 insuring the return flow of the blood along the veins to the heart. 
 Certain veins have no valves, such as the large veins entering the 
 heart, and those of the bones, abdomen, foot, and brain. 
 .. • Veins are normally pulseless, but under certain conditions a 
 pulse-wave may pass through the capillaries into the veins, pro- 
 ducing a venous -pulse. The best physiological example of this 
 form of pulse can be experimentally produced by stimulation 
 of the chorda tympani, a nerve supplying the submaxillary gland 
 with fibres which cause the bloodvessels to dilate. Under 
 stimulation the vessels dilate, the veins pulsate, and the blood 
 coming from them is even arterial in colour. Another form of 
 venous pulse is met with in the great veins at the root of the neck ; 
 the mechanism of the pulsation in these has already been ex- 
 plained (seep. 41). It is abnormal for pulsations to extend 
 any distance up the jugular vein ; when this occurs the explana- 
 tion is pathological, not physiological. 
 
 Mechanics of the Circulation. — At each systole of the ven- 
 tricle a certain amount of blood is forced under great pressure into 
 an already full aorta, and imprisoned there by the closure of. the 
 aortic valves. The aorta dilates to receive this extra blood, 
 because, owing to the friction in the smaller vessels, or, as we 
 shall speak of it, the peripheral resistance, it is impossible for the 
 amount pumped into the aorta at each systole to pass out at 
 once at the periphery; in this way high blood^pressure is pro- 
 duced in the arteries. The increase in the size of the aorta to 
 accommodate this extra blood commences near the heart, and 
 runs as a wave to the periphery ; this wave is the pulse. 
 
 The two important features of the circulation which we have 
 now to consider are blood-pressure and pulse, and to understand 
 these it is necessary that we should study briefly the laws which 
 govern the flow of fluids through tubes.* 
 
 If water be pumped through a rigid tube or pipe, at every 
 stroke of the pump as much fluid passes out at the farther end 
 of the tube as enters it at the other. Between the strokes of 
 
 * The subject of the movement oi fluids in tubes is not only an ex- 
 tremely difficult branch of physics, but one still imperfectly understccd. 
 We have introduced less of it into this chapter than appears in most works 
 on physiology, and, in fact, have touched only on those general principles 
 which have a direct bearing on the circulation. 
 
68 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the pump no fluid issues from the pipe; the jet is produced only 
 at the moment the pump is in action. No more water can enter 
 this rigid tube from the pump end than can leave it at the outlet. 
 If water be now pumped through a short elastic tube, the outlet 
 of which is in no way obstructed, the current of water through 
 it behaves just as if the tube were rigid— viz., a stream of water 
 issues from the outlet during the action of the pump, and nothing 
 more happens until the next stroke. An important alteration 
 can, however, be made to the current through the elastic tube, 
 by offering an obstruction at the outlet to the free passage of 
 the water. The effect of this obstruction is that the elastic 
 tube expands to accommodate the contents, while a stream pours 
 from the partly obstructed outlet which no longer corresponds 
 to the stroke of the pump, but is a continuous stream which 
 issues so long as the pumping is continued. This continuous 
 stream is produced by the elastic recoil of the tube keeping up 
 the pressure which the pump imparted to the fluid, and the 
 reason why the elastic recoil of the tube is now brought into play 
 is the partly obstructed outlet, or, as we have already termed it, 
 the peripheral resistance. If the elastic tube be of sufficient 
 length, a continuous stream will issue in spite of the absence of an 
 obstruction; this is brought about by the internal fluid friction 
 against the walls of the tube, which of course causes a peripheral 
 resistance. In elastic tubes, therefore, the recoil of the tube 
 converts an intermittent into a continuous flow, and the dis- 
 tension of the tube which produces the recoil is caused by the 
 peripheral resistance. 
 
 Whether in a living tube like a bloodvessel, or in a dead tube 
 like a pipe, fluid flowing under a head of pressure meets with 
 resistance; this is due to friction of the fluid particles against 
 the wall of the tube. The amount of resistance is not the* same 
 throughout the length of the pipe; it steadily decreases to the 
 exit; but this assumes that the pipe maintains an even bore 
 throughout. If the pipe were to narrow at any point in its 
 length, the friction would in consequence increase at that par- 
 ticular place. The resistance or pressure due to friction may 
 be estimated at any part of the tube by the introduction of a 
 gauge — viz., a vertical tube placed at right angles to the flow. 
 In this tube the fluid will rise to a certain height, and the weighf 
 of the fluid column indicates the pressure exerted at that par- 
 ticular spot. Such a tube is known as a manometer, and the 
 pressure it gauges is the side or lateral pressure due to frictional- 
 resistance. 
 
 Of the total force or head of pressure engaged in forcing fluid 
 along a tube, the greater part is used up in overcoming the fac- 
 tional resistance, the remainder imparts to the fluid a certain 
 
THE BLOODVESSELS 69 
 
 velocity, and the remaining pressure is known as the pressure 
 velocity. 
 
 The whole mechanics of the circulation can be worked out on 
 a model consisting of a syringe to represent the heart, elastic 
 tubes to represent the bloodvessels, and a few clamps to offer 
 . the needful peripheral resistance. With such a model, if water 
 be forced into the arterial tubes, the clamps being open and the 
 peripheral resistance therefore very small, it is found,' by means 
 of a manometer, that the pressure in the arterial tube rises with 
 each stroke of the syringe, and falls with the free pouring of the 
 contents into the tubes representing the veins. As the peripheral 
 resistance is small, the pulsation set up in the fluid readily passes 
 into the veins, and a manometer will here register nearly the 
 same rise and fall as in the arteries. 
 
 If, however, the vessels be clamped so as to produce a resist- 
 ance, the first stroke of the pump causes the arteries to become 
 distended; they then recoil, and while doing this they receive 
 another stroke from the pump and become still more distended. 
 Once more they recoil on their contents, and are once more dis- 
 tended by the action of the pump, and so on. If during this 
 time the arterial manometer be watched, it will be observed that 
 the mercury or water rises with each stroke of the pump, but 
 instead of falling at once to zero as it did in the undamped tube, 
 it has time to fall only a short distance before a second stroke 
 of the pump sends it still higher than before; this progressive 
 rising is repeated at every stroke of the pump until the water 
 or mercury refuses to rise any higher in the tube, contenting 
 itself by rising to a certain height at each stroke of the pump, 
 and falling to a certain level during the interval between one 
 stroke and another. In other words, a mean pressure has been 
 ■established in the tubes representing the arteries by the peri- 
 pheral resistance, the elastic recoil of the tube, and the pump- 
 ing of the syringe. So long as these factors remain the same the 
 mean pressure will not vary. If, however, the clamped vessels 
 be released, so as to allow fluid to flow more easily into the tubes 
 representing the veins, the manometer at once shows a fall in the 
 mean pressure owing to the removal of a certain amount of re- 
 sistance, and by removing this resistance completely the mean 
 pressure falls to zero. The mean pressure, then, represents the 
 force which is necessary to cause as much fluid to pass through 
 the" periphery as is being pumped into the system of tubes by the 
 syringe; if the peripheral resistance is high the pressure is high, 
 and vice versa. 
 
 A careful study of this experiment places us in complete pos- 
 session of the main facts of the circulation, but even now we have 
 not learnt all the lessons it is capable of teaching. 
 
7o A MANUAL OF VETERINARY PHYSIOLOGY 
 
 If a manometer be placed on the venous side of the model, 
 it will show a very low pressure at the time when the arterial 
 pressure is high. If the arterial tubes be felt it will be observed 
 that at each stroke of the pump they expand, producing what 
 is known in living tubes as the pulse ; this expansion of the tube 
 is greatest nearest to the syringe, dying out entirely at the 
 peripheral resistance. It is evident that, if we loosen the clamps 
 and so feduce the resistance and lower the mean pressure, 
 pulsatile waves will pass over to the venous side of the model, 
 and these can be again obliterated by screwing up the clamp. 
 Lastly, our model, if working at mean pressure, will show the 
 effect of injury to the tubes ; if the arterial tubes be pricked, a con- 
 tinuous jet of water shoots out, the strength of the jet varying 
 with each stroke of the syringe, whilst an injury to the venous 
 side produces no jet of water, but only a trickling flow. 
 
 Practically, this embraces our knowledge of the main facts of 
 the circulation, for all we have found true of syringe, elastic 
 tubes, and clamps, will be found true of heart, bloodvessels, and 
 peripheral resistance. The heart has to keep the arteries full; 
 the innumerable small arteries with their muscular coats supply 
 the peripheral resistance. Under the influence of this and the 
 contr-action of the left ventricle, the pressure in the arteries rises 
 so- high, and their distension is so great, that as much blood 
 passes through the periphery during the contraction of the 
 heart and the ensuing pause as enters the aorta during the 
 contraction of the left ventricle. The elastic nature of arteries 
 insures that an intermittent is converted into a continuous flow, 
 and thus a perpetual pressure is kept up on the mass of blood 
 during the heart's pause. By a contraction of the arterioles 
 the peripheral resistance is increased and the blood-pressure 
 raised ; by a relaxation of the arterioles the peripheral resistance 
 is reduced and the blood-pressure falls. We have stated that a 
 contraction of the arterioles by increasing the resistance raises 
 arterial pressure and as a rule lowers that in the veins. This 
 holds equally true for the pressure conditions in the vessels of 
 any locally circumscribed area of the body as for the vascular 
 system generally. It must not, however, be forgotten that 
 local effects may and do produce general effects. If, for in- 
 stance, one artery alone contracts, this must lead to an increase 
 of arterial pressure, which produces an increased flow oi blood 
 through all the simultaneously uncontracted arteries and through 
 these into the veins. When the contracted artery is small, so 
 that the area it supplies is limited, the local effects are more 
 marked than the general effects. If, on the other hand, the local 
 area affected is at all large, the influence of changes in the arteries 
 of this area on the general blood- pressure may be very obvious. 
 
THE BLOODVESSELS 71 
 
 We shall meet with a striking instance of this when dealing with 
 the action of the depressor nerve on blood-pressure, through, the 
 medium of alterations brought about by means of the splanchnic 
 nerve, in the arteries which supply the splanchnic area. 
 
 Blood-Pressure. — From what has been said, it is hardly neces- 
 sary to define blood-pressure as the pressure exercised upon the 
 blood in the elastic vessels, resulting from the action of the 
 heart and the peripheral resistance. 
 
 If the peripheral resistance is great through a contraction of 
 the arterioles, the amount of fluid passing into the veins is 
 reduced in quantity; a larger quantity of fluid will in conse- 
 quence exist between the pump and its outlets, and the blood- 
 pressure will rise. If, on the other hand, the blood is passing 
 
 Fig. 27. — -Diagram to illustrate the Slope of 'Pressure along the 
 Vascular System (Stewart). 
 A, Arterial; C, capillary; V, venous tract. The interrupted line represents the 
 line of mean pressure in the arteries, the wavy line indicating that the pres- 
 sure-varies with each heart-beat. The line of the veins passes below the ab- 
 scissa axis (line of zero in atmospheric pressure), indicating that at the end 
 of the venous system the pressure becomes negative. 
 
 freely through the dilated arterioles, the blood-pressure falls. 
 When the heart is more active, or when the arterioles contract, 
 the blood-pressure rises ; when the heart is less active or the 
 arterioles dilated the blood-pressure falls. 
 
 The mean pressure in the arteries is highest close to the aorta 
 and lowest in the region of the periphery; the fall in pressure 
 from the aorta to the periphery is gradual. At the minute 
 arterioles the fall in pressure is rapid; in the veins gradual 
 and very slow ; in fact, owing to causes to be dealt with in the 
 chapter on Respiration, a negative pressure may exist in the 
 great veins near the heart. Fig. 27 exhibits in a graphic form 
 the fall in blood- pressure in the different regions of the vascular 
 
 system. 
 
 It has been seen that the amount of side pressure exercised 
 
72 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 by fluid in a tube is ascertained by means of a manometer; 
 • precisely the same principle is employed to ascertain the side 
 pressure in a bloodvessel, here called blood-pressure. It is 
 interesting to observe that the first blood-pressure experiment 
 ever performed was carried out on a horse, a vertical tube being 
 placed in the femoral artery, in which the blood rose to a height 
 of 8 feet 3 inches. It is, of course, for several reasons incon- 
 venient to work with a long tube, and in consequence most 
 manometers are U-shaped tubes containing mercury which is 
 made to balance the pressure to be measured. The greater 
 specific gravity of this metal enables a tube nearly fourteen times 
 shorter to be employed. The inertia of mercury renders it 
 useless for recording delicate or sudden alterations in pressuie; 
 these are obtained by means of spring manometers. 
 
 In Fig. 28 is shown the apparatus employed in determining 
 blood-pressure with the ordinary mercury manometer. 
 
 The Mean Arterial Pressure is increased at each systole of the 
 ventricles, and falls at each diastole; the maximum pressure is 
 therefore known as systolic, the minimum as diastolic. : In the 
 aorta of the dog the systolic pressure may be 168 mm. (6| inches), 
 the diastolic only 100 mm. (4 inches). The average of these two 
 pressures is sometimes spoken of as the pulse pressure or rnean^, 
 pressure. The farther the vessel is situated from the heart,* 
 the less the difference between systolic and diastolic pressures, 
 until finally in the capillaries there is no difference, and conse- 
 quently no pulse. 
 
 In the aorta of the horse the blood-pressure may vary from 
 321 mm. to 150 mm. (12^ inches to 6 inches) ; in the dog, from 
 168 mm. to 100 mm. (6f inches to /\\ inches) ; in the sheep, from 
 206 mm. to 156 mm. (8 inches to 6 J inches). 
 
 In the carotid of the horse the pressure is from 325 to 215 mmr 
 (12 1 inches to 8 inches), equal to a column of blood o,£ feet to 
 13! feet in height. 
 
 Neither size of body nor pulse-rate bear any relation to the 
 amount of blood-pressure present in the arteries of an animal. 
 The pressure in the carotid of the goose is nearly the same as in 
 the carotid of the horse. 
 
 Influence of Muscular Work on blood- pressure is very marked; 
 the pressure is raised because muscular contraction causes 
 mechanical compression of the vessels of the muscles. Every 
 movement of the body, every movement of a limb or part, 
 affects the blood- pressure in it ; in the large arteries the pressure 
 will from this cause be found to be constantly varying, unless the 
 animal be absolutely immobilised. There is also another im- 
 portant cause of variation — viz., the increased force and fre- 
 quency of the heart-beat. 
 
THE BLOODVESSELS 73 
 
 Effect of Gravity on Blood-Pressure.— L. Hill has demonstrated 
 the influence on the blood-pressure of hydrostatic pressure due to 
 gravity. He has shown in man that the arterial pressure is higher 
 in the erect than in the horizontal position; that it is higher 
 in the relaxed leg than in the arm to the extent of the column 
 of blood separating the two points of measurements Whether 
 
 Fig,. 28. — -Arrangements for taking a Blood-Pressure Tracing (Stewart). 
 
 M, Manometer; Hg, mercury; F, float armed with writing-point; A," thread 
 attached to a wire projecting from the drum, and supporting a small weight. 
 The thread keeps the writing-point in contact with the smoked paper and 
 the drum. B is a strong rubber tube connecting the manometer with the 
 artery; C a pinch -cock on the rubber tube, which is taken off when a tracing 
 
 ... is to be obtained.. 
 
 the body is placed in the horizontal, vertical, or inverted position, 
 the blood-pressure in the arteries at the same level as the heart 
 remains little altered, but in the vessels located below the heart 
 the pressure is increased. Constant compensations are occurring 
 in the circulatory system to guard against important alterations 
 in blood-pressure due to gravity. These are effected by a 
 varied output of blood from the heart, but especially by dila- 
 
74 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 tation or constriction of the splanchnic area. Under abnormal 
 conditions, such as ill-health, these compensations are absent or 
 imperfect, and anyone who has been in bed for a few days, and 
 then assumes the vertical position, knows the faintness which 
 follows from the general fall in blood-pressure due to dilatation 
 of the splanchnic area. If the ordinary hutch rabbit be held 
 in the vertical position for a period varying from one-quarterl 
 to three-quarters of an hour, it will in all probability die. The 
 cause of death is cerebral anaemia; under the influence of gravity 
 the blood collects in the splanchnic area and the blood-pressure 
 in the aorta falls. With a wild rabbit, dog, or cat, this experi- 
 ment fails, the explanation being that the tense abdominal 
 walls of the animal leading an active life compress the splanchnic 
 area and prevent the blood collecting in the abdominal cavity. 
 In a dog or cat previously poisoned by chloroform or chloral 
 the experiment succeeds, as the tone of the abdominal walls is 
 lost. When a horse rears up and falls backwards, it may be due 
 to temporary cerebral anaemia resulting from want of compensa- 
 tion in a vertical position, apart from any question of loss of 
 balance. 
 
 The arterial pressure varies, as has been said, with each systole 
 of the ventricle, but besides this there are also certain larger and 
 longer undulations obtainable in graphic records of blood- 
 pressure which hitherto have been regarded by physiologists 
 not as connected with the heart, but as caused by the movements 
 of respiration. It has been observed that inspiration causes a 
 rise and expiration a fall in pressure. This important action of 
 the respiratory pump has never been quite satisfactorily ex- 
 plained, though we now have the view of T. Lewis, who finds 
 that the rise of pressure on inspiration is due to the lessened 
 pressure in the pericardium and consequent increased filling of the 
 heart. If the pericardial sac be opened to the air, no such curves 
 are produced. Lewis therefore concludes that the effect due to 
 changes in the intrapleural pressure is produced entirely by the 
 heart. The question will be again examined in dealing with 
 respiration. The character of the curve produced is seen in 
 Fig. 29. 
 
 The Blood-Pressure in the Capillaries is very difficult to ascer- 
 tain. It is probably J to -f of that in the large arteries, or lies 
 between 20 to 40 mm. of mercury. 
 
 Hill compares the capillary system to a sponge, squeezed and 
 filled continuously by the active motions of the body. The 
 contents of the capillary vessels are constantly being pressed 
 onwards by muscular movements, so that the pressure in them 
 must be continually varying. Hill has shown in the human 
 subject that in an arm held down, the fist being clenched, the 
 
THE BLOODVESSELS 75 
 
 pressure in the arteries and veins is high, but in the capillaries 
 of the fist it is nil, owing to their compression. 
 
 Blood-Presswe in the Veins is r \ or ^ of that in the large 
 arteries. The greater the distance the veins are from the heart, 
 the greater the pressure, so that the highest pressure is in the 
 peripheral veins and the lowest in the jugular. In a sheep the 
 following values were obtained : 
 
 Jugular vein 0-2 mm. ( T J r . inch). 
 
 Facial vem 3 „ ( V f inc h). 
 
 Brachial vein - I2 „ A inch) . 
 
 Crural vein I4 _ (| inch). . 
 
 The venous pressure in a dependent part such as a limb is 
 higher than in a part like the head and neck, where the venous 
 flow is assisted by gravity. During work the venous pressure 
 m the limbs rises, owing to the increased ventricular output, con- 
 striction of the abdominal veins, and compression of the veins 
 
 Fig. 29. — Tracing of Arterial Pressure with a Mercury Manometer 
 
 (Foster). 
 The smaller curves P, P are the pulse-curves due to the heart-beat. The space 
 from r to r embraces a so-called respiratory undulation. The tracing is 
 taken from a dog, and the irregularities visible in it are those frequently 
 met with in this animal. 
 
 of the limbs. A distended condition of the heart also causes the 
 venous pressure to rise, owing to diminished intake. 
 
 In the large veins just as they enter the heart the pressure is 
 very low. The manometer may even at intervals show a nega- 
 tive pressure. In the anterior vena cava of the dog a negative 
 pressure of — 3 mm. (£ inch) may be registered. The nega- 
 tive pressure in the veins entering the thorax is due to the 
 fact that the pressure within that cavity is below that of the 
 atmosphere, so that an aspirating effect is produced upon the 
 blood in these veins. It is this aspirating action which renders 
 operations at the root of the neck in the human subject dangerous 
 should air enter the wounded veins. Judging from the writer's 
 experiments on the horse, blowing air into the veins causes no 
 discomfort until a considerable amount has been introduced; 
 even then only sighing respirations are produced. 
 
 Blood-Pressure in the Pulmonary System. — The variations in 
 pressure in the pulmonary system are very much less than those 
 
7 6 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 occurring in the general circulation, and are believed not to 
 exceed 15 to 20 mm. (f inch) of Hg. 
 
 In the pulmonary artery the pressure is only about one-seventh 
 of that in the aorta. This pressure decreases throughout the 
 entire pulmonary channel up to the left auricle. The resistance 
 offered by the pulmonary capillaries is less than the peripheral 
 resistance experienced in the systemic circulation. 
 
 Influence of Hemorrhage on Blood-Pressure. — The amount of 
 blood which may be removed from the body without lowering 
 the blood-pressure is surprising. This is explained by the fact 
 that the vessels adjust themselves to the reduced amount of 
 fluid in circulation. This adjustment is effected by means of a 
 nervous apparatus to be dealt with presently, by which the blood- 
 pressure is kept up. Experiments show that it is not until two- 
 fifths of the blood in the body have been removed that the 
 blood-pressure begins to fall; after cessation of hemorrhage the 
 pressure again rises, unless the loss of blood amounts to 3 per 
 cent, of the body 'weight, in which case the low pressure becomes 
 dangerously permanent. 
 
 Stewart's observations on dogs show that an animal may 
 recover even after losing more than half its blood. . 
 
 We have previously referred to the question of blood-letting. 
 The boldness with which our forefathers used the lancet, especi- 
 ally those who believed in a heavy bleeding as a preliminary 
 treatment, is justified by the physiological evidence above men- 
 tioned — viz., that it is not until about one-quarter of the blood in 
 the body is drawn off that any effect on the blood-pressure is 
 evident. 
 
 It is astonishing how rapidly a deficiency in the circulating 
 fluid is made good, the fact being that, quite apart from the repair 
 which is being effected through the thoracic duct, the tissues 
 give up their fluid in an endeavour to replace the loss of blood. 
 It is the loss of fluid by the tissues which causes the thirst of 
 haemorrhage. 
 
 HiU has shown that the effect on the blood-pressure of in- 
 creasing the volume of fluid in circulation is very slight. The 
 arterial pressure and the pressures in the venae cavse rise with the 
 injection, but soon fall to the old level, the injected fluid being 
 disposed of in the dilated venous reservoirs, especially those of 
 the splanchnic area. Animals which have been transfused to 
 the extent of 10 or 12 per cent, of the body weight— viz., more 
 than twice the normal amount of blood — suffer from heart 
 failure, the blood-pressure rising and falling as the overloaded 
 heart does its best to cope with the difficulty. Excessive trans- 
 fusion can best be borne if carried on slowly, in order that the 
 large abdominal veins and those of the liver may be given' time 
 to accommodate the increasing bulk of fluid. 
 
THE BLOODVESSELS 77 
 
 The influence of the nervous system on blood-pressure will be 
 studied presently. 
 
 Circulation in the Living Tissues. — The circulation in the 
 living animal may be easily seen in the web of a frog's foot, or 
 in the mesentery of a mammal, and in this way we learn exactly 
 how the corpuscles behave within the vessels. 
 
 In all capillary vessels of small size the corpuscles pass through 
 singly, sometimes revolving in the plasma, traversing certain 
 sections very rapidly, others very slowly. In the vessels larger 
 than -the capillaries, such as the commencement of the small 
 veins, the stream of blood behaves somewhat differently; in 
 each of these the centre of the vessel is occupied by a rapidly 
 moving column of red cells, the axial stream, whilst between 
 these cells and the coats of the vessel is a clear layer or zone, 
 the inert layer, in which may be seen the white corpuscles strolling 
 lazily along the sides, occasionally stopping, then moving for- 
 ward once more. This difference in the behaviour of the cor- 
 puscles is due to the physical facts that the friction against the 
 sides of the vessel is greater than in the centre, and that the red 
 corpuscles, being heavier than the plasma, are drawn into the 
 rapid part of the current. This also explains why the lighter 
 leucocytes hug the walls of the vessel, through which, as pre- 
 viously pointed out, they may pass in order to gain the tissues 
 without. 
 
 Underjhe influence of inflammation the slowly moving leuco- 
 cytes attach themselves to the walls of the capillaries and venules, 
 and pass into the tissues in large numbers. Small numbers of 
 red corpuscles may also pass out. This process is known as 
 diapedesis. Inflammatory changes are essentially due to the 
 cell wall, and not to the blood, and this is proved by the fact that 
 an artificial corpuscular fluid introduced into an inflamed area 
 behaves exactly as does the blood. 
 
 The Pulse.— When the left ventricle contracts it drives a new 
 supply of blood into an already full aorta, and room for it has 
 to be found. This can be effected in one of two ways, either 
 by displacing an equal volume of fluid already in the system, or 
 by temporarily making the artery larger. The latter process 
 is followed as being the more economical in energy. The aorta 
 distends to receive the additional blood ; this distension is followed 
 by an elastic recoil of its walls which drives more blood along 
 another segment of the vessel; this accordingly distends and then 
 recoils, and so the process repeats itself in waves throughout the 
 arterial system. This distension and elastic recoil is the pulse. 
 
 ■Each expansion of the arterial wall coincides with a contraction 
 of the ventricle, and so each beat or throb of the pulse corre- 
 sponds to a contraction of the heart. The rhythmical force of the 
 
78 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 heart is stored up in the arteries as elastic recoil; this inter- 
 mittent expansion and contraction of the arteries gradually 
 becomes less marked at a distance from the aorta, and dies out 
 at the arterioles . 
 
 Attention has previously (p. 68) been drawn to the fact that 
 the elastic properties of the arterial wall, together with the 
 peripheral resistance in the smallest bloodvessels, convert the 
 intermittent flow started by the heart into the continuous 
 stream in the capillaries and veins. In seeking for the cause 
 of the disappearance of the pulse, we find it similarly in the 
 elastic property of the arterial walls. Each inch of the arteries 
 is engaged, by means of its sudden distension after each heart- 
 beat and its more gradual elastic recoil before the next, in shelter- 
 ing the capillaries from the effect of that beat. The oscillations 
 of pressure which give rise to the pulse are, so to say, ' damped ' 
 by the elastic arterial walls, or in other words converted into a 
 steady pressure, a fraction of the pulse being thus actually 
 destroyed by each inch of the arteries. When all the fractions 
 thus destroyed are added together, we can readily understand 
 why the initial 'jerk,' to which the pulse is due, has entirely 
 disappeared just before it would otherwise have reached the capil- 
 laries. If the arterioles dilate considerably, when, in fact, less 
 elastic recoil of their walls is called into play by the lessened 
 peripheral resistance, it may be possible for the ' throb ' to pass 
 not only through the arterioles, but also through the capillaries, 
 and to appear in the veins ; in this way a venous pulse may be 
 produced. An example of this has been given on p. 70 . 
 
 The intermittent expansion of the arteries, called the pulse, 
 travels from the aorta to the periphery, and produces a wave 
 in the arterial system which is spoken of as the pulse-wave. 
 From what has been said it is evident that the height of this 
 wave is greatest nearest the heart, and falls to zero at the capil- 
 laries. The wave travels with considerable velocity, from 4J to 
 9 metres (15 to 30 feet) per second. This may easily be deter- 
 mined by noting the interval between the commencing succes- 
 sive rises of two levers, resting consecutively on the wall of an 
 artery, at a measured distance apart. The length of the pulse- 
 wave is also considerable — viz., about 5| metres -(18 feet). This 
 is arrived at by noting the time each single pulsation, travelling 
 with the previously determined velocity, takes to pass completely 
 under any one lever. Putting these data together, it is evident 
 that the beginning of each pulse-wave is lost in the arterioles 
 before its end has left the aorta. 
 
 No mental confusion should exist as to the difference, and the 
 causes of that difference, between the velocity of the pulse-wave 
 and the velocity of the onward flow of the blood. The factors 
 
THE BLOODVESSELS 
 
 79 
 
 which give rise to them are quite distinct. The pulse-wave runs 
 along the surface of the blood-stream; the blood-current runs, 
 as it were, within the pulse-wave ; the former travels at a high 
 speed, the latter comparatively slowly, at most some 381 mm. 
 (r5 inches) per second. The case is similar to that of a wave 
 seen moving rapidly over the surface of a slowly flowing stream. 
 The pulse- wave can be studied by means of the graphic method ; 
 it is obvious that a lever placed on a pulsating vessel will be 
 moved up and down, and may be made to trace a curve which 
 will record the passage of the pulse-wave under the lever at that 
 particular spot. A tracing thus obtained, known as a sphygmo- 
 gram, simply registers the expansion and recoil of the artery 
 while the wave is passing ; it will not give a tracing of the pulse- 
 wave itself, which, as we have seen, is 18 feet in length, but it 
 gives the details of the form of the wave. It may be at once said 
 
 Fig. 30. — Normal Sphygmogram modified from Dudgeon; Pressure 
 2 Ounces (Hamilton). 
 
 v.e, The period of ventricular systole ; v.d, the period of ventricular diastole ; 
 r, the period of rest ; a, b, c, primary or percussion wave ; d, first tidal or 
 predicrotic wave ; e, aortic notch ; f r dicrotic wave ; g, second tidal wave ; 
 h, termination of pause and beginning of ventricular systole. 
 
 that unless the proper degree of pressure is kept on the vessel, 
 great irregularity in the sphygmograjns will be produced, due to 
 instrumental errors, and not to the true pulse-wave. 
 
 The simplest description of a sphygmogram (Fig. 30) is that it 
 consists of a nearly vertical unbroken upstroke (the anacrotic 
 limb), and an oblique downstroke (the catacvotic limb), which is 
 broken by two or three waves known as catacrotic waves. Of 
 these two or three waves / (Fig. 30) is one of the few which 
 occurs with any regularity, and is known as the die/otic wave. 
 The notch e, described as the aortic notch, is caused by the closure 
 of the aortic valves. The dicrotic wave is produced by a recoil 
 of blood, the result of closure of the aortic valves ; this reflected 
 wave passes from the centre over the whole arterial system. 
 The smaller waves in the catacrotic limb are either 
 vibrations of the arterial wall, or reflections of the pulse-wave 
 from the periphery towards the heart. That the dicrotic wave 
 is a reflection from the aortic valves is shown by the tracing 
 
80 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 in Fig. 31, taken from the facial artery of the horse, A before, 
 and B after destruction of the valves. In B the dicrotic wave 
 has disappeared. A well-marked dicrotic pulse gives a double 
 
 Fig. 31.— Tracing from the Facial Artery of the Horse (Hamilton). 
 A, before ; B, after destruction of the aortic valves. 
 
 beat of the pulse for each single contraction of the heart (see 
 Figs. 32 and 33). Venous pulse- tracings are referred to at p. 46. 
 The pulse-wave distends all the arteries of a part, and by so 
 doing actually increases the volume of any part or organ at the 
 moment of its passage; this has been described as the volume 
 pulse, and it can be recorded in the following manner: A limb, 
 a kidney, or spleen, is placed in a closed chamber containing fluid; 
 at each passage of the pulse-wave the volume of the part, being 
 increased, causes an increased pressure in the fluid, and this, by 
 
 Fig. 32. — Pulse- Tracings (Stewart). 
 Primary elevation ; 2, predicrotic or first tidal wave ; 13, dicrotic wave.' 
 
 The 
 
 depression between 2 and 3 is the dicrotic or aortic notch; 3 is better marked 
 in B than in A. C, Dicrotic pulse, with low arterial tension ; D, pulse with 
 high arterial pressure, summit of primary elevation in the form of an ascend- 
 ing plateau. 
 
 means of any of the methods of registering changes of pressure 
 in a fluid, may be made to furnish a graphic record. An instru- 
 ment for this purpose is known as a plethysmography, and in Fig. 34 
 the apparatus may be seen as applied to the arm. So sensitive, 
 may this method of registration be made, that a tracing of. 
 volume pulse may show not only every beat of the heart, but 
 even the dicrotic wave (see Fig. 35). 
 
THE BLOODVESSELS 
 
 In connection with pulses the term tension has been employed 
 by pathologists ; thus pulses of high and of low tension have been 
 described, and an attempt has been made to distinguish between 
 the pathologits's tension and the physiologist's pressure. If ten- 
 sion be defined as the elastic force exerted by the artery on the 
 blood within, it is evi- 
 dent that this bears 
 some distinct relation 
 to the force distending 
 the artery — viz., the 
 blood-pressure ; a high 
 blood - pressure and 
 high arterial tension 
 describe the same con- 
 ditions. In an artery 
 giving a high tension 
 the dicrotic wave is 
 nearly extinguished ; 
 the vessels, in fact, are 
 so full that the recoil 
 wave makes very little 
 impression on the tense 
 arterial wall •; when 
 blood-pressure is low 
 and the amount of 
 movement in the artery 
 great, the recoil or 
 dicrotic wave is very 
 marked (Figs. 32 and 33). 
 
 The pulse varies in character, according to age, condition, 
 and state of the system; it also differs according to the class of 
 animal. The following table shows the pulse-rate in different 
 animals : 
 
 Elephant 
 Camel - 
 Horse 
 Ox 
 
 Sheep - 
 Pig - 
 Dog 
 
 Certain variations uniform in all animals occur in the pulse- 
 rate It is always much quicker in the young animal than in the 
 adult The heart of a foal at birth beats 100 to 120 per minute, 
 and that of a calf 90 to 130 per minute. As the animal increases 
 in age the pulse-rate drops, and in old age the pulsations are not 
 only reduced in number, but are weaker. 
 
 J 6 
 
 Fig. 33. — Curves of Blood-Pressure taken 
 with A Spring Manometer from the Carotid 
 Artery of a Dog (after Hurthle). 
 
 When. 1 was taken the blood-pressure was high ; 
 2 corresponds to a medium ; 3 to a low ; and 
 4 to a very low pressure ; p is the primary- 
 elevation. This and the succeeding elevations 
 between p and a are called ' systolic wave's.' 
 The systolic waves are followed by a marked 
 elevation, d; which corresponds to the dicrotic 
 wave. 
 
 25 to 
 
 28 beats per mir 
 
 28 ,, 
 
 32 
 
 36 „ 
 
 40 
 
 45 .- 
 70 ,, 
 70 ,, 
 
 50 
 80 
 80 
 
 90 „ 
 
 TOO ,, ,, 
 
82 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The rigid condition of the arterial wall in old age alters the 
 shape and nature of the pulse- tracing. 
 
 Between size of body and pulse-rate there is a distinct con- 
 nection; it varies inversely as the height — in the elephant 27 
 beats a minute, in the mouse 670. 
 
 Fig. 3-j. — Plethysmograph for Arm. 
 
 F, Float attached by A to lever, which records variations of level of the water 
 in B, and therefore variations in the volume-of the arm in the glass vessel C. 
 Or the plethysmograph may be connected to a recording tambour. The 
 tubulure at the upper part of C is closed when the tracing is being taken. 
 
 The heart-rate is rapidly responsive to any outside influence, 
 such as excitement or fear. A harsh word, fear, or timidity will 
 cause the pulse of a nervous animal to register nearly double the 
 number of beats of the heart. To sickness or injury the pulse 
 
 Fig. 35.— Plethysmograph Tracing from Arm (Stewart). 
 The tracing was taken by means of a tambour connected with the plethysmograph. • 
 
 is instantly responsive, and is one of the cardinal aids both in 
 diagnosis and prognosis. Variations of pulse-rate follow as the 
 result of work, so that a marked increase in the number of beats 
 occurs ; this means a larger amount of blood in circulation through 
 tissues which are in a state of activity and consequently in urgent 
 need both of repair and of flushing. 
 
 A relationship exists between the heart-rate and the condition 
 of blood-pressure; when the blood-pressure becomes low, the 
 
THE BLOODVESSELS 
 
 83 
 
 heart-rate increases as the result of reflex stimulation, by which 
 means the output of blood is increased. If the temperature of 
 the blood be raised, the heart-beat increases in frequency, and 
 there appears but little doubt that one cause of the increased 
 pulse-rate in fevers is the actual temperature of the circulating 
 blood. If the temperature of the blood be raised experimentally, 
 it is found that a point is reached at which the heart ceases to 
 beat; in the cat this has been found to be between 44°and 45 °C. 
 (ni° to 113 F.). 
 
 Fig. 36. — Stromuhr of Lud- 
 wig and dogiel. 
 
 A, B, Glass bulbs; a, a metal 
 disc, to which C and D are 
 attached, and which can be 
 rotated on the disc b ; E and 
 "F, cannula; attached to 6, 
 and. connected with the peri- 
 pheral and central ends of a 
 divided_bloodvessel ; m, the 
 mark to which the bulb is 
 filled before rotation. 
 
 Fig. 37.— Chauveau's Ei.ct.ccr/iH. 
 
 A, Tube connected with bleed- 
 vessel; B, metal cylinder in ccn - 
 munication with A. The upper 
 end of B has a hole in the centre, 
 which is covered by a membrane, 
 m, through which a lever, C, passes. 
 C has a small disc, p, at its' end, 
 which projects into the lumen of A, 
 and is deflected in the direction 
 of the blood-stream through A. 
 The deflection is registered by a 
 recording tambour, D, connected 
 with the lever C, and transmitted 
 by E to a drum. 
 
 The Velocity of the Blood varies in the arteries, capillaries, 
 and veins, being greatest in the former, least in the capillaries, 
 and rising again in the veins. 
 
 The velocity of flow is inversely as the sectional area of the 
 tubes ; the total sectional area of the capillaries is greater than that 
 of the aorta, therefore the velocity is reduced ; from the capillaries 
 to the heart the area becomes smaller and the velocity increases. 
 
8 4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The velocity of blood- flow depends on the width of the bed formed 
 by the vessels ; as the arterial system expands the velocity dimin- 
 ishes; in passing through the capillaries with their immense 
 network the velocity is at a minimum; in passing towards the 
 heart the vessels are reduced in number, hence the bed is smaller, 
 and the velocity accordingly increased. The causes of the flow 
 throughout the entire system are the contraction of the left ven- 
 tricle, and the gradual fall in pressure which occurs from the aorta 
 to the right auricle. • 
 
 The vascular system has been compared to two cones. placed 
 base to base, the apex of one being the left ventricle, of the other 
 the right auricle ; where the bases of the two cones meet is the 
 capillary network. The sectional area of this has been estimated 
 by Volkmann as 800 times greater than that of the aorta, while, 
 owing to the syidth of the bed, the rate of the passage of blood 
 through it is 8do times slower than in the aorta. 
 
 Several ingenious instruments have been devised for the 
 purpose of measuring the velocity of the blood. One, introduced 
 by Ludwig, known as the stromuhr (Fig. 36), consists of a bulb 
 of known size, and the length of time taken to fill this bulb 
 of known capacity gives the velocity. The dromogiafih of 
 Chauveau (Fig. 37) is employed for a similar purpose. It consists 
 of a tube placed in the course of the vessel; within the tube is 
 a plate; the blood streaming past this plate deflects a needle, 
 and the angle of deflection may be expressed in terms of velocity. 
 
 By means of these appliances the following velocities have been 
 obtained in the vessels of the horse : 
 
 Carotid artery: 30 to 40 cm. per second (n-8 to 1575 inches). 
 Metatarsal artery: 5-5 cm. per second (2-2 inches). 
 Jugular vein: 22 cm. per second (8-85 inches). 
 
 The velocity in the larger vessels is not constant ; it is greater 
 during systole and less during diastole. Chauveau obtained the 
 following result in the carotid of the horse : 
 
 During systole: 52 cm. per second (20-47 inches). 
 Beginning of diastole: 2175 cm. per second (8-66 inches). 
 During the pause: 15 cm. per second (5-90 inches). 
 
 The mean velocity in the carotid of the dog is 26-5 cm. per 
 second (10 \ inches). 
 
 At the end of systole: 30-5 cm. per second (12 inches). 
 At the end of diastole: 21-5 cm. per second (8} inches). 
 
 The difference between systolic and diastolic velocities becomes 
 less and less as the small arteries are approached, until finally 
 it disappears. We may say, therefore, that the velocity is more 
 uniform in the small than in the large arteries. 
 
THE BLOODVESSELS 85 
 
 The velocity of the blood is not as great as at first sight 
 appears. If the stream in the carotid of the horse were never to 
 diminish in velocity, it would cover nearly one mile in an hour. 
 The velocity of the blood in the arteries farthest from the heart 
 is less than in those nearest the heart; for example, in the above 
 table a horse with a carotid velocity of 30 to 40 cm. per second 
 had a metatarsal velocity of only 5-5 cm. per second. 
 
 In the capillaries the velocity is very low, perhaps not more 
 than 0-5 to 1 mm. ( ¥ V to ■& inch) per second. This reduction, 
 as we have seen, is due to the width of the bed through which the 
 fluid is flowing. As the veins are reached and the bed narrows, 
 the velocity increases, until the heart is approached, when the 
 velocity in the veins, though increasing, is still much less than 
 that in the corresponding arteries. A horse with a carotid 
 velocity of 30-5 cm. (12 inches) gave a jugular velocity of 2275 
 cm. (9 inches). It is generally considered that the velocity of 
 the blood in the large veins entering the heart is about half as 
 great as that in the aorta. 
 
 During work the velocity in the vessels increases. The flow 
 of blood through the carotid of the horse has been observed by 
 Lortet to be five or six times greater during the time the animal 
 is feeding than when at rest. 
 
 In speaking of blood-pressure we drew attention to the fact 
 that when the heart is more active, or the arterioles contract, the 
 blood-pressure rises, and that when the heart is less active, or the 
 arterioles dilate, the blood-pressure falls. These factors also 
 affect velocity. When the heart-beats increase in force, the 
 velocity of the blood is increased; when they are diminished, 
 the velocity is reduced. Further, it is obvious, from what has 
 been said of the influence of the width of the vascular bed, that 
 if the arterioles dilate the velocity of the blood through the 
 capillaries is increased, and if they contract it is diminished. 
 
 In the pulmonary circulation the circulation time is much 
 shorter than in the systemic, only one-fifth or one-sixth of the 
 time being required for a lung circuit as compared with a long 
 body circuit. 
 
 Any attempt made to estimate the velocity of the blood by 
 dividing an artery, and measuring the escape of blood from its- 
 cut end in a given time, would lead to erroneous conclusions, for 
 the velocity in a closed artery and an open one are two different • 
 things. In the undivided artery the peripheral resistance 
 reduces the velocity; in the divided artery the peripheral resist- 
 ance largely disappears, and the velocity is five or ten times 
 greater, so that the carotid of a horse does not bleed with a 
 velocity of 16 inches per second, but of nearly 160 inches per 
 second. Or, to put it in a practical way, if the carotid of the horse 
 
86 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 has a sectional area of o -2 square inch, the amount of blood passing 
 through the unwounded vessel amounts to 2 ounces per second, 
 while if the same vessel be divided the loss of blood would be 
 nearly i pint per second. 
 
 The Duration of the Circulation depends upon the length of 
 time it takes a red corpuscle to travel from a given point and 
 back to it again.* But there are many different paths it can 
 take. For instance, from left heart through coronary vessels to 
 right heart and again back to left heart would occupy a shorter 
 time than a course through the liver, or through the feet or tail, 
 so that a circulation time may mean nothing more than that a 
 certain number of corpuscles have found the shortest cut through 
 the circulation, or, on the other hand, have taken the longest. 
 In a horse with a pulse frequency of 42, the average complete 
 circuit is performed in 31-3 seconds (Hering), equivalent, accord- 
 ing to this observer, to about 28 beats of the heart. In the rabbit 
 with a pulse frequency of 168 per minute, the time occupied in 
 completing the round of the circulation was 779 seconds, or, 
 again, 28 heart-beats; in the dog 167 seconds, or 267 heart- 
 beats. 
 
 Stewart, who introduced the method of electrical conductivity 
 for ascertaining the duration of the circulation, states that the 
 time occupied by the blood in passing through the kidney, spleen, 
 and liver is relatively long and much more variable than that 
 required for a circuit of the lungs. In a dog weighing 13-3 
 kilogrammes the average circulation time in the spleen was 
 10-95 seconds, in the kidney 13-3 seconds, in the lungs 8-4 seconds. 
 The same observer found the circulation time of the stomach 
 and intestines of the rabbit to be comparatively short, not ex- 
 ceeding that of the lungs. The retina and coronary vessels 
 of the heart had the shortest time circulation. 
 
 Influence of the Nervous System. — It is only during the last 
 sixty years that the existence of a set of nerve fibres governing 
 the calibre of the bloodvessels has been known. C. Bernard 
 observed that in the cervical sympathetic of rabbits there were 
 fibres which on stimulation produced constriction of the blood- 
 vessels of the ear; later he found a set of fibres which on stimula- 
 tion produced a dilatation of the bloodvessels. 
 
 The control of the nervous system over the bloodvessels con- 
 sists of these two actions— viz., dilatation and constriction; the 
 nerves inducing the former are called vaso-dilators, and those 
 bringing about the latter vaso-constrictors. Collectively these 
 
 * The circulation time is determined either by injecting an easily dis- 
 tinguishable salt into the blood, or with greater precision by increasing 
 the electrical conductivity of the blood by injecting into it a neutral salt 
 soluti'on. 
 
THE BLOODVESSELS 8 7 
 
 nerves are known as vasomotor. The vaso-constrictor fibres 
 act by causing contraction of the muscular coat of the arterioles. 
 The capillaries are not affected; in fact, as far as is at present 
 known, there is no vasomotor supply to capillaries. The vaso- 
 dilators act by relaxing the muscular coat of the arterioles. 
 
 _ From what has been previously taught it is evident that a 
 high blood-pressure is an essential condition throughout the life 
 of an animal; whether awake or asleep, at work or at rest, a 
 constant watch has to be kept over the vessels of the body, in 
 order to produce not only an effective, but a purposeful circula- 
 tion, for it is evident that the blood-supply to an organ in active 
 secretion must be greater than when the organ is at rest. The 
 nervous system is charged with the duty of controlling the blood- 
 vessels and of regulating the blood-supply to them. 
 
 The system of nerves governing the bloodvessels is not under 
 the control of the will, but belongs to that portion of the nervous 
 apparatus generally known as the sympathetic. The sympa- 
 thetic system has its origin in the brain and spinal cord; it is 
 specially charged with functions relating to plain muscular 
 tissue, cardiac muscle, and glands, so that the plain muscle 
 constituting one of the coats of the arterioles comes under its 
 control. The two opposite changes in the bloodvessels — -viz., 
 contraction and dilatation — -are not caused by the action of the 
 same nerve Lbres. The constrictor nerves operate through the 
 elaborate and complex system to which they' belong — -viz., the 
 sympathetic; but the dilator fibres accompany cranial and 
 spinal nerves, and may or may not have any connection with 
 the sympathetic system, being included, however, in the larger 
 system — -the so-called autonomic (Langley). 
 
 The Constrictor Fibres originate in the medulla at a special 
 part known as the vasomotor centre. The fibres from this centre 
 pass down the spinal cord to end probably around the nerve 
 cells in the intermedio-lateral column of the thoracic region. 
 From these nerve cells medullated nerve fibres pass out in the 
 inferior roots of the spinal nerves, and, leaving the latter by 
 the white rami communicantes, join the lateral sympathetic 
 chain, in which they run either forwards or backwards, depending 
 upon the place of origin of the grey rami communicantes which 
 supply the spinal nerves with their vasomotor fibres. 
 
 Those fibres which supply the vessels of the head and neck 
 pass forwards to end in the superior cervical ganglion from which 
 the non-medullated nerve fibres supplying the arterioles arise. 
 The fibres for the bloodvessels of the limbs pass to the vertebral 
 ganglia from which the non-medullated post- ganglionic fibres 
 pass to the nerves of the axillary and sacral plexuses. Those for 
 the skin of the trunk pass by the corresponding spinal nerves, 
 
&8 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 while those for the abdominal viscera leave the vertebral chain 
 of ganglia by the greater and lesser splanchnic nerves to end 
 around certain prevertebral ganglia in the abdomen from which 
 the non-medullated postganglionic fibres arise. 
 
 The essential feature in all these constrictor fibres is that they 
 originate in the brain or cord, leave the latter as medullated 
 nerves, and enter a sympathetic ganglion, where they terminate 
 by arborising around cells in the ganglion. From these ceUs 
 new fibres arise, which leave the ganglion as non-medullated 
 nerves, and proceed to their destination either direct, as to the 
 head, neck, and viscera, or through the spinal nerves. 
 
 If the spinal cord be divided below the medulla, and life main- 
 tained by means of artificial respiration, the immediate effect of 
 division is a great fall of blood-pressure, due to dilatation of the 
 bloodvessels; in the dog it will drop two-thirds below the normal 
 The effect of division has been to cut off the constrictor influence, 
 which was evidently issuing from some point above the section. 
 
 If in another animal the section be made above the medulla, 
 no effect is produced on the blood-pressure. Evidently, there- 
 fore, the medulla contains a centre presiding over the important 
 functions of maintaining the bloodvessels in the partially con- 
 tracted condition known as tone, and it can readily be shown 
 that this centre lies in the region of the fourth ventricle, and is 
 only a few millimetres in length and still fewer in breadth. To 
 this small area the name vasomotor centre has been given. 
 
 This centre, we have learnt, must be kept in a constant 
 state of activity. This is effected, though perhaps not entirely, 
 by the constant flow of impulses from the periphery to the centre. 
 Impulses passing to a centre from without to within are known 
 as afferent, those passing out from a centre to the periphery are 
 spoken of as efferent impulses, and a collection of nervous matter 
 where afferent impulses are received and efferent discharged is 
 known as a reflex centre. 
 
 The afferent impulses which govern the centre in the medulla 
 are carried to it through the spinal nerves from areas such as 
 the skin and abdominal viscera, for these are the two great 
 vascular areas which maintain and regulate the blood-pressure. 
 On entering the superior root of the spinal nerves, the fibres 
 travelling up the cord by means of collateral branches make 
 connection with the vasomotor centre in the medulla. We have 
 now seen the channels by which this almost microscopic collec- 
 tion of cells in the medulla is brought into connection with the 
 vessels of the whole body. 
 
 The impulses carried by afferent fibres to the vasomotor centre 
 are of two antagonistic kinds ; they either stimulate it and produce 
 contraction of the bloodvessels, with a consequent increase in 
 
THE BLOODVESSELS 89 
 
 blood-pressure, or they diminish the tone of the centre, produce 
 a relaxation of the bloodvessels and a fall in pressure. The 
 first fibres are known as pressor, the latter as depressor, terms 
 which correspond to the effect they produce on the blood-pressure. 
 
 As previously stated, it is in the skin and abdominal viscera 
 that the impulses which regulate the maintenance of normal 
 arterial pressure originate; there are supplemental sources, but 
 the above are the most important. The fibres passing from the 
 skin area are stimulated by the external temperature, a low 
 temperature causing impulses to be transmitted which contract 
 the vessels, a high temperature causing a relaxation of the vessels, 
 accompanied by congestion of the skin. But the splanchnic area 
 is even more actively important than the skin in regulating 
 blood-pressure, especially in those animals such as the herbivora, 
 where the alimentary canal is largely developed. If the splan cli- 
 nics be cut, the intestines become congested in consequence 
 of dilatation of the bloodvessels, and there is a severe fall in 
 blood-pressure. If the peripheral end of the divided nerve be 
 stimulated, the vessels contract, and the blood-pressure at once 
 rises. 
 
 The action of the constrictor fibres of the bloodvessels is 
 always more in evidence than that of the dilators, and the 
 reason of this can be readily understood by remembering that 
 the arterioles have always to be kept in a condition of contraction 
 if blood-pressure is to be maintained. 
 
 Bernard's classical experiment of dividing the cervical sympa- 
 thetic in the rabbit produces not only a remarkable picture" of 
 vasomotor effects, but illustrates the constrictor impulses which 
 are constantly passing to the bloodvessels. On division of the 
 sympathetic, the ear on that side suddenly becomes flushed with 
 blood, hot, and congested, and vessels not previously visible to 
 the naked eye now become very apparent. If the upper end of 
 the nerve be stimulated, so as to imitate roughly the impulses 
 passing along it in an intact condition, the vessels at once con- 
 tract, the flushed appearance disappears, and the ear becomes 
 cooler. 
 
 Since, in the above experiment, mere severance of the nerves 
 which connect the bloodvessels with the central nervous system 
 leads to a dilatation of the arterioles, it is evident that impulses 
 are, under normal conditions, being continually sent out along 
 the nerves from the vasomotor centres. These impulses keep 
 the arterioles normally in that state of medium or partial con- 
 striction which has already been described as arterial ' tone.' 
 Now, inasmuch as the function of the vasomotor nerves is to 
 regulate the blood-supply to any given area of the body, in exact 
 accordance with the varying needs of that area, ' tone 'becomes 
 
go A MANUAL OF VETERINARY PHYSIOLOGY 
 
 a factor of the utmost importance in this regulative mechanism. 
 Without it all the arteries of the body would, in the ordinary 
 passive condition of rest, be dilated to their full extent; hence 
 no increased supply of blood could be provided except by an 
 augmented activity of the heart, which would, of course, affect 
 the body as a whole, and not any one limited part of it. ' Tone ' 
 insures that an arteriole will both dilate and contract, according 
 as it receives less or more of the continuous constricting im- 
 pulses, and thus the regulation of a varying blood-supply is made 
 extremely perfect. 
 
 If the sciatic or brachial nerve be divided in'the dog, as a rule 
 the constrictor influence over the bloodvessels of the limb is 
 
 Fig. 38. — Plethysmograms : Hind-Limb of Cat (after Bowditch and 
 
 Warren). 
 To be read from right to left. On the left hand is shown the effect of slow stimu- 
 lation of the sciatic (1 per second) ; on the right hand the effect of rapid 
 stimulation (64 per second). In the first case the limb swelled owing to 
 excitation of the vaso-dilators ; in the second it shrank through excitation 
 of the vaso-constrictors. 
 
 lost, the foot-pads flush, and the feet rise in temperature. If 
 the central end of the divided sciatic be stimulated, it is generally 
 followed by a contraction of the bloodvessels and a rise in 
 pressure. Occasionally the tone of the centre in the medulla 
 is not! raised, but reduced, and dilatation of the vessels and a 
 fall in blood-pressure results. This experiment suggests that 
 most afferent nerves, such as the sciatic, carry both pressor and 
 depressor fibres, and that the effects which follow experimental 
 stimulation depend upon whichever set of fibres is most effi- 
 ciently stimulated (Fig. 38). 
 
 The best example of a depressor nerve is one we have already 
 studied under that name (p. 60) ; it is the only peripheral nerve 
 
THE BLOODVESSELS 91 
 
 the stimulation of which invariably reduces blood-pressure. 
 This nerve, it will be remembered, is capable of regulating the 
 work of the heart by taking off strain through the medium of 
 the abdominal venous cistern. Stimulation of the depressor 
 acts like division of the splanchnic— viz., the abdominal vessels 
 fill with blood and the blood-pressure falls. 
 
 Experimental inquiry shows that, in spite of the enormous 
 importance of the vasomotor centre in the medulla, if the cord be 
 divided in the lumbar region the vessels of the hind-limb dilate 
 and the blood-pressure falls, but if the animal be kept alive the 
 blood-pressure probably returns to the normal, though it is once 
 Sgain lost by destroying the already divided cord. These results 
 are explained by saying that the cord possesses vasomotor sub- 
 centres, and that, given time, these are capable of carrying on 
 the work unaided. It is even possible to carry the inquiry a 
 stage farther, and by destroying all nervous connection, isolate 
 the vessels from their innervation ; even then the vascular tone 
 may be recovered, and it is supposed that it is developed in the 
 wall of the vessel itself, possibly in response to the stimulating 
 influence of variations of the internal blood-pressure. 
 
 It is possible that the nature of the stimulus conveyed by 
 afferent nerves to the vasomotor centre may determine the 
 nature of the reflex which follows, and this is considered probable 
 because the electrical stimulation of muscle always produces a 
 rise in blood-pressure, whereas the mechanical stimulation by 
 rubbing always causes a fall. The effect of stimulating the 
 divided sciatic was recently referred to as producing constriction 
 of the bloodvessels and a rise in pressure ; but if the sciatic nerve 
 be cooled and then stimulated, exactly the converse effects are 
 produced — viz., a dilatation of the arterioles and a fall in pres- 
 sure. Not only are the results obtained dependent on the 
 nature of the stimulus, but the state of the centre itself is of the 
 utmost importance in determining the results of reflex vaso- 
 motor stimulation. When an animal is deeply under the influence 
 of chloroform or chloral, the functions of the vasomotor centre 
 appear to be reversed, and excitation of pressor fibres causes a 
 fall instead of a rise in pressure. If the centre be under the 
 influence of strychnine, stimulation of the depressor nerve, 
 instead of causing a fall in the general blood-pressure, produces 
 a rise. 
 
 The vasomotor centres, both in the medulla and cord, are 
 extremely sensitive to the varying amounts of carbon dioxide 
 in the blood, and in this way they are believed to be capable 
 of a good deal of self-regulation apart from the afferent impulses 
 which flow in to their aid. An increased venous condition of 
 blood leads to a constriction of the arterioles and a raising of 
 
92 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the blood-pressure. In asphyxia the arterioles remain con- 
 stricted under the influence of the intensely venous blood as it 
 stimulates the vasomotor centre to unwonted activity, and 
 though the initially high blood-pressure subsequently falls to 
 zero, it does so, not because the arterioles have relaxed, but 
 because the heart has failed. 
 
 It has been supposed that one cause of surgical shock is a de- 
 ficiency of carbon dioxide in the blood. The prevention of this 
 complication by avoiding a too free ventilation in the lungs, 
 and so maintaining a carbon dioxide blood-pressure, has been 
 recommended as a means of combating shock. The alarming 
 depression which after a prolonged operation not infrequently 
 follows the removal of the closed chloroform mask employed in 
 chloroforming horses is doubtless due to the fall of blood- 
 pressure caused by less Cu 2 circulating in the vasomotor centre. 
 
 Fjte. 39. — Blood-Pressure Curve of a Rabbit recorded on a Slowly Moving 
 Surface to show Traube-Hering Curves (Foster). 
 
 The heart-beats are the closely situated up and down strokes, readily seen by 
 means of a lens. The next curves are those generally regarded as due to 
 respiration, the large bold undulations being the Traube-Hering curves. In 
 each Traube-Hering curve there are about nine respiratory curves, and in 
 each respiratory curve about nine heart-beats. 
 
 Rhythmical Activity of the Vasomotor Centre. — Under certain 
 conditions, such as asphyxia and haemorrhage, the vasomotor 
 centre transmits to^the vessels rhythmic constrictor impulses, 
 which result in the appearance, on a simultaneous record of 
 blood-pressure, of undulations known as Traube-Hering curves 
 (Fig. 39). They can, of course, only be detected by taking a 
 tracing of the blood-pressure. The existence of these waves is 
 indicative of abnormal excitation of the vasomotor centre. 
 
 The Dilator Nerves. — Hitherto we have mainly dealt with 
 the constrictor influence exercised over the bloodvessels, but the 
 nervous system likewise produces a dilator effect. In contrast 
 to the constrictor influence, the dilator is not tonic in its action. 
 It might be supposed that a dilator effect would naturally follow 
 as the result of removing a constrictor influence from a vessel, 
 
THE BLOODVESSELS 93 
 
 without the intervention of a separate or antagonistic nerve 
 supply; and this is exactly what does happen in most cases. 
 But it is equally certain that special vaso-dilator nerves exist, of 
 which the chorda tympani is one of the best examples. This 
 nerve supplies the bloodvessels of the submaxillary gland with 
 dilator fibres; if the nerve be cut no evident change in the blood- 
 vessels occurs, but if the end in connection with the gland be 
 stimulated the vessels dilate, the arteries throb, and the blood 
 passes through the gland with such rapidity that the venous 
 blood becomes arterial in appearance. Much the same pheno- 
 menon occurs when the nervi e'igentes, through which erection of 
 the penis is produced, are brought into activity. If these nerves 
 be divided there is no effect, but if the peripheral end be stimu- 
 lated the organ swells as the result of its enormously increased 
 blood-supply, and the blood flowing through the dorsal vein may 
 now be fifteen times greater in amount than in the quiescent 
 condition. 
 
 Though it is clearly established that true vaso-dilator nerves 
 exist, there is still great difference of opinion as to the extent 
 of their distribution. In most peripheral nerves, such as in the 
 nerves of the limbs, both dilator and constrictor effects may, 
 as has been mentioned, be obtained on stimulation, rapid induc- 
 tion shocks bringing about constrictor effects, slower shocks 
 producing dilator effects. If the nerve — say the sciatic — -be 
 divided, the subsequent degeneration which follows affects the 
 constrictor fibres earlier than the dilator. There is good reason 
 for believing that the view originaUy held regarding muscles 
 possessing dilator fibres may have to be modified in the light 
 thrown on the matter by Bayliss, who has shown that there are 
 fibres in the superior roots of the cord, where the axillary and 
 lumbar nerves come off, which on stimulation produce dilatation 
 of the vessels of the limbs. As these fibres are running from the 
 limbs into the cord, difficulty has been experienced in explaining 
 why stimulation should cause an impulse to travel in a direction 
 opposite to the normal. However, the only explanation at 
 present offered is that the fibres act in a double capacity, con- 
 veying to the cord sensory impulses from the limbs, and by the 
 same fibres conveying efferent or outgoing impulses from the 
 cord to the vessels, resulting in their dilatation. 
 
 The dilator nerves in their distribution— and we here refer to 
 those whose existence is undoubted— are very different from the 
 constrictor fibres. They leave the brain or cord by any cerebro- 
 spinal nerve, and may or may not pass into a sympathetic 
 ganglion before distribution. In contrast to the constrictors, 
 they pass direct to their destination instead of taking a round- 
 about course, and they do not lose their medulla until near their 
 
94 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 termination. There is no positively known centre in the medulla 
 governing the vaso-dilator fibres. There is, however, un- 
 doubted evidence of such a centre in the cord, for erection of 
 the penis will still occur as a normal reflex after the cord has been 
 divided above the lumbar region. 
 
 All the ' depressor ' influences exercised on the circulation are 
 not produced through inhibition (withholding) of constrictor 
 impulses. Some must occur through stimulation of dilator 
 nerves, but of this subject very little is positively known. 
 
 It is evident from what has been said that the knowledge of 
 dilator nerves for the body tissues generally is still in a very 
 uncertain condition. 
 
 We have seen the general effects produced by vaso-constrictor 
 and vaso-dilator nerves, and indicated some of the gaps which 
 exist in our knowledge. The main features of the theory appear 
 to admit of no doubt. The size of the vessels has to be auto- 
 matically regulated according to the demand made for blood, 
 so that a double set of fibres is provided, one to constrict and one 
 to dilate them. A general contracted condition of the vessels 
 must be maintained if the circulation is to continue; conse- 
 quently, constrictor impulses appear always more in evidence 
 than dilator. These constrictor effects are assisted by the con- 
 tractile reaction of the arterial wall, the peculiar property of 
 smooth muscle, and this contractile reaction is brought into 
 play, even apart from nervous impulses, by the thrust given to 
 the blood at each contraction of the heart. Substances circu- 
 lating in the blood, the secretion of such glands as the adrenals, 
 of which we shall learn more presently, act on the nerve plexus 
 in the walls of the bloodvessels, causing them to contract and 
 raising the pressure. The heart centre in the medulla is being 
 constantly informed of what is occurring in the two vast systems 
 — skin and splanchnic — which between them regulate the blood- 
 pressure of the body. Equally, impulses are constantly stream- 
 ing out from this centre to the vessels, some of a constrictor, 
 others of a dilator nature, resulting either from the afferent im- 
 pulses received or from the changes in the composition of the 
 blood circulating through the medulla itself. 
 
 Absence of Vasomotor Nerves. — Some vessels have no vaso- 
 motor nerves. They have not been satisfactorily demonstrated 
 in the vessels of the heart, nor in those of the lungs or brain. 
 They are not found in veins (excepting the portal, which is 
 really an artery), and are only provisionally accepted as occur- 
 ring in muscles. If muscles possess vasomotor nerves, they must 
 be essentially dilators, and it is easy to understand that these 
 fibres may be brought into operation when the muscles contract, 
 and thus provide a flow in proportion to the activity of the part. 
 
THE BLOODVESSELS 95 
 
 It has, however, been suggested that the dilatation in the blood- 
 vessels of muscles during activity may be due to the chemical 
 action of acid metabolic products on the vessels themselves. 
 
 Surgical Shock. — The condition known as shock, which 
 follows certain operations, is intimately connected with the vaso- 
 motor apparatus. This has been suggested in speaking of the 
 influence of carbon dioxide in the blood in maintaining blood- 
 pressure. In surgical shock ther.e is a marked depressor effect 
 on the circulation, and a f allin g blood-pressure i s one of the earliest 
 indicat : ons of its onset. The maintenance of blood-pressure is 
 the cardinal principle in combatirg shock, yet the physiologist 
 does not regard shock as beginning, as has been suggested, in 
 the vasomotor centre, in consequcr.ee of the violence of sensory 
 stimulation, but as originating more probably in the paralysis 
 of the nervous connections with that centre. The cutting off 
 of the afferent impulses which stimulate the vasomotor centre' 
 is the initial move towards the production of shock. 
 
 Aids to the Circulation. — The distribution of the blood-supply 
 in accordance with the requirements of the various organs is the 
 special duty of the system of nerves we have been studying. 
 The requirements vary from time to time, even from moment to 
 moment. Active digestion diverts the blood to the splanchnic 
 area ; active work causes the stream to pass into the muscles, and 
 later into the skin, in order to get rid of the excess of heat. 
 Some glands, such as the kidneys, are secreting constantly; 
 others, like the pancreas, intermittently . A failing heart requires 
 more blood, one struggling against an ever-increasing load re- 
 quires less, and so on. Throughout the whole life of the animal 
 this delicate balance has to be maintained, and it is effected by 
 means of the nervous system acting through the medium of the 
 heart and bloodvessels. 
 
 It is impossible in connection with this question of an efficient 
 circulation to avoid once more referring to the heart, and it is 
 impossible to avoid being struck by the close similarity of the 
 nervous arrangements for the care, management, and control of 
 these two communicating systems— heart and bloodvessels . The 
 nerve fibres which constrict the bloodvessels, and those which 
 cause a more forcible contraction of the heart, are of the non- 
 medullated variety. They excite muscular action and increase 
 wear and tear. The fibres which dilate the bloodvessels, and 
 those which slow the heart, are both medullated, muscle-restrain- 
 ing and exciters of repair rather than of disintegration. The 
 impulses affecting heart and bloodvessels pass from the periphery 
 to the centre— viz., they pass from skin and muscle, viscera and. 
 glands, and even from the heart and bloodvessels themselves, to 
 the nerve centres presiding over these two important systems. 
 
96 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 They are true reflex effects, and without these impulses the 
 centres themselves are powerless to effect regulation. 
 
 The delicacy of the balance must not be displaced. The body 
 does not consist of a set of isolated functions working in se- 
 quence ; three or four or a dozen may be in operation at the same 
 moment, and to each of these a full blood-supply must be guaran- 
 teed for so long and no longer than is necessary, or in such re- 
 duced or increased quantity as may be needed. Whether the 
 animal be awake or asleep, this remarkable give and take, this 
 perfect adjustment to all requirements, is taking place from 
 birth to death. 
 
 There has been reason- to think that a certain amount of self- 
 help has been obtained from the tissues themselves. For in- 
 stance, the lactic acid and carbon dioxide formed during mus- 
 cular activity may cause dilatation of the vessels and an increased 
 blood-supply. This chemical help has for some time been recog- 
 nised, but it is only within recent years that we have learnt 
 that the body is capable of elaborating substances which, when 
 introduced into the blood, greatly affect the calibre of the blood- 
 vessels and the rhythm and tone of the heart itself. 
 
 The Chemical Stimulus in Blood- Supply .—The study of the 
 so-called ductless and functionless glands of the anatomist has 
 given us one of the romances of physiology. The adrenals and 
 pituitary bodies — tissues so insignificant in size that it was but 
 rarely that- more than a glance was given them during dissection 
 — are known to furnish to the blood-stream chemical substances 
 of the utmost importance to the circulation. 
 
 An extract of the medulla of the adrenals, when introduced 
 into the circulation, increases the rhythm, force, and tone of the 
 cardiac muscle (see p. 62), strongly stimulates the contrac- 
 tion of the arteries, except those of the heart, and causes a con- 
 sequent great rise in blood-pressure. There is evidence that the 
 internal secretion of the adrenals finds its way into the blood, 
 for a larger amount of pressor substances may be recovered 
 from the blood leaving an adrenal than from ordinary blood. 
 Removal of the glands is followed by great weakening of the 
 heart and a low blood-pressure ; fatal results ensue. 
 
 Extracts of the posterior lobe of the pituitary body introduced 
 into the circulation greatly increase the tone, but not the rhythm, 
 of the heart muscle (see p. 62), and, with the exception of those 
 of the kidney, cause constriction of the arterioles of the body. 
 This gland also secretes a depressor substance which diminishes 
 the force of the heart and permits the vessels to dilate. The 
 extract obtained from the pituitary body under experimental 
 conditions contains much less of the depressor than of the pressor 
 substances, but it may well be that under physiological condi- 
 
THE BLOODVESSELS 97 
 
 tions they are passed into the circulation in the required propor- 
 tion, and act, as in the case of the adrenals, as a direct chemical 
 stimulus. 
 
 Muscular Aids.— Abdominal respiration assists the return of 
 blood from the splanchnic area. If an incision be made in the 
 posterior vena cava in front of the diaphragm, then with every 
 retreat of that muscle, pressure is exercised on the abdominal 
 contents, and the blood spurts out of the vessel. This pressure 
 on the contents of the abdominal cavity is one of the most im- 
 portant mechanical aids to the circulation. The abdominal 
 muscles are assisted by the stout elastic fascia which plasters 
 their external surface, the anus is tightly closed, and the pressure 
 within the abdomen raised at each inspiration. Failure to secure 
 this pressure results from a relaxed condition of the anus. This 
 is frequently seen in the last stage of some debilitating diseases 
 in the horse. The anus remains open, and allows the air to rush 
 in and out of the bowel at every respiratory effort. In the veins 
 of the hind-limb the whole course of the blood is directly against 
 gravity until the posterior vena cava is reached. In that vessel 
 gravity is in favour of the contents, for the vein falls from the 
 spine to the heart, and the respiratory help above described 
 completes the work as the heart is approached. The squeezing 
 to which the liver is subjected in the abdomen is a most valuable 
 means of assisting its circulation. 
 
 Muscular movements of the body and limbs are of the utmost 
 importance as aids to the circulation. Muscles, by their con- 
 traction, squeeze the blood along the veins, regurgitation being 
 prevented by the valves. HiU has shown that the column of 
 blood in the vessels of the limbs may be broken up into segments, 
 and by muscular action the stream to a part turned on or cut off, 
 and the supply regulated according to its requirements. The 
 importance of muscular action as an aid to circulation is seen 
 every day, especially in the case of the horse. The swollen 
 legs which are caused by standing are due to transudation, 
 which is itself due to pooling of blood under the influence of 
 gravity, and the absence of the venous massage which results 
 from muscular activity. We see this condition also m the feet, 
 where the absence of exercise means loss of the needful stimulus 
 to the circulation in the lamina;, and the feet become congested. 
 
 It is impossible for a man in an upright position to remain 
 absolutely immovable. The slightest change in the pressure 
 on the limbs brings muscles into play which force the blood 
 along the veins. The same thing occurs in the horse, an animal 
 that may not lie down for days or weeks together (and some 
 never lie down). The weight imposed upon the hiM-hmbs is 
 never equally distributed during repose. First one leg, then the 
 
98 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 other, takes the weight, and at every change in posture the 
 muscles compress the veins and force the blood along. In the 
 fore-leg these alternate changes in position are not necessary; the 
 veins are short and near the heart. 
 
 The experiment of holding the domestic rabbit in the vertical 
 position referred to at p. 74 is a good example of the manner 
 in which the blood will pool under the influence of gravity in the 
 absence of muscular support. 
 
 It is true that in the larger quadrupeds changes of posture are 
 of a far simpler character than in the biped, the most frequent, 
 perhaps, being that of the head in grazing. This imposes a 
 column of venous blood in the head and neck, from which it has 
 to ascend against gravity. This is fully provided for by the 
 venous cisterns beneath the masseter muscle, which at every 
 masticatory act mechanically force the collected blood into and 
 along the jugular, the valves preventing regurgitation. In the 
 days of bleeding, when jugular obliteration was not uncommon, 
 horses so affected could not be grazed, owing to the vertebral 
 veins being unable to compensate for the posture. 
 
 The left ventricle is capable of driving the blood throughout 
 the entire vascular system, but it needs assistance. Under the 
 conditions of a normal circulation it drives it principally as far 
 as the arterioles, then the skeletal muscles and valves in the veins 
 afford the venous system the aid necessary to it, and the respira- 
 tory pump, as the heart is approached, completes the emptying 
 of the veins and nils the right heart. 
 
 Peculiarities in the Circulation through various tissues occur 
 as the result of their special functions. Peculiarities are observed 
 in the brain, erectile tissues, etc. The great vascularity of the 
 brain necessitates that the blood should pass to it with a degree of 
 uniformity which will insure the carrying out of its functions. 
 It must never be left without blood, or immediate unconscious- 
 ness would occur. We see this provided for by the frequent 
 arterial anastomoses — for example, the Circle of Willis and 
 the Rete Mirabile of ruminants, which insure that not only 
 does the blood enter with diminished velocity, but that if a 
 temporary obstruction occurs in one vessel its work is readily 
 performed by the others. The rete mirabile alluded to, which 
 forms the arterial plexus on the base of the brain of ruminants, 
 is considered by some to regulate the flow of blood to the brain 
 when the head is depressed during grazing, and, it is said, 
 accounts for the absence of cerebral haemorrhage in these animals. 
 It is probable that this may be one of its functions, but the 
 horse possesses no rete, and his head is depressed during grazing 
 for more hours out of the twenty-four than is the case withj 
 ruminants. It has probably, therefore, some other function to! 
 perform. 
 
THE BLOODVESSELS gg 
 
 The free anastomoses of the vessels of the brain guarantee to 
 it an uninterrupted blood-supply. In the dog both internal 
 carotids and both vertebrals have been ligatured without causing 
 unconsciousness or death, the supply of blood being kept up 
 through the anterior spinal artery. The veins of the brain 
 empty their contents into venous sinuses which by their struc- 
 tures are well protected from compression. There are no valves 
 to the openings into the sinuses, nor are there valves in the 
 thin- walled small veins of the brain. 
 
 The circulation in the brain is peculiar, inasmuch as its sub- 
 stance is contained within a closed unyielding cavity. A small 
 space formed by the membranes exists between the skull and 
 the brain, and in this is found a little fluid, which may pass into 
 the interior of the brain or backward into the spinal canal. The 
 cerebral fluid acts as a water-pad, and this fluid may be rapidly 
 absorbed, for it readily passes into the veins at any pressure 
 higher than that in the venous circulation. Nevertheless, the 
 space which this renders available is very small, so that the 
 brain may be regarded as incompressible. Special provision 
 must, therefore, exist in its circulation to guard against a rise 
 in blood-pressure, and this is furnished by dilatation of any one 
 set of vessels producing a constriction of the other set. By this 
 means no increase in size nor increase in blood-capacity occurs ; 
 all that is affected is the velocity of flow. If the arterial pressure 
 of the brain rises and that in the veins remains constant, there is 
 an increased velocity of flow; if the arterial pressure is constant, 
 and the venous pressure rises, there is a diminished velocity of 
 flow. 
 
 The brain presses against the cranium with a pressure equal 
 to the blood-pressure in the capillaries. The brain-pressure, 
 cerebro-venous pressure, and cerebro- spinal fluid pressure have 
 been shown by Hill to be one and the same. 
 
 If the brain be exposed it is observed to rise and fall synchron- 
 ously with the respiratory movements. Expiration causes the 
 brain to rise by hindering the return of blood, while inspiration 
 causes it to fall by facilitating its flow. Owing to the incompres- 
 sibility of the brain substance in the cranial cavity the pulse is 
 transmitted through the brain substance to the veins, and causes 
 the blood to issue from them in pulses synchronous with the 
 arterial pulses. 
 
 There is no vasomotor nerve supply proved to exist in the 
 
 brain. 
 
 The singular arrangement of the venous plexuses of the corpus 
 cavernosum penis admits of this organ attaining a great increase 
 in size, a condition which in the case of the brain every measure is 
 adopted to prevent. The considerable size of the venous plexuses 
 of the penis, their frequent intercommunication, the muscular 
 
ibo A MANUAL OF VETERINARY PHYSIOLOGY 
 
 pressure to which the veins leading from the sinuses are exposed, 
 produce, under the direction of the vasomotor nervous system a 
 considerable increase in the volume of this part. 
 
 In some other organs the distribution of the bloodvessels is 
 also peculiar. It is not known why the spermatic artery and 
 plexus of veins should take such remarkably tortuous courses. 
 Possibly, in some way or other, it may be concerned with the 
 secretion of the glands, but its use is far from clear. On the 
 other hand, tortuous vessels in the walls of hollow viscera, such 
 as the stomach and intestines, perform a very evident function. 
 We have only to think of the difference in size between a col- 
 lapsed and of a full stomach in the horse to recognise the neces- 
 sity for some arrangement existing to prevent over-stretching 
 of the vessels or interference with the blood-supply. 
 
 The vast venous and arterial plexuses of the foot of the horse 
 are a peculiarity in the circulation dealt with in the chapter 
 devoted to the Foot. 
 
 Pathological. 
 
 A man is considered to be as old as his arteries, but a horse may 
 be stated to be as old as his feet and legs. 
 
 It is a remarkable fact that very little of the strain of the hard life 
 of a horse falls on his arteries; -with age the vessels become more 
 rigid, but no sudden strain produces aneurisms, such as might be 
 expected from the class of work performed ; this is probably due to the 
 fact that he does not suffer from syphilis. There is, however, one 
 kind of strain which arises in the hunting field, or under similar cir- 
 cumstances, in which the walls of the external and internal iliac 
 arteries suffer; in consequence of this a thrombus forms in the vessels, 
 which become partly or completely obliterated. Collateral circula- 
 tion suffices in a state of repose, during which not a sign of any circu- 
 latory trouble is evident, but as soon as the animal gets to work 
 sudden and painful muscular cramps occur, and finally temporary 
 paralysis follows. These symptoms, accompanied by intolerable 
 pain, completely pass away with rest and return with work. 
 
 Parasitic trouble of the vessels is very common, the main seat being 
 the anterior mesenteric artery, which is rendered rigid and aneur- 
 ismal, and has its lumen obliterated by Strongylus armatus. It is 
 remarkable how very little interference with the intestinal circulation 
 occurs in consequence of this parasitic invasion, and it is equally 
 astonishing how few horses are free from this infection. It is 
 probably the most widely spread equine parasite. 
 
 Pulse. — The older physicians studied the pulse with care; at the 
 present day it does not receive the same amount of attention. It is 
 not sufficient to know the number of pulsations; the important 
 thing is the character of the wave. 
 
 A pulse may be quick or slow. Either of these may be strong, 
 weak, hard, or soft. Strong and weak refer to the force of the 
 ventricular contraction; hard and soft refer to the tension as judged . 
 by the finger — viz., the amount of pressure required to obliterate the 
 pulsations. A further division of pulses is into large and small; 
 this group refers to the volume of the artery. There is no pulse 
 specially indicative of any given affection, but the character of the 
 pulse in the prognosis of disease is of the utmost clinical value. 
 
CHAPTER IV 
 
 RESPIRATION 
 
 Section i. 
 
 The Lungs. 
 
 The Chest Wall. — The chest is formed of the moderately rigid 
 case furnished by the ribs and the flexible wall provided by the 
 diaphragm. The contents of the chest are vascular and respira- 
 tory, and the provision made for their protection is not quite the 
 same in' each case. The vascular contents are situated mainly in 
 the anterior portion, for here the heart is found with its blood- 
 vessels, while relatively speaking there is only a small amount of 
 lung. The horizontal spine of all animals requires the ribs to 
 perform a function not required in the biped. It would exceed the 
 space at our disposal to deal with the peculiarities of the chest 
 wall in all animals. That of the horse may be selected as a type, 
 more particularly on account of its practical importance. 
 
 It is usual to speak of the ribs as true and false, but this tells 
 us nothing as to their function. The true or sternal ribs form 
 a case for the vascular organs, and help to support the horizontal 
 spine. To enable them to do this they are shorter, stouter, and 
 straighter than the false, or, as we would prefer to call them, 
 respiratory ribs. Further, they are all inserted into the sternum 
 either directly, as the first pair, or indirectly by means of carti- 
 lage, as the remaining seven pairs. The chest is very narrow 
 between the two first ribs, but rapidly increases in width, as may 
 be seen in the following measurements, taken at the widest place 
 between each pair of ribs in a riding-horse of medium height : 
 
 
 
 
 
 
 Mm 
 
 ist pair of ribs 
 
 greatest 
 
 distance 
 
 apart 
 
 90 
 
 2nd 
 
 , n 
 
 , , 
 
 - 
 
 "5 
 
 3rd 
 
 , 
 
 j a 
 
 ,, 
 
 1- 
 
 160 
 
 4th 
 
 , 
 
 , .. 
 
 ,, 
 
 ,'- 
 
 191 
 
 5th 
 
 
 , 
 
 
 - 
 
 230 
 
 6th 
 
 , , 
 
 , , , 
 
 ,, 
 
 
 280 
 
 7th 
 
 , , 
 
 , " 
 
 ,, 
 
 - 
 
 355 
 
 8th 
 
 
 
 ,, 
 
 
 420 
 
 9th 
 
 , 
 
 , 
 
 .. 
 
 
 470 
 
 
 Increase from 
 
 
 Rib to Rib. 
 
 Inches. 
 
 Inches. 
 
 3i 
 
 
 
 4* 
 
 I 
 
 6 ? 
 7i 
 
 I| 
 
 J f 
 
 9 
 
 If 
 
 n 
 
 2 
 
 14 
 
 3 
 
 i6£ 
 
 i* 
 
 i8| 
 
 2 
 
102 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 This table shows how rapidly the chest increases in width from 
 front to rear, and the rate of that increase from rib to rib. If a 
 vertical section of the chest be made, say between the fifth and 
 sixth ribs, the thoracic cavity will be found egg-shaped, the 
 broad part being above, the wall curving inward to obtain a 
 hold in the sternum. From the first to the sixth rib the main 
 function of the chest wall is to form a case for the vascular 
 apparatus and to support the horizontal spine. The lateral 
 movemnts of the sternal ribs are ofgno importance from a respira- 
 
 Fig. 40- — First Pair of Ribs 
 in the Horse (after 
 Schmaltz: Atlas d. Anat. d. 
 Pferdes). 
 
 Note the way in which the 
 head and tubercle are dis- 
 posed in order to prop up 
 the spine. 
 
 Fig. 41. — Head of the Eighth and 
 Sixteenth Rib of the Horse (after 
 Schmaltz: Atlas d. Anat. d, Pferdes). 
 
 A is the eighth rib in normal position; 
 the tubercle is still above the head. B 
 is the sixteenth rib in position; the 
 tubercle now lies behind the head, and 
 cannot be seen from the front. 
 
 tory point of view. Some, like the first pair (Fig. 40), have 
 no lateral movement, and it is very limited in the others, though 
 it increases farther back in the chest. Under forced respiration 
 the fifth rib may take a limited share in respiration, but practically 
 none of those anterior to it have any movement. 
 
 The relatively rigid cage formed by the first to the sixth ribs 
 articulates with the spine, and it might be supposed that some 
 rib movement favourable to respiration must occur, or the ribs 
 
RESPIRA TION 
 
 103 
 
 would not form a joint with the spine; but this series of joints 
 is of far more importance in securing flexibility of the spine than 
 in respiratory functions, while the manner in which they articu- 
 late shows that they are propping the spine up — a feature which 
 is in marked contrast to the false ribs. If the true ribs be ex- 
 amined, it will be found that both the head and tubercle are large 
 and well developed, the tubercle being disposed above the head 
 (Fig. 41, A), and both so arranged towards the vertebrae that they 
 prop it up. From the first to the seventh rib the substance and 
 strength of rib is very evident; from the eighth backwards the 
 ribs decrease in size, both in width and depth, and the decrease 
 in substance becomes progressively greater. From the first to 
 the thirteenth rib the tubercle is above the head, but from the 
 
 Fig. 42. — Vertical Section of the Body through the Eighth and 
 Seventeenth Dorsal Vertebra. 
 
 A, Eighth dorsal; B, seventeenth dorsal section. 
 
 fourteenth backwards the tubercle is placed lower and lower, 
 and finally is in the same horizontal plane as the head (Fig. 41, B) . 
 The tubercle begins to descend from the point where the ribs 
 are so far removed from the sternum that no further propping 
 up of the spine is possible. 
 
 The anterior part of the thorax is narrow in order to accom- 
 modate the fore-legs, which are, as it were, plastered on to it. 
 
 The false ribs have no sternal insertion, but each is connected 
 with the one in front by means of a piece of cartilage; while 
 rigidity of the walls is characteristic of the anterior chest, mobility 
 is characteristic of that portion now being considered. The false 
 ribs are narrow, thin, and curved. The curve gives the barrel 
 shape to the side of the chest. Further, these ribs before they 
 curve downwards have a relatively level or flat surface on their 
 
io 4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 upper part, and on this the actual width of the back depends. 
 From the ninth to the eighteenth the ribs decrease in length. 
 The respiratory portion of the thorax extends from the seventh 
 to the eighteenth rib. The movement of the ribs is mainly of 
 two kinds. At their upper part they are partly everted and 
 pulled forwards, below they are thrust outwards. The curva- 
 ture of the rib is of value when the process of eversion is 
 
 attempted, for by means 
 of this curvature very little 
 eversion represents a dis- 
 tinct increase in the trans- 
 verse diameter of the chest 
 cavity. The semicircle 
 formed by the cage is well 
 seen in vertical section in 
 Fig. 42, A and B, made at 
 the anterior and posterior 
 part of the thorax. 
 
 A front to rear section 
 of the chest (Fig. 43) is 
 roughly egg - shaped, the 
 narrow end of the egg in 
 front, while the broad end 
 is cut off obliquely for a 
 considerable distance by the 
 diaphragm. Of the latter 
 we shall speak in detail 
 
 Fig. 43. — Horizontal Section of the ,, -f j. -x . -u„ 
 
 Horse's Chest looked at from Above Presently, but it may be 
 
 (Sussdorf-Ellenberger). noted that the movements 
 
 a, Right lung; b, left lung; 1, position of of the diaphragm and the 
 
 the diaphragm during deep expiration; c, other muscles of inspiration 
 
 liver during deep expiration ; d, stomach i_i j • ; ± 
 
 during deep expiration; e, spleen during are capable, during quiet 
 
 deep expiration; 2, position of diaphragm respiration, of lengthening 
 
 during deep inspiration ; c, position of the front to rear diameter 
 
 liver; d' , of stomach; e', of spleen during t x u t- it, ,„ ± ~ ~ 
 
 deep inspiration ; /, posterior vena cava ° f the cheSt h Y I0 to I2 Cm ' 
 as it passes through the diaphragm. (4 to 5 inches), while at 
 
 the widest part of the chest 
 wall, viz., between the eleventh and twelfth ribs, the increase 
 in side to side diameter is about 4 cm- (i| inches). 
 
 Movements of the Diaphragm.— The vast musculo- tendinous 
 sheet placed between the abdominal and thoracic cavities merits 
 detailed consideration, owing to its importance in the respiratory 
 mechanism. This muscle is placed obliquely forward in the 
 body, and extends from the region of the loins to the sternum. 
 It roughly corresponds to the borders of the false ribs on both 
 sides. In the right of its centre it is penetrated by the posterior 
 
RESPIRATION 105 
 
 vena cava; to the left and above its centre it is perforated to 
 receive the oesophagus. Hung on to it are the liver wholly, and 
 the stomach partly, and if it were not for two powerful tendons 
 and muscles let into it from above, known as the crurae, these 
 weights could not be supported. Between the branches of the 
 crurae the posterior aorta escapes from the chest. The tendon 
 of the diaphragm is centrally placed, and forms a dense feltwork 
 of fibres, while the circumferential portion of the organ is mus- 
 cular. The diaphragm is convex towards the thorax, resembling 
 an open umbrella. This convexity is due to the pull exerted on 
 it by the diminished pressure in the air-tight thorax, supple- 
 mented by the pressure of the abdominal viscera from behind. 
 The diaphragm maintains its dome shape towards the thorax so 
 long as the chest is air-tight. Even under pathological condi- 
 tions, when the muscle is burdened by the effusion of gallons 
 of fluid poured into the chest, no flattening of the diaphragm 
 occurs. 
 
 The cavity of the chest is not cut off cleanly and sharply by 
 the diaphragm. The central part of the latter is thrust forward 
 into the chest, and by its projection separates the two lungs 
 at their posterior part, which in consequence rest on or wrap 
 themselves around the diaphragm, and as it were envelop it. 
 The lungs, when fully distended, do not reach within 8 cm. 
 (3 inches) of the cartilages of the false ribs. These points may 
 be seen in Fig. 44, which gives a side view of the horse's chest. 
 AFE indicates the margin around which the diaphragm is 
 attached; the dotted line suggests the central part of the dia- 
 phragm thrust forward into the cavity of the chest on either 
 side. This is covered by the lungs to within a short distance of 
 the false ribs. If, therefore, the chest were punctured transversely 
 anywhere below the dotted line in the diagram, and the instru- 
 ment pressed through to the opposite side, the structures would 
 suffer in the following order : lung, diaphragm, abdominal cavity, 
 diaphragm, lung . To state these facts briefly, the largest respira- 
 tory area of the lungs lies on an obliquely placed dome-shaped 
 table formed by the diaphragm, which projects into the chest. 
 
 The diaphragm recedes on inspiration, being pushed back into 
 the abdomen, but the retreat does not occur evenly over its 
 whole area. The central part of the diaphragm is naturally 
 restrained by the posterior vena cava, the upper part and ribs 
 move freely, while in the lower half— viz., from the vena cava to 
 the sternum— the movement is again somewhat restricted. As 
 the diaphragm recedes it compresses and carries back the whole 
 of the abdominal viscera, more especially the liver, stomach, and 
 spleen. The diaphragmatic curve of the colon is also carried 
 back to a limited extent. But the chief result arising from the 
 
io6 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 backward movement of the diaphragm is that on the circulatory 
 system. Blood-pressure in the posterior vena cava is increased, 
 the phrenic veins are emptied, and the portal vein is filled. The 
 to-and-fro action of the muscle is a most important aid to the 
 circulation. The limitation in movement of the central portion 
 of the diaphragm suggests that no flattening of it occurs during 
 inspiration, and this view is supported by X-ray inspection in 
 smaller animals. • The viscera behind the diaphragm are affected 
 
 Fig. 44. — Diagram of the Extent of the Chest in the Horse and Position 
 of TjHE Diaphragm. 
 
 The area B, C, D, E, is under the scapula and its muscles, and practically not 
 available for auscultation ; the surface A, B, E, F, is the available area of 
 the chest wall. The lung reaches to within a hand's breadth of the false 
 ribs. A, F represents the last rib; B, E runs parallel to the posterior edge 
 of the triceps; C, D corresponds to the position of the first rib. 
 
 The diaphragm bulges into the chest centrally, thus separating the two 
 lungs. The curved dotted line falling from A to E represents the central 
 line of the diaphragm, and shows the extent to which it encroaches on the 
 chest. 
 
 by each of its backward and forward movements (see Fig. 43). 
 The abdominal contents bulge forward, as it were, into the chest, 
 and so behave, to quote Keith's apt description, like a piston 
 being driven to and fro (see Fig. 45). 
 
 The lungs contain more air during a deep than during an ordi- 
 nary inspiration, so that the chest cavity is automatically reduced 
 in size during an ordinary inspiratory effort because of the 
 reduction in the amount of lung inflated. Fig, 46, slightly 
 
RESPIRATION 107 
 
 modified, from Ellenberger and Baum, shows the position of 
 the lungs during normal inspiration.* The lungs are resting on 
 the diaphragm, and it will be observed that this muscle may be 
 seen passing to the ribs for attachment. During ordinary in- 
 spiration the portion of the thoracic cavity not occupied by the 
 lungs is temporarily obliterated by the abdominal viscera being 
 thrust forward, and thus pressing the diaphragm against the ribs 
 and so closing the potential space. In Fig. 46, if the lungs were 
 fully distended during the forced inspiration of work, they would 
 extend to where the diaphragm is attached to the ribs — viz., 
 about 4 inches above the edge of the cartilages of the false 
 ribs. In Fig. 44, A, the dotted line under the loin indicates the 
 limiting position of the lungs during a forced inspiration . During 
 the ordinary inspiration of repose the dotted line would end 
 farther forward. 
 
 The pressure behind the diaphragm can be realised by an in- 
 spection of Ellenberger's figures (Figs. 45 and 46). When this 
 pressure against the diaphragm, especially during fast or heavy 
 work, is considered, there is no difficulty in understanding why 
 this muscle so frequently ruptures in the horse, and, as we shall 
 see later on, this same cause, pressure, may help to explain the 
 far more common lesion of ruptured stomach.' Ellenberger) 
 represents the lungs as occupying a relatively very small portion 
 of the thoracic cavity during expiration. His figure is not re- 
 produced, as we think the representation of the collapsed lung 
 is likely to prove misleading. 
 
 Briefly, the chest cavity of the horse is so arranged that it 
 accommodates itself to the needs of respiration; during repose 
 a large portion of it can be completely blotted out by the muscu- 
 lar rim of the diaphragm adhering to the pleura. During work 
 this potential space, indicated at 3, 3 (Fig. 46), is filled by^the 
 now greatly enlarged lungs. 
 
 The Lungs Fill with Air. — Air is never completely absent from 
 the healthy lungs, even after death. On life ceasing they retract, 
 especially from the diaphragmatic region, and shrink upwards 
 towards the spine. If they be now distended artificially, the 
 organs gradually fill, those portions nearest the bronchus dis- 
 tending first. There is no sudden or unusual swelling, but rather 
 a gradual invasion of different but neighbouring areas, which 
 gradually come nearer and nearer together, and finally meet. 
 The parts to expand last are the edges of the lungs and the sur- 
 face in contact with the diaphragm, especially with its upper half. 
 It is not intended to infer from these post-mortem observations 
 that the distension of the lungs during life is identical, because 
 during life they are never collapsed to the same degree, and, 
 * The writer's view is that the lung is not shown sufficiently distended. 
 
A MANUAL OF VETERINARY PHYSIOLOGY 
 
 3 3 M " 
 w °JSB| 
 
 H S 
 
 a s 
 
 5-2 
 P e 
 ~ So 
 
 w S 
 
 > m 
 
 Q a 
 
RESPIRATION 
 
 109 
 
 it ; ^ (. 
 
 V 
 
 D . 
 
 61 
 
 1 
 
 •a m 
 
 5<V 
 
 H >4 
 
 2 S 
 h a 
 
 •< S 
 
 w Q 
 o *• 
 
 2 §> 
 >■§■ 
 
 -< .. 
 
 S s 
 
 S 3 
 
 O < ' 
 
 s m 
 
 « 
 
 5 « 
 
 ■< w 
 o 
 
 s* "^ 
 g u . 
 
 5 ■ 
 
 P s 
 
 o « 
 
 w 
 
 z i . 
 
 •J 
 
 2 3 
 
 •a ,g 
 
 >_ 
 
no A MANUAL OF VETERINARY PHYSIOLOGY 
 
 further, when distended after death.Jthey inflate in the line of 
 least resistance. Nevertheless, all parts of the lung are not 
 equally distensible. The part situated around the roots of the 
 lungs must necessarily, from the obstruction offered by the 
 bronchi, be less distensible than those parts farther removed. 
 The anterior lobes, in consequence of the rigid nature of this 
 part of the thoracic cage, can hardly move at all, though, of 
 course, they. are kept distended. The greatest amount of ex- 
 pansion occurs where there is the greatest amount of spongy 
 lung-tissue and the least amount of bronchial ramification, and 
 this necessarily exists in the outer zone of the lungs, particularly 
 towards that portion facing the diaphragm, and corresponding 
 to that part of the chest where the greatest movement occurs. 
 
 From this it must not be inferred that the lung in other places 
 does not expand. As a matter of fact, the lungs fill up in the 
 living animal every crevice not occupied by heart or vessel. 
 There is no such thing as a pleural cavity in the healthy chest. - 
 The lungs lie close against the ribs, and never leave ribs or 
 diaphragm during the whole life of the animal so long as health 
 exists. There is, of course, a space between the costal and 
 pulmonary pleura, but in health it is only a potential space, 
 and there is a second potential space during repose, as already 
 described, between the muscular rim of the diaphragm and the 
 margins of the ribs. A finger introduced into the chest finds 
 the lung, close up against the opening which has been made, 
 and if an attempt be made to inject fluid into the potential 
 pleural sacs, the close application of the lung against the chest 
 wall prevents it as effectually as it prevents a piece of soft 
 rubber tubing from being passed along between the costal and 
 pulmonary surfaces. The lung exposed by this small opening may 
 even bulge outwards under the pressure of the inspiratory act. 
 
 These facts must not be confused with those which may be 
 observed under pathological conditions, when, the lungs having 
 collapsed under hydrostatic pressure, as in the case of effusion 
 into the chest, air readily enters the cavity through the cannula 
 when the level of the fluid is lowered by tapping; for the lung 
 is no longer in contact with the chest wall, having collapsed 
 upwards towards the spine. 
 
 The areas affected in pneumonia might indicate which are the 
 essential and which the non-essential portions of the lung in 
 ordinary respiration. Pneumonia always affects the lower lobes 
 of the lung, and nearly always the anterior lobes as well. The 
 latter, owing to the curve of the spine, are on a lower plane than 
 the upper lobes. It might be supposed from this that the upper 
 part of the lung was of more respiratory importance than the 
 lower, for with this surface of lung left the pneumonic patient 
 
RESPIRATION m 
 
 can live for days. But in the upright animal the lower part of 
 the lung is also the first in which pneumonia appears, and the 
 lower part in the biped is the extreme upper part in the quad- 
 ruped, from which it would appear that it is not necessarily the 
 least efficient portion of the lungs in which pneumonia begins, 
 though doubtless in the horse the least useful portion of both 
 lungs is the anterior lobes. 
 
 Though we have attempted to indicate that some parts of the 
 lung are of more use than others, and that these parts are mainly 
 filled during ordinary respiration, it must not be inferred either 
 that the remaining portions are quiescent or that the lungs are 
 firmly anchored within the chest. As a matter of fact, the roots 
 of the lungs are carried back with each inspiration, and come for- 
 ward at each expiration, and it is this which explains the up-and- 
 down tracheal movement of the horse in hurried respiration. 
 
 A small hole into the pleural sac of the horse does not lead to 
 collapse of the lungs, but a large opening does. Even then the 
 lungs continue partly to distend and recoil, but only with diffi- 
 culty, and finally asphyxia occurs. In all animals in which no 
 communication between the pleural sacs exists, the collapse of 
 the one lung under the influence of a large opening into the chest 
 is followed by the opposite lung doing extra work, and even 
 bulging across the middle line of the chest in the mediastinal 
 region. 
 
 In dealing with the distension of the lungs, the condition of 
 repose has been assumed. If work be done, the respirations are 
 increased. They are shorter and shallower, and may vary from 
 20 to ioo or more per minute, depending upon the nature of the 
 work and the fitness of the animal. In such cases the lungs 
 remain more fully distended than during repose, but the actual 
 amount of air entering at each inspiration is less, owing to the 
 fact that the respirations are more numerous, the rule being that 
 when the rate of breathing is increased the depth is decreased. 
 
 Inspiration. — We have seen that during life the lungs occupy 
 the cavity of a rigidly air-tight chest. So long as this air-tight 
 condition is maintained, any movement which tends to increase 
 the size of the che&t causes distension of the lungs. By disten- 
 sion, the air within them becomes rarefied, and thus a differ- 
 ence in pressure occurs between the outside air and that already 
 inthelungs. Inconsequence, air rushesin to restore equilibrium, 
 and this inrush is inspiration. 
 
 An increase in the size of the chest in its two diameters is 
 caused by the movements of the diaphragm and the false ribs. 
 The former retreats and carries with it the abdominal viscera, 
 which are in consequence exposed to compression. The ribs 
 generally are pushed outwards, while above they are drawn 
 
ii2 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 forwards, and their posterior edge everted, by which means the 
 natural curve of the rib increases the side-to-side diameter. At 
 the same time the intercostal spaces widen. By these means 
 the capacity of the chest is increased, and the lungs at once fill 
 the space thus created. 
 
 The tendency of the lungs is to recoil, owing to the highly 
 elastic nature of their tissues, and the fact that they are normally 
 in a state of distension. In the post-mortem condition they 
 are much smaller than the chest. During life the chest cavity, 
 being air-tight and larger than the lungs, the latter are distended 
 by the atmospheric pressure within them. 
 
 During forced inspiratory movements the elbows are turned 
 out, which brings auxiliary muscles into play, and allows a 
 certain number of true ribs to take part in the chest movement. 
 
 Expiration. — The chest having been filled with air, the next 
 process is its expulsion, and physiologists are not entirely 
 agreed upon the mechanism at work here. At one time expira- 
 tion was considered to be a purely non-muscular act, the elastic 
 recoil of the lungs and of the abdominal muscles being the im- 
 portant factors ; but if the forward movement of the abdominal 
 contents is to be credited with any influence in expiration, it is 
 evident that the action of the abdominal muscles cannot be of the 
 nature of an elastic recoil. There is some experimental evidence 
 suggesting that a reflex nervous co-ordination exists between 
 the inspiratory and the abdominal muscles, so that, as the 
 former cease to act, the antagonists at once come into operation. 
 It will be shown later that this holds true of the muscles of 
 locomotion, and there seems no reason why it should not apply 
 to the muscles of inspiration and expiration. At any rate, 
 the fact is undoubted that the contents of the abdomen travel 
 chestward, and there seems no great reason why this should 
 be brought about by the elastic recoil of the lung, and of the 
 abdominal muscle, when a purposive movement, for which 
 every facility exists, would be more to the point. As the abdo- 
 minal contents are forced backwards in inspiration, the gaseous 
 contents of the intestine are submitted to compression. This 
 may be turned to useful advantage during the period of recoil. 
 There is also the elastic recoil of the cartilages of the false ribs, 
 seeking to return to their position of rest after inspiration ; but as 
 material contributions to the effort of expiration, the two elastic 
 recoils are not of any great value. 
 
 Expiration is considered by some physiologists as a passive 
 act, and this view is partly based on the fact that it still con J 
 tinues after all nervous supply to the muscles is cut off. If the 
 cord be divided below the origin of the phrenic nerves, all the 
 muscles of the body posterior to that section are paralysed, 
 
RESPIRATION 113 
 
 but expiration still continues, owing to the elastic recoil of the 
 lungs. 
 
 Costal and Abdominal Respiration.— In the human subject 
 two types of respiration have been described, one costal, as in a 
 woman, and one abdominal, as in a man. It is now generally 
 admitted that the costal type in the woman is an artificial con- 
 dition, the result of compression. In the lower animals the 
 breathing is essentially abdominal. 
 
 The Foetal Lung contains no air, and therefore sinks in water. 
 The first few inspiratory gasps at birth distend the alveoli, but 
 for some time the conditions present in the adult, viz., the 
 negative pressure in the pleural cavity, and the collapse of the 
 lungs on opening the chest, are not found in the very young 
 animal. They occur only when the cavity of the thorax is 
 larger than the lung in a state of collapse. In the foetus the 
 lungs exactly fill the chest in the condition of expiration, and it 
 is not until the chest cavity grows, as it were, too large for the 
 lungs that a negative pressure in the thorax is produced. Later 
 on (p. 142) the cause of the first act of breathing will be ex- 
 plained. Thoracic development in young animals is very rapid; 
 a foal will increase 4 cm. (i| inches) in circumference within 
 the first few hours after birth ; when this absolute increase in 
 chest capacity is established, a negative pressure in the pleural 
 cavity is obtained. 
 
 Muscles of Respiration. — The action of the muscles' of the 
 chest during respiration has been much disputed. The external 
 intercostals doubtless, from the direction taken by their fibres, 
 pull the ribs forward, and by so doing increase the transverse 
 diameter of the chest; in this respect they are regarded as in- 
 spiratory muscles. The interna! intercostals, the fibres of which 
 run in an opposite direction to those of the external, draw the 
 ribs backwards and act as muscles of expiration; and speaking 
 generally, we may say that those muscles which draw the ribs 
 forward are inspiratory, whilst those which draw them back 
 are expiratory, The following table shows the inspiratory and 
 expiratory muscles of the chest : 
 
 Muscles of Inspiration. Muscles of Expiration. 
 
 Diaphragm. Abdominal muscles. 
 
 External intercostals. Internal intercostals. 
 
 Serratus anticus. Transversalis costarum. 
 
 Leva tores costarum. Serratus posticus. 
 
 Serratus magnus (during Triangularis sterm. 
 
 laboured respiration). 
 Latissimus dorsi (during 
 
 laboured respiration) . 
 Scaleni (during laboured 
 
 respiration). 
 
H4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 In most quadrupeds the sternum is fixed to the ribs and under- 
 goes little or no movement ; even the most powerful respiratory 
 movements in the horse give rise to no sternal movement. On 
 the other hand, there is a moderate amount of movement be- 
 tween the last two or three sternal ribs and their cartilages, 
 and a good deal of movement between the false ribs. During 
 laboured respiration any muscle which can assist in advancing 
 the ribs directly or indirectly is brought into play. This is well 
 marked in dyspnoea. 
 
 After the expiratory act there is a pause before the next 
 inspiration. In the horse at rest the period of expiration is as 
 a rule longer than that of inspiration, though the 1 proportion 
 between the two is not invariable. During work the duration 
 of the inspiratory and expiratory acts is about equal. 
 
 Intrapulmonic and Intrathoracic Pressure. — During in- 
 spiration a slight negative pressure exists in the trachea, and 
 during expiration a slight positive pressure. A strong inspira- 
 tion, with the glottis closed, such as can be made by a voluntary 
 effort in man, rarefies the air in the respiratory passage and the 
 pressure falls. Conversely a strong expiratory effort, with the 
 glottis closed, as in coughing, defsecation, or parturition, raises 
 the pressure in the respiratory passages considerably, and thereby 
 increases the intrathoracic pressure. The increased pressure 
 within the respiratory passages and thorax influences blood- 
 pressure, for the veins leading to the heart are partly obstructed, 
 as may be witnessed, for example, in man in a violent fit of 
 coughing, and thereby the venous flow to the heart is lessened. 
 In the pleural cavity a negative pressure is always present, due 
 to the tendency of the elastic lungs to collapse. The value of 
 this pull of the lungs on the chest wall has been ascertained for 
 the sheep to be about 3 mm. (| inch) of mercury, and during 
 dyspnoea 9 mm. (f inch) . In the dog during inspiration the nega- 
 tive pressure in the pleural sac is 6 mm. (£ inch) of mercury, 
 whilst during expiration 3 mm. (£ inch) has been observed. In 
 the horse 6 mm. (J inch) has been registered during a powerful 
 expiration, and 28 mm. (i£ inches) during a powerful inspira- 
 tion. The negative pressure can be recognised post mortem by 
 the rush of air immediately the chest is punctured. ' 
 
 (- When the chest is opened during life, the atmospheric pressure 
 within and without the lungs is equal, and in consequence the 
 lungs shrink and in time collapse. In the horse a large wound of 
 the chest cavity is seripus, for the reason that the two pleural 
 sacs communicate, and therefore both lungs shrink and finally 
 collapse. The collapse is gradual and leads to death by asphyxia, 
 unless the chest wound can be perfectly sealed, in which case 
 the air in the chest cavity is absorbed, and the negative thoracic 
 
RESPIRATION n 5 
 
 pressure restored. On the other hand, a small penetrating 
 wound of the chest does not cause collapse of the lung, for the 
 organ distends and pneumatically closes the pleural space. 
 
 The^N umber of Respirations varies with the class of animal; 
 as a rule, the larger the animal the slower the respirations. 
 
 Horse 8 to 10 per minute. 
 
 ° x " 12 „ 15 ,, 
 
 Sheep and goat 12 ,, 20 
 
 D °g - 15 ,, 20 "„ "„ 
 Pl S 10 ,, 15 ,, 
 
 Rumination increases the frequency of, and causes irregu- 
 larity in, the respiration, and in all animals muscular exertion 
 at once causes it to rise. In experiments on respiration this is 
 most marked; walking a horse will nearly treble the number 
 of respirations, but the rate of breathing begins to fall immediately 
 the horse stops, though it does not reach the normal for a few 
 minutes. 
 
 The ratio of heart-beats to respiration has been placed at 
 4 : 1 or 5 : 1. 
 
 The Effect of Respiration on the Circulation. — We have 
 previously alluded to the influence of respiration on the circula- 
 tion, and have pointed out that the negative pressure in the 
 thorax assists the circulation by favouring the return of blood 
 by the veins. This aspiratory action of the thorax mainly 
 affects the anterior vena cava, but the contents of the posterior 
 vena cava, we have seen, are also influenced during inspiration 
 by the contraction of the diaphragm and the consequent com- 
 pression to which the vessels are sub j ected . These two important 
 features of the respiratory system have the effect of increasing 
 at each inspiration the flow and velocity of the blood to the heart. 
 Aspiration of the thorax favours the filling of the right heart, 
 while the distension of the lungs in inspiration is generally be- 
 lieved to dilate the vessels, and so favour the flow of blood through 
 these organs. 
 
 In dealing with the question of blood-pressure (p. 74), it 
 was pointed out that the curves obtained show certain undula- 
 tions which are generally regarded as due to the influences of 
 respiration, for it is found, if the blood-pressure and respiration 
 curves be simultaneously obtained and superposed, that the 
 undulations in the latter correspond very closely to the undula- 
 tions in the blood-pressure curve. On comparing the curves, it 
 is found that the blood-pressure rises with inspiration and falls 
 with expiration; further, it is observed that the rise does not 
 take place immediately inspiration begins, nor does the fall 
 occur immediately expiration starts, but shortly after in both 
 cases, as may be seen in Fig. 29. These results have never 
 
n6 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 been adequately accounted for ; until the question was reopened 
 by Lewis in recent years, the following was the explanation given 
 of the respiratory rise and fall of blood- pressure: 
 
 If the curves be closely examined, it will be found that during 
 inspiration the pulse frequency is increased, while during ex- 
 piration it is decreased. This increase appears to hold good for 
 all animals, and is considered to be due to a diminished activity 
 of the cardio-inhibitory mechanism, while the reduction in 
 pulse-rate is believed to be caused by a stimulation of the same 
 centre. Any increase in the action of the heart results in more 
 blood being thrown into the aorta, and so raises the blood-pressure, 
 winch is further raised by the aspiratory action of the right auricle 
 in sucking in blood from the veins. So far, then, the general 
 causes of a rise in blood-pressure during inspiration can be ex- 
 plained. The next thing to account for is the delay in the rise 
 at the beginning of inspiration, and this is considered to be due 
 to the dilatation of the lungs causing an increase in the capacity 
 of the pulmonary vessels. The result of the increase is that they 
 have to be filled, and the delay thus caused explains the delay 
 in the rise of blood-pressure. In expiration the pulmonary 
 vessels become smaller, and the delayed fall in pressure is 
 attributable to these vessels having not yet returned to their 
 expiratory capacity. 
 
 Such, then, was the explanation accepted by most physiolo- 
 gists, but in recent years the investigation of the phenomena 
 by T. Lewis has resulted in considerable light being thrown on 
 the matter. This observer shows that the respiratory curves of 
 blood-pressure are of very complex origin, and unless the char- 
 acter of the respiratory act be known, it is not possible to say 
 what the effect on the blood-pressure will be. In a man trained 
 to breathe with his ribs, the blood-pressure falls during inspira- 
 tion and rises during expiration; when abdominal breathing is' 
 practised, there is a rise during inspiration and a fall during 
 expiration; and with costal and abdominal respiration combined, 
 intermediate results are obtained. 
 
 The secondary inspiratory rise of blood-pressure which 
 follows the fall, where respiration is slow and the costal type of 
 breathing is present, is, according to Lewis, due to lessened 
 intrapleural pressure, which causes lessened pressure on the 
 heart, and in consequence an increased filling of the organ. He 
 was able to show, by introducing or withdrawing air from the 
 pericardial cavity, how sensitive the heart is to an increase or 
 reduction of intrapericardial, and therefore of intrapleural, 
 pressure. 
 
 The Nostrils. — Before the air reaches the lungs it is warmed 
 by passing through the nasal cavities, so that it enters the 
 
RESPIRATION 117 
 
 trachea at nearly the body temperature. The incoming air also 
 becomes saturated with water vapour, this saturation likewise 
 occurring in the nasal chambers. In the majority of animals air 
 may pass through either the nose or mouth to enter the trachea, 
 but in the horse, owing to the length of the soft palate, nasal 
 respiration alone is possible; in this animal we therefore find 
 the nasal chambers with their inlets and outlets well developed. 
 The opening into the nostrils of the horse is large, funnel-shaped, 
 and capable of considerable dilatation ; it is partly cartilaginous 
 and partly muscular. Immediately inside the nostril is a large 
 blind sac, termed the false nostril, and one of its uses appears 
 to be to increase the capa- 
 city of the nasal opening by 
 allowing considerable and 
 
 rapid dilatation. Another s ***£' : $F£^k r 
 
 use is the production of . , 
 
 the peculiar snorting sound 
 made by a horse either 
 when he is alarmed or very 
 ' fresh.* 
 
 During forced inspiration 
 the nostril expands, espe- 
 cially the outer segment — 
 
 J ., , . ° . Fig. 47. — Nostril of Horse. 
 
 viz., that part m communi- ... ,, «. 
 
 •ii r ii 1 1 j. -i In, The inspiratory portion; Ex, the ex- 
 
 CatlOn With the false nostril piratory portion. 
 
 — and the air is rapidly 
 
 drawn up through the nasal chambers; during expiration the 
 outer segment of the nostril collapses, but the inner segment, 
 composed principally of the cartilaginous ala, dilates. Thus 
 the movement of the outer and upper part of the nostril is 
 principally inspiratory, of the lower and inner part mainly 
 expiratory, producing a peculiar double motion of the nostrils, 
 well seen after a gallop or in acute pneumonia (Fig. 47). The 
 dilatation of the inner segment of the nostril is brought about by 
 muscular contraction and by the rush of expired air; striking 
 the cartilaginous wing of the nostril, the current is directed 
 outwards at an obtuse angle to its course down the nostrils, 
 as may be well seen on a frosty morning when a horse is 
 respiring rapidly. 
 
 The nasal chambers are remarkable for their great depth and 
 narrowness; the cavities are partly filled by the turbinated 
 bones, which nearly touch the septum on each side, so that a 
 deep but thin column of air passes through the chambers; this 
 arrangement insures that the air is saturated with vapour and 
 raised to the proper temperature. The nasal chamber is divided 
 into two parts, the lower or respiratory and the upper or olfac- 
 
n8 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 tory. The latter will be dealt with under the Senses. It com- 
 prises the upper portion of the superior turbinated bone, the 
 ethmoid cells* and a portion of the middle meatus ; the respira- 
 tory channel, on the other hand, lies in the inferior part of the 
 nasal chamber, and comprises the inferior meatus, inferior tur- 
 binated bone, part of the superior, and part of the middle meatus. 
 The superior meatus may be said to exist for the purpose of smell, 
 the inferior for respiration, while the middle connects the sinuses 
 of the face with the nasal passages. 
 
 The termination of the two turbinated bones and their con- 
 nection with the external nostril is not as a rule fully described 
 in anatomical works. The soft, fleshy appendix of the superior 
 turbinated bone, having divided and then reunited, is inserted 
 into the cartilage of the nostril. In Fig. 47 the portion of the 
 nostril marked Ex would have to be lifted upwards and out- 
 wards to show the seat of insertion of this bone. The lower 
 turbinated bone also terminates in a fleshy appendix much 
 larger than that of its neighbour. It divides: one branch is 
 inserted into that portion of the nostril indicated Ex in Fig. 47, 
 close, but external to, the appendix of the upper bone ; while the 
 inferior division is inserted into the floor of the nostril. In order 
 to show its insertion, the portion of the nostril In (Fig. 47) 
 would have to be pulled outwards. The opening into the nasal 
 > ^tfambefs is formed, therefore, by the two fleshy terminations 
 of the fnferior turbinated bone, and in forced respiration these 
 two divisions separate widely. 
 
 The Glottis. — -The air having been warmed by passing over 
 the septum and turbinated bones, enters the glottis, the ary- 
 tenoid cartilages being separated to a greater or less extent to 
 enlarge the opening. In quiet respiration this enlargement of the 
 glottis is not very marked, but during work the cartilages are 
 drawn powerfully upwards and backwards, and the V-shaped 
 glottis fully opened (Figs. 54 and 56, pp. 149, 151). It is a 
 remarkable fact that the laryngeal opening should be relatively 
 so small, considering the diameter of the trachea and the size of 
 the nasal openings. 
 
 During inspiration the larynx and trachea descend slightly, 
 while they ascend during expiration. This is particularly well 
 seen in horses during the hurried respirations of disease, pro- 
 ducing as it does a well-marked rhythmical movement of the 
 laryngeal region and base of the tongue. 
 
 The Facial Sinuses are cavities in the face communicating with 
 the nasal chambers; they are of considerable size, occupying 
 nearly the entire facial region, and they give the needful bulk 
 to the head without adding to its weight ; they are lined by a 
 membrane which is continuous with that of the nose. These 
 
RESPIRATION 
 
 119 
 
 sinuses are filled with air which enters them through a foramen 
 at the posterior part of the middle meatus; during every act of 
 respiration air is passing into or out of them. At first sight it 
 would appear that air ought to enter the sinuses during inspira- 
 tion, but the reverse is the case ; it is only during expiration that 
 they are filled, whilst during inspiration air is sucked out. Con- 
 sidering the position of the common inlet to these sinuses, it is 
 difficult to understand why they should fill during expiration, 
 though the advantage of hot instead of cold air entering is 
 evident. 
 
 Respiratory Changes in the Air and Blood. — The changes"' 
 which air undergoes on passing into the lungs must now be con- 
 sidered. 
 
 Atmospheric Air contains in 100 Parts at 0° C. and 760 Mm. 
 
 (30 Inches)— 
 
 Oxygen 
 
 Nitrogen 
 
 Carbon dioxide 
 
 Argon 
 
 Helium 
 
 Hydrogen 
 
 By Volume. 
 20-96 
 78-09 
 00-03 
 
 00-94 
 
 Traces 
 
 Traces 
 
 The proportion of carbon dioxide is small; it is a natural 
 impurity in the air, though essential to plant life. The atmo- 
 sphere also contains moisture, the amount of which depends 
 upon the temperature; the higher the temperature the greater 
 the amount of water which the air can contain as vapour, and 
 the lower the temperature the less the amount. Air may be 
 dry or saturated, the latter term implying that it contains as 
 much vapour as it can hold at the observed temperature; it 
 generally contains about 1 per cent, of moisture, and is spoken 
 of as dry if it contains J per cent. The air which passes from the 
 lungs is always saturated with moisture. 
 
 When air is taken into the lungs it alters in composition: it 
 loses a proportion of its oxygen and gains in carbon dioxide, 
 as may be seen in the following table : 
 
 
 Nitrogen. 
 
 Oxygen. 
 
 Carbon 
 Dioxide. 
 
 Composition of inspired air 
 „ „ expired air 
 
 78-09 
 78-09 
 
 20-96 
 
 16-02 
 
 °'°3 
 4-38 
 
 -4' 94 
 
 + 4' 35 
 
120 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The volume of oxygen absorbed is slightly greater than that 
 of the carbon dioxide which takes its place, so that if dried and 
 reduced to standard barometric pressure and temperature, the 
 volume of dry air expired is slightly less than that of the air 
 inspired. But since expired air is usually warmer than in- 
 spired,* and is saturated with aqueous vapour, the volume ex- 
 pired is actually greater. 
 
 Respiratory Quotient. — The proportion which the volume 
 of oxygen absorbed bears to the volume of oxygen returned as 
 carbon dioxide is termed the Respiratory Quotient. It shows 
 the proportion of. oxygen required to oxidise carbon, and is 
 
 expressed as — - . , ' The quotient on a carbohydrate did, 
 
 such as starch, is high, though rarely greater than I, and generally 
 about 0-9; for in starch the oxygen and hydrogen of the molecule 
 exist in the proportion to form water, so that only the carbon 
 remains to be oxidised. The quotient may, as has been said, 
 be unity, or nearly so, depending upon whether the whole, or 
 nearly the whole, of the 2 is returned as C0 2 . If a purely 
 carbohydrate diet could support life, the-R.Q. would be 1 exactly, 
 as there would be as much of the 2 returned as C0 2 , as there was 
 2 taken in ; for example — 
 
 C 6 H 12 0»+60 2 = 6C0 2 +6H 2 |=i. 
 
 (Dextrose) ° 
 
 On a fat diet the opposite condition exists: the fat molecule 
 is poor in oxygen, and cannot supply from itself more than 
 one-sixth of the amount required to oxidise the hydrogen to 
 water; the remaining five-sixths have to be found from the in- 
 coming air, and in consequence less of the oxygen inspired is 
 returned as C0 2 . Thus; 
 
 C 3 H 5 (C 18 H3 3 2 )3+ 8o0 2 = 57 C0 2 + 5 2H 2 §2 = 0-71. 
 
 (Olein) 00 
 
 Consequently the R.Q. on a fat diet may be as low as 071. 
 On a protein diet, which, in the matter of R.Q., comes midway 
 between the carbohydrate and the fat diet, the oxygen in the 
 molecule is still in insufficient quantities to oxidise its own 
 hydrogen to water ; and the elements other than carbon, such as 
 sulphur, also require to be oxidised. It can contribute nearly 
 half the amount required, but the remainder must be abstracted 
 from the oxygen inspired. The R.O. for protein varies, from 
 circumstances which need not detain us here, but is generally 
 taken at 0-8. 
 
 The R.Q. in animals depends, therefore, upon the nature of 
 
 * Not always so in the Tropics. 
 
RESPIRATION 121 
 
 the diet : on one largely consisting of carbohydrates it approaches 
 unity; on a diet of flesh it approaches the figure given for pro- 
 tein. 
 
 In herbivora the R.Q. is 0-9 to i-o. 
 
 In camivora the R.Q. is 0-75 to o-8. 
 
 In omnivora the R.Q. is 0-87. 
 
 The value of the R.Q. lies in its being a measure of the oxi- 
 dation occurring in the body as a whole. As a rule the amount 
 of carbon dioxide formed is less than that of the oxygen absorbed, 
 but there are exceptions to this, as in hibernating animals, where 
 the processes of oxidation are at their lowest. In such cases the 
 R.Q. shows the possibility of oxidation occurring without the 
 production of carbon dioxide, and in other cases of carbon dioxide 
 being formed without a sufficient absorption of oxygen to account 
 for its production. Respiratory quotients as low as 0-3 have 
 been found in hibernating animals, while during the period of 
 their active life, when laying up or storing fat for winter use, the 
 R.Q. may be greater than unity — >even as high as 1-39. Such 
 high quotients are not unknown among non-hibernating animals 
 after a diet rich in carbohydrates, as in the case of fattening 
 animals. 
 
 In these cases the carbon dioxide excreted is in excess of the 
 oxygen absorbed. The carbohydrates are being stored up as fat, 
 which is poor in oxygen ; more oxygen is consequently rendered 
 available from the food for the production of C0 2 , which is split 
 off from the carbohydrate molecule when converted into fat. 
 
 The R.Q. is not altered by muscular work, provided the work 
 is not excessive or carried to the point of fatigue, (see Table, 
 p. 145). More oxygen is absorbed and more carbon dioxide 
 given off during work, but the ratio is the same as during repose. 
 The inference from this is that the material which is being used 
 up by the muscles both during rest and during work is the same. 
 With fatiguing work the R.Q. may fall, and this suggests that 
 the body fat is being drawn upon. 
 
 In a starving animal the R.Q. diminishes, the creature lives 
 on its own tissues, and as the body contains only a trifling amount 
 of carbohydrate, it is on fat and protein that the animal lives. 
 During starvation the output of carbon dioxide falls off at a 
 greater rate than the consumption of oxygen. 
 
 There are other gases returned from the lungs besides oxygen 
 and carbon dioxide, but very little is known about them. Both 
 hydrogen and marsh gas are given off in the expired air of 
 ruminants, having been absorbed by the bloodvessels of the 
 intestinal canal. The amount has been placed at 3 litres (183 
 cubic inches) in twenty-four hours. The nitrogen of the air is 
 returned unaltered. 
 
122 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Ventilation of the Lungs.— The lungs of an average horse 
 contain, when fully distended, 42 litres (i£ cubic feet) of air, and 
 at each inspiration 4 litres (250 cubic inches) are drawn in 
 during a condition of repose. As the animal during repose 
 breathes ten times a minute, the whole lung is ventilated about 
 once a minute. 
 
 The column of air in the respiratory passages extends from 
 the nostrils to the infundibular cavities. The air existing in the 
 alveoli is spoken of as the alveolar air ; it is the important air 
 in respiration, and carries out the effective respiratory changes. 
 From above the air-sacs to the nostrils is the ' dead space ' — that 
 is to say, it contains air which is for the time being functionless 
 from a respiratory point of view. The amount of the ' dead 
 space ' has not been determined for all animals ; in man one- 
 third of an inspiration fills it, while the remaining two-thirds 
 go to the alveoli. This proportion must necessarily vary in 
 different animals, depending on the length of the head and 
 neck. 
 
 The composition of the air in the respiratory passages is 
 obviously not the same throughout; it grows progressively 
 poorer in oxygen and richer in carbon dioxide from nostrils to 
 alveoli. During expiration the air from the dead space is 
 the first to leave, and some of the alveolar follows. During 
 inspiration the incoming air is diluted with that already in the 
 lungs, and its chemical composition altered. If it takes a 
 minute to ventilate the lungs— viz., to replace entirely the air 
 they originally contained — it is obvious that, though some of the 
 inspired air may reach favourably placed infundibular cavities 
 at once, the bulk can reach them only gradually. In man it is 
 estimated that about one-eighth of the alveolar air is changed 
 at each respiration. How far an ordinary inspiration travels 
 it is difficult to determine. Under the most favourable circum- 
 stances part of the axial stream of the current might reach 
 the alveoli, especially those of the anterior lobes, but the greater 
 part of it will get no farther than the bronchi. Some of this may 
 effect deeper penetration at the next inspiration, while some of 
 it will be expelled unchanged. In the infundibula the air is 
 changed by the process of diffusion. 
 
 The composition of the air in the alveoli of the lungs has for 
 years been a difficult question to determine, yet in order to ex- 
 plain pulmonary respiration this knowledge is essential. Hal- 
 dane, to whom we are indebted for a method of obtaining the 
 alveolar air, showed that in man the percentage of carbon dioxide 
 is almost constant for the same individual, while the oxygen 
 percentages are liable to great variation. The mean amount of 
 carbon dioxide in alveolar air was found to be 5-16 per cent, in 
 
RESPIRATION 123 
 
 men; it was less in women. The total volume of air inspired 
 is regulated by the amount of carbon dioxide in the arterial 
 blood circulating through the respiratory centre, and the per- 
 centage in arterial blood is identical with that found in the air 
 of the alveoli of the lungs. 
 
 During fast paces the respirations increase in frequency, 
 and as they do so they decrease in depth. The actual bulk of 
 an inspiration is less during fast paces than during repose, 
 though the total amount of air respired in a given time is very 
 much greater, owing to the increase in the number of respirations. 
 
 The influence of work on chest-ventilation in increasing the 
 frequency and decreasing the depth of the respiration is dealt 
 with on p. 143. 
 
 The Respiratory Exchange. — The respiratory exchange which 
 takes place is of two kinds : an external respiration between the 
 air and the blood through the medium of the lungs, and an 
 internal respiration between the blood and the tissues. Both 
 these are complex questions, and far from being settled. 
 
 We have seen that the air leaving the lungs has gained over 
 4 per cent, in carbon dioxide, and lost nearly 5 per cent, of its 
 oxygen. The excess of oxygen absorbed over carbon dioxide 
 produced is explained by the fact that oxygen is required for the 
 purpose of oxidising not only carbon to carbon dioxide, but also 
 hydrogen to water, and sulphur to sulphuric acid. In addition 
 to the above changes, the expired air is found to be warmer than 
 the inspired, and to be saturated with water vapour. This indi- 
 cates a loss of body heat and likewise a loss of water.. We have 
 also learnt that in the alveoli of the lungs the composition of the 
 air is not the same as in the dead space, and that in effect the air 
 in the alveoli contains more CO a and less 2 . It is in the alveoli 
 that the blood gets rid of its carbon dioxide, and takes up 
 its oxygen, and this process must be examined in some little 
 detail. 
 
 The Gases of the Blood are oxygen, carbon dioxide, and 
 nitrogen. These may be extracted from the blood in the 
 vacuum of a mercurial pump, or more conveniently by chemically 
 expelling the oxygen by means of potassium ferricyanide, and 
 the carbon dioxide by means of tartaric acid, and estimating 
 their amounts. The proportion of gases found is liable to con- 
 siderable variation, depending on the condition of the animal, 
 the vessel from which the blood is drawn, the activity of the 
 tissues, and even the length of time elapsing between collecting 
 and analysing the sample. 
 
 The gases of venous and arterial blood are necessarily different. 
 The figures given in the following table are only convenient 
 averages, and represent the gases in 100 volumes of blood 
 
I2 4 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 measured at o° C. (32" F.), and 760 mm. (30 inches) baro- 
 metric pressure : 
 
 
 Oxygen'. 
 
 Carbon 
 Dioxide. 
 
 Nitrogen. 
 
 Arterial blood 
 Venous blood - 
 
 20 
 
 12 
 
 43 
 5° 
 
 I'2 
 I - 2 
 
 Blood, in passing from arterial to venous, loses 8 parts of 
 oxygen and gains 5 parts of carbon dioxide, the nitrogen re- 
 maining unchanged. This suffices as a broad general statement, 
 but for the purpose of ascertaining the respiratory changes in 
 the blood, the mixed venous blood of the body — viz., that found 
 in the right ventricle of the heart — should be examined. An 
 analysis of this, from the dog, compared with arterial blood, 
 is given in the following table : 
 
 Venous Blood from Right Ventricle. 
 
 Arterial Blood. 
 
 Oxygen. 
 
 Carbon 
 Dioxide. 
 
 Nitrogen. 
 
 Oxygen. 
 
 Carbon 
 Dioxide. 
 
 Nitrogen 
 
 Mean. 
 
 hi; 
 
 Mean. 
 
 44 3| 3 8-8 
 
 Mean. 
 
 2-OJ4 -01 
 
 Mean. 
 
 Mean. 
 
 Mean. 
 
 2 . 7 |5*50 
 2 'lr62 
 
 
 The mean and the extremes are shown in the above table, 
 which gives a good idea of the variations in the amounts of gases 
 especially in venous blood; and this variation doubtless holds 
 good for all animals . Both nitrogen and argon are found in blood . 
 It is believed they are in simple solution and of no physiological 
 value. 
 
 Oxygen in the Blood. — If the oxygen in blood were held in 
 simple solution, as it is in water, and the blood introduced into 
 a mercury pump, with every reduction of pressure in the pump 
 oxygen would be given off from the blood. But in effect this is 
 not so. It is not until the pressure in the pump stands at one- 
 third of an atmosphere that oxygen begins to escape, and when 
 the pressure is reduced to one-sixth of an atmosphere, the oxygen 
 is suddenly and vigorously liberated. It is evident that its union 
 with something has been suddenly broken, and this something is 
 haemoglobin. Further evidence that the oxygen is not simply 
 absorbed by blood is afforded by the experiment of shaking 
 
RESPIRATION 
 
 125 
 
 air and blood, and oxygen and blood, together. The blood 
 absorbs more oxygen from the air than an equal bulk of water 
 would, while it absorbs no more from an atmosphere of pure 
 oxygen than it does from atmospheric air. The explanation is 
 that the oxygen and haemoglobin form a definite chemical 
 compound. 
 
 A, The blood-bulb; B, the froth- 
 chamber; C, the drying-tube; D, 
 fixed mercury bulb; E, movable 
 mercury bulb connected by a 
 flexible tube with D; F, eudio- 
 meter; G, a narrow delivery tube; 
 1, 2, 3, 4, taps, 4 being a three-way 
 tap. A is filled with blood by con- 
 necting the tap 1 by means of a 
 tube with a bloodvessel. Taps 1 
 and 2 are then closed. The rest of 
 the apparatus from B to D is now 
 exhausted by raising E, with tap 4 
 turned so as to place D in com- 
 munication with G only, till the 
 mercury fills D. Tap 4 is now 
 turned so as to connect C with D, 
 and cut off G from D, and E is 
 lowered. The mercury passes out 
 of D, and air passes into it from 
 B to C. Tap 4 is again turned so 
 as to cut off C from D, and connect 
 G and D. E is raised, and the 
 mercury passes into D, and forces 
 the air out through G, the end of 
 which is not yet placed under F. 
 This alternate raising and lowering 
 of E is continued till a manometer 
 connected between C and 4 indi- 
 cates that the pressure has been 
 sufficiently reduced. The tap 2 is 
 now opened. The gases of the 
 Wood bubble up into the froth- 
 chamber, pass through the drying- 
 tube C, which is filled with pumice- 
 stone and sulphuric acid, and enter 
 D. The end of G is placed under 
 the eudiometer F, and by raising 
 E with tap 4 turned so as to cut off C, the gases are forced out through G and 
 collected in F. The movements required for exhaustion can be repeated several 
 times till no more gas comes off. The escape of gas from the blood is facilitated 
 by immersing the bulb A in water at 40° to 50 C. 
 
 The available oxygen in blood is carried in two forms : a very 
 small quantity, amounting to 0-65 volumes per cent., is simply 
 dissolved in the plasma; the other and larger portion is the 
 oxygen chemically combined with haemoglobin, which may be 
 conveniently taken at 20 volumes per cent. As fast as the oxygen 
 in free solution is used up by the tissues it is replaced in the plasma 
 by the oxygen contained in the haemoglobin of the red cells, for 
 
 itH'<f4 
 
 y ' ■ ( 
 
 ( V *■ i 
 
 G 
 
 > 
 
 F 
 
 J 
 
 3 
 
 TIG. 4^. — iCHEME OF Gas rUMP. 
 
126 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the reason that haemoglobin is an easily dissociated* compound 
 and readily gives up free oxygen. The dissociation of oxygen in 
 the blood from the haemoglobin is very rapid, and occurs in about 
 one second ; similarly, the taking up of oxygen in the vessels of 
 the lungs by the haemoglobin is equally rapid. 
 
 When a pure solution of haemoglobin is exposed at body tem- 
 perature to a mixture of oxygen and nitrogen, the percentage 
 saturation of the haemoglobin with oxygen will increase with a rise 
 in the oxygen pressure in the following order : 
 
 Partial Pressure of Oxygen in Mm. o 
 
 l 
 5 | i° 
 
 20 
 
 72 
 
 5° 
 87 
 
 IOO 
 
 94 
 
 Percentage of oxyhaemoglobin - - o 
 
 37 
 
 55 
 
 This is graphically represented in Fig. 49, Curve o. In the circu- 
 lating blood itself the conditions are different from a pure solu- 
 tion of haemoglobin, the main disturbing factor in the former being 
 the carbonic acid pressure. In the presence of a carbonic acid 
 pressure of 40 mm. Hg, the blood loses its oxygen rapidly 
 whenever the oxygen pressure falls below 50 mm., while in the 
 absence of carbonic acid, even when the oxygen pressure has fallen 
 to 20 mm., the percentage of oxyhaemoglobin still remains 
 very high (80 per cent.). The effects produced by varying the 
 carbonic acid pressures are shown clearly in Fig. 49. 
 
 Under certain pathological conditions there is a defective 
 saturation of the arterial blood with oxygen, designated by 
 Haldane f anoxaemia, or insufficient supply of oxygen to the 
 tissues, and among other causes of this condition he points out 
 that, when the partial pressure of the carbonic acid in the blood, 
 is diminished, the dissociation of haemoglobin is diminished in 
 consequence of the haemoglobin holding on more tightly to its 
 oxygen. It has already been pointed out that the facility with 
 which haemoglobin parts with its oxygen is likewise affected by 
 the presence of certain salts in the blood, and these act as 
 carbonic acid acts — i.e., by means of their acidity. 
 
 The facility with which haemoglobin parts with its oxygen is 
 affected by the temperature, for under a given pressure less 
 oxygen is taken up by haemoglobin the higher the temperature. 
 
 * The process termed dissociation may be defined as the tendency which 
 certain gases have to leave the substances with which they are united 
 when the surrounding pressure becomes reduced. In the animal body 
 it is not confined to the union of oxygen with haemoglobin. The same pro- 
 cess is at work in liberating the carbon dioxide in the lungs trom the bodies 
 with which it is chemically combined. 
 
 f British Medical Journal, July 19, 191 9. 
 
RESPIRATION 
 
 127 
 
 The blood in the lungs comes into contact with the air in the 
 alveoli, the oxygen pressure of which is 100 mm. of mercury, or 
 about 13-2 per cent, of an atmosphere; there the haemoglobin 
 becomes nearly saturated (93 per cent.) with oxygen. The 
 blood returns to the arterial system and enters the capillaries 
 with 20 volumes of oxygen per cent, at a partial pressure of 
 100 mm. It leaves the tissues in a second or two with only 13 
 volumes of oxygen, having yielded up 8 volumes, or 40 per 
 cent., of its original amount, 
 in consequence of the fact 
 that the partial pressure of 
 oxygen in the tissues is 
 practically nil. Dissocia- 
 tion of the oxyhemoglobin 
 has therefore occurred, and 
 undoubtedly this has been 
 assisted by the presence 
 of carbon dioxide (see Fig. 
 49). The oxygen which 
 has passed into the tissues 
 is that in free solution in 
 the blood, and the loss of g—^- m 30 4 - 60 60 , ao 90 
 
 this is at Once restored by Fig. 49.— Dissociation Curves of Blood 
 
 the passage of oxygen from at 37 (Barcroft). 
 
 the haemoglobin of the red The curves show the effect produced on 
 rplk into thp nlasma dissociation by a rise in C0 2 pressure, the 
 
 ceilS into me piasma. blood having been exposed to the different 
 
 Taking advantage of the oxyg en pressures. 
 
 fact that only a trifling ( x ) with no carbonic acid - - o 
 percentage of the oxygen (2) With C0 2 at 40 mm. pressure 40 
 in blood is dissolved, While (3) With C0 2 at 9 o mm. pressure . 90 
 
 the hulk is chemicallv Com- Abscissa = 2 pressure m mm of Hg. 
 me DUiK IS cnemicdiiy uim 0rdlnates= percentage of Oxyhemoglobin. 
 
 bined, Haldane was able (Barcroft.) 
 
 to show that the oxygen 
 
 capacity of the blood varied with the colouring power, and thus 
 established a method of clinically estimating the haemoglobin 
 in any sample. < . . 
 
 Carbon Dioxide in Blood.— This gas is held in blood both in 
 simple solution and in chemical combination, the latter account- 
 ing for the larger part of the contained gas. In a gas-pump the 
 blood gives up all the carbon dioxide it contains; but if, instead 
 of using blood, serum be so extracted, it is found impossible to 
 obtain the whole of the carbon dioxide from the latter without 
 the use of an acid. It has been generaUy accepted in the past 
 that carbon dioxide in blood is carried by the sodium bicarbonate. 
 According to Buckmaster sodium bicarbonate takes practically no 
 share in the process; he considers that carbon dioxide is mainly 
 
128 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 carried by the haemoglobin, being united to the protein portion 
 of the molecule, thus leaving unaffected the iron moiety by 
 which the oxygen is carried. Haemoglobin is capable not only 
 of absorbing large quantities of carbon dioxide, but of giving 
 it off in the lungs by dissociation. The amount taken up by 
 haemoglobin is one-tenth greater when the haemoglobin is 
 reduced than when it is saturated with oxygen, so that venous 
 blood takes up more than- arterial at the same partial pressure. 
 
 The amount of carbon dioxide in venous blood is about 46 
 volumes per cent., of which about 5 per cent, is in solution; 
 but the fluid is not saturated, for it can be shown that blood will 
 dissolve as much carbon dioxide as water which at normal 
 temperature and pressure is about 100 per cent. 
 
 The source of the carbon dioxide is the blood in the tissues, 
 where it arises as the result of oxidations. By its presence in 
 blood the normal reaction of that fluid is preserved. It is also 
 the normal stimulus to the respiratory centre, and it insures the 
 liberation of oxygen from the haemoglobin. This explains why 
 venous blood takes up more oxygen than arterial at the same 
 partial pressure. These effects are produced by virtue of its 
 acid properties. 
 
 Circumstances influencing Respiratory Exchange- 
 Respiratory exchange is influenced by food, being not merely 
 increased as the result of the considerable muscular effort in- 
 volved during mastication, but directly excited by the nature 
 of the food. Thus maize causes in horses a greater excretion 
 of carbon dioxide and absorption of oxygen than an equal weight 
 of oats. A protein diet causes a larger respiratory exchange 
 than one of carbohydrate or fat . Respiratory exchange is greater 
 in the young than in the old, less during sleep than when awake, 
 greater in warm-blooded than in cold-blooded animals, greater 
 in small than in large animals, and greater at work than at rest. 
 Some of these points will be examined at length in the chapter 
 devoted to their consideration. 
 
 The Tension of the Gases in the Blood. — In order to arrive 
 at a knowledge of the laws which govern the exchange of car- 
 bonic acid and oxygen between the alveolar air and the pulmonary 
 capillary blood, it is necessary to determine the tension of these 
 gases in the blood. If a gas mixture containing definite quanti- 
 ties of oxygen, carbonic acid, and nitrogen were brought into 
 contact with a thin film of blood at body temperature, and if it 
 were found that, after sufficient time for the establishment of 
 gaseous equilibrium had been given, the composition of the gas 
 mixture remained unaltered, then one might conclude that the 
 tensions of the gases in the blood corresponded to their partial 
 pressures in the gas mixture. Instruments, called aerotoHio- 
 

 Carbonic 
 
 Oxygen. 
 
 Acid. 
 
 m. Mercury. 
 
 Mm, Mercury 
 
 110 I 
 
 A40- 
 
 100 
 
 
 40+ 
 
 40 1 
 
 
 45 
 
 oV 
 
 
 60 
 
 RESPIRATION 129 
 
 meters, have been used to determine the gas tensions in this 
 way. The partial pressure or tension of the oxygen is highest 
 in the alveolar air and lowest in the tissues, while the carbonic 
 acid is highest in the tissues and lowest in the alveolar air, the 
 differences in the former case being great — namely, from about 
 14-5 per cent, of an atmosphere (no mm. Hg.) in alveolar air to 
 zero in the tissues, and in the latter much less marked— namely, 
 from about 8 per cent, of an atmosphere in the tissues (60 mm. 
 Hg) to slightly over 5 per cent, of an atmosphere (40 mm. Hg) in 
 the alveolar air. The tensions of the gases in the blood and in 
 the tissues vary, but the following table gives an approximate 
 idea of the pressures which occur in the important situations. 
 
 Tensions of Gases. 
 
 Alveolar air 
 Arterial blood 
 • Venous blood 
 Tissues 
 
 Gaseous Exchange in the Lungs. 
 
 There are two theories put forward to account for the transfer 
 of oxygen from the external air to the blood, and the passage of 
 carbon dioxide from the blood to the external air. The first 
 and oldest view is a physico-chemical one. If the partial pres- 
 sure of the oxygen in the air-cells is higher than in the blood, 
 oxygen will pass into the blood. If the pressure of the carbon 
 dioxide in the blood is greater than in the alveolar air, carbon 
 dioxide will be given off to the latter. Until recent years the 
 acceptance of this explanation had been very general. When, 
 however, the air in the alveolar cavities was examined, facts were 
 discovered which threw grave doubt on the accepted view. It 
 was found by certain observers that the pressure of oxygen in the 
 arterial blood might be above that of the oxygen in the alveoli, 
 and that the pressure of the carbon dioxide in the blood might be 
 below that in the pulmonary alveoli. Under these circumstances 
 it was held that the theory of diffusion was no longer able 
 to explain the respiratory exchange in the lungs, and a fresh 
 explanation had to be found. The view held by those who reject 
 the physical theory is that the respiratory exchange is carried 
 out by a process of secretion, and this in the present state of our 
 Tchowledge of physics and chemistry is very difficult to under- 
 stand. We must look at both views more closely. 
 
 The diffusion theory is outlined above. The blood, robbed of 
 
 9 
 
130 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 40 per cent, of its oxygen in the tissues, and carrying over 16 
 per cent, increase of carbon dioxide, makes its way back to the 
 lungs in a partially reduced condition. Here it circulates 
 through the vast capillary system spread over the alveoli of these 
 organs, and is brought as closely as possible into contact with the 
 alveolar air in the ultimate air-passages. Nothing but the capil- 
 lary wall and the delicate moist membrane of the alveolus 
 separates the blood from the air, and through this wall the oxygen 
 instantaneously passes, in consequence of its partial pressure in 
 the blood being low and in the alveoli high. In the blood it is 
 taken up by the plasma, and then absorbed by the hsemoglobin 
 of the red cells, with which, as we have already seen, it forms 
 a weak chemical compound. Concurrently with this, carbon 
 dioxide, by the process of diffusion, passes out of blood, in which 
 its partial pressure is high, into the air of the .alveoli, where its 
 partial pressure is low.* 
 
 We have said that for some time past the law of diffusion and 
 chemical dissociation has been held by some authorities in- 
 adequate to explain the pulmonary exchange, the reason being 
 that there are many observations, none of which, perhaps, are 
 entirely free from error, which go to show that the alveolar air 
 may contain a higher partial pressure of carbon dioxide than 
 is found in the blood, while the oxygen tension of the alveolar 
 air may be below that of the blood. Under these circumstances 
 it is, held that oxygen is secreted. 
 
 It is no part of a work of this character to examine the rival 
 physical and vital theories of pulmonary respiration, but no 
 matter how difficult it may be to explain the secretion of oxygen, 
 the fact remains that it occurs in fishes. 
 
 * If a mixture of gases be absorbed by a fluid, it is found that the 
 volume of each gas forming the mixture is absorbed as perfectly as if it 
 were the only gas present ; no more and no less is absorbed, whether the 
 gas be alone or whether it form only a proportion of the mixed gases 
 present. This is explained as resulting from the fact that one gas does 
 not exercise any pressure upon the other gases with which it forms a mix- 
 ture. The term used by Bunsen to define the pressure exerted by one gas 
 in a mixture of gases is ' partial pressure.' For example, 100 volumes of 
 air contain at freezing-point n° C, and standard barometric pressure 
 760 mm. (30 inches), 21 volumes of oxygen and 79 volumes of nitrogen. 
 What is the partial pressure exerted by each gas in this mixture ? 
 
 760 x 21 _ 159-6 mm. (6-3 inches) of mercury; which is the 
 
 100 partial pressure of the oxygen; 
 
 and 
 
 760 x 79 ^ 600 mm. (23-7 inches) of mercur}', the partial pres- 
 
 100 sure of the nitrogen. 
 
 The term ' partial pressure ' occurs so constantly in all discussions of 
 the nature and causes of gaseous exchanges of the animal body, that the 
 above explanation has been given to make the matter clear. The term 
 tension is employed when the gases are dissolved, pressure when they are 
 in the gaseous state. 
 
RESPIRATION 
 
 J 3I 
 
 The so-called ' swim bladder ' of deep-sea fishes contains a 
 high percentage of oxygen. Cods brought up from a depth of 
 45 feet have shown 52 per cent, of oxygen in their gas reservoir, 
 and gas drawn off on succeeding days has shown 79 per cent, 
 and 84 per cent, of oxygen in the bladder. The production of 
 this gas by secretion is the only possible explanation of its exist- 
 ence. As a matter of fact, there is a vascular area in the swim 
 bladder by which it is formed ; and to render the secretory theory 
 more complete, nerves supplied by the vagus have been found, 
 which on section prevent the further accumulation of gas. 
 
 In the researches of Haldane and Priestley the alveolar air 
 was found to contain a remarkably constant percentage of carbon 
 dioxide. This constancy is maintained by the increase in alveo- 
 lar ventilation, which results from the increased production of 
 carbon dioxide. The respiratory centre, these observers dis- 
 covered, is exquisitely sensitive to the slightest increase in the 
 alveolar carbon dioxide pressure, a rise of 0-2 per cent, of an 
 •atmosphere being sufficient to double the amount of alveolar 
 ventilation during rest. It is an astonishing fact that the lungs 
 should be able to arrange for their effective ventilation. 
 
 Internal or Tissue Respiration. — This is infinitely more 
 obscure than pulmonary respiration — in fact, the means by 
 which the tissues utilise the oxygen given to them is quite un- 
 known. 
 
 We have previously seen that in the passage of blood through 
 the capillaries of the tissues it loses in about one second 40 per 
 cent, of its oxygen. This loss occurs in consequence of the dis- 
 sociation of the oxyhemoglobin in the presence of tissues whose 
 oxygen tension is practically nil. The blood, when it left the 
 pulmonary capillaries, was nearly saturated with oxygen; it 
 held, perhaps, 19 c.c. of oxygen for every 100 c.c. of blood, equal 
 to a partial pressure of 100 mm. of mercury. In the tissues, the 
 oxygen being used up or fixed by living substance as fast as it is 
 supplied, its partial pressure is nil. Under these circumstances, 
 there is no difficulty in accounting for the transfer of oxygen 
 from the blood to the tissues on purely physical and chemical 
 grounds, the dissociation of haemoglobin being assisted by the 
 presence of carbon dioxide in the tissues, for, as previously men- 
 tioned, it is known that an increased tension of carbon dioxide 
 in blood favours the giving up of oxygen to the tissues. 
 
 It is evident, in consequence of the above changes, that there 
 is a reduction in oxygen tensions from the blood to the tissues ; 
 the tension being highest in the blood-cell, is less in the plasma, 
 still less in the vessel wall, lower in the lymph, and at its lowest 
 in the tissue elements. In fact, these various tissues deal with 
 the haemoglobin in order to obtain its oxygen, exactly in the 
 
132 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 same manner as we deal with it outside the body in order to 
 obtain its gases — viz., expose it to reduced pressure, as, for ex- 
 ample, in a mercurial pump. The blood, as we have mentioned 
 earlier, does not give up all its oxygen in the tissues, probably 
 on account of the rapidity of its passage. On no occasion, 
 excepting under the condition of asphyxia, is the whole of the 
 haemoglobin reduced . There may be more oxygen returned to the 
 lungs unused than has been abstracted by the tissues. 
 
 The whole of the carbon dioxide in blood is not got rid of in 
 the lungs ; the tension of the gas in arterial blood on leaving the 
 pulmonary capillaries is equal to 5 per cent, of an atmosphere 
 Or 35 mm. of mercury. On the other hand, the pressure of carbon 
 dioxide in the tissues is high — 50 mm. to 70 mm. In conse- 
 quence, there is no difficulty in explaining on physical and 
 chemical grounds the transfer of carbon diojcide from tissues to 
 blood. 
 
 The medium in which this transfer occurs is the lymph which 
 bathes the tissues. This is the carrier between the blood and 
 the cell, for it is in the cell that tissue respiration occurs, and 
 not in the blood. 
 
 As evidence of the important fact that the oxidations take 
 place in the tissues, and not in the blood, two interesting experi- 
 ments may be quoted. If the blood in a frog's body be replaced 
 by saline solution, and the animal kept in an atmosphere of pure 
 oxygen, it will continue to produce carbon dioxide. It is obvious 
 in this case that the carbon dioxide has been produced from the 
 tissue, as the animal is without blood. A more telling experi- 
 ment is the following : Methylene blue is a comparatively stable 
 oxygen-holding substance — more so than oxyhemoglobin — and 
 is likewise an extremely powerful dye. If a solution be injected 
 into the circulation and the animal destroyed, the blood is found 
 dark blue in colour, but the tissues, especially the muscles, are 
 normal in appearance. When, however, they are exposed to the 
 air, they turn a vivid blue. The explanation is that the tissues 
 robbed the methylene blue of oxygen, and formed a colourless 
 reduction product, which on exposure to the air took up oxygen 
 and again formed methylene blue. No more remarkable example 
 of the instantaneous absorption of oxygen by the tissues could 
 be furnished. 
 
 In spite of the imperative need for oxygen which exists in the 
 body, it is remarkable how little can be obtained from it. From 
 muscle none can be obtained; from the various secretions — 
 lymph, bile, urine, milk, saliva — only a very little can be ob- 
 tained, though in all these there is an abundance of carbon 
 dioxide. 
 In the respiration of muscle, which can be readily studied, the 
 
RESPIRATION 133 
 
 muscle preparation of a frog will go on contracting until ex- 
 hausted in an atmosphere of hydrogen, producing all this time 
 carbon dioxide, thereby showing that it is using oxygen, yet neither 
 in the muscle nor in the atmosphere surrounding it does any 
 oxygen exist. The question, therefore, is, How does the oxygen- 
 free muscle obtain oxygen for the production of carbon dioxide ? 
 Few things in the whole range of physiology are more difficult to 
 understand, but it is supposed by some that when the oxygen 
 reaches the muscle it is at once stored up in a compound or com- 
 pounds among the muscle molecules ; hence the substance used 
 by the muscle has been termed intramolecular oxygen. In this 
 connection it is important to note that Bayliss reminds us that 
 there is no experimental evidence of the existence of intra- 
 molecular or other form of cellular oxygen available for oxidation. 
 The changes which the oxygen undergoes from the time it leaves 
 the blood until it reappears as carbon dioxide in the tissues are 
 completely unknown. It has been suggested that the nature of 
 the oxidative changes occurring in living tissues may be due to 
 the presence of oxidising ferments in the body — oxydases, as 
 they have been termed. Such ferments have been demonstrated 
 to exist in nearly all the tissues. 
 
 The phenomena of tissue respiration may be studied in living 
 tissues by ascertaining the composition of the blood before and 
 after circulation through them. Such an experiment has been 
 carried out on the horse by Chauveau and Kaufmann, who 
 utilisedthe levator muscle of the upper lip during feeding. The 
 consumption of oxygen and the production of carbon dioxide 
 was many times greater during activity than during rest . Glands 
 such as the submaxillary have been similarly examined, while the 
 respiratory changes occurring in the lungs during rest and work 
 have formed the subject of many experiments. All furnish the 
 same evidence, though not all with the same clearness. One 
 remarkable example of tissue respiration is furnished by L. Hill's 
 examination of the respiratory exchange in the living brain, which, 
 compared with that in skeletal muscle, was found to be very low. 
 
 It is impossible here to examine further the question of respira- 
 tion in the tissues, but the matter will be referred to in detail 
 when its importance warrants it, as in respiratory exchange, 
 muscular work, animal heat, and general body metabolism. 
 
 Deficiency in Oxygen. — When an animal is compelled to 
 breathe the same air over and over again, there is a gradual loss 
 of oxygen and an increase in carbonic acid, and though death will 
 ultimately ensue unless the air be renewed, it is remarkable that 
 before this occurs nearly the whole of the oxygen will have been 
 consumed from the atmosphere. This is further evidence, if any 
 be needed, that the oxygen is not simply absorbed by the blood, 
 
134 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 and that its absorption does not obey the ordinary laws of pres- 
 sure. Experimental inquiry has proved that animals may live 
 in an atmosphere containing only 14 per cent, of oxygen ; that 
 distress appears at 11 per cent., for at or below this pressure the 
 haemoglobin cannot take up its full amount of oxygen, and that 
 rapid asphyxia follows when the oxygen falls to 3 per cent. 
 
 In poisoning by carbon monoxide, the latter gas turns the 
 oxygen out of the blood- cells, yet although the whole of the red 
 cells are converted into carriers of carbon monoxide, the animal 
 may still be kept alive in an atmosphere of pure oxygen under 
 pressure, the amount of oxygen in solution in the plasma at an 
 oxygen pressure of two atmospheres being sufficient to carry on 
 the functions. 
 
 We have already referred to Haldane's recent work on de- 
 ficiency of oxygen in the tissues designated anoxaemia. He 
 points out that this condition may be brought about by defective 
 saturation of the arterial blood with oxygen ; by a slowing of the 
 circulation, so that an excess of oxygen is used up in the tissues; 
 by a defective proportion of haemoglobin in the blood, and by 
 alterations in the dissociation of oxyhaemoglobin, so that oxygen 
 is given off less readily to the tissues. 
 
 When the supply of oxygen to the tissues is less than their 
 requirements, there is an increase in the number of respirations, 
 which has the effect of bringing in more oxygen and getting rid 
 of the excess of carbon dioxide in the lungs. Where, however, 
 carbon dioxide is not in excess, the increased respirations wash 
 out an abnormal amount of this gas, and so increase the trouble 
 by removing from the body the natural acid stimulus to the 
 respiratory centre. The blood becomes abnormally alkaline in 
 anoxaemia, and the condition known as alkalosis is produced. 
 In this state the liver and kidneys respond by getting rid of 
 alkali. In connection with alkalosis it is convenient to refer 
 to acidosis, which, until recently, was believed to be associated 
 with a deficiency of oxygen. Haldane at one time thought 
 that, owing to this deficiency, lactic acid was produced in the 
 body and remained in the blood, thus helping to stimulate the 
 respiratory centre. This hypothesis he no longer believes to be 
 necessary. 
 
 Hyperpnoea is the term applied to the slightly increased 
 amplitude and frequency of the respiratory movements, such 
 as occur in gentle exercise, as the immediate result of any com- 
 mencing defective oxygenation of the blood, or of other cause 
 which acts as a stimulus to the respiratory centre. When the 
 stimulus is strong or continued, a further increase in the force 
 and frequency of the respiratory movements takes place, 
 and this condition is known as dyspnoea. The later stage 
 
RESPIRA TION 
 
 135 
 
 of dyspnoea is characterised by the respiratory movements 
 becoming ' convulsive ' in their activity, and this finale to 
 dyspnoea marks the onset of true asphyxia. 
 
 If the air supply be entirely cut off, asphyxia and death 
 rapidly ensue. Asphyxia has been divided into three stages. 
 In the first the attempts at breathing are laboured and painful, 
 deep and frequent, and all the respiratory muscles, including the 
 supplemental ones, are brought into play. Convulsions occur, 
 and the blood-pressure rises. In the second stage the inspira- 
 tory muscles are less active, the expiratory still powerful, and 
 the convulsions cease. In the third stage the animal lies uncon- 
 scious, occasional violent inspiratory gaspings occur, the mouth 
 is open (even in the horse), the pupils dilated, the pulse barely 
 perceptible or absent. During this stage the blood-pressure 
 rapidly falls. Death occurs in from five to six minutes from 
 the commencement of the first stage. Young animals are less 
 easily asphyxiated than adults for the reason that their tissue 
 respiration is much less. The length of time necessary to drown 
 puppies and kittens is evidence of this, and they may recover 
 even after prolonged immersion. 
 
 Excess of Oxygen. — It has been shown that oxygen above a 
 certain pressure is very poisonous. From 3 to 5 atmospheres of 
 oxygen, corresponding to 15 to 2.5 atmospheres of air, suffice to 
 kill seeds, to hinder the development of eggs, and to produce 
 convulsions in warm-blooded animals. In these latter cases the 
 amount of extra oxygen in the blood is 10 volumes, so that 
 there are 30 instead of 20 in every 100 volumes of blood. This 
 extra oxygen is not carried in the red cells, but in solution by 
 the plasma. All animals are instantly killed by a pressure of 
 50 atmospheres of oxygen. The reason of the toxic nature of 
 oxygen at high tensions is unknown. The blood cannot, by our 
 increasing the amount of oxygen above that contained in air, 
 be made to take up much more oxygen than if the normal 
 amount only were present. A pressure of 10 atmospheres causes 
 an increase of only 3-4 per cent, absorbed, so that the blood 
 contains 23-4 per cent, of oxygen instead of 20 per cent. The 
 practical application of this fact in the treatment of certain 
 diseases by the inhalation of oxygen is interesting. If we double 
 the amount of oxygen in the air, less than 1 per cent, of the extra 
 addition is absorbed. Either the small amount of extra oxygen 
 thus absorbed must be very valuable, or we must find some other 
 explanation of the undoubted advantage of oxygen inhalation 
 in disease. 
 
 The physiology of the matter is, in effect, this : The air con- 
 tains 20 per cent, of oxygen, which is more than enough for the 
 needs of the body; even the venous blood returns to the lungs 
 
136 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 with from 10 to 12 volumes of oxygen per cent, unused; and if 
 the oxygen in the air be doubled, less than 1 per cent, of the 
 extra is absorbed. It may, however, be that the excess of oxygen 
 in the alveolar air of the lungs during oxygen inhalation enables 
 the tissues to obtain their normal amount more easily. Oxygen 
 inhalation is of special value when the barometric pressure is 
 very low or the pulmonary absorbing surface is diminished. 
 
 Apncea is the term applied to a condition in which no respira- 
 tory movements of any kind are made. It is in the main a 
 condition artificially produced in the laboratory, though it may 
 also be met with in surgical operations. Apncea may be pro- 
 duced artificially by blowing air into and sucking it out of the 
 lungs at a more rapid and forcible rate than the ordinary respira- 
 tory rhythm of the animal. Two conditions have now been 
 brought about, either of which separately will produce apncea. 
 In the first place, there is a very free lung ventilation, and in 
 consequence a change in the composition of the gases in the alveoli 
 of the lungs, and so in the blood of the medulla. Secondly, the 
 partial pressure of the carbon dioxide in the blood will be lowered, 
 and as a result of the natural stimulus of the respiratory centre 
 being withdrawn, the centre will remain inactive. This is not 
 due, as was at one time supposed, to an excess of oxygen in conse- 
 quence of free lung ventilation, as it will occur when the lungs 
 are distended with hydrogen. 
 
 If the lowered pressure of carbon dioxide in the blood is the 
 cause of the suspended breathing, the introduction of this gas 
 into the blood should abolish the apnceic condition. This is 
 found to be so . A blast of carbon dioxide cuts short apncea in the 
 rabbit, while a blast of air has no such effect. 
 
 Apncea may also be produced in another way, by stimulation 
 of the afferent pulmonary fibres of the vagus by the repeated 
 distension of the lungs. This condition cannot be produced if 
 the vagi be divided. 
 
 The real value of the experimental production of apncea lies 
 in the light which it throws on the normal respiratory process, 
 and it is very remarkable that the normal rate of breathing 
 suitable to the animal's immediate requirements should be 
 regulated by the carbon dioxide circulating through the medulla. 
 
 There is a surgical condition known as shock, common after 
 prolonged operations or excessive haemorrhage. In these cases 
 there are a diminished amount of carbonic acid in the blood, a 
 dangerous fall in blood-pressure, and a deficient oxygen supply. 
 With this last is associated the production of lactic acid, and 
 acidosis has been considered to be present. The view held 
 at the present time is that no acidosis is present, as it would 
 be neutralised by the sodium bicarbonate of the blood, and 
 
RESPIRATION 137 
 
 that the symptoms are due to diminished bulk of fluid in 
 circulation and a deficient oxygen supply. In connection with 
 the diminution in the total volume of blood, Bayliss showed that 
 a solution of gum arabic in 0-9 per cent, saline remained in the 
 vessels, and was capable of saving many lives, while ordinary 
 saline only acted temporarily, and rapidly passed through the 
 vessel wall into the tissues. Intravenous injections of gum 
 together with oxygen inhalation constitute the modern method 
 of dealing with surgical shock. 
 
 The Nervous Mechanism governing Respiration. — A large 
 number of nerves is required for the production of respira- 
 tory movements. The facial dilates the nostrils, the vagus 
 supplies the larynx, the phrenic the diaphragm, certain spinal 
 nerves supply all the muscles of the trunk engaged in respiration, 
 and besides these nerves with' purely motor functions, there is an 
 enormous number of sensory nerves connected with respiration. 
 Such extensive ramifications require for the proper discharge of 
 their functions a central co-ordinating mechanism, and this is 
 known to exist in the medulla. The position of this centre has in 
 certain animals been very accurately defined, though it is not 
 represented histologically by any special group of cells. In 
 general terms it may be spoken of as being situated close to the 
 deep-seated origin of the vagus, and in front of the vasomotor 
 centre. The fibres which pass from it down the cord end in 
 motor nuclei in the grey matter of different levels, corresponding 
 to the points of outflow of the nerves connected with the muscles 
 of respiration. There is also evidence of decussation of the fibres, 
 or at any rate of a connection between those on the right and left 
 sides of the cord, as we shall see presently in dealing with the 
 phrenic. 
 
 The respiratory centre consists of two halves, each of which 
 is capable of working independently. By some it has also been 
 considered to consist of two parts — an expiratory and inspira- 
 tory. There is no definite indication of the former division, but 
 the latter is well marked. Of the two acts, the inspiratory is the 
 more important, for, as we have already seen, expiration may, 
 under some circumstances, be a purely passive act. Nevertheless, 
 there are special expiratory functions of central origin, such 
 as the act of straining, as in parturition, micturition, defsecation, 
 or coughing — actions which are certainly not of a passive char- 
 acter, and which might be considered to originate in a special 
 expiratory centre. 
 
 It is known that, besides the motor nerves previously described 
 as being connected with the process of respiration, the respira- 
 tory centre is connected extensively with sensory nerves— prob- 
 ably with every sensory nerve in the body, for the centre may be 
 
138 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 readily stimulated through any sensory branch. Further, the 
 cortex of the brain is connected with the respiratory centre, for 
 the animal may at will increase or withhold its respiration, 
 while psychical events produce their effects through some such 
 channel, as may be witnessed in the increased respiration of 
 nervous apprehension. A perfectly orderly sequence of events 
 occurs in normal respiration, beginning at the nostrils and end- 
 ing at the flank, the entire smoothness and regularity of which 
 is dependent on the nervous connections of the respiratory centre 
 with the outside. 
 
 The centre is automatic, i.e., it is within itself that the dis- 
 charges are generated which issue forth as respiratory impulses; 
 indeed, it is as automatic in its working as the heart, for if every 
 nervous connection leading to it be divided, the centre continues 
 notwithstanding to act rhythmically . If the chief path by which 
 these impulses gain the exterior be interrupted — as, for instance, 
 by dividing the spinal cord behind the medulla— death from 
 paralysis of respiration at once ensues. Such a section does not 
 affect the facial muscles of respiration, and an animal that has 
 ceased to breathe and is virtually dead may still continue to 
 make powerful inspiratory facial gasps, the nerves supplying 
 these muscles being derived from a point anterior to the 
 section. 
 
 The nature of the impulses which issue from the respiratory* 
 centre depends upon the character of the impulses received from 
 without, which stimulate their production reflexly. Thus the 
 breathing may be hastened or slowed down, quickened in rhythm 
 and decreased in depth, or increased in both rhythm and depth. 
 From the cortex of the brain impulses may 'be transmittedj 
 voluntarily increasing the respirations, as in sniffing, or with- 
 holding them entirely, as when the head is under water. Through ' 
 the medium of the nasal branch of the fifth pair of nerves im- 
 pulses may be transmitted from the nostrils inhibiting inspiraj- 
 tion; from the skin impulses of two kinds are received — viz., 
 those increasing and those diminishing respiration. It has even 
 been supposed that different nerve fibres are concerned in the 
 transmission of stimulating and inhibiting impulses to the respira- 
 tory centre, though these are not exclusively channels for respira- 
 tory purposes. From the larynx through the superior laryngeal! 
 nerves impulses inhibiting inspiration and stimulating expiration| 
 are transmitted, and similarly impulses are sent through the sen- 
 sory fibres of the glossopharyngeal, which inhibits respiration 
 at the moment of swallowing. In the diagram (Fig. 50) the chief 
 nervous connections of the respiratory centre are shown; the sign 
 denotes whether they convey impulses which stimulate or inhibit, 
 the respiratory centre. 
 
RESPIRATION 139 
 
 The Influence of the Vagus on Respiration. — The vagus 
 is the most important afferent channel by which the lungs are 
 brought into connection with the medulla; the sensory fibres 
 are distributed over the area from the glottis to the alveoli of 
 the lungs. If impulses from the lungs to the medulla are cut off 
 
 Fig. 50. — Diagram to illustrate the Chief Nervous Connections of the 
 Respiratory Centre (after Waller). 
 
 by dividing the vagi, the respirations become slower and deeper 
 (Fig. 51, C) ; in the horse they have been known to fall to five 
 per minute ; the inspirations particularly are deep and prolonged. 
 If one vagus only be cut, the effect just described may not occur, 
 
 p IG . 5I . — Respiratory Tracings: Dog (Stewart). 
 A, Normal; B, effect of stimulation of the central end of vagus; C, effect of 
 section of both vagi. Time-tracing, seconds. 
 
 or be of only a partial character. From this it is evident there 
 are impulses passing up the vagus from the lungs to the medulla 
 which maintain the normal respiratory rhythm, and these are 
 'permanently lost by section of the nerves. If the end of the 
 divided vagus which is connected with the brain be stimulated 
 
140 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 below the origin of the superior laryngeal nerve, there is arrest of 
 respiration if the stimulation be strong, but with moderate 
 stimulation respiration is quickened (Fig. 51, B). The strength 
 of the stimulating current and the condition of the centre at the 
 time of the experiment appear to be important factors in deter- 
 mining the exact results which will follow. But the interpreta- 
 tion of the results of these experiments shows that there are two 
 kinds of fibres in the vagus conveying impulses from the lungs 
 to the medulla, and affecting it in opposite ways : (1) Fibres carry- 
 ing impulses which cause inspiration and inhibit expiration ; and 
 (2) fibres conveying impulses which inhibit inspiration, and so 
 cause expiration. These sets of fibres are in alternate activity, 
 and the cause of their normal stimulation is believed to be the 
 alternate distension and collapse of the air vesicles. If, for 
 example, air be pumped into the lungs, expiration is excited, 
 and if it be sucked out, inspiration follows; from which it is 
 argued that an inspiration, by distending the air vesicles, excites 
 expiration, and the contraction of the air vesicles on expnation 
 excites inspiration. It can be shown experimentally that electri- 
 cal changes in the divided vagus indicate a current of a particu- 
 lar character throughout each inspiration, and another' of a dif- 
 ferent character during expiration. If, however, the respiratory 
 centre be regarded as primarily automatic, the inspiratory fibres 
 found in the vagus may be regarded as increasing the rate of 
 respiration, the expiratory as inhibiting or controlling inspiration, 
 and thus producing expiration. If this view be adopted, the act 
 of inspiration proceeds from an automatic centre, which requires 
 no other stimulus than that which is generated within itself, 
 while expiration proceeds from the stimulation caused by dis- 
 tension of the air vesicles. 
 
 The nature of the internal stimulus which provokes the respira- 
 tory centre has for some time been the subject of controversy. 
 All were agreed that the stimulus lay in the blood gases, but 
 whether this was to be attributed to their richness in carbon 
 dioxide or to their poverty in oxygen could not for some time 
 be decided. It is now generally admitted that the richness in 
 carbon dioxide is a more potent stimulant than the poverty in 
 oxygen, and Haldane's and Priestley's researches show the extra 1 
 ordinary delicacy of the respiratory centre's response to minute 
 increases in carbon dioxide, a 0-2 per cent, increase leading to an 
 increased pulmonary ventilation of 100 per cent. It should be 
 noted that it is not the carbon dioxide in the venous blood which 
 stimulates the medulla, but that which is in the arterial blood. 
 
 During muscular work, especially that of a severe nature, there 
 is lactic acid in the circulating blood. To this condition the term 
 acidosis is applied, but in Haldane's recent work he points out 
 
RESPIRATION 
 
 141 
 
 that lactic acid is rapidly oxidised in the system, and that his 
 earlier interpretation of the influence of this acid in stimulating 
 the respiratory centre cannot now be held to be true. Should 
 blood become slightly too acid, the liver produces ammonia 
 instead of urea, which neutralizes the acid and is excreted by the 
 kidneys as the ammonium salt. 
 
 Respirations are increased in frequency as the result of such 
 sensory stimulation as occurs, for instance, in painful operations ; 
 sensory stimulation of the abdominal wall may be employed as a 
 means of starting an inspiration in chloroform-poisoning. 
 
 Animals which do not sweat, pant after work in order to get 
 rid of the surplus heat by warming a larger volume of air. This 
 question will be considered again under Animal Heat, but the 
 possible mechanism involved may here be glanced at. It is 
 conceivable that the congested condition of the skin may send 
 impulses to the respiratory centre, but this will not account for 
 the panting of animals in ' show condition.' Here there is pre- 
 sumably no increase in the carbon dioxide in their alveoli, and 
 the stimulation to increased respiratory activity would appear 
 to be obtained reflexly from the skin, though the increased tem- 
 perature of the circulating blood in the medulla may account for 
 it, as appears to be the case in fever. 
 
 Influence of the Phrenic Nerves on Respiration. — The 
 phrenic is essentially an inspiratory nerve. We have referred to 
 the cutting off of the respiratory centre by dividing the cord above 
 the phrenic. If the cord be divided below the point of exit of 
 the phrenics, the connecting channel between the respiratory 
 centre and lungs via the spinal cord is not interfered with, but 
 the resulting paralysis of the abdominal and intercostal muscles 
 necessitates that the action of the diaphragm should be more 
 powerful . If one phrenic nerve be divided, half the diaphragm is 
 paralysed; if both be divided, the whole diaphragm is paralysed, 
 and in most animals death ensues. In the horse division of the 
 phrenic nerves is not fatal; it leads to difficulty in breathing, 
 increased heart action, and the collection of faeces in the rectum; 
 but in about twenty-four hours these symptoms pass away, and 
 if the animal be worked, no appreciable difficulty in breathing is 
 subsequently observed. 
 
 We are not prepared to offer any explanation of this re- 
 markable exception to experimental division of both phrenic 
 nerves. Sometimes the fact may be demonstrated in surgical 
 practice, for though as a rule in the horse fracture of any one 
 of the four upper cervical vertebrae, i.e., fracture above the origin 
 of the phrenic nerves, is immediately fatal, yet there are many 
 exceptions to the rule, and death may be delayed for some time. 
 
 The phrenic nuclei in the cord are connected by crossed fibres, 
 
142 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 so that, if the cord be half cut through above the nuclei, both 
 sides of the diaphragm are still able to contract, the explanation 
 being that the impulses cut off from one half of the diaphragm 
 are transmitted through the crossed channel. 
 
 Division of Seventh Pair. — Colin has shown that if the 
 seventh pair of nerves be divided in the horse, and the animal 
 worked, asphyxia results. This nerve dilates the nostrils; 
 when it is divided, the paralysed flaccid nostrils are drawn in : 
 ward at each inspiration, and so close the opening. 
 
 Cause of First Inspiration. — The cause of the first act of 
 inspiration in the newly-born animal is that the placental circula- 
 tion being cut off, the respiratory centre of the foetus becomes 
 stimulated through the increased venous character of the blood 
 now circulating through it ; as a result of this, inspiration is auto- 
 matically produced. But it is also assisted by reflex impulses 
 carried from the surface of the skin due to handling and drying; 
 handling the skin of the foetus while still in utero with the placen- 
 tal circulation intact may provoke respirations, and in aU animals 
 the very first act of the mother is to dry the body of the young 
 and stimulate the skin by licking. 
 
 The Amount of Air Required. — Numerous respiration experi- 
 ments have been made on all animals to determine the amount 
 of air they require and the gases of respiration. It is, perhaps, to 
 a knowledge of the respiration of the horse that the greatest 
 practical interest attaches, although a knowledge of the respira- 
 tion of other animals is also of value. 
 
 The lungs of a horse will contain about 42-5 litres (i£ cubic 
 feet) of air at the end of a deep inspiration. During ordinary 
 repose he draws into them 2,265 to 2,548 litres (between 80 and 
 90 cubic feet) of air in the hour, though considerable variation 
 may be found even in the same animal. 
 
 An average inspiration in the horse during repose amounts 
 to about 4-1 litres (250 cubic inches), and the amount of air 
 which flows in and out during ordinary quiet respiration is known 
 as the tidal air. Speaking roughly, it is only one-tenth of what 
 the lungs can contain ; the remaining nine-tenths are made up of 
 complemental, reserve, and residual air. The complemental air 
 is that over and above the tidal which can be taken in by a forced 
 inspiration, while the reserveis a somewhat similar amount which 
 can be expelled by a forced expiration. The most powerful 
 expiratory effort is unable to remove from the lungs all the air 
 they contain, and this remainder is known as the residual air. 
 
 The great variations which have been observed in the amount 
 of air taken in by the same animals, under apparently identical 
 conditions, cannot be adequately explained; the slightest dis- 
 turbing influence alters both the rhythm and depth of the 
 
RESPIRATION 
 
 143 
 
 respirations. Under the influence of work, the amount of air 
 required is greater, and as a rule, the faster the pace the more 
 air needed; but many disturbing factors occur which render 
 experiments on this subject very contradictory, and give results 
 of the greatest variation. During severe work, such as a gallop, 
 a horse is taking air into his lungs at the rate of 24,067 litres 
 (850 cubic feet) per hour at least, and probably more; the respira- 
 tions, from being 9 to 10 per minute during repose, may now be 
 anything between 70 and 100 per minute. The effect of taking 
 in all this extra air is that the pulmonary ventilation is increased. 
 It is calculated that in man a deep inspiration more than doubles 
 the capacity of the alveoli by distending them. In such paces 
 as the canter, trot, and walk, the amount of air used is corre- 
 spondingly less ; immediately the pace slackens or the horse stops, 
 the respirations fall, and the amount of air inspired becomes 
 reduced. This is one of the great difficulties attending respiration 
 experiments on horses under natural conditions. 
 
 In the following table is shown the mean amount of expired 
 air obtained from horses performing actual work, and collected 
 in the apparatus shown in Figs. 52 and 53 : 
 
 
 Air expired in Litres 
 per Hour. 
 
 Air expired in Cubic Feet 
 per Hour. 
 
 Repose - 
 
 Walk - 
 
 Trot ... 
 
 Canter - 
 
 Gallop - 
 
 2099-75 
 
 3780-80 
 
 8149-70 
 
 II069-2I 
 
 24038-02 
 
 74'17 
 I33-55 
 287-87 
 
 391-00 
 849- 10 
 
 A horse in a state of repose, according to Zuntz and Lehmann, 
 produces 85 litres (3 cubic feet) of C0 2 per hour, and absorbs 
 nearly 99 litres (3§- cubic feet) of oxygen; the expired air is 
 found to have lost 4 per cent, of its oxygen and gained 3J per 
 cent, of C0 2 . This is very much more than we found,* but it 
 agrees pretty closely with observations made on other animals 
 and on man. It may be noted that even in animals which, 
 from their small size or from other causes, lend themselves to 
 exactitude in experimentation, the most divergent results have 
 been obtained, and the same divergence is observed in experi- 
 ments in man. 
 
 * ' The Chemistry of Respiration in the Horse during Rest and Work,' 
 Journal of Physiology, vol. xi., 1890. It is now considered that samples 
 of air are not sufficient to determine respiratory exchanges ; the C0 2 has 
 a tendency to accumulate in the tissues, and an apparatus such as was em- 
 ployed by Zuntz and Lehmann, which admits of prolonged observation, 
 is necessary. 
 
i 4 4 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Fig. 52. — Respiration Apparatus. 
 
 The face mask; 2, rubber connections with 6 and 7; 3, pneumatic collar to 
 render mask air-tight above; 6 is the inlet to the face-piece, and 7 the outlet 
 to the bag; 8, valve-box through which the expired air passes to 10, a rubber 
 bag of 20 cubic feet capacity. After an experiment, the tap 9 is closed and 
 the air pressed out of the bag, passing through n, where it is measured in 
 the meter. 4, A chamber containing a tray of coke saturated with caustic 
 potash, through which the inspired air passes and is robbed of its C0 2 before 
 entering 5 and 6. 
 
 Fig, 53 f — Horse in Position on Respiration Apparatus. 
 
RESPIRATION 
 
 HS 
 
 Muscular work has a profound influence over the respiratory 
 exchange; it increases the amount of oxygen absorbed and the 
 proportion of carbon dioxide given off. The faster the pace, the 
 greater the amount of exchange which occurs, though experi- 
 ment has failed to prove definitely an immediate relationship 
 between the amount of oxygen absorbed and the amount of 
 work produced. 
 
 The following table is taken from the experiments of Zuntz 
 and Lehmann on horses :* 
 
 
 Air expired per 
 Hour. 
 
 Carbon Dioxide 
 discharged per Hour. 
 
 Oxygen absorbed 
 per Hour. 
 
 Respiratory 
 Quotient. 
 
 Litres. 
 
 Cubic 
 Feet. 
 
 Litres. 
 
 Cubic 
 Feet. 
 
 Litres. 
 
 Cubic 
 Feet. 
 
 Rest 
 Walk 
 Trot 
 
 2,640 
 10,620 
 19,980 
 
 93' 2 
 375'i 
 705' 7 
 
 88-68 
 260-52 
 450-96 
 
 3"i3 
 
 9- 20 
 
 15-90 
 
 96*06 
 
 285-96 
 485-58 
 
 3 - 39 
 
 IOIO 
 
 17-15 
 
 0-92 
 090 
 O93 
 
 The table on p. 146 taken from Colin gives a general view of 
 the respiratory changes in animals. It represents the mean of 
 observations made many years ago by different authorities, but 
 Laulanie considers the figures to be too high ; they certainly are 
 so far as. the horse is concerned, but are of interest from a com- 
 parative point of view. 
 
 The column showing the respiratory changes per kilo of body 
 weight illustrates the fact, to which attention will subsequently 
 be drawn, that the changes among small animals are greater 
 than among large ones, owing to their having, relatively to their 
 weight, greater body surface. The discharge of carbon dioxide 
 by a mouse, for example, is relatively sixteen or seventeen times 
 greater than that of an ox. 
 
 The Influence of Work on Respirations.— During moderate 
 work the increase of carbon dioxide in the alveoli of the lungs is 
 sufficient, as we have seen, to increase the pulmonary ventilation ; 
 but with prolonged and especially with severe work it is probable 
 that there are formed in the muscle fibre some products of its 
 metabolism, which may, by circulating through the blood, either 
 stimulate the respiratory centre, or render it still more sensitive 
 to the percentage of carbon dioxide circulating through it. 
 
 * The work performed in these experiments was carried out on a plat- 
 form revolving at different speeds, and the animal was thus kept in con- 
 stant communication with the respiratory apparatus from which samples 
 of the exoired air were taken for analysis. It has been shown that this 
 is a necessary condition to insure accuracy. The writer s observations 
 were carried out on horses which actually performed, under natural con- 
 ditions, the various paces, but the samples of air were unavoidably collected 
 for too brief a period. 
 
146 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 
 Horse. 
 
 Cow. 
 
 Ass. 
 
 Pig. 
 
 Sheep. 
 
 Dog. 
 
 Body Weight : 
 
 
 
 
 
 
 
 Kilos 
 
 45° 
 
 45° 
 
 150 
 
 75 
 
 45 
 
 20 
 
 Pounds 
 
 990 
 
 990 
 
 330 
 
 165 
 
 99 
 
 44 
 
 Amount of air in- 
 
 
 
 
 
 
 
 spired in 24 hours: 
 
 
 
 
 
 
 
 Litres 
 
 95.591 
 
 78,800 
 
 3 1 .495 
 
 34.444 
 
 20,400 
 
 8,441 
 
 Cubic feet 
 
 3376 
 
 2783 
 
 1112 
 
 1216 
 
 720 
 
 298 
 
 Grammes of oxygen 
 
 
 
 
 
 
 
 consumed per kilo 
 
 
 
 
 
 
 
 of body weight in 
 
 
 
 
 
 
 
 24 hours 
 
 13-272 
 
 11-040 
 
 I3-577 
 
 29-698 
 
 29-314 
 
 28-392 
 
 Amount of oxygen 
 
 
 
 
 
 
 
 consu med in 
 
 
 
 
 
 
 
 24 hours: 
 
 
 
 
 
 
 
 Litres 
 
 42516 
 
 3456o 
 
 1417-3 
 
 1550-0 
 
 giS'o 
 
 397-86 
 
 Cubic feet 
 
 150-0 
 
 1220 
 
 50-0 
 
 54-7 
 
 32-4 
 
 14-0 
 
 Grammes of carbon 
 
 
 
 
 
 
 
 dioxide produced 
 
 
 
 
 
 
 
 per kilo of body 
 
 
 
 
 
 
 
 weight in 24 hours 
 
 50S 
 
 4128 
 
 5-08 
 
 ii- 166 
 
 7-638 
 
 7-621 
 
 Amount of carbon 
 
 
 
 
 
 
 
 dioxide produced 
 
 
 
 
 
 
 
 in 24 hours : 
 
 
 
 
 
 
 
 Litres 
 
 4285-55 
 
 346575 
 
 142752 
 
 1562-12 
 
 641-17 
 
 29176 
 
 Cubic feet 
 
 1510 
 
 1223 
 
 50-4 
 
 55'i 
 
 22'6 
 
 I0'3 
 
 The next table is given by Munk : 
 
 Animal. 
 
 Body Weight. 
 
 Oxygen absorbed Daily. 
 
 Carbon Dioxide given 
 off Daily. 
 
 Kilos. 
 
 Pounds. 
 
 Litres. 
 
 Cubic: Feet. 
 
 Litres. 
 
 Cubic Feet. 
 
 Horse 
 Ox 
 Sheep 
 Dog- 
 
 45° 
 
 600 
 
 70 
 
 *5 
 
 990 
 1320 
 
 *54 
 33 
 
 4270-0 
 
 5565-0 
 
 605-2 
 
 2975 
 
 150-800 
 196-500 
 
 21-377 
 
 IO5OO 
 
 4867 
 
 5548 
 580 
 224 
 
 171-90 
 
 I96-00 
 
 2048 
 
 79I 
 
 Evidence that the panting respirations of work may be due to 
 the presence of a chemical substance circulating in the blood 
 is afforded by the experiment of dividing the spinal cord in the 
 dog, and stimulating the muscles of the hind limbs. The animal, 
 of course, is unconscious of any movement, but the respirations 
 are increased as if it had been running for some distance. What 
 the substance is that gives rise to this is not known. Lactic acid 
 has been suggested, though this is now known to be rapidly 
 oxidised, and dilute acids injected into the blood give rise to 
 much the same condition . Hurried respira tions may also be pro- 
 
RESPIRATION 
 
 147 
 
 duced through the circulatory system. In an animal in training 
 the breathlessness which it is one of the objects of training to get 
 rid of, is due, not only to the need for oxygen, but also to the fact 
 that more blood is brought to the lungs than can be disposed of. 
 If the right heart pumps into the lungs more blood than the lungs 
 can return to the left heart, breathlessness follows. The gallop 
 by which an animal gets its ' wind ' and ' staying ' power pro- 
 duces its effects by operating on the circulatory system. For- 
 tunately, the vessels of the lungs are capable of considerable 
 adjustment; they hold more blood during inspiration than ex- 
 piration, and in this way may be regarded as a safety-valve to the 
 heart: The important practical questions of work, ' condition,' 
 and fatigue will be again referred to in the chapter dealing with 
 the Muscular System. 
 
 Air vitiated by Respiration was at one time believed to be 
 poisonous, either on account of its deficiency in oxygen, its 
 increase in carbon dioxide, or because of the organic matter mixed 
 up with it. It is now generally admitted that the ill-effects of 
 vitiated air are mainly due to the stagnation of the air and to the 
 warm and humid atmosphere, by which the respiratory exchange 
 and body metabolism are affected. Even the number of bacteria 
 in the air is no criterion of its purity ; there may be fewer in ex- 
 pired air than in the same air before inspiration, in consequence 
 of their being arrested in the lungs. Nevertheless, modern in- 
 quiry supports the principle contained in the old view, of the evil 
 resulting from breathing atmosphere charged with C0 2 . When 
 that gas accumulates to the extent of 4 per cent., rapid breathing 
 and general distress begin to be evident. 
 
 The calculation of the amount of air required for effective 
 ventilation' is conveniently based on the amount of permissible 
 impurity, as judged by the proportion of carbon dioxide present. 
 Many observations show that when 0-02 per cent, of carbon 
 dioxide is present, in addition to that in the air as a normal 
 impurity — viz., 0-03 per cent. — the ventilation may be regarded 
 as effective. On this basis, if the rate of C0 3 production by any 
 given animal is known, it is easy to calculate the number of times 
 the air of the building should be changed, in order to maintain 
 it in a pure condition. 
 
 Respiratory Murmur. — An accurate acquaintance with the 
 normal respiratory murmur is essential to the physician. The 
 air sounds both of inspiration and expiration should be heard 
 all over the chest, the inspiratory murmur being louder and 
 better marked than the expiratory; in fact, in many perfectly 
 healthy chests the expiratory murmur can scarcely be heard. 
 The normal murmur, whether inspiratory or expiratory, is soft 
 in character; there is no harshness. The sound is best repre- 
 
148 A MAMUAL op vetekwaky physiology 
 
 sented by the noise made by the stream of air which issues from 
 a pair of hand-bellows when gently blown. 
 
 The respiratory murmur, also known as the vesicular murmur, 
 is caused by the friction of the air entering the alveoli. Laulani£ 
 states that after division of the vagi in the horse all vesicular 
 murmur is lost. In those portions of the lung lying close to the 
 bronchi and larger tubes there is, in addition to the vesicular 
 murmur, a sound produced by the trachea and glottis. This is 
 not distinct from the vesicular sound, but is added to it, the result 
 being that the respiratory murmur over the tubes is louder than 
 elsewhere. The expiratory sound is weaker and shorter than the 
 inspiratory — that is to say, the sound is not continued to the end 
 of expiration, but dies away before that is reached. The ex- 
 piratory murmur immediately follows the inspiratory without a 
 pause, but there is a marked pause between the end of one ex- 
 piration and the beginning of the next inspiration. 
 
 The ordinary murmur is best heard where the chest wall is 
 thin; if the ribs be covered with fat or any great thickness of 
 muscle, the sound may be entirely lost. It is also important 
 to note that there are some chests perfectly healthy where, for 
 no apparent reason, the respiratory murmur is weak or even un- 
 detectable. 
 
RESPIRATION 
 
 149 
 
 Section 2. 
 The Larynx. 
 
 The larynx serves a twofold purpose— viz., for respiration and 
 for phonation; in animals the former is the more important, 
 the voice-producing function being of a very subordinate char- 
 acter. 
 
 The larynx may be described as a cartilaginous box placed 
 at the summit of the trachea, the opening into it being capable 
 of increasing or decreas- 
 ing in size, and so of 
 allowing a larger or 
 smaller amount of air to 
 enter the lungs. Within 
 the larynx are two elas- 
 tic cords arranged in 
 V shape, the function of 
 which is solely the pro- 
 duction of sound (Fig. 
 54, 3). Both the respira- 
 tory and vocal functions 
 require that the several 
 parts of the larynx 
 should move — viz., that 
 the mouth of the organ 
 should be widened or nar- 
 rowed, or that the cords 
 should be approximated, 
 drawn apart, tightened, 
 or slackened. These 
 movements are brought 
 about by certain groups fig 
 of muscles, those which 
 approximate the walls of 
 the glottis being known 
 as the adductors, whilst 
 those which widen it are 
 known as the abductors. „..,.. A ,., t 
 
 The Muscles of the Larynx may be divided into those of 
 respiration and phonation. As the most important feature in 
 respiration is the opening or dilating of the glottis the term 
 respiratory muscle might be confined to the dilators of the glottis 
 while the constrictors would represent the vocal muscles; but 
 
 54. — The Laryngeal Opening during 
 Ordinary Respiration. 
 The epiglottis ; z, margin of arytenoids ; 3, 
 vocal cord ; 4, pharynx laid open. The 
 V-shaped slit is the glottis. Note how 
 much wider the epiglottis is than the open- 
 ing it has to cover. 
 
15° 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the constrictors are not entirely without respiratory function, 
 as, for example, in coughing, so that in the following table they 
 are included under this head: 
 
 Respiratory Muscles. 
 Dilator or abductor, Crico-arytenoideus posticus. 
 
 Constrictors or adductors of the Crico-arytenoideus lateralis, Ary- 
 glottis, tenoideus, and Thyro - aryte- 
 
 noideus. 
 
 Phonatory Muscles. 
 Muscle which relaxes the vocal Thyro-arytenoideus 
 
 cords, 
 Muscle which renders the cords 
 
 tense. 
 Muscles which bring the cords 
 
 together, 
 Muscle which moves the cords 
 
 apart, 
 
 These muscles are shown in Fig. 55. 
 
 The crico-arytenoidei, lateralis and posticus, are direct an- 
 tagonists; the lateralis depresses the arytenoid cartilages and 
 
 anterior and 
 especially posterior fasciculus. 
 Crico-thyroid. 
 
 The respiratory adductors. 
 
 The respiratory abductor. 
 
 Fig. 55. — The Position of the Muscles of the Larynx in the Horse. 
 a., Epiglottis; b, opening leading to the glottis; c, portion of the arytenoid car- 
 tilage; d, position of the joint formed between the cricoid and arytenoid 
 cartilages; e, the trachea. The wing of the thyroid cartilage has been re- 
 moved, so as to expose the constrictor muscles; 4, 4, represents its cut edges. 
 1 and 2, Thyro-arytenoideus: 1, anterior; 2, posterior fascicules. The 
 space /between these two muscles indicates the position of the ventricle of 
 the larynx. 3, Crico-arytenoideus lateralis; 5, crico-thyroid muscle, half of 
 which lies inside the thyroid cartilage, and cannot, therefore, be seen; 
 6, crico-arytenoideus posticus; 7, portion of thyroid cartilage; 8, portion 
 of cricoid cartilage ; 9, arytenoideus muscle. 
 
 closes the entrance into the glottis, the posticus swings the 
 arytenoids upwards and outwards and enlarges the glottis. 
 
RESPIRATION 
 
 151 
 
 ■***%» 
 
 The entrance to the larynx is formed by the two arytenoid 
 cartilages, the epiglottis, and the ary epiglottic folds; beyond 
 these is the glottis proper— viz., the V-shaped opening formed 
 by the vocal cords. When the laryngeal opening dilates, the 
 vocal cords pass towards the walls of the cavity and render the 
 V-shaped space wider; when the larynx closes, the cords are 
 approximated and the space rendered narrower (Figs. 54 and 56). 
 During ordinary respiration there is very little, if any, alteration 
 in the shape and size of the glottis; but "during exertion every 
 inspiratory movement _" ^ 
 
 is accompanied by an &** ,r "'"""■-. 
 
 increase in size, every 
 expiration by a de- 
 crease. At each ex- 
 piration the vocal 
 cords pass towards the 
 centreline, and at each 
 inspiration return to 
 the walls of the larynx. 
 The closure of the 
 larynx, as during the 
 act of swallowing, is 
 a powerful movement, 
 and if the finger at 
 this moment be intro- 
 duced into the cavity 
 and placed between 
 the arytenoids, it ex- 
 periences considerable 
 pressure. The closure 
 of the larynx is brought 
 about by the depres- 
 sion and approxima- 
 tion of the arytenoid 
 cartilages and the ap- 
 proximation of the 
 vocal cords ; in addition, during the act of swallowing, the base 
 of the tongue presses the epiglottis over the arytenoids and renders 
 the part both air- and water-tight. 
 
 The Epiglottis is much larger than the opening it is intended 
 to seal during a condition of laryngeal repose; it is, as just 
 described, carried, during the act of swallowing, backwards by the 
 base of the tongue and pressed over the arytenoids; the larynx 
 at the same moment advances, with its arytenoid cartilages 
 closely approximated. After the act of swallowing, the tongue 
 advances, the larynx recedes, and the epiglottis returns to its 
 
 Fig. 
 
 56. — The ^Laryngeal Opening during 
 Hurried Respiration, seen in a State of 
 Dilatation. 
 Epiglottis f z, margin of arytenoids ; 3, vocal 
 cord; 4, pharynx laid open. Note the size and 
 shape of the glottal opening as compared with 
 the same in Fig. 54. 
 
152 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 position by means of its elastic recoil. It is not essential to a 
 food- or water-tight condition of the larynx that the epiglottis 
 should exist ; it has been removed both by disease and experi- 
 mentally, and its place is then taken by the base of the tongue. 
 Nor is an arytenoid cartilage essential to safety in swallowing. 
 
 The Nervous Mechanism of the Larynx is peculiar. In the 
 majority of animals sensation is supplied to the mucous lining 
 membrane, and motor power to the crico-thyroid muscle by the 
 superior laryngeal branch of the vagus, this nerve containing 
 both sensory and motor fibres. In the horse the motor fibres 
 in the superior laryngeal are, it is said, derived from the first 
 cervical nerve and not from the vagus. All the other muscles; 
 both abductor and adductor, are supplied with motor 'power by 
 the inferior or recurrent laryngeal branch of the vagus. It is 
 strange that both abductor and adductor muscles should have 
 the same source of nerve supply, and one naturally asks what it 
 is which determines that only the opening or only the closing 
 muscles shall act at any given moment ? The explanation lies 
 in the law of ' reciprocal innervation,' demonstrated for the limb 
 muscles by Sherrington, which will be considered later in the 
 chapter on the Nervous System. Both dilator and constrictor 
 fibres run in the recurrent laryngeal nerve, and are quite distinct; 
 in some animals the different bundles have been experimentally 
 isolated and injured — injury to the dilator fibres producing ab- 
 ductor paralysis, and injury to the fibres going to the muscles 
 which close the larynx producing adductor paralysis. 
 
 If the recurrent laryngeal be cut and the peripheral end 
 strongly stimulated, the glottis almost invariably is found to 
 close ; in other words, only the adductor fibres appear to be acted 
 upon. If a weak stimulation be applied, the glottis opens — i.e., 
 the abductor muscles are affected. 
 
 Another curious fact in the history of the recurrent nerves 
 is furnished by pathology. In the disease of horses known as 
 ■ roaring,' there is paralysis of the left abductor muscle of the 
 larynx — viz., the crico-arytenoideus posticus, the wasting, and 
 fatty degeneration due to paralysis being very marked. It is 
 not unusual to find the adductor muscles normal in appearance, 
 or presenting Very little sign of disease, and, even if they are 
 pale and wasted, the degree of degeneration cannot be compared 
 with that of the abductor muscle. This is a difficult fact to 
 explain; one would think that as both abductor and adductor 
 muscles receive the same nerve -supply, equal wasting would 
 occur in both groups. Again, it is observed when the left re- 
 current has been divided experimentally, that the abductor muscle 
 lbses its irritability long before the adductors, and the same fact 
 may be observed in post-mortem stimulation of the nerves. If 
 
RESPIRATION 153 
 
 the recurrent laryngeal nerves be divided under ether, and the 
 peripheral ends stimulated, adduction of the larynx is obtained; 
 but if the ether narcosis be pushed to a dangerous extent and 
 the nerves then stimulated, the glottis dilates— that is, abduction 
 follows. These and other observations have furnished a law 
 which is of clinical significance— viz., that in functional disturb- 
 ance of the larynx the adductor muscles are first affected, but 
 that in changes accompanied by organic lesions the abductor 
 muscles are the first to suffer. 
 
 When one recurrent laryngeal nerve is divided, the vocal 
 cord ori that side remains immovable and therefore cannot 
 approach its fellow; the healthy cord endeavours to com- 
 pensate for the weakness of its companion by passing beyond 
 the middle line of the larynx in its attempt to come into con- 
 tact with it. 
 
 The inspiratory distress occasioned in ' roaring ' is not caused, 
 as has been said, by a paralysed vocal cord flapping about, for 
 the elastic nature of the cord, and the fact that the only muscle 
 never affected with paralysis is the one which keeps the vocal 
 cord tense, negative this. The sound is produced by the para- 
 lysed left arytenoid cartilage being drawn into the glottis at 
 each inspiration, and this is the explanation why the noise which 
 accompanies the disease is always inspiratory and never ex- 
 piratory. The drawing into the glottal opening of the paralysed 
 arytenoid cartilage is due not only to the inrush of air, but also 
 to the unbalanced action of the antagonists of the dilator muscle. 
 
 Phonation.— Voice is produced by the approximation and 
 vibration of the vocal cords, the, pitch of the voice being produced 
 by the tension of the cords, whilst the quality is due to the shape 
 of the cords — viz., their thickness or thinness. The position of 
 the resonant chambers, such as the mouth, pharynx, posterior 
 nares, and even the nasal chambers, also importantly affects the 
 quality of the voice. It is obvious that the chief alterations in 
 the larynx during phonation have reference to the vocal cords; 
 these are approximated by the adductor muscles, and separated by 
 the abductor muscles, whilst they are relaxed by the thyro-ary- 
 tenoideus and tightened by the crico- thyroid. The latter muscle 
 has a peculiar action ; it lowers the thyroid cartilage, on to the 
 cricoid and swings the wing of the thyroid outwards, thus render- 
 ing the cords tense. These changes in the vocal cords produce 
 changes in the shape of the V-shaped glottal opening; in a high 
 note the glottis is reduced to a mere slit, in deeper notes the cords 
 are separated. If air be forced through the larynx of a dead 
 horse, and the tension of the cords altered, a sound remarkably 
 like a neigh may be produced. The ventricles of the larynx and 
 the cavities of the mouth, nose, pharynx; etc., act as resonators. 
 
154 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Being filled with air, they effect the needful alterations in the 
 quality of the voice, and assist in giving it its distinctive char- 
 acter ; thus the false nostrils furnish the ' snort ' of the frightened 
 or ' fresh ' horse, the nasal chambers the whinny and neigh of 
 pleasure, the mouth and pharynx the neigh of impatience, loneli- 
 ness, excitement, etc. We do not consider that the guttural' 
 pouches act as resonators, and Colin obtained no alteration in the 
 character of the neigh by opening them. 
 
 The voice of each species of animal — horse, ass, ox, sheep, and 
 pig — is so distinctive that we may recognise the animal without 
 seeing it ; yet though the larynx in all these animals is more or 
 less different, the difference is not sufficient to offer any explana- 
 tion as to why the sounds emitted are so entirely distinct. The 
 voice of male and female animals differs in intensity. The 
 wild neigh of the stallion is very different from the neigh of the 
 mare, and the bellowing of the bull is distinct from the ' lowing ' 
 of the cow. The operation of castration has a remarkable effect 
 on the voice, the neigh of the gelding resembling that of 
 the mare. 
 
 In the horse the voice is used during sexual and ordinary 
 excitement, also during fear or especially loneliness, during pain, 
 anger, and as a mark of pleasure. It is not possible to convey 
 in words the difference in the notes produced, but the various 
 sounds are easy to recognise. The horse is essentially a sociable 
 animal; when accustomed to be in the company of others he dis- 
 likes separation, and shows it by persistent neighing, which is 
 perhaps more noticeable amongst army horses than amongst 
 any others. The neigh of pleasure is often spoken of as the 
 ' whinny ' ; the word rather conveys some idea of the sound made. 
 Sounds which can be described only as ' screams ' are often evoked 
 during ' horse-play ' and temper, or by mares during oestrum. 
 It is not a scream as we know it in the human subject, but no 
 other word conveys an idea of its shrillness. If a horse cries 
 from pain (a very rare occurrence), as during a surgical operation, 
 the cry is a muffled one and short ; it is a groan rather than a cry. 
 
 In the cerebral cortex voice is represented in the praecrucial 
 and neighbouring gyrus in the dog, and in corresponding regions 
 in other animals. Stimulation of this region leads to bilateral 
 adduction of the cords, which suggests that both sides of the 
 larynx are represented in each hemisphere. There is no region 
 in the cortex of the dog which, on stimulation, leads to abduc- 
 tion of the cords, though such a region is found in the cat. It 
 would appear that adduction of the cords is represented in the 
 cortex, as the muscles producing it are especially associated with 
 the production of voice, which is under the influence of the will. 
 Respiration, on the other hand, is automatic, and the abductor 
 
RESPIRATION 155 
 
 muscles being essentially respiratory, their centre is found to 
 exist in the medulla. 
 
 Neighing in the horse is produced by an expiration, partly 
 through the nostrils and partly through the mouth ; braying in 
 the ass is both inspiratory and expiratory, nostrils and mouth 
 each taking a share in it. The ventricles of the larynx are large 
 in the horse and relatively still larger in the ass and mule; they 
 act as resonators and allow of free vibration of the vocal cords. 
 Their obliteration as a surgical treatment for roaring is said to be 
 productive of marked benefit, but this can only be due to cicatri- 
 cial tissue limiting the movement of the paralysed arytenoid 
 cartilage, for the ventricles take no share in the cause of the 
 disease. According to Chauveau both ass and mule have the 
 subepiglottic sinus provided with a thin membrane capable of 
 vibrating. In the ox, sheep, and goat, the larynx is very simple ; 
 there are only rudimentary vocal cords and no ventricles. The 
 bellowing of the ox and the bleating of the sheep are expiratory 
 efforts through the mouth. The dog and cat have the larynx 
 something like that of the horse,, but the ventricles are shallow ; 
 the voice is produced almost entirely through the mouth, though 
 both growling and purring may occur through the nostrils. 
 
 Castration in early life infuences the voice of both geldings 
 and oxen, as the larynx remains imperfectly developed. 
 
 Yawning is a deep, slow inspiration followed by a short ex- 
 piration; the air, even in the horse, is taken in by the mouth, 
 which is widely opened, the jaws being crossed. 
 
 Sneezing and Coughing are expiratory efforts. The former 
 occurs solely through the nose, and, excepting in the dog and 
 cat, is unaccompanied by the peculiar sound attending this act 
 in the human subject. If snuff be introduced into the nostrils 
 of the horse, a peculiar though well-known vibration of the 
 nostrils occurs as if the animal were blowing its nose, and this 
 is, in fact, what it accomplishes. It is an entirely nasal sound; 
 the mouth takes no part, in the act. Coughing occurs through 
 the mouth, the long palate in the horse being raised for the pur- 
 pose. Before coughing can occur the lungs must be filled with 
 air and the glottis closed; a forcible expiration follows, the glottis 
 opens, and the air is expelled through the mouth. 
 
 Hiccough is due to a sudden contraction of the diaphragm. 
 While the air is rushing into the lungs the glottis closes, and 
 the incoming air, striking the closed glottis, produces the sound. 
 The condition known as spasm of the diaphragm in the horse 
 is very different from a human hiccough, and is referred to more 
 fully_on p. 156. 
 
156 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Pathological. 
 
 Pneumonia and Pleurisy in the horse are very common in early 
 life, and attended by a high mortality. The lungs and pleura, 
 separately or combined, may suffer a degree of inflammation varying 
 from small localised trouble to general and extensive inflammation 
 of the pleura and lungs. The whole of the lung tissue is never 
 affected; even in the most severe cases of pneumonia there is some 
 breathing area available; the upper portion of both lungs generally 
 escapes. Effusion of fluid into the cavity of the thorax is a common 
 sequel to pleurisy in the horse. 
 
 Both the above pathological conditions as well as their progress 
 are diagnosed by auscultation and percussion; there are many de- 
 partures from the normal respiratory murmur, each of which has its 
 significance. 
 
 Apoplexy of the Lungs arises as the result of overwork, especially 
 in hot weather; but it may also occur in the winter. Horses ridden 
 to death in the hunting field, in the name of ' sport,' die as a rule 
 from pulmonary apoplexy; the lungs cannot get rid of their abnormal 
 burden of blood to the left heart. 
 
 Bronchitis is a disease probably rarely distinct from pneumonia. 
 
 ' Broken Wind' is one of the most interesting of the various chest 
 diseases of the horse; it is a condition peculiar to this animal, liable 
 to occur suddenly, and frequently traced to errors in dieting. To 
 state the case shortly, the lungs lose their power of elastic recoil, and 
 do not collapse even after death; the respirations are greatly in- 
 creased, the expiratory effort being powerful,- characteristically 
 irregular, and prolonged. A chronic typical cough becomes estab- 
 lished, and the animal unfit for anything but slow work. On post- 
 mortem examination the lungs are found to fill the chest entirely; 
 they cannot collapse, for all elastic recoil has left them. One of 
 the fundamental errors in veterinary pathology is to attribute this 
 condition to asthma. If healthy lungs be powerfully and repeatedly 
 distended immediately after death, subpleural emphysema is pro- 
 duced of even the pleura ruptured in consequence of the rupture of 
 the air cells; the lungs have largely, though not entirely, lost their 
 power of elastic recoil; in other words, an artificial ' broken wind ' has 
 been produced. We must certainly look upon violent and prolonged 
 exertion especially on a full stomach as the chief, if not the sole, cause 
 of the disease. 
 
 Roaring is a nervous affection, to which sufficient allusion has been 
 made in the section dealing with the larynx. One point may be 
 emphasised, in consequence of the frequency with which the larynx 
 is now surgically treated, and that is, an injury, however slight, 
 to a cartilage of the larynx is always followed by thickening and ossifi- 
 cation, with consequent reduction in the lumen of the larynx. 
 The logical treatment of the disease is to establish a fresh nerve- 
 supply. The writer many years ago demonstrated on the horse 
 that if the left recurrent nerve of a healthy larynx be divided and the 
 distal end sutured to the central end of the spinal accessory, the 
 dilator muscle could be innervated from this new source. A new 
 axis cylinder has to grow through the degenerate nerve, so that the 
 process of restoration is a slow one. 
 
 Spasm of the Diaphragm is another respiratory affection due to 
 disordered nervous supply. The sound emitted is quite unlike that 
 in the human subject; it appears to come from within the chest or 
 
RESPIRATION 157 
 
 abdomen, and is represented by a dull ' thud ' like a magnified 
 heart-beat, which, in its frequency and regularity, it closely resembles, 
 and for which it may easily be mistaken. 
 
 Rupture of the Diaphragm is a common lesion frequently due to 
 disorders of the digestive canal, the gas generated in the intestine 
 being sufficient to burst the diaphragm. Falls are by no means an 
 uncommon cause; for example, an animal falls on to its head, and 
 the abdominal viscera are propelled against the diaphragm. The 
 diaphragm rarely gives way below, almost always above, and in the 
 tendinous substance rather than in the muscular. This point is of 
 physiological interest. 
 
 Catarrh. — As the horse can breathe only through the nostrils, 
 obstruction of these passages from catarrh renders the animal unfit 
 for work, even when no other symptoms of importance are present. 
 In the facial sinuses collections of pus are frequent and troublesome. 
 
 Laryngitis is frequently the result of strangles infection, rarely 
 of ordinary cold. In the former condition local oedema and dyspnosa 
 are not infrequent, and arise suddenly. 
 
 In the ox pneumonia is rare, with the exception of the specially 
 highly infectious type, constituting one of the animal plagues. 
 Practically none of the other diseases mentioned above as affecting 
 the horse are found in any ruminant. 
 
 The number and character of the respirations are not only a trust- 
 worthy guide during the onset and progress of disease, but their 
 character may be absolutely diagnostic. Broken wind and hydro- 
 thorax may be determined by a glance at the flanks; stertorous 
 breathing suggests cerebral compression; while any noise in repose 
 accompanying the inspiratory act in the horse is suggestive of com- 
 mencing oedema of the pharynx or larynx pr nasal passages, as in 
 acute glanders, especially of the mule. 
 
CHAPTER V 
 DIGESTION 
 
 Section i. 
 
 Digestion in the Mouth. 
 
 Prehension of Food. — The methods by which animals convey 
 food to the mouth differ according to the species. In the horse 
 the lips play an important part, for which purpose they are thick, 
 mobile, remarkably strong, and endowed with acute sensation; 
 in the ox they serve a subordinate function, being rigid and 
 wanting in mobility; in the sheep the upper lip is cleft in such 
 a manner as to divide it completely into two parts, "each possess- 
 ing independent movement ; in the pig the lower lip is pointed 
 and the upper one insignificant. 
 
 A horse, when grazing, advances one fore-leg to enable the 
 mouth to reach the ground; the jugular veins and those of the 
 face distend, and the parotid glands bulge, owing to the enlarged 
 condition of the veins beneath them. With the mouth on the 
 ground the eyes are roughly on a level with the knees, but no 
 animal with the eyes laterally placed is able to see what it is 
 eating ; it is guided entirely by the sense of smell. * 
 
 In manger feeding the horse collects the food with the lips,, 
 but in grazing cuts off the grass with the incisor teeth, drawing 
 the lips back in order that he may bite more closely to the ground. 
 In the ox the tongue is protruded and curled round the grass, , 
 which is thus drawn into the mouth and taken off between the 
 incisor teeth and the dental pad. In the sheep the divided 
 upper lip allows of the incisors and dental pad biting close to 
 the ground, so that animals of the sheep and goat class can live 
 on land where others such as the horse and ox would starve. 
 In whatever way the food is cut off, it is carried back by the 
 movements of the tongue to the molar teeth, there to undergo 
 a more or less complete grinding. 
 
 In the ox and sheep the incisor teeth move freely in their 
 sockets; the object of this is to prevent injury to the dental 
 
 158 
 
DIGESTION 
 
 159 
 
 pad, for which purpose also they are placed very obliquely in 
 the jaw. In the horse the incisor teeth in early life are very 
 upright, but become oblique with age. The molars in all her- 
 bivora are compound teeth; in the horse they are very large, 
 especially those in the upper jaw. Being composed of materials 
 of different degrees of hardness, they wear with a rough surface, 
 which is very essential to the grinding and crushing they have 
 to inflict on grasses and grain. The teeth in herbivora, both 
 incisors and molars, are constantly, though slowly, being pushed 
 out of the sockets which hold them; in this way wear and tear 
 is compensated^ for, whilst the fang of the tooth becomes corre- 
 
 M. \W-M-Mf \lf 
 
 Fig. 57- — Schematic Transverse Section of the Upper and Lower Jaws 
 of the Horse between the Third and Fourth Molars, showing the 
 Position of the^Tables of the Teeth during Rest and Mastication. 
 
 UJi Upper jaw; LJ, lower jaw; RM, right molar; LM, left molar; RLM, right 
 lower molar; LLM, left lower molar. 1, The position of the teeth during 
 rest, the outside edge of the lower row in apposition with the inside edge of 
 the upper. 2, The jaws fully crossed masticating from left to right; the 
 tables of both upper and lower molars now rest on each other. 3, The posi- 
 tion halfway through the act of mastication, the outer half of the lower teeth 
 wearing against the inner half of the upper. 
 
 spondingly reduced in length. It is owing to this fact that the 
 incisor teeth alter in shape and direction, and so enable the age 
 to be determined. The tables of the molar teeth are not flat, 
 but oblique ; this is especially well seen in the horse, where the 
 cutting surface is chisel-shaped, the upper teeth being longest on 
 the outside, while those of the lower row are longest on the inside 
 (see Fig. 57). This arrangement produces sharp teeth, which 
 are a constant source of trouble and loss of condition in horses. 
 
 The movements of the tongue are important. In the ox and 
 dog they are very extensive, the former animal having no diffi- 
 culty in protruding the tongue and even in introducing the tip 
 into the nostrils. It is not a very common habit in horses to 
 
i6o A MANUAL OF VETERINARY PHYSIOLOGY 
 
 protrude the tongue except when yawning, but they have con- 
 siderable power in withdrawing it in the mouth. A great differ- 
 ence exists between the tongue of the horse and that of the ox, 
 the former is flabby, broad and flat at the end, constricted oppo- 
 site the frenum, and swelling out at the apex; it is comparatively 
 smooth on its surface. The tongue of the ox narrows from base 
 to apex, the latter being pointed ; it is very rough, which prevents 
 it from losing its hold on the food, protects it from such injury 
 as might be inflicted by coarse grasses, and is also of value to the 
 animal in cleaning its body. The tongue is supplied with motor 
 power by the hypoglossal nerve and with sensation by the lingual 
 branch of the fifth, which supplies the anterior two-thirds of 
 the mucous membrane, the posterior third being supplied by 
 the lingual branch of the glosso-pharyngeal ; the same nerve 
 also supplies the sense of taste to this part of the organ, while 
 taste for the anterior two-thirds is supplied by the chorda 
 tympani of the seventh pair. Division of the hypoglossal nerve 
 prevents the ruminant from grazing, the dog from lapping, and 
 in all animals causes the tongue to suffer injury by its being 
 unable to avoid the incisor teeth. 
 
 The inside of the mouth of the ox is covered with long papillae, 
 which point backwards ; these would appear to be of use in pre- 
 venting the food from falling out of the mouth. In the horse 
 no such papillae exist; in fact, the lining membrane of this part 
 is remarkably smooth. The majority of animals have grooves 
 in the palate ; they are well marked in the horse, ox, sheep, and 
 even in the dog. Their function is probably to assist the tongue 
 to pass the food back in the mouth. 
 
 Drinking is performed by the animal drawing the tongue 
 backwards, and thus using it as the piston of a suction-pump; 
 this action produces a vacuum in the front of the mouth, as the 
 result of which the cheeks are drawn inwards, the lips at the 
 same time being closed all round, excepting a small space in 
 front which is placed under water. Such is the method in both 
 horse and ox; in the former animal the head is extended while 
 drinking, the ears are drawn forward at each swallow, and during 
 the interval fall back. The cause of this motion is not clear, but 
 is probably due to the movement of air in the guttural pouches. 
 A thirsty horse will swallow from 150 to 250 grammes (5 to § 
 ounces) of fluid at each gulp. In drinking off the ground the 
 fluid has to travel against gravity. Lapping in the dog is per- 
 formed by curling the tongue in such a way as to convert it into 
 a spoon. Sucking, like drinking, is produced by the animal . 
 creating a vacuum in the mouth by closing the lips, decreasing ; 
 the size of the tongue in front and increasing it behind, the dorsum 
 being applied to the roof of the mouth. The foal places the 
 
DIGESTION 161 
 
 tongue beneath the nipple and curls it in from each side ; by this 
 means he protects it from the lower incisors and gets a better 
 hold. 
 
 Mastication. 
 
 Mastication is performed between the molar teeth ; the move- 
 ments which the jaws undergo, to admit of this being carried 
 out, depend upon the class of animal. In the dog they are very 
 simple, being only a depression and elevation of the jaw; this 
 motion means a simple temporo-maxillary articulation, and such 
 is met with in this animal. In the horse and ox the movement 
 is not only up and down, but lateral, and some say even from 
 front to rear. This necessitates a complex joint capable of 
 affording a considerable amount of play, and this is provided 
 by a disc of cartilage, placed in the articulation, which accommo- 
 dates the joint to the varying movements required of it in the 
 horse, ox, and sheep, and also saves the part from jar. In her- 
 bivora, therefore, we find the cartilage extensively developed, 
 whilst in carnivora it is small and simple. In Fig. 57, 1, the 
 position of the molar teeth at rest is shown ; it is obvious that the 
 first act of mastication is to bring the teeth into apposition, and 
 to do this the lower jaw has to be rotated, or, as the expression 
 is, 'crossed' (see Fig. 57, 2). The muscle which effects this 
 rotation is the internal pterygoid, the left muscle pulling the 
 jaw to the right, the right muscle pulling it to the left ; meanwhile, 
 in the temporal articulations, the disc of cartilage glides forward 
 on one side and backwards on the other, while the contents of 
 the. orbital fossae are observed in the horse to be alternately 
 ascending and descending. This movement is due to the coro- 
 noid process, which pushes up the fossa as the process comes for- 
 ward and retracts it as the process retires. It is evident that the 
 advancing of the coronoid process, the temporal fossa ascending 
 and the cartilage of the joint gliding forward, are simultaneous 
 actions occurring on that side, right or left, from which the jaw 
 is working, while the retreat of the coronoid process, the descent 
 of the fossa, and the retirement of the cartilage of the joint occur 
 on that side on which the molars are finally brought into apposi- 
 tion. In the hinge-like movement of opening the mouth without 
 mastication occurring both condyles and cartilages come forward. 
 
 The temporal joint of the ox is relatively small as compared 
 with that of the horse, and permits of greater lateral movement. 
 In the pig the jaw undergoes very little lateral movement, but 
 extensive protraction and retraction, and the temporal joint is 
 much longer to aUow of this. In the dog, hinge-like movement 
 predominates and lateral movement is limited. The herbivora 
 can masticate only on one side at a time; when tired on one side 
 
t62 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the process is reversed, and the opposite molars take on the 
 crushing. It is surprising how long an animal will carry on masti- 
 cation on one side ; even as long as an hour has been observed in 
 the horse by Colin. This physiologist noticed that in the ox and 
 sheep the first stroke of the molars is in the opposite direction 
 to the regular action which follows; thus, if masticating from 
 right to left the first stroke is made from left to right. It is 
 important to note that in those animals where a single-sided 
 lateral or rotatory movement in mastication is necessary, the 
 upper jaw is always wider than the lower ; this we can understand, 
 for if both were the same width the molar teeth would not meet 
 each other when the jaws were crossed for lateral mastication. 
 This extra width of the upper over the lower jaw, in conjunction 
 with the peculiarity of mastication, explains why the molar teeth 
 of the horse and other herbivora wear with sharp chisel edges (see 
 Fig- 57)- 
 
 When the animal is grazing, very little molar mastication is 
 performed, as the incisors are kept in constant cutting action; 
 cutting and grinding cannot occur simultaneously, but the nature 
 of the food ' at grass ' is such that very little mastication is re- 
 quired. The animal remains for long periods with the mouth 
 to the ground, swallowing almost as rapidly as he is cutting off 
 the food, which travels against gravity to the stomach. 
 
 Mastication in manger-fed horses is a slow process, though 
 " greedy ' feeders are not unknown. The grinding is very thor- 
 oughly performed. The resulting semi-liquid mass weighs from 
 50 to 100 grammes (1 to 2 ounces), takes about half a minute to 
 produce, and necessitates as a rule about 40 crushings between 
 the molars before being fit for swallowing. This data gives some 
 notion of the amount of work performed by the masseter muscles 
 in eating, say, 2 kilogrammes (4-4 pounds) of hay. Colin, whose 
 results are given above, shows that 5 horses took the following 
 times to eat 2 kilogrammes of hay: 1 hour, ij hours, 1 hour 
 12 minutes, 1$ hours, if hours, the last being a very small 
 horse, while the second animal in the series was a big one. The 
 average rate of crushing is 70 to 80 per minute, while the amount 
 of work performed by the jaws working at 420 to 480 times an 
 hour constitutes a distinct source of daily loss. Colin shows 
 that when the animal is very hungry he will prepare and swallow 
 30 balls in 15 minutes, but as he gets satisfied he does not make 
 more than 10 or 12 swallows in the same time. If the flow 
 of saliva be reduced in amount, the length of time occupied by 
 mastication is naturally increased. 
 
 According to the writer's observation, it takes a horse 15 to 
 20 minutes to eat 1 pound of hay, and 5 to 10 minutes to eat 
 1 pound of corn. 
 
DIGESTION 163 
 
 With the ox the first mastication is imperfectly performed, 
 and is three times quicker than in the horse. When, however, 
 the material is brought back for remastication, the process is 
 slow. In the dog mastication of the naturaLfood, meat, is im- 
 perfectly performed; after a few hasty snaps of the jaw the 
 material is swallowed. 
 
 Opening the mouth is equivalent to depressing the lower 
 jaw, for the upper takes no share in the process. The muscles 
 which open the mouth are comparatively small, for very little 
 effort is required ; the stemo- and stylo-maxillaris and digastricus 
 perform this function. On the other hand, the closing of the 
 jaws in mastication is a difficult task, and for this purpose very 
 powerful muscles exist; they are the masseters, temporals, and 
 pterygoids. In the dog the temporal muscles are considerably 
 developed, whilst in herbivora the masseters are the largest. 
 
 The nerves employed in mastication are the sensory fibres 
 of the fifth, which convey to the brain the impulses resulting from 
 the presence of food in the mouth, while the motor fibres of the 
 same nerve supply the needful stimulus to -all the muscles of 
 . mastication excepting the digastricus, which receives its motor 
 supply from the seventh pair. 
 
 Deglutition. 
 
 The process of swallowing is usually described as occurring in 
 three stages. The first stage comprises the carrying of the food 
 back to the base of the tongue and pressing it against the soft 
 palate; it is a simple process readily understood. In the second 
 stage the act is complex, for the bolus or fluid has to cross the air- 
 passage, and must be prevented from falling into the nasal 
 chambers, or finding its way down the trachea. To accomplish 
 this in the horse the soft palate is raised and so closes the nasal 
 chambers, the tongue at the same time being carried backwards, 
 while the larynx and pharynx are advanced. This movement 
 causes the base of the tongue to press on the epiglottis and close 
 the larynx, which is further secured by the arytenoid cartilages 
 and vocal cords coming close together. The bolus, liberally 
 coated with mucus from the large mucous crypts in this locality, 
 or the fluid, can now safely pass towards the pharynx, being 
 grasped by the pharyngeal muscles and pressed into the oesopha- 
 gus. In the third act of swallowing the food is carried down the 
 oesophagus by a continuous wave of contraction, which starts at 
 the pharynx and ends at the stomach. Chauveau points out 
 that owing to its extreme length, the soft palate of the horse 
 passes completely into the pharynx during the second act of 
 deglutition The length of the soft palate in this animal prevents 
 
i6 4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 food or water being returned by the mouth when once they have 
 entered the pharynx, so that in vomiting, or in cases of sore 
 throat, the food, water, or other material is returned by the 
 nostrils. 
 
 I* It is now considered that in some animals the constrictor 
 muscles of the pharynx take less share in the process of swallow- 
 ing than was at one time supposed, and that the sharp contraction 
 of the mylo-hyoid muscles of the tongue, together with a back-- 
 ward movement of the organ, exerts pressure on the bolus, and 
 shoots the latter through the pharynx into the oesophagus . From 
 the time the mylo-hyoids act until the entrance of the food 
 into the oesophagus, only a second passes. We are not inclined 
 to think these observations can at present be applied to swallow- 
 ing in the horse. If the hand is placed in the pharynx there is 
 no suggestion of any such shooting movement, and material, 
 such as a bolus, placed far back on the tongue, is frequently 
 very deliberate in entering the oesophagus. Arloing proved 
 that the passage of fluid along the oesophagus was not due to 
 muscular contractions of that tube, but to the pharyngeal 
 muscles pumping it along the oesophagus. As a matter of fact, 
 fluid scarcely requires one second to reach the stomach, so that 
 its velocity is, as we shall presently see, ten times greater than 
 the rate of oesophageal contraction. 
 
 The action of the epiglottis in the closure of the glottis has been 
 much discussed. In the horse it is forced over the opening- 
 by the base of the tongue and the advancing larynx; but the 
 epiglottis is not essential to swallowing, for an animal can swallow 
 when it has been removed, and even when one of the arytenoid 
 cartilages has been excised. Placing the finger in the larynx 
 will easily demonstrate that the part closes tightly and forcibly 
 during the second stage of swallowing, the vocal cords and ary- 
 tenoids being brought so close together that the glottis is perfectly 
 air-tight. It has been pointed out that animals usually swallow 
 with a flexed neck, as in this position the epiglottis is behind the 
 soft palate, and in the most favourable position to be applied 
 over the glottis. It has also been shown, however, that when the 
 head is extended the epiglottis is in the mouth — viz., anterior 
 to the soft palate. We have found it in this position in the horse, 
 and judging from the fact that in a state of nature the horse and 
 ox swallow with an extended and not with a flexed neck, it is 
 probable that in feeding off the ground the epiglottis is anterior 
 to the soft palate. During the third stage of deglutition the 
 bolus can be seen slowly travelling down the channel of the neck; 
 if liquid, however, be passing, the movement is very rapid, for 
 as many as sixty swallows may be made in a minute. As pre- 
 viously noticed, both in eating and drinking the third act of de- 
 
DIGESTION 165 
 
 glutition can occur against gravity ; this is because it is a muscu- 
 lar act. The whole process of deglutition is considerably assisted 
 by the salivary secretion. When this has been experimentally 
 diverted, swallowing occurs only with difficulty and very slowly. 
 
 The oesophagus of the horse is found to differ considerably 
 from that of most other animals. It is composed for the greater 
 part of its length of red striated muscle, while at and near its 
 termination the previously thin muscular coat becomes very 
 thick and rigid, and the red gives way to pale, non-striped 
 muscle; further, the lumen of the tube becomes very narrow. 
 The thick terminal end of the oesophagus of the horse is always 
 closely contracted, so that if the oesophagus is cut through close 
 to the stomach no material can escape from this organ ; this is 
 one explanation why horses vomit with such difficulty. In the 
 ox, sheep, and dog, the tube is composed of red muscle throughout ; 
 it terminates in a dilated end at the stomach, and, owing to its 
 thin, distensible walls, even bulky material can pass along it; 
 what the ox and dog can swallow with ease would certainly 
 ' choke' the horse. 
 
 Chauveau showed that in the horse the peristaltic contractions 
 of the oesophagus originate in the pharynx, and that, once started, 
 they continue irresistibly to the stomach. A bolus introduced 
 into the oesophagus by a fistula in the neck remains in situ ; it 
 must start from the pharynx. If by means of a fistula the bolus 
 is discharged externally, the wave of contraction in the oesopha- 
 gus continues. If the bolus be experimentally arrested, the 
 muscular wave leaves it behind and continues on its journey. 
 
 Arloing ascertained the rate of progress of a bolus in the 
 oesophagus of the horse, and found it to be 0-2 metre (7! inches) 
 per second in the red portion of the tube and 0-05 metre (1-96 
 inches) per second in the pale portion. This is evidence that 
 the bolus slows down considerably before it enters the stomach. 
 This observer found the mean time of transit through the oeso- 
 phagus to be ten seconds. The wave of contraction in the oeso- 
 phagus is helped by the immediately preceding contraction of the 
 longitudinal muscle, which thereby shortens, and so tends to 
 dilate the tube for the reception of the bolus. 
 
 The first stage of deglutition is voluntary, but the remaining 
 processes are quite involuntary, and are brought about by the 
 stimulation of a centre in the medulla known as the swallowing 
 centre. By means of ingoing or afferent nerves supplied by 
 branches of the fifth and by the superior laryngeal, the centre is 
 made acquainted with the fact that food is present in the fauces. 
 ATeflex act is now set up in the centre, and an impulse conveyed 
 by outgoing or efferent nerves, furnished by the pharyngeal 
 plexus (composed of the vagus and glossopharyngeal) for the 
 
166 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 constrictor muscles of the pharynx, by the hypoglossal for the 
 tongue, and by the recurrent laryngeal for the muscles which close 
 the glottis. The glosso-pharyngeal is the inhibitory nerve of 
 deglutition ; if the central end be stimulated it is impossible to 
 produce the act of swallowing. Further, it is the nerve which 
 immediately inhibits respiration during swallowing, no matter 
 at what phase of the act — viz., inspiration or expiration — the 
 stimulus is applied. Swallowing may be induced without the 
 presence of food in the fauces; touching the rim of the glottis 
 will produce it; so also will pouring a fine jet of fluid into the 
 trachea, or even touching the interior of the trachea as far down 
 as the bronchi. 
 
 Stimulation of the mucous membrane of the pharynx excites 
 reflex movements of the oesophagus, but stimulation of the 
 mucous membrane of the oesophagus itself is ineffective in this 
 respect. 
 
 The swallowing centre also presides over the oesophagus, and 
 the peristaltic wave from the pharynx to the stomach is pro- 
 duced by impulses sent out from this centre through the vagus. 
 This wave is, therefore, not due to the nerve handing on a con- 
 traction by direct conduction from one layer of the muscular 
 wall of the oesophagus to the next. Hence, when once started, 
 it is not arrested either by ligaturing or dividing the oesophagus, 
 though section of the oesophageal nerves prevents it. The con- 
 traction wave which sweeps along the oesophagus is not interfered 
 with even by excising a portion of the tube; the wave, having 
 reached the point from which the upper segment has been cut out, 
 appears in due course at the point from which the lower end was 
 removed. 
 
 It is not uncommon in watching a bolus pass down the neck 
 of the horse to see it suddenly come to a standstill, and then 
 slowly pass on again after, probably, an attempt to ascend. This 
 is generally due to absence of saliva. In rumination and in 
 vomiting the wave runs upward from the stomach to the pharynx. 
 Division of the vagus interferes with the passage of food along 
 the oesophagus, which in consequence becomes blocked. 
 
 The Saliva. 
 
 During the process of mastication the food becomes mixed 
 in the mouth with a fluid known as saliva, the secretion of which 
 occurs in three distinct pairs of glands. The method by which 
 it is formed is important to understand, as much the same pro- 
 cess occurs in other secretory glands which we have not the same 
 opportunity of watching during their activity. 
 
 Classification of Salivary Glands. — The three glands which 
 secrete saliva are the parotid, submaxillary, and sublingual; 
 
DIGESTION 
 
 167 
 
 these are structurally divided into two groups, mucous and 
 serous (or albuminous) glands, the submaxillary and sublingual 
 being types of the first, the parotid the type of the second. 
 
 The following table from Colin shows the relative percentages 
 of the glands in Various animals : 
 
 
 Horse. 
 
 Ox. 
 
 Sheep. 
 
 Pig- 
 
 Dog. 
 
 Parotid 
 Submaxillary 
 Sublingual - 
 
 78-00 
 17-00 
 00-05 
 
 45 -00 
 48-00 
 00-07 
 
 52-00 
 
 43-00 
 
 00-05 
 
 8roo 
 1 6- 00 
 0003 
 
 48-00 
 52-00 
 
 The table shows that, excepting the pig, the horse has the 
 best-developed parotid system, while his submaxillary glands are 
 very small. In the ox the parotid and submaxillary glands 
 are nearly equal in weight. Colin showed that the size of a sali- 
 vary gland does not determine its secretory power; the parotid 
 of the horse secretes from fifteen to twenty times more saliva 
 than the submaxillary, while it is only about five times heavier. 
 Similarly, the parotid of the ox secretes four times more saliva 
 than the submaxillary, though they are of nearly equal weight. 
 
 All the salivary glands belong to the class known as ' compound 
 tubular.' The parotid is regarded as the type of serous (or 
 albuminous) gland, and this holds good in all animals ; but the 
 submaxillary is sometimes of the mucous type, as in the dog 
 and cat, and sometimes mixed, as in man. The sublingual may 
 also be a mixed gland, though with mucous cells predominating. 
 
 Mucous glands are generally characterised by the presence of 
 peculiar crescent-shaped cells known as demilunes, lying beneath 
 the basement membrane, and away from the central lumen 
 of the tube. Great difference of opinion has existed as to the 
 special function of these crescents of Gianuzzi, some believing 
 that they replace worn-out mucous cells, others that they possess 
 specific functions. 
 
 It has been supposed that the activity of the three glands 
 depends upon the character of the food substances, and in the 
 dog there is experimental proof of the correctness of this view. 
 Pavlov, whose work on digestion has opened up a new field, 
 has shown that in the dog— and the f ollowing remarks refer solely 
 to this animal— the submaxillary gland responds to the sight of 
 food, to the chewing of meat, and to the action of acids ; while 
 the parotid responds to dry food, such as dry powdered meat, 
 bread, or biscuit. 
 
 The selective power of the glands and their adaptability to 
 the class of food in the mouth is very remarkable. Dry bread 
 
68 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 excites the parotid, since water is required to moisten it, and the 
 submaxillary, since mucin is needed in order to lubricate it, but 
 moist bread stimulates only the submaxillary. Fresh meat 
 requires no parotid, but only submaxillary saliva. Pebbles 
 placed in the mouth excite little or no secretion, but pebbles 
 reduced to sand excite an abundant secretion. It has been 
 suggested that this abundant secretion is to wash out the sand, 
 while pebbles require no washing out, as they can be dropped. 
 What is still more remarkable, it has been shown that the sight 
 of food causes an abundant secretion of saliva ; Claude Bernard 
 demonstrated that this was also true of the horse. 
 
 From this it is evident that no stimulation of the buccal mucous 
 membrane is essential to secretion, though there can be little 
 doubt that the adaptative mechanism is frequently provoked 
 through this channel. 
 
 Colin in opposition to Bernard stated that in the herbivora the 
 secretion of saliva was uninfluenced by the sight or smell of food; 
 nor could he obtain any secretion from the parotids by the 
 employment of excitants to the mucous membrane of the mouth; 
 salts, acids, aromatic substances, all gave equally negative results 
 in this respect. The submaxillary and sublingual glands, on the 
 other hand, actively responded to these stimuli. In the horse he 
 found that oats produced a greater secretion of saliva than hay, 
 though the amount of fluid absorbed by oats is one-quarter that 
 by hay. 
 
 It is well known that fear or anxiety gives rise in the human 
 subject to impulses inhibiting secretion of saliva; the mouth 
 becomes dry, and the tongue refuses to move. In the horse an 
 identical condition is produced by abdominal pain. During an 
 attack of colic the mouth is quite dry, which symptom is of the 
 utmost value. The dryness is not due to thirst, for the animal 
 never drinks while the pain lasts ; a moist condition of mouth and 
 a desire for water are two favourable indications of the utmost 
 value in prognosis. 
 
 Physical and Chemical Characters. — Mixed saliva is an alka- 
 line, opalescent, or slightly turbid fluid which readily froths 
 when shaken. On standing exposed to the air, it throws down 
 a deposit of carbonate of lime due to the loss of its carbonic acid. 
 It has a specific gravity of 1-005 in the horse, and i-oio in the ox. 
 Examined microscopically, saliva is seen to contain epithelial 
 scales and salivary corpuscles. The latter are small round 
 granular cells which seem to be altered leucocytes, and ape prob- 
 ably derived from the soft palate. About o-6 per cent, of the 
 saliva consists of mineral matter, and 0-2 per cent., more or less, 
 of organic matter, the latter consisting of mucin (which gives 
 saliva its well-known viscidity and ropiness), and small amounts 
 
DIGESTION 
 
 169 
 
 of protein substances the nature of which has not been exactly 
 determined. Mucin belongs to a peculiar group of protein bodies 
 combined with a carbohydrate, for which see Chapter XX. 
 Ptyalin or salivary diastase is the most interesting organic con- 
 stituent of saliva in man, and belongs to a group of ferments 
 known as unorganised . It is doubtful if it exists in the herbivora, 
 and under no circumstances has its amount been determined. 
 Ptyalin is also absent from the saliva of the dog. The salts of 
 saliva are principally carbonate of lime, alkaline chlorides, 
 and phosphates of lime and magnesia. Sulpho-cyanates are 
 found in minute quantities in the saliva of the human subject, 
 but are absent from that of the horse. The gases of -the saliva 
 are principally carbonic acid, with traces of oxygen and nitrogen ; 
 there is no body fluid which contains so much carbonic acid as 
 saliva (65 volumes per cent.). The three salivas have different 
 physical properties. Parotid saliva is watery, clear, and free from 
 mucin, but contains a small quantity of protein ; submaxillary 
 and sublingual salivas are viscid, especially the latter, owing to 
 their richness in mucin. The watery saliva has to impregnate 
 the food and prepare it for digestion, but the viscid salivas are 
 concerned principally in swallowing. 
 
 Amount of Secretion. — Colin observed some remarkable facts 
 regarding the secretion of the various salivary glands in the 
 herbivora. When an animal is masticating, the parotid on that 
 side is secreting, while its fellow is almost at rest. When the 
 rhythm of mastication changes over to the opposite side, the 
 parotid on that side becomes active, while the other passes into 
 the condition of comparative rest. The parotid of the side on 
 which the animal is masticating will secrete two, three, or four 
 times as much saliva as its fellow. The following table shows 
 one of Colin's experiments on the horse: 
 
 
 Right Parotid. 
 
 Left I'arotid. 
 
 1 
 Direction of Mastication. , ! 
 
 3 minutes 
 
 6 
 
 4 
 
 5 
 
 Grammes. 
 
 5° 
 200 
 
 30 
 200 
 
 Grammes. 
 IIO 
 
 5° 
 IOO 
 
 30 
 
 To the left 
 To the right 
 To the left 
 To the right 
 
 So far, these observations apply both to the horse and to the 
 ruminant, but differences soon occur. When the horse is no 
 longer eating, the parotids stop secreting, but with the ruminant 
 this is not the case ; the parotids continue to act, not only during 
 rumination, but during abstinence. During rumination the secre- 
 tion is unilateral in the sense that very much more is obtained 
 from the side on which the jaws are active. 
 
170 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The secretion from the submaxillary and sublingual glands 
 behaves quite differently from that of the parotid. It is constant 
 from both sides in all herbivora, no matter what the direction 
 of mastication may be. The submaxillary glands cease to secrete 
 during rumination ; the sublingual is not affected by this function. 
 
 The actual amount of fluid secreted daily from these glands in 
 the large herbivora is astonishing. Colin put it down at 5 to 
 6 kilogrammes (8| to 10 \ pints) hourly for the horse during 
 the seven hours he is engaged in feeding. He gives the total 
 
 Fig. 58. — Apparatus employed by Colin in Experiments on the Secretion 
 of Parotid and Submaxillary Saliva. 
 
 daily secretion for this animal at 42 kilogrammes (9^ gallons), 
 while for the ox it is still higher — -56 kilogrammes (12 \ gallons), 
 of which from 800 grammes (1-4 pints) to 2,400 grammes (4J 
 pints) per hour are produced during the intervals of feeding. 
 
 The submaxillary secretion is placed at 280 grammes (1 J pints) 
 per hour, and the sublingual at 18 to 20 grammes (f ounce) per 
 hour. 
 
 It is obvious that the amount of secretion depends upon the 
 character of the food- only half the daily mean is secreted if 
 
DIGESTION i 7 i 
 
 green food be given, and only one-third if roots form the diet. 
 On the other hand, oats increase the amount secreted. Hay 
 absorbs four times its own weight of saliva, oats a little more than 
 their weight, and green fodder half its weight. 
 
 The use of the saliva in herbivora is to assist in mastication 
 and swallowing, in stimulating the nerves of taste, and in rumi- 
 nants to assist in rumination. According to the writer's observa- 
 tions on the horse, confirmed by R. J. Seymour in 1917 and C. E. 
 Hayden in 1918,* saliva has no chemical action on the raw starch 
 of the food, and this is not surprising when we remember that the 
 starch grains are enclosed in an envelope of cellulose, a sub- 
 stance on which saliva has no action. So intimately, however, 
 is salivary secretion associated with starch conversion, that it is 
 not possible to pass over without further notice the action pro- 
 duced on starch in man, and, according to some observers, in 
 horses and cattle, by the presence of ptyalin in the saliva. 
 
 The starch found in plants exists in the form of granules 
 which are of shapes peculiar to the various species ; these granules 
 are enveloped in tough envelopes of cellulose; before the true 
 starch, the granulose contained in the cellulose envelope, can be 
 reached the cellulose must be traversed. For this reason some 
 animals, like man, cannot digest raw starch, but by being cooked 
 the starch (granulose) is liberated and made free to be acted upon ; 
 on the other hand, all the herbivora are capable of digesting raw 
 starch, perhaps because they can digest cellulose. 
 
 If boiled starch be mixed with filtered human saliva and 
 kept at a temperature of 95° F., in a short time the characteristic 
 reaction of a blue colour with iodine disappears on the addition 
 of this reagent, and a reddish colour is formed, indicating the 
 presence, of a substance known as erythrodextrin. The fluid, 
 which before was sugar-free, contains distinct evidence of the 
 presence of this substance ; by continuing the action of the saliva 
 it is shortly found that the red colour on the addition of iodine 
 has disappeared, and the fluid gives evidence of containing a 
 considerable proportion of sugar. But analysis shows that for 
 the amount of/starch employed the full amount of sugar has not 
 been obtained; in other words, there is a second substance 
 present besides sugar, which is produced as the result of the 
 action of the saliva, and to this the name achroodextrin has been 
 given; it is formed from the erythrodextrin. The sugar formed 
 from starch by the action of saliva is not grape-sugar, but 
 maltose; glucose (dextrose or grape-sugar) being found only in 
 small quantities, if at all. This action of the saliva on starch is 
 described as the Amylolytic action; it is due to the presence of 
 
 ptyalin. 
 
 * American Journal of Physiology. 
 
172 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Ptyalin, the active principle of starch-converting salivas, is 
 an unorganised enzyme, and, like all ferments, is of unknown 
 chemical nature. It is capable of converting cooked starch 
 rapidly and raw starch slowly into maltose and dextrin, the 
 ferment acting by hydrolysis — i.e., the molecule of starch takes '[• 
 up water, and undergoes cleavage into simpler bodies. Between 
 starch and maltose there are doubtless many other bodies, but 
 the chemistry is not agreed upon. The amylolytic action is 
 permanen tly destroyed by a high, inhibited by a low temperature, 
 retarded by a slightly acid or alkaline medium, and destroyed 
 by free hydrochloric acid. If starch be boiled with a dilute acid, 
 conversion into sugar occurs. The difference between the action 
 of boiling acid on starch and of saliva is that the latter can pro- 
 duce only maltose, whereas the acid produces dextrose. 
 
 The view we hold as to the non-amylolytic action of the saliva 
 of the horse is not supported by other observers; Ellenberger* -fjjj 1 
 distinctly states that both the parotid- and submaxillary secre- 
 tions of the horse and ox can convert starch into sugar, but in 
 the case of the horse it is only the saliva first secreted by the glands 
 after a rest which possesses this property ; ,as secretion proceeds 
 the power is nearly lost. In the pig, according to this observer, '.< 
 all the salivary glands are starch-converting; in the rabbit the 
 submaxillary has no action, while the parotid is energetic ; in the 
 cat, dog, horse, sheep, and ox the action is very feeble or entirely : 
 absent. Meade Smithf states that the saliva of the horse will , 
 convert crushed raw starch into sugar in fifteen minutes, and :| 
 that the process begun in the mouth is continued in the stomach; 
 he further adds that the saliva of the horse will convert cane into 
 grape sugar. 
 
 The amylolytic properties of equine saliva have been inquired 
 into recently by Lieut. -Colonel Watkins Pitchford, C.M.G., 
 R.A.V.C, at the Central Veterinary Research Laboratory at 
 Aldershot. A washed sponge inserted into the mouth of the 
 horse becomes rapidly saturated with the salivary secretion, 
 which is readily collected in this manner in considerable quan- 
 tities. 
 
 Observation with the saliva thus obtained from a series of ten i 
 horses showed in all cases the presence of an amylolytic ferment f 
 existent both before and after feeding. In these experiments ] 
 the starch was either partly converted into maltose and ery- 
 throdextrin or completely into maltose, Fehling's test showing 
 the presence of sugar in abundance in all cases. This amyloly- 
 tic action was noted to be less marked than in the case of human 
 saliva, but it was evident in dilutions of one part of saliva to 
 
 * ' Physiologie der Haussaugethiere.' 
 f ' Physiology of the Domestic Animals.' 
 
 
DIGESTION 
 
 173 
 
 five parts of boiled starch solution, within a time limit of thirty 
 minutes at 37° C. With raw oat starch the action is much 
 slower and less complete. (Communicated.) 
 
 In man starch conversion, brought about by the action of 
 ptyalin, is now recognised as taking place in the stomach from 
 the swallowed saliva — in fact, the bulk of the conversion must 
 necessarily take place there, and not in the mouth. There is an 
 abundance of sugar in the stomach of the horse, which the writer 
 suggests is probably formed by a ferment pre-existing in the 
 grain. Pitchford's observations show that with some horses it 
 may originate from the saliva swallowed. 
 
 Secretion of Saliva. — The mechanism concerned in the secre- 
 tion of saliva deserves careful attention, for the reason that it 
 throws considerable light on other secretory processes. The 
 subject has been worked out by so many competent observers 
 that the leading points are beyond all doubt. The submaxillary 
 gland of the dog has mainly afforded the desired information, 
 and there is reason to believe that the same process of secretion 
 holds good for the parotid and other glands, both of this animal 
 and of herbivora. 
 
 The chief point in the secretion of saliva is that it is controlled 
 by the nervous system, and is not directly dependent upon any 
 mere increase in the blood- pressure in the gland. Afferent 
 nerves — viz., the gustatory division of the fifth and the glosso- 
 pharyngeal — convey from the mouth to the medulla a certain 
 impulse, which, by means of efferent nerves, is conveyed to the 
 gland, and secretion results. The efferent nerve of the sub- 
 maxillary gland of the dog is supplied by the chorda tympani, a 
 small branch given off by the seventh cranial nerve, which enters 
 the gland at its hilum, and supplies the vessels with dilator and 
 the cells with secretory fibres. The second nerve supplying the 
 submaxillary gland is a branch of the sympathetic, which spreads 
 out and invests with constrictor fibres the walls of the artery 
 supplying the part (Fig. 59). Thus the chorda tympani supplies 
 the gland with secretory fibres and the walls of the vessels with 
 dilator fibres, while the sympathetic supplies the vessels with 
 constrictor fibres, and only a few secretory fibres. 
 
 If the tongue or the lingual branch of the fifth or glosso- 
 pharyngeal nerves be stimulated, secretion of saliva results; if 
 the sympathetic nerve be divided and the tongue then stimu- 
 lated, secretion follows; but if the chorda tympani be previously 
 divided, no secretion follows on stimulation of the tongue, lingual, 
 or glosso-pharyngeal nerves. If the chorda be stimulated, the 
 vessels dilate, the gland becomes red, the blood flowing from the 
 veins is arterial in tint, and the veins pulsate; in addition to this, 
 there is an abundant secretion of watery saliva poor in solids. 
 
174 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 When the sympathetic is stimulated, exactly the reverse is 
 observed— : viz., the vessels constrict, in consequence of which 
 the gland becomes pale, only a small quantity of extremely viscid 
 saliva flows, which is rich in solids, the blood in the veins becomes 
 very dark in colour, and the blood-stream slows to such an extent 
 that if the veins leading from the gland be cut, the flow from 
 
 \ 
 
 Jh 
 
 __ ffl.cer.s. 
 
 .1 Willi ill 
 
 
 
 / 
 
 
 |fegiS^!5? 
 
 VJ ' ? /V^ 
 
 -r.sm.jx v. s.m. 
 
 Fig. 59. — Diagrammatic Representation of the Submaxillary Gland of 
 the Dog, with its Nerves and Bloodvessels (Foster). 
 
 (The dissection has been made with the animal on its back, and is very 
 diagrammatic). 
 
 The submaxillary gland {sm.gld.) occupies the centre of the figure; the blood- 
 vessels supplying it, derived from the carotid artery, a.car., are seen on the 
 left, whilst the duct from the gland sm.d., in which a cannula is inserted, 
 is on the right of the figure. 
 
 The chorda tympani nerve, ch.t"., running in company with the lingual branch 
 of the fifth n.l'., is seen to the right and below; after running together the 
 two nerves separate, the chorda tympani, ch.t., running along the submaxil- 
 lary duct to the gland. Close to where the two nerves separate is the sub- 
 maxillary ganglion, sm.gl. 
 
 The sympathetic nerve-supply is shown in the figure to the left and above, the 
 fibres being derived from the superior cervical ganglion, gl.cer.s., and cours- 
 ing along the bloodvessels to enter the gland. 
 
 The bloodvessels leading from the gland fall into the jugular vein, v.j. 
 
 The arrows indicate the direction of the nervous impulses during the reflex act, 
 ascending to the brain by the lingual, and descending by the chorda. 
 
 them is less than from a gland at rest. That the increased flow 
 of blood to the gland produced by stimulating the chorda is not 
 the essential cause of the secretion is proved by the fact that 
 the pressure of the saliva in the duct of the gland is higher than 
 the blood-pressure within the vessels. Further, if before stimu- 
 
DIGESTION 175 
 
 lation of the chorda some atropine be injected, stimulation of the 
 nerve still produces to the full all the vascular changes, but not 
 a trace of saliva is secreted. Hence, secretion is not due merely 
 to increased blood-pressure. This atropine experiment proves 
 the existence in the chorda of two sets of nerves — viz., secretory 
 and vaso-dilator ; owing to the action of atropine, the secretory 
 nerves are paralysed, while the vaso-dilators are not. In the 
 sympathetic two sets of nerves can similarly be demonstrated— 
 secretory and vaso-constrictor — though it is most likely that 
 in the majority of animals the secretory fibres in the sympathetic 
 are few in number. Pilocarpine is antagonistic to atropine, 
 and produces a profuse flow of saliva. 
 
 A peculiar phenomenon is observed in connection with salivary 
 secretion after division of the chorda. Though the gland is 
 cut off from its secretory nerve, yet one or two days after section 
 a secretion appears, and may continue for some weeks until the 
 gland undergoes atrophy. This is known as ' paralytic secretion.' 
 The cause of paralytic secretion is not definitely known. The 
 cut portion of nerve in connection with the gland degenerates; 
 it has been suggested that the secreting cells are controlled by 
 inhibitory fibres in the nerve, and that the cells, in consequence 
 of a local nervous mechanism in the gland, continue secreting. 
 Very little of a definite nature is known of the paralytic pheno- 
 menon, and it is curious to observe that it is not limited to the 
 gland of which the nerve has been divided, but affects both sides. 
 Langley, who has described this, refers to the continuous secre- 
 tion from the unoperated gland as antiparalytic or antilytic. 
 
 Heidenhain's view of the action of secretory nerves is that a 
 gland is supplied with a trophic or nutritive nerve which excites 
 the formation of the organic constituents of the secretion, and 
 a secretory nerve, which controls the secretion of water and 
 inorganic salts. The cranial nerves are chiefly secretory, while 
 the sympathetic are trophic, or building up ; hence stimulation 
 of the chorda yields the water and salts of the saliva, while 
 stimulation of the sympathetic produces the organic substances 
 and ferment. 
 
 Trophic fibres are supposed to effect a breaking-down of the 
 complex living substance of the gland and conversion into simpler 
 bodies, and that chemical changes of importance are occurring 
 is undoubted from the large amount of C0 2 found in saliva, which 
 indicates active oxidation. Saliva contains more C0 2 than any 
 other secretion of the body— more, even, than venous blood. 
 Side by side with the above destructive changes construction 
 must also be going on, though there are no known nerve fibres 
 responsible for the process. As to the manner in which the 
 secretory nerves act very little is known; the blood loses more 
 
176 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 water than can be accounted for by the amount in the saliva, 
 the difference being supposed to be represented by an increase 
 in the lymph flow from the fluid ; but in what way nerve impulses 
 can cause the cells to secrete is at present inexplicable. 
 
 Fig. 60. — Changes in the Cells of the Living Parotid (Serous Gland) 
 
 during Secretion (Foster, after Langley). 
 A, At rest; B, in the first stage of secretion; C, after prolonged secretion. 
 
 The method by which secretion in the parotid gland is carried 
 out differs in no essential respect from that in the submaxillary. 
 The nerves supplying the parotid are the glosso-pharyngeal (the 
 action of which corresponds to the chorda of the submaxillary) 
 and the sympathetic. In the glosso-pharyngeal are dilator 
 
 fibres, and in the sympathetic 
 constrictor fibres for the blood- 
 vessels, while both trunks contain 
 secretory nerves. 
 
 The influence of atropine and 
 pilocarpine on the gland-cells 
 has previously been mentioned; 
 it remains to notice the effect of 
 nicotine, which prevents the secre- 
 tion of saliva — not, however, by 
 its action on the gland-cells, but 
 by paralysing the connections of 
 the nerve fibres in such ganglia as 
 the submaxillary, Langley's, and 
 the superior cervical. This action 
 of nicotine, which essentially con- 
 sists in paralysing the nerve-cells 
 in the ganglia, and not the fibres, 
 was discovered by Langley, and has largely helped in building up 
 our knowledge of the sympathetic system. 
 
 The changes occurring in the cells of the salivary glands 
 during secretion depend upon the type of gland. We select 
 Langley's observations, since he examined the living gland, and 
 not one simply hardened and stained. During the stage of rest 
 in a living serous gland, such as the parotid, the cells are found 
 
 Fig. 61. — Cells from Mucous 
 Gland (Submaxillary Gland 
 of the Dog) (Foster). 
 
 a, From loaded gland; b, from dis- 
 charged gland; a', b', treated 
 with dilute acetic acid; a', from 
 loaded ; b', from discharged gland. 
 
DIGESTION 177 
 
 to be filled with a quantity of granular material, and the outline 
 of each individual cell is indistinct ; the lumen of the gland is also 
 occluded, and no nucleus can be observed in the cells ; in other 
 words, the gland is charged with its secretory products (Fig.6o,A). 
 During activity the cells get rid of their granular material, 
 which gradually passes towards the centre of the acinus or 
 lumen, leaving each cell with a clear outer edge, while that edge 
 next the lumen is still granular (Fig. 60, B). In an exhausted 
 condition the cells are smaller and remarkably clear, only a 
 few granules being left in them on the inner edge, while the 
 lumen is now distinct and large, and the nuclei are clearly seen 
 occupying a central position (Fig. 60, C) . 
 
 If a mucous gland, such as the submaxillary, be examined at 
 rest under like conditions, the cells are found filled with granules 
 much larger than those of a serous gland, and a nucleus is seen 
 occupying one edge of the cell (Fig. 61, a). During activity the 
 granules are passed into the lumen of the gland, but they do 
 not leave behind them in the cells the same clear space seen in 
 the serous cell (Fig. 61, b) . If the cells, while in an active con- 
 dition, be acted upon by water or dilute acetic acid, the granules 
 swell up and become transparent owing to the mucin they con- 
 tain, and a delicate network is seen to pervade the cell (Fig. 61, a') . 
 A similar appearance is produced in the exhausted cell (Fig. 61, b') , 
 excepting that less transparent mucin is seen and more granu- 
 lar substance, while the nucleus of the exhausted irrigated gland 
 is seen passing towards the centre of the cell instead of remaining 
 close to the outer wall. Though we have spoken of these granules 
 as mucin, in the gland they are not really mucin, but the mother- 
 substance of it — viz., mucigen — which during the act of secretion 
 is converted into mucin. The same holds good for the serous 
 type; the granules in the resting gland are the precursors of the 
 ferment or the zymogen of the secretion, from which the secretion 
 is actually formed at the moment it is poured out. 
 
 The outcome of the changes above described proves that the 
 organic elements found in the salivary secretion are manufactured 
 by the cells in the glands; the inorganic constituents are either 
 the result of filtration or of secretion. Experiments made by 
 Langley and Fletcher go to prove that even water and salts are 
 the result of cell secretion, and not of mere transudation. 
 
178 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Section 2. 
 Stomach Digestion. 
 
 Important digestive changes in the food of the lower animals 
 take place in the stomach. It is not a matter for surprise to find 
 that the size and shape of this organ varies with the species of 
 animal; we should expect a simple stomach in the dog, and com- 
 plex arrangement in vegetable feeders. It seems remarkable 
 that any animal should possess a laboratory capable of converting 
 grass, hay, and grain into muscle and fat; and it is evident that 
 the conversion of vegetable into animal tissues must be a more 
 complex process than the conversion of animal tissues into the 
 living structure of an animal body. But it is curious to observe 
 that a complex stomach is by no means a necessity for a vege- 
 table feeder ; the stomach of the ruminant and the simple stomach 
 of the horse could not be in greater contrast, while the laboratory 
 processes of digestion are practically identical. So far as vege- 
 table food is concerned, it does not matter whether the solution 
 and absorption of its readily soluble matters comes before macera- 
 tion, or whether maceration precedes the extraction of the readily 
 soluble substances. If maceration comes first, as in ruminants, 
 bulky gastric compartments are provided for the purpose, and 
 the subsequent intestinal canal is short. If the simple stomach 
 comes first, bulky intestines for the purpose of maceration follow; 
 in both cases ample provision is made in the stomach for the 
 maceration necessary for the solution of the cell- wall and fibrous 
 portion of plants. The dog, with its simple stomach and simple 
 intestines, offers no difficulty to our understanding. He lives on 
 flesh, and converts it into flesh; it is not' very clear why he has 
 both a stomach and intestines, for the whole process of digestion 
 is simple, and could be readily carried out by the intestines alone. 
 In fact, the stomach of the dog has been removed experimentally, 
 and the animal remained in health. 
 
 For simplicity in construction the stomach of the dog occupies 
 one end of the scale, for complexity the gastric reservoirs of the 
 ox occupy the other, while between the two comes the stomach of 
 the omnivorous pig, partaking of some of the characteristics of the 
 carnivora and of some of the ruminant, and belonging to neither. 
 
 Stomach Digestion in the Horse. 
 
 The subject of stomach digestion in the horse has been worked 
 out by means of feeding experiments, as it has been found 
 impossible to establish a gastric fistula in this animal owing to 
 
DIGESTION i 79 
 
 the distance of the stomach from the abdominal wall; pure 
 gastric juice has, therefore, never been obtained from the horse. 
 
 The first peculiarity to be noticed in soliped digestion is that 
 the stomach is rarely empty ; it is only when horses have pur- 
 posely been deprived of food for not less than twenty-four hours 
 that an empty stomach can be obtained. On the other hand, 
 feeding experiments show that very shortly after food arrives 
 in the stomach it commences to pass out, and the difficulty thus 
 presented to the observer in reconciling these opposed facts is at 
 first sight considerable. It is perfectly true that food does pass 
 out early, it is equally true that it is long retained, these opposite 
 conditions being the result of the "periods of digestion. When 
 food enters an empty stomach it passes towards the pylorus, 
 where it meets with a fluid of an alkaline or neutral reaction 
 which has come from the mouth. As more food is consumed an 
 acid fluid is secreted in the stomach, and material commences 
 to pass out at the pylorus into the bowel, the amount passing 
 out not equalling at present the amount passing in. Thus the 
 stomach becomes gradually distended, and when two-thirds full, 
 which is the condition in which the most active digestion occurs, 
 the amount passing out will, if more food be taken, equal the 
 amount being swallowed, so that we have a stream of partly 
 peptonised chyme streaming out of the right extremity, while a 
 corresponding bulk of ingesta is entering the inert left sac. In 
 fact, the stomach may during feeding allow two or three times 
 the bulk of food to pass out which remains in it when the meal is 
 finished. Nc sooner is the feed finished than the passage of 
 chyme into the duodenum ceases, or becomes so slowed down 
 that only small quantities of food pass out, and so gradual 
 is this passing out that it will be many hours before the stomach 
 is really empty, though, had the process continued as it com- 
 menced, the stomach would not have contained anything at the 
 end of an hour. This condition of stomach digestion in the horse 
 may be variously modified, according to the nature of the food, 
 the quantity given, the form in which it is given, the order in 
 which one food follows another, and whether water be given before 
 or after feeding. All these are points requiring our attention, 
 but first we must briefly look at the stomach itself. 
 
 The mean capacity of a horse's stomach is, according to Colin, 
 from 15 to 18 litres (3 to 4 gaUons) ; these figures were obtained 
 from a very large number of observations, and give the extreme 
 size of the organ when distended. The viscus is under the best 
 physiological conditions for digestion when it contains about 
 10 to 12 litres (2 to 2-5 gaUons), or is distended to two-thirds of 
 its capacity. The mucous membrane of the stomach of the 
 horse is peculiar ; one portion of it, practically half, is a continua- 
 
i8o 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 tion of the membrane of the oesophagus; this ends abruptly, and 
 is succeeded by the villous coat, which extends to the pylorus: 
 It is in this latter coat that a true digestive juice is secreted, 
 though not from the entire surface, for on examination of the 
 villous membrane it is found to show great variations in appear T 
 ance, the fundus being channelled, furrowed, and velvety, whilst 
 the pyloric portion is smooth. It is in the fundus only that true 
 gastric juice — viz., pepsin and acid — is secreted; in the smooth 
 pyloric mucous membrane pepsin only is formed. The area of 
 the secreting fundus surface is about i square foot. Fig. 62 
 shows the relative positions of the various parts of the mucous 
 membrane of the stomach of the horse; the drawing accurately 
 
 PYL. 
 
 CARD. 
 
 L.S. 
 
 CUT. 
 
 VIL 
 
 R.S. 
 
 BOU 
 
 FUN D. 
 
 Fig. 62. — Longitudinal Section of the Stomach of the Horse, 
 card., Cardia; fyl., pylorus; l.s., left sac; r.s., right sac; cut., cuticular coat; 
 vil., villous coat; bou., boundary-line between the cuticular and villous 
 portions; fund., fundus of the stomach. The dotted surface indicates the 
 area for the secretion of gastric juice. 
 
 indicates the shape of the stomach, the positions of the inlet and 
 outlet, and the directions and positions of the various areas. In 
 Fig. 63, after Ellenberger and Baum, is indicated the position of 
 the organ in the living animal. 
 
 There is a want of agreement in the matter of stomach nomen- 
 clature. The human physiologist describes the fundus of the 
 stomach as being close to the cardia, whereas we have spoken of 
 it as being at the bottom or on the floor of the stomach, as if it 
 were in the position the name ' fundus ' would assign it.* In 
 Fig. 64 are shown the parts of the human stomach according to 
 modern nomenclature. To the left of the figure is seen the 
 
 * The fundus of an organ is the rounded base of a viscus ending in a 
 neck, and having an aperture. It is the bottom of anything. 
 
DIGESTION 
 
 181 
 
 antrum pylori, marked off by a fissure, IA, and a transverse 
 band for the main body of the organ. The main body is divided 
 into fundus and prepyloric region; it will be observed that the 
 position assigned to the fundus of the human stomach is not that 
 found in the horse. On the other hand, the antrum pylori of 
 the horse is as well 
 marked as in the 
 human subject, pos- 
 sessing both the trans- 
 verse band and the 
 sacculated condition 
 towards the duode- 
 num. 
 
 A very remarkable 
 amount of mucin is 
 secreted by the villous 
 sac of the stomach, 
 and forms over the 
 inner surface of the 
 viscus a thick gelatin- 
 ous, firmly adherent 
 coating like white of 
 egg, which cannot be 
 washed away even by a 
 powerful jet of water. 
 
 The pyloric orifice 
 of the stomach is usu- 
 ally large and open, 
 and there is a distinct 
 pyloric ring ; behind 
 this the duodenum is Fig 
 dilated, and the gut 
 comports itself in such g 
 a singular manner 
 (which has a very 
 important bearing on 
 the pathology of the 
 organ) that mention 
 must be made of it 
 
 here. From the pylorus . ,;„.,.. 
 
 the duodenum curves down and then up again, forming the letter 
 U • so much does this remind one of a well-known form of trap 
 used in drainage that we have described it as the syphon trap of 
 the duodenum (Fig. 65). The use of this trap appears to be to 
 regulate the passage of material from the stomach into the 
 Sines. The writer's observations have shown that its pres- 
 
 5 , _ Position of the Stomach of the 
 Horse in the Living Animal (after Ellen- 
 berger and baum). 
 Vertical transverse section of the frozen body 
 made through the fourteenth dorsal vertebra, 
 showing the position occupied by the stomach; 
 c oesophagus; d, duodenum; a, cuticular area; 
 b villous area. As a perfectly vertical section 
 would not give a complete picture of the 
 stomach, owing to its obliquity, the above has 
 been schematically amended. The observer is 
 looking towards the animal's head. 
 
1 82 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 ence in all probability influences rupture of the stomach, for the 
 more distended the large bowels become, the greater the pressure 
 exercised on the duodenum, and in cases of severe tympany 
 the passage from the stomach to the intestines is completely cut 
 
 fjr/ori/s 1 V. 
 frfrsverse tenet ^^ 
 
 fv/XZu* 
 
 /rt/-grmeGf/4te 
 
 or />ref>y/ar'C rep'OB 
 
 Fig. 64. — Schematic Figure of the Human Stomach (Howell, after 
 
 Retzius). 
 
 off. Should fermentation still continue in the stomach, the con- 
 tents can neither escape into the oesophagus nor into the bowel, 
 and the coats of the viscus may be completely ruptured under the 
 intense strain. Fig. 63 demonstrates the position the greater 
 curvature of the stomach occupies towards the left ribs. There 
 is no space left into which the stomach can swell. It is held back 
 by the diaphragm, pushed forward by the intestines, bound down 
 
 uo. 
 
 Fig. 65. — Longitudinal Section of the Stomach of the Horse, showing 
 the Syphon Trap of the Duodenum. 
 
 ce, (Esophagus; py., pylorus; d, left sac; v, fundus; duo., duodenum. 
 
 by the ribs, and rupture of the greater curvature follows. It was 
 mentioned on p. 165 that the oesophagus of the horse near its 
 termination changes from red to pale muscle, and for several 
 inches increases enormously in thickness, It is this thickened 
 
DIGESTION 183 
 
 contracted end of the oesophagus which completely seals the 
 stomach anteriorly; nothing can be forced out by this passage, 
 not even after death or under great pressure. 
 
 The physiological points of interest in the structure of the 
 horse's stomach are: (1) That it is small; (2) that it is not in 
 contact with the abdominal wall, but rests on the colon ; (3) that 
 the outlet and inlet are situated close together ; (4) that the cardia 
 is tightly constricted; (5) that only a portion of its surface is 
 capable of secreting a digestive fluid; (6) that there are remark- 
 able differences in the character and nature of the various regions 
 of its mucous membrane. 
 
 We can now consider the stomach digestion of the two chief 
 foods used for horses — viz., hay and oats. 
 
 Digestion of Hay. — Hay, as has been stated, mixes in the 
 mouth with four times its bulk of saliva, and after a very perfect 
 grinding passes into the stomach. If the stomach }\e empty it is 
 of no size, and the material lies in the lesser curvature and pyloric 
 region; as the viscus gradually fills, the greater curvature is 
 occupied, the gastric juice begins to act, and chyme commences 
 to pass into the intestines probably in a very imperfectly elabor- 
 ated form. Let us assume the animal to have finished eating 
 hay ; we find the output into the intestine now becomes small and 
 slow. The gastric juice has an opportunity of acting more thor- 
 oughly upon the ingesta, which turn yellow on that surface which 
 is in contact with the villous wall, the compression of the stomach 
 on the contents causing them to become distinctly moulded into a 
 mass the shape of the viscus. Owing to gravity there is more 
 fluid towards the pylorus than elsewhere, and for the same reason 
 the greater curvature in all probability is fuller than the lesser. 
 The material in the stomach is perfectly comminuted, resembles 
 firm green and yellow faeces, and the smell is peculiar, like that 
 of sour tobacco. The yellowness is due to the gastric juice, and 
 ,is consequently more marked towards the pylorus; the portion 
 coloured green is the part as yet unacted upon by the juice. The 
 entire surface of the stomach and its contents are now acid, 
 excepting at the cardia, where they may occasionally be alkaline 
 from swallowed saliva ; the acidity is greater at the fundus than 
 at the cardia. This general acidity shows that a diffusion of the 
 gastric juice must have been going on. There is no evidence of 
 any churning motion ; the cake-like condition into which the hay 
 is compressed is produced by the simple compression of the 
 stomach walls. 
 
 The duration of hay-digestion in the stomach is very .variable. 
 In examining a series of digestions hour by hour, such as will 
 be mentioned presently, moderately uniform results may be 
 obtained, but even these are sure to be here and there broken by 
 
1 84 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 excessive, or conversely by very small, digestions which cannot 
 be explained. 
 
 No one can say with any degree of certainty how much hay a 
 horse will digest in any given period, as may be seen from the 
 following observations: 
 
 Amount digested . 
 
 per Cent. * 
 
 After 4! hours' digestion - - - - - 17 
 
 »» 5 •» ,. .... 66 
 
 >» 5i » .» ..... 64 
 
 ..6 » „ ..... 44 
 
 1. 9 .... ..... 58 
 
 It is known, however, that as a rule more will be digested in the 
 first hour, less in the next, still less in the third, and so on until 
 the stomach empties itself, or until what remains in the stomach 
 is actually pushed out by the arrival of the next feed. Assuming, 
 however, that the animal is purposely starved, the stomach may 
 not empty itself for fifteen, eighteen, twenty-four, or even thirty- 
 six hours. It is impossible to say, either at the end of twelve 
 or of thirty-six hours, that the stomach has passed the whole of 
 its contents into the intestine. The writer has found hay in the 
 stomach fifteen and eighteen hours after being given, and under 
 identical experimental conditions the stomach at the fifteenth 
 hour has been found empty. 
 
 Colin's elaborate researches furnish very complete data on 
 the question of hay digestion in the horse. In one experiment 
 carried out on fourteen animals he divided them into two groups, 
 each horse receiving 2-5 kilogrammes (5-5 pounds) hay. One 
 group had long, the other chaffed hay. The following table 
 shows the percentage digested at each period : 
 
 
 Long Hay. 
 
 Chatted Hay. 
 
 
 
 Per Cent. 
 
 Per Cent. 
 
 Amount 
 
 digested at second hour 
 
 6l 
 
 56 
 
 
 ,, third hour - 
 
 69 
 
 77 
 
 
 „ fourth hour - 
 
 73 
 
 64 
 
 
 „ fifth hour 
 
 78 
 
 91 
 
 
 „ sixth hour 
 
 74 
 
 83 
 
 
 seventh hour 
 
 73 
 
 79 
 
 »» 
 
 „ eighth hour- 
 
 88 
 
 81' 
 
 This shows that the rate of digestion falls off after the second 
 hour, so that even at the end of eight hours there is still food left 
 in the stomach. Only one animal digested as much as go per 
 cent., and this was at the fifth hour. With this exception the 
 results are fairly uniform, and they further demonstrate that 
 chaffing hay does not increase its rate of digestion. 
 
DIGESTION 
 
 i8 b 
 
 The influence of water on the digestion of ' hay was tested 
 by Colin. The following table shows the results, and also 
 furnishes detailed information, hour by hour, of the rate of 
 stomach digestion: 
 
 Duration of 
 Digestion. 
 
 Hay without 
 
 drinking Water 
 
 (per Cent, digested). 
 
 Duration of 
 Digestion. 
 
 Hay with drinking Water 
 (per Cent, digested). 
 
 Hours. 
 
 
 Hours, 
 
 
 
 i 
 
 3° 
 
 1* 
 
 4 1 
 
 mean of 2 experiments^ j 
 
 i 
 
 45 
 ,f 37 (mean of") 
 
 2 
 
 4 b 
 
 >> 4 »> 
 
 2 
 
 -J 4 experi- J- 
 .1 ments) J 
 
 3 
 
 bi 
 
 >> 2 ,, 
 
 3 
 
 52 
 
 4 
 
 (34 
 
 ji 5 »» 
 
 5 
 
 77 
 
 5 
 
 88 
 
 3 
 
 6 
 
 66 
 
 6 
 
 79 
 
 3 
 
 7 
 
 6 4 
 
 7 
 
 «7 
 
 
 8 
 
 72 
 
 9 
 
 82 
 
 ,, 2 ,, 
 
 10' 
 
 83 
 
 10 
 
 92 
 
 2 
 
 ii 
 
 95 
 
 11 
 
 95 
 
 
 12 
 
 95 
 
 12 
 
 «5 
 
 »> 2 •• 
 
 14 
 
 96 
 
 13 
 
 92 
 
 
 18 
 
 96 
 
 J 4 
 15 
 16 
 
 94 
 97 
 98 
 
 
 On the whole, during the earlier hours of digestion the horses 
 receiving water digested hay better than those kept without it, 
 but there is no difference after the ninth or tenth hour succeeding 
 feeding. 
 
 Digestion of Oats.— Oats take up their own weight of saliva, 
 and during digestion behave much as does hay— viz., while the 
 animal is feeding the contents of the stomach begin very early 
 to pass into the intestine, but the rate is considerably reduced 
 the moment no more material arrives in the organ. 
 
 Colin 'fed six horses on 2-5 kilogrammes (5-5 pounds) oats; 
 three horses received uncrushed, three crushed corn, and the 
 following table shows the percentage digested: 
 
 Duration of Digestion. 
 
 2 hours 
 
 4 » 
 6 „ 
 
 Crushed Corn 
 (per Cent, digested). 
 
 48 
 56 
 62 
 
 Uncrushed Corn 
 (per Cent, digested). 
 
 45 
 61 
 
 54 
 
 The crushing of the corn made no difference to the rate of diges- 
 tion— a point of practical importance in dietetics. Comparing 
 
186 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 this table with the one on p. 184, it would appear that hay is 
 easier of digestion than oats. 
 
 The digestion of oats in individual horses is just as irregular 
 as that of hay. A horse to which only 2 pounds of oats /were 
 given was destroyed twenty hours later, and the stomach was 
 not completely empty. In another to which 1 pound had been 
 given 6 ounces were recovered after four hours' digestion. In the 
 following table the results of some experimental feedings are 
 recorded : 
 
 Amount digested 
 per Cent. 
 
 3 hours - 45 "O 
 
 3 » - - 57-o 
 
 3 ,, - 70-0 
 
 3 .. 78-6 
 
 These horses were fed under similar conditions, yet there is a 
 good deal of variation in the amounts digested. In the next 
 table the period of observation was increased in order to see 
 whether greater uniformity would result : 
 
 Amount digested 
 per Cent. 
 
 4 hours Nil 
 4 ., 18-0 
 4 -. - 54-5 
 4 .. - 5°-o 
 4 .< 75-o 
 
 These results are less regular than those of the first series; 
 one horse digested nothing, and that is explained by a practical 
 fact capable of being turned to clinical account. The animal 
 was of a very nervous disposition, and the experiment was 
 carried out in a strange stable, the mare being alone, whereas 
 she was used to the company of Other animals. The second 
 horse in this series digested only 18 per cent. ; this also illustrates 
 a practical point in feeding — namely, the influence of a sudden 
 change in diet. This animal had not received oats for eighteen 
 months, having been fed on a patent food; the sudden change 
 to oats for the experimental observation explains why only 
 18 per cent, was digested. 
 
 Arrangement of Food in the Stomach.- — If a horse be fed on 
 oats, maize, hay, etc., in succession, the stomach, on examina- 
 tion, will be found to contain these substances quite unmixed, 
 and arranged in strata in the order of their arrival, the first food 
 being in the pylorus and greater curvature, the last in the cardia 
 and lesser curvature. We have pointed out that the contents of 
 the stomach of the horse are squeezed and pressed, but never 
 churned. In successive feeding one material never mixes with 
 another, excepting at the pylorus, but keeps together from the 
 
DIGESTION 187 
 
 time of its arrival until it passes out of the stomach, a sharp line 
 of demarcation distinguishing it from its neighbour. If the 
 stomach be empty or greatly contracted, the first food to arrive 
 is lodged in the lesser curvature and pylorus; gradually, as new 
 material arrives, this passes over to the greater curvature in 
 order to make room for itself. If a horse be fed with hay, fol- 
 lowed by oats, the presence of the oats causes the hay to pass out 
 more rapidly than it would have done had it been given alone. 
 
 Ellenberger has shown that when hay and oats are given in 
 the order named, a portion of the oats may pass into the bowel 
 by the lesser curvature without entering either the left sac or 
 fundus of the stomach (see Fig. 66, I.). When oats followed by 
 hay are given, the oats, as the first arrival, naturally commence 
 to pass out first, but the presence of the hay hurries the rate of 
 progress, and the oats pass more quickly into the intestines 
 than they otherwise would do. The regular arrangement of food 
 in layers is disturbed when a horse is watered after feeding; 
 half the food may in this way be washed out of the stomach, 
 for the water which a horse drinks does not stop in the stomach, 
 but passes directly through it on its way to the caacum. Hence 
 we have the golden rule of experience that horses should be 
 watered first and fed afterwards. 
 
 These facts may be summarised by saying that in a succession 
 of foods the first consumed is the first to pass out. That is 
 not to say that the whole of it passes out before any portion of 
 the succeeding food enters the bowel, for we have shown that 
 after a time, at the pylorus, they mix and pass out together; 
 but the actual influence of giving a food first is to cause it to pass 
 out first. The practical application of this fact, according to 
 Ellenberger, is that when foods are given in succession, the least 
 albuminous should be given first. This appears distinctly to 
 reverse the English practice of giving oats first and hay after- 
 wards, but perhaps only apparently so, for experiment shows 
 that the longer digestion is prolonged, the more oats and the less 
 hay pass out, so that some hay (under ordinary circumstances 
 a moderate quantity) is always left in the stomach until the 
 commencement of the next meal. The presence of this hay 
 from the previous feed may prevent the corn of the succeeding 
 meal from passing out too early. According to Ellenberger, in 
 order that horses may obtain the fullest possible nutriment from 
 their oats, hay should be given first, and then water; this carries 
 some of the hay into the bowel, and after a time the oats are 
 given. The remaining hay now passes into the bowel, and the 
 oats remain in the stomach . This does not accord with English 
 views of watering and feeding fast-working horses, views which 
 have stood the test of prolonged practical experience, 
 
1 88 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The appearance of the food after it has been in the stomach 
 depends upon the period of digestion. We have previously, 
 drawn attention to the fact that an hour or two after hay has. 
 
 III. 
 
 Fig. 66.— Longitudinal Section of the Horse's Stomach, showing the 
 Arrangement of the Food according to the Order in which it was 
 received (Ellenberger). 
 
 -Iii each. case a> is the oesophagus; py., pylorus; d, the left sac; v, the fundus. 
 I. Hay first, followed by oats: b, the hay; a, the oats; the latter are passing 
 along the lesser curvature and escaping with the hay at the pylorus. II. Oats 
 first, followed by hay,: a, theoats; b, the hay. III. The order of three sue 
 cessive feeds: c, the first feed; 6, the second; a, the third. 
 
 been taken the material is found in a finely-chopped condition, 
 .firm— one may almost say dry— in places, though towards the ' 
 jpylorus it is liquid. This hay contains between four and five 
 parts of saliva; it is yellow in colour where the gastric juice has 
 
DIGESTION 189 
 
 attacked it, but of rather a greenish tint elsewhere, and it has 
 a peculiar odour. Several hours after feeding, the stomach is 
 found to contain a variable quantity of watery fluid discoloured 
 by the hay which is left behind, part of which may be found 
 floating on the fluid. At other times, when the stomach is 
 empty, the fluid is viscid, contains numerous gas bubbles, and 
 is of an amber or yellow tint ; this particular fluid is no doubt 
 saliva and mucin, with possibly a little bile, the result of a 
 reflux from the bowel. When oats alone have been given, the 
 contents of the stomach are found to be liquid, the fluid being 
 creamy in consistency and colour; the oats are swollen, soft, with 
 the interior exposed; towards the end of digestion the creamy 
 fluid is replaced by the frothy yellow one. With both hay and 
 oats, and also other foods, there is a peculiar sour-milk-like 
 smell from the contents of the stomach, more marked with bran 
 and oats than with hay, the latter, as previously mentioned, 
 smelling like sour tobacco. 
 
 The reaction of the contents of the stomach is strongly acid ; 
 this acid reaction may be obtained on the cuticular as well as 
 on the villous portion of the lining, and is very persistent; the 
 cuticular membrane, even after prolonged washing, gives an 
 acid reaction. The acidity is derived entirely from the juice 
 secreted by the villous membrane of the fundus. Our observa- 
 tions on this subject do not agree with those of Ellenberger, 
 who says that during the first hour of digestion the contents of 
 the stomach may be alkaline; acidity, he states, then commences 
 in the fundus and extends to the cardia, though for some time 
 the fundus acidity is three or four times greater than that of the 
 cardia; in the course of five or six hours the proportion of acid 
 throughout the stomach is equal. When the stomach is empty, 
 as after a few days' starvation, its reaction is neutral or alkaline. 
 We have observed extreme alkalinity towards the pylorus under 
 these conditions, due, no doubt, to the. regurgitation of bile and 
 pancreatic fluid. 
 
 Alkalinity of the contents may be met with at the cardiac end 
 of the stomach; in such cases it is due to the swallowed saliva, 
 and the stomach contents at this part exhibit a marked sugar 
 reaction. It is difficult to say whether this alkalinity is invari- 
 able; in the writer's experience it has seldom been met with; 
 but the stage of digestion may explain it, for it is easy to under- 
 stand that during the early period of digestion the stomach 
 acids will not have penetrated to the upper part of the stomach, 
 and in this way have neutralised the immense quantity of sali- 
 vated food of an alkaline reaction which is swallowed. 
 
 The Stomach Acids. — It is not necessary here to enter into 
 any detail as to the nature of the gastric acids; a considerable 
 
igo A MANUAL OF VETERINARY PHYSIOLOGY 
 
 amount has been written to prove that the acidity, both in the 
 horse and man, depends upon lactic or upon hydrochloric acid, 
 and it is possible that both these views may be reconciled. Ellen- 
 be'rger and Hofmeister are of opinion that shortly after a meal 
 lactic acid predominates in the horse's stomach, to be replaced ' 
 by hydrochloric acid some four or five hours after the commence- 
 ment of feeding. These observers found that the nature of the 
 acid depended upon the region of the stomach, the length of 
 digestion, and the character of the food; oats induced an out- 
 pouring of hydrochloric acid, whilst hay favoured the organic 
 acids. 
 
 The following are Ellenberger's views on the nature of the 
 stomach acids: in the contents of the stomach, hydrochloric, 
 lactic, butyric, and acetic acids may be found, the two latter in 
 insignificant quantities only. In flesh-feeders HC1 predominates, 
 and lactic acid is found in small quantities. In vegetable feeders 
 lactic acid at first predominates, and later HC1 is present in small 
 quantities ; lactic acid exists throughout the whole stomach, but 
 predominates in the right and left sacs, whilst hydrochloric 
 acid principally exists in the fundus region. Lactic is the first 
 digestive acid employed, but towards the end of a long digestion 
 hydrochloric exists throughout the whole stomach. The amount 
 of lactic acid found in the stomach of the horse during the first 
 hours of digestion is considerable. 
 
 It has been suggested that the presence of lactic acid in the 
 stomach of herbivora may be due to the fermentation of the 
 carbohydrates of the food, and this cannot be rejected as a con- 
 tributory cause. In the dog, in which animal the acid is doubt- 
 less hydrochloric, it is well known that no free HC1 may be 
 obtained for the first hour or two of digestion, owing to the acid 
 combining with the protein of the food, and to the diminution it 
 undergoes in neutralising the alkali of the saliva. 
 
 The writer's experience regarding the presence of hydrochloric 
 acid and organic acids in the stomach contents is that, no matter 
 at what period of digestion observations have been made, on 
 only two or three occasions has he succeeded in finding hydro- 
 chloric acid in the stomach of the horse, and he is convinced that 
 lactic is the chief, if not the sole, digestive acid in this animal. 
 
 The Secretion of Gastric Juice is accomplished in certain 
 glands known as the gastric. In man these are divided into 
 cardiac and pyloric, each having not only a different structure, 
 but a separate function. In the horse cardiac glands are impos- 
 sible owing to the presence of the cuticular coat ; but it has been 
 shown that the villous coat contains glands corresponding to 
 cardiac, which are principally situated in the greater curvature, 
 at the fundus of the stomach, and extend over a limited area, 
 
DIGESTION 
 
 191 
 
 described on p. 180 as not larger than 1 square foot (Fig. 62). 
 The two kinds of gland employed in the production of gastric 
 juice are both found in the villous coat, the one in the fundus, 
 the other in the pyloric portion, though Ellenberger states that 
 he has found fundus glands in the pyloric region. They are 
 simple or divided tubes lying side by side, and opening, generally 
 in groups, on the surface of the mucous membrane by means of 
 
 Duct. 
 
 Chief Cells. 
 
 -Parietal Cells. 
 'Cland. 
 
 Gland. 
 
 Pvlokic Gland. Cardiac or Fundus Gland. 
 
 Fig. 67.— The Gastric Glands after Heidenhain (Waller). 
 
 a shallow depression in the coat. These depressions can readily 
 be seen studded over the tunic of the fundus, giving it a rough 
 appearance owing to the elevation of the mucous membrane 
 between the openings of the glands; in the pyloric region the 
 membrane is as smooth as that found in the intestine. Each 
 gland consists of a body, neck, and mouth, and is lined with 
 cells; it is in respect of the cellular contents that the pyloric 
 and fundus glands differ. 
 
192 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The cells of the fundus gland (Fig. 67) are small, polyhedral, 
 granular, and nucleated, and line the lumen of the gland; they 
 are called the principal, central, or chief cells. Scattered amongst 
 the principal cells, but existing in larger numbers at the neck 
 of the gland than at its base, are found certain large cells (oval, 
 granular, and nucleated), which from their position relative to 
 the lumen of the gland are called parietal, marginal, or border cells. 
 These cells are distinctive of the fundus glands, and they stain 
 readily with. aniline blue. 
 
 The pyloric gland (Fig. 67) below its neck has but one variety 
 of cell — -viz.,; the cylindrical — containing a nucleus at its attached 
 edge. The duct is lined, above the neck, by the ordinary epi- 
 thelium of the stomach, and the same remark applies to the 
 fundus glands; it is from this epithelium that the mucus is 
 secreted. The important distinction between the fundus gland 
 with principal and parietal cells, and the pyloric gland with 
 principal cells only, is that the former, since the acid is separated 
 from the blood by parietal cells, whilst pepsin only is formed by 
 principal cells, secretes both the pepsin and the acid of the gastric 
 juice, whereas the pyloric glands, made up of principal cells, 
 secrete only pepsin. 
 
 We have previously mentioned that the cells of the salivary 
 glands undergo certain changes in appearance, as the results of 
 rest and of activity; the same remark applies to the gastric 
 follicles, in which the general type of change during secretory ; " 
 activity is very closely allied to those already described for the 
 salivary glands. Langley has found that in the active state the 
 granules decrease in number, the cell becoming clear, and capable 
 of differentiation into a clear outer and a granular inner zone, 
 just as we have seen in the parotid gland; during rest the entire 
 cell becomes granular. The parietal cells during digestion were 
 found to increase in size, but characteristically did not- lose their 
 granules. The central cells secrete both the pepsin and rennin 
 ferments, neither of which exist as such in the cells, but as the 
 mother-substances or zymogens of the ferments. The formation 
 by the parietal cells of a free acid from the alkaline blood is a 
 special chemical change, the result of selective powers possessed 
 by the cells. In animals, such as the dog, which yield hydro- 
 chloric acid, the cells very possibly form it by the interaction of 
 the sodium chloride and the sodium dihydrogen phosphate of 
 the blood; but no satisfactory explanation of how the neutral 
 chlorides are broken up with the formation of hydrochloric 
 acid has yet been given. 
 
 Mucin is secreted by mucous glands found in the deep layers 
 of the villous membrane, especially in the region of the fundus; 
 the epithelial cells lining the excretory ducts of the gastric glands 
 
DIGESTION 193 
 
 also take part in the process. The amount of mucin formed in 
 the stomach of the horse is remarkable ; it adheres to the villous 
 coat like unboiled white of egg, and cannot be washed away, 
 even by a powerful jet of water. The amount secreted is un- 
 known, but must be considerable; less is formed during hunger 
 than during activity, and there is less in ruminants than in 
 horses. 
 
 Gastric Juice. — -It is only lately that a pure sample of gastric 
 juice (but not from the horse) has been available for analysis. 
 Most of the secretions previously examined have been mixtures 
 of saliva, gastric juice, and perhaps other substances. Pavlov 
 devised a method by which the stomach of the dog could be 
 rendered available for physiological inquiry, and a pure secretion 
 was obtained (see Figs. 68 and 69). 
 
 Py lorus ^S K \{W / /Oesop hagus 
 
 flatus gastncusS v\/~ Al / Plexus gastrieus 
 
 anunor vegj ~V ^— — -«^k / poster ,lr Jag" 
 
 Fig. 68. — Pavlov's Stomach Pouch. 
 A, B, Line of incision; C, flap for forming the stomach pouch. At the base of 
 the flap the serous and muscular coats are preserved, and only the mucous 
 membrane divided, so that the branches of the vagus going to the pouch are 
 not severed. 
 
 Pure gastric juice in the dog is as colourless as water, and 
 possesses a specific gravity of 1-002 to 1-004; it is thin— that 
 is to say, is not mucinous — and of a strongly acid reaction. 
 Chemically it consists of acid and enzymes ; it contains 0-4277 per 
 cent, of dried material, of which 0-1325 is ash. The ash con- 
 tains 24 per cent, potassium, 19 per cent, sodium, and 0-18 per 
 cent, calcium. The acid, which is hydrochloric, amounts to 
 0-46 to 0-56 per cent., gastric juice being the only secretion in 
 the body containing a free acid. Pavlov states that the total 
 chlorine contents of the secretion are twice the chlorine found in 
 blood, and that he has obtained 5 grammes chlorine from the 
 secretion in 3^ hours — as much as in the entire blood. There 
 
 13 
 
194 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 are two, perhaps three, enzymes present in the juice — viz., 
 pepsin, rennin, and possibly lipase ; the first is unable to act, 
 excepting in an acid medium, and furnishes the only example 
 in the body of this necessary combination of acid and enzyme. 
 Exclusive of the ferments, protein is present . How far the gastric 
 juice of other animals resembles in its composition that of the 
 dog we have no means of knowing, inasmuch as a pure secretion 
 has not been obtained from any other animal; but in all, cases 
 an acid and an enzyme are present. The enzyme is invariably" 
 pepsin, but the acid is not always hydrochloric. The amount of 
 
 Ser osa 
 Mus cularis 
 
 Mucosa 
 
 sy 
 
 Fig. 69. — Pavlov's Stomach Pouch. 
 ,S, The completed pouch; V, cavity of the stomach; A, A, the abdominal wall 
 
 juice secreted is uncertain; in the dog some 700 c.c. (24^ ounces) 
 have been collected in a few hours, from which we may perhaps 
 imagine that a considerable amount is formed in the stomach' 
 of the larger animals. 
 
 Pavlov's experiments on the dog show that the amount of 
 juice secreted is directly proportional to the amount of foodi 
 eaten; for example, 100 grammes of meat required 26 c.c. of 
 juice, and 400 grammes needed 106 c.c. The same fact was 
 observed on a mixed diet of meat, bread, and milk ; if these were 
 increased by half or doubled, the juice was increased in the same 
 ratios. 
 
DIGESTION 195 
 
 The gastric juice of the dog withstands putrefaction for a long 
 time; it may be kept for months without undergoing any im- 
 portant change. Not so that of herbivora; the mixed gastric 
 fluids of the horse rapidly putrefy. The antiseptic properties 
 of the dog's juice are attributed to its hydrochloric acid; if this 
 is so, it is additional evidence against the stomach acid of her- 
 bivora being hydrochloric. There appears to be no reason why 
 lactic acid should not be formed in herbivora by the marginal 
 cells of the fundus glands, but a source of lactic supply is the 
 carbohydrate of their food. 
 
 Pepsin is of a protein nature, though very little is known of it 
 chemically. It exhibits its action best at the temperature of 
 the interior of the body (37° to 40 C.) ; a low temperature retards 
 its activity, while a high one destroys it. The ordinary com- 
 mercial product is very impure; it is an extract of the mucous 
 membrane of the stomach to which starch or milk sugar has been 
 added. In physiological work a glycerin extract of the mucous 
 membrane of the stomach suffices; to it is added some dilute 
 HC1. Glycerin has the power of extracting the ferments both 
 from the stomach and from other portions of the digestive tract, 
 such as the pancreas. The pepsin is formed, as we have seen, 
 in the chief cells of the gastric glands as a zymogen or propepsin, 
 which becomes pepsin after secretion. The action of pepsin is 
 almost wholly, if not entirely, confined to the protein constituents 
 of food. It converts the insoluble proteins into soluble ones; 
 not by direct transformation, but by several stages. The pro- 
 ducts intermediate between protein and peptone have received 
 certain names suggestive of differences in their chemical nature, 
 but as to the exact manner and character of these changes a good 
 deal of doubt and of speculation exists. 
 
 In the following table the various stages of conversion are 
 indicated in the order in which they are found to occur as deter- 
 mined by slight differences, such as solubility or colour reaction, 
 in the peptonised product, as shown by chemical tests. The table 
 is the one drawn up by Kuhnei: 
 
 1. The protein as consumed, or native_ albumin. 
 
 2. Acid albumin or s'yntonin. 
 
 3. Primary proteoses. 
 
 4. Secondary proteoses. 
 
 5. Peptones. 
 
 The proteins, having reached the stage of peptones, are now 
 capable of being absorbed. But the conversion from protein to 
 peptone is a most complex one, during which the large protein 
 molecule is converted into simpler products of an infinitely 
 smaller molecular weight, while so great is the complexity that 
 
196 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the resulting product, peptone, is in all probability not a single 
 compound, but rather a group of compounds, which resemble 
 each other only in their solubility and their definite reaction to 
 certain chemical tests. There is good reason for thinking that 
 even peptone may not represent the final end-product of the 
 stomach, but that still simpler bodies, to which the name 
 peptids has been given, may be formed. 
 
 Rennin is the second enzyme found in the gastric juice; it is 
 produced in the chief cells of the gastric glands as a prorennin, 
 and is subsequently, on secretion, changed into rennin under the 
 influence 'of the acid present. This enzyme can be readily ob- 
 tained from the true stomach of the calf, but not from the pyloric 
 end. Commercially, rennin is used in the manufacture of cheese, 
 a good extract causing very rapid clotting of milk. This process 
 closely resembles the clotting of blood, and, like it, is followed by 
 a contraction of the clot, resulting in a solid substance known as 
 curds, and a yellowish fluid, whey. The above process further 
 resembles blood-clotting in requiring the presence of a calcium 
 salt. The conversion of the casein of milk into curd is believed 
 to involve two distinct processes: first, the formation of a sub- 
 stance known as paracasein by the action of rennin; and, 
 secondly, the action of lime salts on paracasein, by which means 
 the curd is produced. The curdling of milk brought about by 
 the lactic acid organism may be familiarly recognised when milk 
 ' turns sour ' ; it is not the same as clotting, but represents the 
 precipitation of casein by an acid. 
 
 There would appear to be no necessity for animals to possess 
 this ferment after weaning, as milk does not form an article of 
 diet for animals, if we except the chemically altered milk given 
 to the pig. Once the curd is formed, the process of digestion is 
 carried on by pepsin, rennin taking no further part. 
 
 Rennin has been described as existing in the pancreas, testis, 
 and in vegetable tissues, but little is definitely known. It is 
 stated that an antirennin may be formed in the blood by im- 
 munising an animal by injections of rennin, and that the sub- 
 stance produced prevents milk clotting. 
 
 Other ferment actions of the gastric juice have been described,, 
 such as fat- and starch-splitting. A fat-splitting ferment, allied 
 to pancreatic lipase, may, it is said, be extracted from the gastric^ 
 mucous membrane of young animals, but of the existence of 
 this ferment there is very little evidence. Protein digestion is 
 the essential duty of the stomach, but in all vegetable feeders 
 maceration of the vegetable fibres is begun also in the stomach 
 as a preliminary measure. A stomach is not essential to life in 
 all animals ; in the dog, for example, it may be removed experi- 
 mentally, for, as we shall see later on, protein digestion is provided 
 
DIGESTION 197 
 
 for elsewhere. But in the herbivora, especially in ruminants, a 
 stomach is essential. The chief value of the stomach in those 
 animals which can be proved to live without it lies in the prepara- 
 tion of the food for subsequent digestion in the small intestines, 
 for it is quite undoubted that protein previously acted upon 
 by gastric juice is far more thoroughly treated by the pancreatic 
 fluid than protein not so previously acted upon. 
 
 Influence of the Nervous System on the Secretion of Gastric 
 Juice. — -By means of Pavlov's fistula, it has been proved that the 
 secretion of gastric juice is under the control of the nervous 
 system, the secretory fibres being contained in the vagus. 
 Stimulation of the peripheral end of the divided nerve causes 
 secretion after a slight delay; the cause of the latent period is 
 unknown. No secretion results in consequence of mechanical 
 stimulation of the stomach wall, which is contrary to all pre- 
 viously accepted views. The sight of food, its smell, taste, 
 mastication, and swallowing, are direct excitants of secretion. 
 A secretion so obtained is known as psychical. Pavlov's experi- 
 ment of ' sham feeding ' a dog with a divided oesophagus, the 
 upper end of which was brought outside, has placed this psychical 
 secretion beyond doubt. The animal ate a meal which never 
 entered the stomach, but the effect of which could be ascertained 
 through the fistula. In addition to this reflex secretion, there 
 is another produced by the action of some of the food substances 
 themselves; such substances, known as secretagogues, directly 
 stimulate the production of gastric juice/though all foods do not 
 possess this power. Bread, starch, fat, and white of eggs, intro- 
 duced surreptitiously into a Pavlov's fistula so that the animal 
 does not see them, do not excite secretion ; while, on the contrary, 
 meat extract is most effective. 
 
 Secretagogues in the products of digestion may also influence 
 the production of gastric juice. This chemical secretion is 
 difficult to explain; it occurs after the vagus and all other nervous 
 connections are cut, which suggests that it does not operate 
 through a nervous reflex. It has been shown by Edkins that 
 the injection into the blood of an extract of pyloric mucous 
 membrane, specially prepared, causes a marked secretion of 
 gastric .juice. And it is suggested that the chemical secretion 
 above referred to may be produced by the action of the secreta- 
 gogues upon the pyloric mucous membrane, resulting in the 
 formation of a gastric secretin, or gastrin (analogous to pan- 
 creatic secretin, yet to be studied), which, when absorbed into 
 the blood, acts as a chemical excitant, and stimulates the pro- 
 duction of gastric juice. The secretins belong to the class of 
 bodies now referred to as hormones . 
 
 Pavlov believed that -the quantity and quality of the gastric 
 
198 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 juice will be found to depend on the character of the food, so 
 that while in some cases an average production is obtained, 
 in others a stronger or weaker fluid is poured out, depending upon 
 the work to be done, the regulation of which is probably a specific 
 action on the part of the food itself. 
 
 Each article of diet in a meal of meat, bread, and milk, taken 
 separately, produces not only a definite rate of secretion of gastric 
 juice, but also an alteration in the quality of the ferment suited 
 to the work it has to perform. Thus flesh or bread causes a 
 maximum secretion during the first hour of digestion; milk 
 during the second or third hour. The greatest digestive powers 
 of the gastric juice are found to occur with a meat diet, while 
 the weakest proteolytic action occurs with milk. 
 
 If an animal fed for weeks on a bread-and-milk diet be sud- 
 denly placed on meat, the power of dealing with protein is at 
 first weak; the juice is in sympathy with a starchy rather than 
 with a protein diet. If another change be made when the meat 
 diet is being satisfactorily dealt with, and the animal put back on 
 bread and milk, the whole process has to be reversed, and the 
 glands this time brought into accord with a starchy rather than 
 with a protein diet. 
 
 It is true this observation has been made on the pancreatic 
 juice, but doubtless what is true for it is true for other digestive 
 secretions. Further, we have overwhelming proof in everyday 
 management of horses that sudden changes in diet are productive 
 of disease. Whether the above facts regarding digestive juices 
 and changes in diet are explained by what we know of the function 
 of chemical excitants — ' hormones ' — remains to be proved. 
 
 Such is the modern account of stomach digestion. We appear 
 to have approached appreciably nearer to a better understanding 
 of the circumstances attending digestive troubles in the horse, 
 and, as we shall have to point out again in dealing with pan- 
 creatic secretion, Pavlov's work explains why a sudden change 
 of a long-continued diet is bad, and is, as we know, followed in 
 horses by disastrous consequences. 
 
 Starch Conversion. — There are other changes occurring in the 
 stomach independently of peptonisation or of gastric juice. If a 
 horse be fed on oats and the stomach fluid examined, it will be 
 found to contain an abundance of sugar. The sugar, we suggest, 
 is produced from an enzyme in the starch of the grain, and is not, 
 according to our observations, caused by the action of saliva. 
 Abundant saliva exists in the stomach, but it will be remem- 
 bered that in the horse we have never succeeded in getting it to 
 give any evidence of starch conversion. The question, therefore, 
 is, What is the cause oi this formation of sugar ? Pitchford's 
 observations prove that in some horses it may be due to the 
 
DIGESTION 199 
 
 saliva, but in those whose secretion is non-amylolytic a ferment 
 in the grain explains the observed facts, and a starch converting 
 enzyme has been shown to exist in cereals. The whole of the 
 starch is not converted in the stomach, for distinct evidence of 
 unaltered starch can be obtained in the first portion of the smaU 
 intestines. Further, some of the starch is no doubt converted 
 into lactic acid, and the presence of this acid in the proportion of 
 2 per. cent, does not in any way inhibit the amylolytic action. 
 If oats provide their own starch-converting enzyme, we have here 
 the strongest argument against giving boiled food for horses — a 
 practice we believe to be deleterious, or even dangerous. 
 
 Fats are not acted upon in the stomach, though the envelope 
 surrounding the fat globule is digested, and the fat set free. 
 
 Cellulose fermentation is considered by Tappeiner to occur in 
 the left sac of the stomach, and when marsh-gas has been found 
 in this organ, it is the result of cellulose decomposition. Brown* 
 has shown that the destruction of the cell wall of oats and barley 
 occurs in the stomach, where the wall is dissolved by a cyto- 
 hydrolytic ferment pre-existent in the grain , the changes occur 
 with extraordinary rapidity in the stomach of the horse. The 
 researches of this observer on a cellulose-dissolving ferment are 
 of the greatest interest to the veterinary physiologist, and of con- 
 siderable practical importance. 
 
 Periods of Stomach Digestion in the Horse. — Stomach 
 digestion in the horse has been divided by Ellenberger and Hof- 
 meister into certain periods corresponding to definite chemical 
 changes in the food. For example, it is said that during the two 
 first periods, which between them last two and three hours, 
 starch conversion, lactic acid fermentation, and protein conver- 
 sion to a limited extent take place. In the third period mixed 
 digestion of starch and protein occurs, while in the fourth and 
 last period only protein digestion takes place. The third and 
 fourth periods may together last four hours and upwards. We 
 must be careful to avoid regarding the account of these periods 
 as based on some rigid law ; they are very variable in duration, 
 owing to causes we have previously considered, and run imper- 
 ceptibly into each other. With this caution we give the follow- 
 ing periods at which gastric digestion is said by Ellenberger and 
 Hof meister to be at its maximum in the horse : 
 
 After a moderate feed digestion is at its height in 3 or 4 hours, 
 full ,, ,, ,, „ 6 to 8 „ 
 
 an immoderate ,, delayed still longer. 
 
 * ' On the Search for a Cellulose-dissolving Enzyme,' H. T. Brown, 
 F.R.S., Journal of the Chemical Society, 1892, p. 352, 
 
A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Stomach Digestion in Ruminants. 
 
 The stomachs of ruminants are divided into three compart- 
 ments and one true stomach. The compartments are the 
 rumen, reticulum, and omasum. The true stomach is the 
 abomasum (see Fig. 70). 
 
 The Rumen is divided into four sacs by the constrictions 
 produced in its wall by large muscular bands, which on the in- 
 terior of the organ are of immense thickness, and well deserve 
 the name of pillars. The interior of the organ is lined with a 
 well-developed mucous membrane, covered with pointed papillae, 
 3 to 9 mm. in length, excepting where the muscular pillars are 
 most prominent. A few small glands are described as existing 
 in this mucous membrane, but they form no digestive secretion. 
 The rumen communicates freely with the reticulum; by means 
 of the oesophageal groove it is connected with the omasum, and 
 it naturally connects with the oesophagus. Material in the 
 rumen can do one of three things — pass into the reticulum, 
 into the omasum, or into the oesophagus. The way to the 
 omasum and oesophagus is via the oesophageal groove. 
 
 The (Esophageal Groove (see Fig. 71) is a canal which runs 
 from the termination of the oesophagus on the inner wall of the 
 reticulum to the omasum. Guarding it on the aspect presented 
 towards the rumen are two fleshy lips, thick and overlapping 
 below, thin above. These lips approximate so closely that the 
 groove can only be discovered by separating them. The object 
 of the lips is to open and permit the passage of material direct 
 from the oesophagus to the rumen or reticulum, or from these to 
 the oesophagus or omasum. Further details are given on p. 210. 
 
 The capacity of the rumen is enormous : 100 litres (22 gallons) 
 can be stored in it in the ox; 4 to 6 litres (8| gallons) in the 
 sheep. Its muscular bands are arranged in two diameter-like 
 girdles, and their function is very obviously to contract on the 
 contents. All splid food, on being received from the mouth, 
 enters the rumen; all fluid substances may enter any or all of 
 the compartments, or they may proceed to the abomasum 
 via the omasum, as is the case in the calf, or they may go to the 
 rumen or reticulum, as most fluids — viz., saliva and the water 
 consumed— do in the adult. Colin, having placed his hand in 
 the stomach, assured himself, through a window in the abdominal 
 wall, that during the first mastication very little passes to the 
 omasum and abomasum, and that fluid only. Without fluid 
 the rumen can do no work; cut off the supply, and rumination 
 ceases. This explains the necessity for the enormous salivary 
 secretion in these animals. The rule appears to be that the 
 
DIGESTION ioi 
 
 greater part of the fluid arriving in the stomach is divided 
 between the first and second compartments, the overflow from 
 these passing in to the omasum and abomasum. 
 
 As food arrives in the rumen it passes into what Colin has 
 described as the lower storey, a part which never empties itself. 
 It consists of the usual coarse ingesta and fluid. As the stomach 
 fills, the mass extends into the upper storey, the stomach dilating, 
 and finally the reservoir becomes full to the roof, though not 
 tightly packed. The last arrival in the rumen is naturaUy re- 
 ceived in the anterior extremity of the left sac, and from here 
 it passes through all hemispheres of the rumen. Material from 
 the posterior part of the rumen makes its way forwards towards 
 the oesophageal opening for remastication, and there can be no 
 doubt that the mass in these sacs is actuaUy revolved, for material 
 capable of identification, if introduced into the stomach through 
 a wound in the abdominal wall, gradually disappears, and returns 
 to the same place in from twelve to twenty-four hours. 
 
 Colin, through the window in the abdominal wall, was able, 
 by illumination, to inspect the interior of the rumen during diges- 
 tion, and while food was still arriving in the compartment. He 
 observed that the level of the mass varied from moment to 
 moment, rising and falling alternately ; he could even see a portion 
 rise up out of the mass, detach itself, and pass backwards. At 
 •other times the detached portion moved forwards ; in either case 
 it disappeared into the body of the organ, and was churned up 
 again later on. During rumination or when fluids are swal- 
 lowed this oscillation of the mass is most energetic, and, as Colin 
 expresses it, the flow and return flow are most interesting to 
 witness. They induce the most complete mixing of the contents, 
 that which was at the top passing to the bottom, while the 
 material at the bottom passes to the top. Evidence of the 
 completeness of the churning movements of the rumen is afforded 
 by the perfectly spherical ' hair balls ' which are sometimes 
 found there. Stimulation of the mucous membrane of the rumen 
 provokes contraction of the walls, especially of the muscular 
 pillars. 
 
 The contents of the rumen are alkaline generally, more 
 markedly so in the lower than in the upper parts, where, in fact, 
 a slightly acid reaction may be obtained from food fermentation. 
 Very frequently the nature of the reaction is doubtful. The alka- 
 line reaction is due to the saliva, for there is no secretion from 
 the wall of the rumen. 
 
 The Function of the Rumen is to retain the food for re- 
 mastication, to macerate all fibrous substances and render them 
 fit for cellulose digestion, and to act to a limited degree on carbo- 
 hydrate and nitrogenous matters. Cellulose is by far the most 
 
202 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 important food substance dealt with; no cellulose ferment is at 
 present known, but there are ferments probably contained in 
 the food which, together with decomposition, under the influence 
 of heat, moisture, and bacteria, act energetically on cellulose.. In 
 consequence of its digestion gases' are formed — carbonic acid, 
 sulphuretted hydrogen, hydrogen, nitrogen, and marsh-gas in 
 quantities. In excess it is these gases which produce the patho- 
 logical condition known as tympanites. The carbohydrates 
 a,re attacked by organisms which convert appreciable quantities 
 of starch and cane sugar into glucose, while, under the influence of 
 microbes of putrefaction, fermentation is produced in the nitrpg^ 
 enous substances of the food, slowly converting them into 
 peptone. The acid found in the lower sac results from lactic acid 
 
 Fig. 70. — The Gastric Compartments and True Stomach of Ruminants 
 
 (Colin). 
 
 C, The oesophagus; A, A, left hemisphere, B, B, right hemisphere of the rumen; 
 D, the reticulum ; E, the omasum; F, the abomasum. 
 
 fermentation, while acetic, butyric, and other acids' ate present. 
 The acids are always limited in amount, excepting under digestive 
 derangement, when they become excessive and are associated 
 with tympanites. 
 
 The rumen never empties itself ; even after prolonged starva- 
 tion it contains food. In young ruminants digestion occurs 
 principally in the abomasum, the other compartments being 
 rudimentary ; when the young animal is placed on solid food, it is 
 remarkable how soon the rumen, reticulum, and omasum are 
 developed, and the process of remastication established. 
 
 The Reticulum, or second gastric reservoir, is a small .one 
 placed anteriorly and inferiorly resting, in fact, on the sternun) 
 -(see Fig. 72). Its capacity' in the ox is 2 litres (3 J pints), and in 
 
DIGESTION 203 
 
 the sheep 0-2 litre (0-35 pint). It communicates with the 
 rumen by a large opening formed by a constriction of its walls ; 
 the bulk of the organ lies below this opening. Over the lip which 
 separates the two chambers fluids and solids may pass in either 
 direction. The reticulum communicates with the oesophagus via. 
 the oesophageal groove, under the opening of which it is placed 
 like a sac, while by means of the same groove it communicates 
 with the omasum. The muscular coat of the reticulum is largely 
 striated, being a continuation of the oesophagus, and, in conse- 
 quence, the movements of the compartment are not only power- 
 ful, but energetic. The mucous membrane on its interior, 
 being arranged in polyhedral cells 10 to. 15 mm. in depth, closely 
 resembles a honeycomb . In these cells stones, gravel, and foreign 
 bodies may frequently be found, the latter in many instances 
 penetrating the heart. 
 
 The contents of this compartment are fluid and alkaline, the 
 fluid being derived from that swallowed, and from the rumen; 
 the alkaline reaction is due to the saliva, for, so far as we know, 
 the mucous membrane possesses no secretory activity. The 
 fluid in the reticulum is of use in rumination, and is forced into 
 the oesophagus by a contraction of the walls of the viscus; it 
 may also be forced into the rumen. Colin, with his hand in this 
 sac— the presence of which produced energetic contractions— 
 found the fluid poured over his hand, flooding the contents of the 
 rumen. In order that fluid may be retained in this bag, the 
 openings out of it are situated considerably above the base of the 
 organ ; and, further, the reticulum is so situated relatively to the 
 rumen that it receives the overflow of fluid from that compart- 
 ment when the latter contracts. 
 
 Ellenberger is of opinion that the reticulum regulates the 
 passage of food from the first to the third compartment, and 
 from the rumen to the oesophagus. In transferring the contents 
 of the rumen to the omasum, the reticulum contracts and forces 
 the material into the oesophageal groove. Flourens showed that 
 the reticulum was not essential to rumination, for he excised it 
 in a sheep, and rumination was not interfered with. 
 
 The Omasum, or third compartment, is quite different from 
 those hitherto examined. Placed between the reticulum and 
 abomasum, the viscus does its work with its contents inverted. 
 The openings into and out of a stomach or diverticulum are 
 generally above the portion holding the contents, but in the 
 omasum the contents are above the openings which lead into and 
 out of it. The omasum defies the laws of gravity. 
 
 This third compartment communicates anteriorly with the 
 second and posteriorly with the fourth, by means of a canal 
 three or four inches long, which runs obliquely backwards 1 , 
 
20 4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 downwards, and to the right . It is over this canal that the struc- 
 ture of the omasum is placed. The interior arrangement of the 
 organ is most singular, being composed of several large leaves 
 running the length of the organ ; between these are smaller leaves, 
 and between these a third and then a fourth series. Alto- 
 gether about one hundred laminae of variable size are found 
 in the compartment. The leaves are papillated, the papillae 
 at the reticulum end being large, horny, and pointed; towards 
 the omasum end they lose the pointed character and become 
 warty. Ellenberger, who has specially worked out the physi- 
 ology of this organ, and whose description we intend to follow, 
 describes the anterior papillae as resembling the teeth of a harrow, 
 the posterior papillae those of a file. In all cases the papillae are 
 arranged with their free ends pointing towards the abomasum, 
 the object of this being to direct the food towards the abomasum, 
 and prevent its reflux towards the reticulum when the walls of 
 the organ contract. 
 
 The food may find its way into the omasum, either directly 
 from the oesophagus after remastication, or from the first or 
 second compartments. It is probable that its chief source of 
 supply is directly from the oesophagus, the omasum being drawn 
 forwards towards it by a contraction of the pillars of the oeso- 
 phageal groove, by which means communication with the rumen 
 and reticulum is cut off. Normally the reaction of the contents 
 of the omasum is neutral ; if found acid, this is due to regurgita- 
 tion from the true stomach. 
 
 We have the authority of Ellenberger for saying that the 
 organ secretes no digestive fluid, nor does it absorb. It is 
 peculiar in possessing a separate source of nerve-supply, stimula- 
 tion of the pneumogastric producing contraction of all the 
 compartments but this. Further evidence of an independent 
 nerve-supply is furnished by the fact that after death the walls 
 of the other stomachs relax, but that of the omasum remains 
 contracted. 
 
 When the food arrives at the omasum the opening leading to 
 the abomasum is closed, and by means of the horny papillae at 
 the reticulum end of the organ the material is divided and directed 
 towards the leaves, the oesophageal groove at the same time 
 closing in order to prevent a reflux. The presence of the food 
 then causes a reflex contraction of the leaves, by which they are 
 rendered rigid and tense, and advance to meet the mass. The 
 leaves having been drawn forward, the fibres now relax, by which 
 movement the food is carried back with them. The direction 
 of the papillae on the leaves prevents the material from falling 
 out of the chambers formed between the leaves, while the curve 
 of the organ enables the leaves to give each other mutual support, 
 
DIGESTION 205 
 
 like the bricks in an arch, so that no muscular effort is required 
 to maintain the food in position. The waUs of the omasum are 
 strong, the circular fibres being three or four times as thick and 
 powerful as those longitudinally placed; contraction of the walls 
 is almost constantly occurring. In this way the contents of the 
 omasum are moved towards the fourth stomach, the position 
 and direction of the papillae preventing them from travelling in 
 the other direction. Each contraction drives the ingesta not 
 only towards the omasum, but more completely between the 
 laminae of the deeper system. These leaves take no part in 
 emptying the omasum; their function is to advance and meet 
 the incoming matter, then to raise it to the system of channels 
 formed by the series, and finally, by means of their papillae, to 
 further grind down and reduce insufficiently remasticated food. 
 This grinding or rasping down is effected by the leaves becoming 
 shorter, thicker, and stiff er; the papillae on them are drawn 
 through the mass like the teeth of a harrow. The movements of 
 the leaves are not simultaneous, but successive; while one is 
 passing in one direction, its fellow is travelling in the opposite 
 direction, so that rasping of the food is continually going on, 
 and this is evident from the fact that the contents are much finer 
 at the abomasum end than at the reticulum end of the organ. 
 Ellenberger, therefore, speaks of the third compartment as a 
 triturating apparatus, a masticatory stomach, the contents of 
 which are dry because the fluid part is being constantly 
 strained off. 
 
 With illness rumination ceases, and the chief supply of fluid 
 to the omasum is then cut off — viz., the saliva, which is re- 
 swallowed during rumination. As no fresh ingesta and very 
 little fluid is entering the omasum, the contents rapidly become 
 dry and caked. 
 
 The Abomasum, or true digestive stomach, is the only com- 
 partment secreting gastric juice. In the abomasum proteins 
 are converted into peptones, the region of the cardia being in 
 this respect more active than that of the pylorus. Ellenberger 
 states that starch is also digested, and that this precedes protein 
 digestion. In the fourth stomach of the calf a milk-curdling 
 ferment (rennin) exists, which has already been dealt with. 
 
 Stomach Digestion in the Pig. 
 
 The stomach of the pig is peculiar; it is a type between the 
 carnivorous and ruminant, and is divided by Ellenberger and 
 Hofmeister into five distinct regions, which do not all possess 
 the same digestive activity. 
 
 The gastric juice of the pig contains for the first hour or two 
 
3o6 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 of digestion lactic, and afterwards hydrochloric, acid ; pepsin is 
 present, and, it is said, a ferment which converts starch into 
 sugar. In the pig, according to the above observers, the process 
 of digestion is not the same in all regions of the viscus; one 
 may contain hydrochloric acid, another lactic; one may be 
 abundant in sugar, while this may be absent elsewhere. The 
 first stage of digestion is starch conversion; the second is the 
 same, only more advanced ; the third stage is starch and protein 
 conversion, both processes occurring at the cardia, but only pro- 
 tein conversion taking place at the fundus ; lactic acid is present 
 in the cardia, and both lactic and hydrochloric acid in the fundus. 
 In the fourth stage starch conversion is nearly complete, while 
 hydrochloric acid predominates in all the regions, and protein 
 conversion is general. 
 
 Stomach Digestion in the Dog. 
 
 Very complete knowledge of the physiology of the dog's 
 stomach exists, for nearly all the work undertaken to elucidate 
 the physiology of the human stomach has been effected on the 
 dog. We have summarised some of this knowledge in the pre- 
 vious pages in treating of gastric juice. 
 
 In the stomach of the dog and cat digestion occurs mainly in 
 the pyloric dilatation or antrum, which is from time to time cut 
 off from the rest of the stomach by the pyloric ring or transverse 
 band (not to be confused with the pyloric sphincter), so that the 
 general body of the stomach acts mainly as a receptacle, passes, 
 from time to time, material into the antrum for chyme con- 
 version, and then ejects it into the intestines. The length of 
 time material lies in the stomach depends upon its nature and 
 consistence; both carbohydrates and fats escape into the intes- 
 tines more readily than proteins. The time appears to depend 
 also upon the strength of acid present. A weaker acid sufficing 
 for non-protein food is more rapidly neutralised in the small intes- 
 tines, and the opening of the pylorus appears to depend upon 
 impulses passing from the duodenum as soon as the last received 
 acid supply has been neutralised in that bowel. 
 
 A flesh diet requires very little saliva and practically no masti- 
 cation, but its digestion is slow, in spite of the fact that it is 
 taken in a form closely allied to that in which it is assimilated- 
 Colin states that it taker a dog twelve hours to digest an amount 
 of meat which it could eat at one meal. The substances most 
 difficult of digestion are tendons and ligaments, but of these 
 digestion is facilitated by boiling ; liver and flesh are best given 
 raw, as cooking interferes with their digestibility. The gastric 
 juice of the dog is fully described at p. 193. 
 
DIGESTION 207 
 
 Absorption from the Stomach. — The needful changes occur- 
 ring in the stomach having been considered — we now refer 
 principally to the stomach of the horse — the next step is to 
 inquire into the proportion of food so altered as to be rendered 
 fit for absorption. 
 
 ,* Experiment shows that in the stomach 40 to 50 per cent, 
 of the carbohydrates has been converted into sugar, whilst 
 40 to 70 percent. of the proteins has been converted into peptones. 
 When food has been long in the stomach, not more than 10 per 
 cent, of the proteins escape being peptonised. In ruminants 
 probably the greater part of the digestion of the food substance 
 is carried out in the gastric compartments and stomach, leaving 
 comparatively little for the intestines to perform. 
 
 In spite of the changes which occur in the stomach, it has been 
 proved by the experiments of Colin that no absorption occurs from, 
 this organ in the horse. It would be useless to recapitulate all 
 his experiments; strychnine was generally employed, and he 
 found that, so long as the pylorus was securely tied, no symptoms 
 of poisoning occurred when the alkaloid was introduced into the 
 stomach, ho matter how long it was left there, but that when the 
 ligature was untied, and the contents of the stomach passed into 
 the intestines, poisoning rapidly followed. These remarkable 
 results were obtained by him so often, and under such varying 
 conditions, -as to leave no doubt as to the accuracy of the observa- 
 tions. Strychnine experiments are not altogether free from 
 objection, but in view of these results we can only surmise that 
 no absorption of sugar or peptones occurs in the stomach. On 
 the other hand, what becomes of the peptones is certainly very 
 mysterious ; the writer has never found any in the stomach con- 
 tents, no matter at what period of digestion the examination 
 was made, and if they are not absorbed in the stomach they 
 must pass very rapidly into the intestines and enter the vessels 
 at once, as no peptone can be found in the small intestines. 
 Colin attributes the absence of absorption from the stomach in 
 the horse to the small area of the mucous membrane, which, he 
 says, cannot be secreting gastric juice and absorbing at the 
 same time. He attributes the non-absorption of poisons in the 
 empty stomach to the thick layer of tenacious mucus which, 
 as we have previously mentioned, covers the villous stomach of 
 the horse. Colin's experiments also show that there is little or 
 no absorption from the abomasum of ruminants. On the other 
 hand, there is absorption from the stomachs of the dog and pig. 
 Recent experiments on the dog show that absorption does not 
 take place readily from the stomach. Water taken alone is, 
 practically speaking, not absorbed at all; sugars and peptones 
 are absorbed only when in sufficient concentration, while fats 
 are not absorbed. 
 
208 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Self-Digestion of the Stomach.— A question which for a long 
 time gave rise to energetic discussion was the reason why the 
 stomach during life does not digest itself, seeing that the action 
 of its secretion is so potent that portions of living material, legs 
 of frogs, ears of rabbits, etc., if introduced into it, are readily 
 digested, and that post-mortem digestion of the stomach in 
 some animals is far from rare. The walls of the stomach are 
 not singular in possessing a specific resistance to a digestive 
 fluid. The small intestine is immune to trypsin, which, if in- 
 jected under the skin, digests the tissues and produces ulcera- 
 tion. Faeces and urine are very irritating, unless, so to speak, 
 in their own home. The bladder never shows any sign of irrita- 
 tion from the presence of urine, but if it be paralysed, and the 
 urine, in dribbling away, falls on the skin, the part in time 
 ulcerates, and very early becomes excoriated. Even tears 
 running over the face, in cases of obstruction of the duct, cause 
 the skin to ulcerate. In spite of the action of gastric juice, 
 ' bots ' live in the stomach for months, and, unfortunately, are 
 never digested. Parasites of the digestive tract enjoy the same 
 immunity from digestion that the membrane of the canal 
 possesses. 
 
 It is now generally believed that the immunity enjoyed by 
 the tissues in contact with active or irritating secretions is due 
 to the local formation of an antibody. That in the stomach 
 has been named antipepsin, and its function is to neutralise 
 the digestive action of the gastric juice on the living wall. 
 
 We have never yet met with post-mortem digestion of the 
 stomach in the horse; whether this be due to the horse's acid 
 being mainly or wholly lactic cannot be definitely stated. 
 
 The Gases of the Stomach. — The nature of these largely 
 depends upon the food — for example, green food is most pro- 
 ductive of gas, owing to the active fermentation it undergoes. 
 Traces of oxygen, a quantity of carbonic acid, and variable 
 amounts of marsh-gas, sulphuretted hydrogen, hydrogen, and 
 nitrogen, are found. The oxygen and nitrogen are derived from 
 the swallowed air, the carbonic acid is derived from the fer- 
 mentation of the food and the action of acids on the saliva, 
 while the marsh-gas is obtained by the decomposition of cellu- 
 lose. 
 
 The gases from the intestines of the horse and rumen of the 
 ox are very commonly inflammable, and burn with a pale blue 
 flame. This is due to marsh-gas, which may be readily ignited 
 when mixed with a due proportion of oxygen. 
 
DIGESTION 209 
 
 Vomiting and Rumination. 
 
 Vomiting amongst solipeds and ruminants is rare, but the 
 act is common in the dog and pig. 
 
 The explanations given as to why the horse does not ordinarily 
 vomit are various: (1) The thickened and contracted cardiac 
 extremity of the oesophagus; (2) the oblique manner in which 
 the latter enters the gastric walls ; (3) the dilated pylorus lying 
 close to the contracted cardia, so that compression of the 
 stomach contents forces them into the duqdenum; (4) the 
 cuticular coat thrown into folds over the opening of the cardia ; 
 (5) muscular loops encircling the cardia, the contraction of 
 which keeps the opening tightly closed; (6) the stomach not 
 being in contact with the abdominal wall. All these and other 
 reasons have been assigned as the cause of non-vomiting in 
 the horse. Yet on turning to ruminants, which also normally 
 do not vomit, we find the stomach, gastric compartments, and 
 oesophagus freely communicating, the largest reservoir lying in 
 contact with the abdominal wall, the cardia freely open, the 
 oesophagus of great size, and, still stranger, the animal possessing 
 the ability, under the control of the will, to bring up food from 
 the stomach as a normal condition, and yet unable to vomit. 
 It is evident, therefore, that all these theories, taken together, 
 are not sufficient satisfactorily to account for the absence of 
 vomiting, and we are bound to suppose that the vomiting centres 
 in the medulla of both horse and ox are either only rudimentary 
 or very insensitive to ordinary impressions. 
 
 Vomiting in the horse is no doubt seriously interfered with by 
 the thickened oesophagus, contracted cardia, and the arrange- 
 ment of the muscular fibres. The folds of mucous membrane 
 filling up the orifice could offer no serious obstruction to a 
 distended stomach, for it is known that even when this mem- 
 brane is dissected away post mortem, a stomach will burst 
 rather than allow fluid or air pumped in at the pylorus to escape 
 at the cardia, unless the muscular fibres surrounding it be partly 
 divided. Vomiting in the horse is generally indicative of rup- 
 tured stomach, and much has been written as to whether it occurs 
 before or after rupture. From no inconsiderable experience of 
 these cases, the writer has arrived at the conclusion that it may 
 occur at either time, and that a horse may vomit though a rent 
 7 or 8 inches long exists in the stomach wall. 
 
 Dilatation of the cardia and oesophagus is essential to the 
 act of vomiting in the horse, and in all cases where vomiting 
 occurs during life the cardia is so dilated that two or three 
 fingers may readily be introduced into it. It is perfectly possible 
 for a horse to vomit and recover (showing that it had not a 
 
 14 
 
4io A MANUAL OF VETERINARY PHYSIOLOGY 
 
 ruptured stomach), and it is not unusual to have attempted 
 or even actual vomition when the small or large intestines are 
 twisted. Vomiting in the horse is not as a rule attended by any 
 distressing symptoms ; the ingesta dribble away from one or both 
 nostrils ; occasionally an effort is made on the part of the patient, 
 the head being depressed to facilitate expulsion, but more 
 than this is very rarely seen.* 
 
 It is important to notice in connection with the subject of 
 vomiting that agents such as tartar emetic, ipecacuanha, and 
 apomorphia, which excite vomiting by their action on the cere- 
 bral centre, have no effect on the horse or on ruminants, nor does 
 the horse vomit as the result of sea-sickness, though he suffers 
 extremely from it. Why vomition should be more frequent in 
 horses with a ruptured stomach than when the organ is whole 
 is a fact we cannot explain. 
 
 In those animals where vomiting is a natural process, the three 
 important factors are — The dilatation of the cardia by active 
 contraction of the longitudinal fibres of the oesophagus, pressure 
 on the walls of the stomach by a contraction of the diaphragm 
 and abdominal muscles, and closure of the pylorus. But there 
 is some evidence to show that the stomach itself is not passive; 
 it is true Majendie produced vomiting after he had replaced the 
 stomach by a bladder, but under normal conditions there appears 
 no reason why the stomach wall should remain quiescent, and 
 in the cat it has been observed that during vomiting a strong 
 contraction of the pyloric end of the stomach occurred, shutting 
 it off from the cardiac portion. We may here have one explana- 
 tion of ruptured stomach in the horse, which is supported by 
 the view put forward on p. 181 of the influence of the duodenal 
 trap. 
 
 The oesophagus in ruminants has its inner layer of fibres 
 spirally arranged in double obliquity; the tube is wider at its 
 termination than in the anterior part. On entering the gastric 
 reservoirs, it forms a groove, previously described (see description, 
 Fig. 71), which brings it into connection with all the compart- 
 ments. In this way material coming down the oesophagus may 
 enter any of the reservoirs, the choice being determined by the 
 condition in which it is swallowed. 
 
 The lips or pillars of the oesophageal groove are composed 
 of involuntary muscular fibres, arranged longitudinally and 
 transversely by means of which arrangement the groove can 
 be shortened and constricted. By a contraction of the pillars, 
 
 * The only case of vomiting the writer has seen in the horse which re- 
 sembled that presented by the human subject was in a case of volvulus of 
 the small bowels. The horse was lying on his chest with the muzzle 
 tended, the ingesta gashed in a stream from both nostrils, and a sound" 
 accompanied the effort. 
 
DIGESTION 411 
 
 the omasum may be drawn forward and brought nearly into appo- 
 sition with the oesophagus. By relaxation of the pillars the oeso- 
 phagus is made to communicate with either the first or second 
 reservoir. Fig. 72 shows the rumen and reticulum in position. 
 
 Fig. 71. — The Oesophageal Groove from the Reticulum. The Groove is 
 shown with its Lips separated. From a Photograph. (Sisson.) 
 
 A, opening of oesophagus. The arrow below points to the reticulo-omasal orifice. 
 The groove is about 7 inches long, and though vertically placed in the reti- 
 culum, there is a slight spiral twist in the canal. „The groove is lined by 
 mucous membrane similar to that of the oesophagus, and on this membrane 
 at the lower part there are some horny papillas. 
 
 The reticulo-omasal opening is about 1 J inches in diameter. The groove continues 
 from here and passes along the floor of the omasum to the abomasum. The 
 length of this channel, which gradually increases in width, is about 3J inches, 
 and it opens into the abomasum by an orifice 4 inches in diameter. As the 
 abomasum lies on the floor of the abdomen its course is backwards, downwards, 
 and to the right. 
 
 The grooveirom the cardia to the abomasum may be regarded as a portion of modified 
 oesophagus, provided with a slit in its side capable of opening and closing, and 
 its further portion as an open channel or gutter on the floor of the omasum. 
 
 The oesophageal groove is represented as open, though normally 
 its lips cover it so perfectly that it requires looking for. The 
 close proximity of the reticulum to the heart will be observed, 
 and in this connection see the remarks on p. 203. At one time 
 it was considered that the oesophageal groove took an important 
 
212 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 share in rumination, but the question was decided by Colin in 
 his usual thorough manner. He stitched the pillars together, 
 and found this did not interfere with the process. Prior to this 
 experiment it was believed that the lips formed the bolus, and 
 then passed it into the oesophagus. Colin, with his hand in the 
 stomach, introduced ingesta into the canal, but could not suc- 
 ceed in getting the lips to grasp and carry it into the oesophagus. 
 Additional proof of. this point is furnished by the llama, which 
 ruminates and has but one pillar to the groove. 
 
 After the food has remained sufficiently long in the rumen, 
 exposed to a slow churning movement and to the action of the 
 various bacteria, which cause softening and ultimately digestion 
 of the cellulose, it is returned as a bulky mass to the mouth for 
 remastication. That which is ready is slowly pressed forward 
 towards the oesophagus, the diaphragm by contracting forms a 
 partial vacuum in the chest, and material is drawn into the 
 oesophagus under the influence of this vacuum, assisted, perhaps, 
 by a contraction of the abdominal walls, but not by the walls of 
 the rumen, as supposed by Colin. Chauveau was responsible for 
 this theory, and Toussaint placed it to the test, of experimental 
 inquiry by means of various air-chambers introduced into the 
 oesophagus and respiratory passage, which recorded their move- 
 ments on a revolving drum.* In this way it was found that im- 
 mediately preceding a regurgitation the glottis closed, the dia- 
 phragm contracted, and a partial vacuum was formed in the chest. 
 According to these observers, the contraction of the abdominal 
 muscles is purely passive, and depends upon the displacements 
 caused by the oscillation of the diaphragm. It is certainly a 
 fact that the sudden contraction of the abdominal wall is scarcely 
 seen in the ox, though it is somewhat clearer in the sheep, and 
 very evident in deer, so that, as opposed to the original view of 
 Colin, and what the observer can see for himself, the contraction 
 of the abdominal muscles would appear to take no active share 
 in the phenomenon. 
 
 If a tracheotomy be performed, rumination is not prevented, 
 though the glottis can no longer close ; the thoracic walls, which 
 are ordinarily passive during the phenomena, now take on a 
 vigorous inspiratory activity, and so assist the diaphragm. If the 
 phrenics be divided the diaphragm is paralysed, but rumination 
 is not abolished; other inspiratory muscles intervene, and the 
 process is carried out, though with difficulty. If the distal end 
 of the divided phrenic be stimulated, the diaphragm contracts arid 
 regurgitation takes place, while if at the moment of stimulation 
 the nostrils and mouth be momentarily closed, there is a violent 
 
 * ' 6l6ments de Physiologie,' F. LaulaniS, 1905. 
 
DIGESTION 213 
 
 expulsion of ingesta to the distance of several feet.* The 
 modern theory of rumination, then, is that it is produced by 
 aspiration within the thorax causing a negative pressure in the 
 oesophagus, and thus ' drawing out ' the ingesta from the rumen, 
 the walls of that organ taking no share. In fact, during the act 
 of regurgitation, they are quiescent. 
 
 In this way the semi-solid ingesta from the rumen, together 
 with fluid from the reticulum, is sucked into the oesophagus; 
 here the bolus is cut off by the cardiac end of the tube, and by 
 reversed peristaltic action the bolus is conveyed to the mouth, 
 the soft palate being raised, thus cutting off the posterior nares. 
 In passing under the velum the liquid portion is squeezed out, 
 and is at once reswallowed, passing to the third compartment, 
 while the solid material is ground. If the left abdominal wall 
 be auscultated during rumination, certain sounds may be heard. 
 One of these Colin describes as resembling a crepitant lung rale, 
 and is due to the disengagement of gas in the rumen ; another is 
 a friction sound resembling that heard in pleurisy, due to the 
 gliding motion of the gastric reservoirs against the abdominal 
 wall and diaphragm ; and a third sound is heard at the moment 
 the fluid returns from the mouth. Gurgling sounds may also be 
 heard over the oesophagus in the neck at the moment of swallow- 
 ing. 
 
 The bolus in the mouth may weigh from 100 to 120 grammes 
 (3 to 4 ounces), and it is projected into this cavity with such 
 force that if the mouth is open it may drop out. Its remastica- 
 tion occupies about fifty seconds; it is then reswallowed, and 
 passes to the third compartment, or, if not sufficiently com- 
 minuted, it returns to the rumen, and is once more remasticated. 
 
 The ascent and descent of the bolus in the neck may be 
 readily seen, and Colin has estimated that each of these requires 
 1-5 seconds, while an equal period is required for the formation 
 of a fresh bolus. On this data, from the time the bolus is pre- 
 pared until its return must occupy 54-5 seconds. One minute 
 may be taken as sufficiently near. Rumination is, therefore, 
 a slow process, and occupies at least seven hours out of the 
 twenty-four. The length of time an animal will continue when 
 ruminating to use the jaws in one direction is remarkable; 
 from the effort involved frequent changes of direction might have 
 appeared to be necessary, but such is not the case. One-quarter 
 to half an hour's mastication on one side may be performed. 
 
 The animal prefers a recumbent position for the process, 
 but draught oxen may be seen ruminating in the yoke, and camels 
 while carrying their burdens. 
 
 The share taken by the reticulum in rumination is mainly the 
 
 * Laulanie, op. cit. 
 
214 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 supplying of fluid to the mass, but rumination can occur even 
 after the removal of the reticulum. Unlike the rumen, it is cap- 
 able of sudden contractions. A certain distension of the rumen 
 is absolutely necessary; though this organ never empties itself, 
 it must contain a fair amount of food before rumination begins: 
 
 fellfm 
 
 
 
 
 p 13 
 
 
 
 
 rt o 
 
 
 
 
 &,« 
 
 
 O 
 
 
 i/) +-< 
 
 
 
 
 ., &, 
 
 < 
 
 H 1J 
 
 
 t3 
 
 
 
 7 
 
 a ja 
 
 > 
 
 j=j ^ 
 
 
 
 
 
 
 ■£ « 
 
 53 
 
 I- tj 
 
 7, 
 
 £ g 
 
 W 
 
 to 
 
 
 ,yi Jh 
 
 
 
 o 
 
 
 Crf 
 
 
 ' k 
 
 &■ s 
 
 
 rt 
 
 
 
 < 
 
 CO - 
 
 
 rj Ci3 
 
 n 
 
 o — 
 
 < 
 
 *3 & 
 
 e £ 
 
 JS ^ 
 
 .2 "I 
 
 a ° 
 
 o br -* 
 
 a -as 
 
 'Pi- 
 
 further, it must contain a considerable quantity of fluid. If the 
 salivary flow which so largely contributes to this fluid be diverted, 
 remastication becomes difficult, though for a day or two rumina- 
 tion may be carried on ; later it begins to fail, and it is only by a 
 great effort that food can be passed into the oesophagus. Colin's 
 experiment in this direction showed that by the thirteenth day 
 
DIGESTION 215 
 
 rumination was no longer possible. It has been shown that the 
 parotids of the ruminant are always in a state of activity, and 
 the object of this continuous secretion is to insure the function 
 of rumination being duly carried out. 
 
 An animal alarmed or disturbed at once ceases to ruminate, 
 and one of the first signs of ill-health is that the function is 
 suspended. 
 
 Rumination is a reflex nervous act, the centre for which 
 probably lies in the medulla. If, as we have seen, the phrenics 
 be divided, rumination is carried out with great difficulty, and 
 only by an extra effort of the inspiratory muscles; if the vagi 
 be divided, or the spinal cord in the mid-dorsal region, rumina- 
 tion is no longer possible. 
 
 Movements of Food in the Stomach. 
 
 A good deal of exact information as to the manner in which the 
 stomach behaves in dealing with its contents has been obtained 
 by means of direct experimental inquiry. Colin, in the case of 
 the ruminant, introduced his arm into the organ through an 
 external opening. In the horse, carefully conducted feeding 
 experiments have helped to show what occurs from hour to 
 hour, but it was not until the X rays were employed that 
 the process could be seen occurring in the intact living animal. 
 
 If a cat be fed on a diet liberally mixed with bismuth, the 
 process of digestion can be watched by X rays, owing to bismuth 
 being opaque to these rays. Within a few minutes of the entry 
 of food into the stomach, contractions, which begin at the middle 
 of the organ and end of the pylorus, are started. As digestion 
 advances the contractions become stronger and regular, occurring 
 at intervals of about ten seconds, and travelling to the pylorus, 
 which is reached in twenty seconds. Periodically the pylorus is 
 relaxed, and chyme forcibly propelled into the duodenum ; after 
 each discharge it closes, and in course of time the process is 
 repeated. All idea of a churning motion occurring in simple 
 stomachs has now been abandoned; it was difficult to disprove, 
 except in the case of the horse, until X rays were employed. In 
 the horse the arrangement of the food in the stomach in strata 
 as received never left any doubt that churning took no part in 
 the process of stomach digestion in this animal. It will be 
 observed that the cardiac end of the stomach of the cat takes 
 apparently no share in the movement, and it is probable that in 
 most animals this end plays but a passive part. In the hor?e, 
 in fact, it may almost be regarded as an oesophageal dilatation 
 
 If the abdominal cavity in a recently destroyed horse be rapidly 
 opened, it may be possible to see the stomach at work. Its 
 
216 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 peristalsis is slow, rather more deliberate and less energetic 
 than intestinal peristalsis, and not infrequently hour-glass con- 
 strictions may appear, though not to a marked degree. Many 
 observations may be made before gastric peristalsis is seen; 
 unlike intestinal peristalsis, it is not always present after death. 
 The mechanism controlling the opening and closing of the 
 pylorus is by no means clear; solid bodies are denied a passage 
 for some time, and even for chyme the pylorus does not open 
 with every contraction wave which passes over the stomach. A 
 bullet administered as a bolus was found in the stomach thirty- 
 six hours later, though the animal had in the meantime been fed, 
 from which it seems certain that the pylorus is capable of de- 
 ciding what should and what should not pass. Yet in the case 
 of the horse some modification in this statement must be made, 
 for, as has been shown, owing to the small size of the stomach 
 and the bulky nature of the food in an organ already filled, an 
 amount passes out at one end equal to that being taken in at 
 the other, and where this mechanism fails disease at once arises. 
 Liquids rapidly pass out of the stomach of the horse; the 
 large quantities of fluid required by this animal could not be 
 contained in the stomach. In fact, if auscultation be practised 
 while an animal is drinking, water may be heard passing along 
 the duodenum below the right kidney. 
 
 Little of what has just been said applies to ruminants. The 
 movements of the stomach wall in these animals are complex; 
 the immense muscular pillars of the rumen are capable, as we have 
 seen, of mixing, churning, and revolving the contents of the organ. 
 That rotation occurs is certain from the formation of hair balls 
 from material swallowed by the animal in licking its body, or, in 
 the case of sheep, from wool torn out when scratching them- 
 selves and subsequently swallowed. The rumen and omasum are 
 constantly at work, the rumen contracting a little oftener than 
 three times a minute; the reticulum, on the other hand, has 
 periods of rest, likewise the true stomach. In the latter the 
 movements must be of a most simple character, as in the single 
 stomach of other animals. 
 
 The movements of the stomach are excited by the presence of 
 food, or by any irritation applied to the mucous membrane. 
 These movements are rendered more energetic by stimulation of 
 the vagus, but even when all the nerves going to the part are 
 divided, the stomach can still contract, which is probably due 
 to the ganglia contained in its walls. The stomach is, then, an 
 automatic organ. Nevertheless, it is supplied by both pneumo- 
 gastrics, the nerves being non-medullated ; in addition, it obtains 
 sympathetic fibres from the solar plexus, to which the right vagus 
 also sends some fibres (see Fig. 80, p. 236). In the wall of the 
 
DIGESTION 217 
 
 stomach ganglia are found with which both the vagus and sym- 
 pathetic communicate . The vagus may be regarded as the motor 
 nerve of the stomach, while the sympathetic is mainly inhibitory ; 
 stimulation of the vagus leads to contraction of the stomach walls, 
 stimulation of the sympathetic causes dilatation of a contracted 
 stomach and relaxation of the pylorus. The vagus supplies the 
 bloodvessels with dilator fibres, whilst the sympathetic supplies 
 them with constrictor fibres. Section of the vagus in the horse 
 causes paralysis of the stomach. In other animals, if the move- 
 ments are not abolished, they are certainly diminished. The 
 result of stomach paralysis is that nothing passes on to the in- 
 testines, so that in the horse even large poisonous doses of strych- 
 nine may thus fail to cause death by lying inert in the stomach. 
 This experiment demonstrates the uselessness of giving medicine 
 by the mouth in many cases of digestive trouble in the horse ; the 
 material lies in the stomach owing to paralysis of the organ, and 
 is not absorbed. 
 
 The nervous supply of the stomach of ruminants is derived 
 mainly from the vagus, excepting in the case of the third com- 
 partment, which has a separate and, at present, unknown source 
 of supply. Stimulation of the vagus was found by Ellenberger 
 to produce energetic contraction of the reticulum, slow kneading 
 movements of the rumen, and after an interval slower and peri- 
 staltic contractions of the abomasum, but no contraction of the 
 omasum. Section of both vagi was found to cause paralysis of 
 the oesophagus, rumen, and reticulum, followed by tympany of 
 the rumen. Ellenberger could not obtain any effect on the 
 stomach movements by stimulating the sympathetics. 
 
 The influence of the mind over stomach digestion is well known 
 in man, and we have already noted its effect in the horse in the 
 feeding experiment related on p. 186. Pavlov's fistula has 
 enabled this point in the lower animals to be settled with accuracy. 
 One of his dogs, while actively secreting gastric juice under the 
 influence of a ' sham meal,' was made angry and excited by being 
 restrained from chasing a cat ; the gastric glands at once ceased 
 secreting. Similarly by means of X rays the movements of the 
 cat's stomach have been observed to cease when the animal 
 became angry and excited. 
 
2i 8 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Section 3. 
 Intestinal Digestion. 
 
 The chyme which is poured from the stomach into the small 
 intestines meets there with three digestive fluids— viz., the 
 succus entericus, the bile, and the pancreatic juice. 
 
 The Succus Entericus is prepared by the glands of the small 
 intestines; the glands of Brunner are found in the duodenum, 
 and the follicles of Lieberkiihn throughout the whole of the small 
 and large intestines. Lieberkuhn's crypts supply a considerable 
 proportion of the intestinal juice, while the secretion from the 
 glands of Brunner is scanty. Brunner's glands, which are very 
 large in the horse, are arranged on the same principle as the gastric 
 glands, while those of Lieberkiihn are tubular glands, amongst 
 the cylindrical epithelial cells of which numerous mucus-forming 
 goblet cells may be found. 
 
 The total daily amount of succus entericus secreted by the 
 horse is given by Colin as 10 litres (17 pints). 
 
 At one time it was considered that the succus entericus was a 
 comparatively unimportant fluid, the chief function of which 
 was to neutralise the acid chyme; Colin, however, showed that 
 in the horse it has a distinctly digestive effect. The Lieberkiihn 
 fluid is considerable in amount and alkaline in reaction owing to 
 the presence of carbonate of soda. It is now known that though 
 a pure secretion of Lieberkuhn's crypts has little or no digestive 
 action excepting oh starch, an extract of the intestinal wall and 
 juice squeezed from it have a most important function. The 
 intestinal extract, on the other hand, contains three types of 
 enzymes, and in addition a peculiar chemical substance of re- 
 markable properties. The enzymes are: 
 
 1. Enterokinase, which converts the trypsinogen, the mother 
 substance of the pancreatic proteolytic enzyme, into trypsin. 
 
 2. Erepsin, a proteolytic ferment, which supplements the 
 work of trypsin, acting on deutero-albumoses and peptones, 
 breaking them up into amino-acids and hexone bases. 
 
 3. Inverting Ferments, converting double sugars which 
 cannot be utilised by the tissues into single sugars which can. 
 Of inverting ferments there are three : 
 
 1' Maltose, converting maltose and dextrin into dextrose,^ 
 
 Invertase, converting cane sugar into dextrose and levulose^ 
 j Lactase, converting milk sugar into dextrose and galactose^ 
 I Finally, the intestinal fluid contains secretin, which is not a 
 
DIGESTION 
 
 219 
 
 ferment, but a chemical substance (hormme) found in the walls of 
 the small intestines; this, when taken into the blood, possesses 
 the singular property of causing the secretion of pancreatic juice. 
 Enterokinase, erepsin, and secretin will be dealt with in 
 considering the pancreas. 
 
 Intestinal Digestion in the Horse. 
 
 Immediately or shortly after the ingesta leave the stomach 
 the contents are neutralised by the pancreatic and biliary secre- 
 tions. So rapid is this change that on the duodenal side of 
 the pylorus the reaction of the previously acid chyme is neutral, 
 and a few inches along the duodenum it is alkaline; this alkaline 
 reaction is at first faint, but becomes more marked as the ileum 
 is approached. Ellenberger describes the contents of the small 
 intestines as being acid in the first two-thirds of their length, 
 then neutral as far as the ileum, where they become alkaline ; we 
 have only once found them other than alkaline throughout. 
 He further states that in the fasting horse the contents are alka- 
 line, but that in the digesting animal, whether horse, ox, or sheep, 
 they are acid, the acidity decreasing after passing the common 
 duct, and the contents becoming decidedly alkaline at the 
 posterior portion of the small intestine. This, as has been said, 
 does not agree with the writer's experience in the horse; it is 
 usual to find the contents of the duodenum next the pylorus 
 neutral, and from this point the bowel contents are faintly alka- 
 line, the reaction increasing in intensity up to the ileum, where 
 the contents are always markedly alkaline. We have only once 
 found the small bowels acid in the horse, no matter what diet has 
 been given, or at what period of digestion the examination has 
 been made ; a neutral or faintly alkaline reaction in the anterior 
 part of their course, and marked alkalinity in the posterior 
 portion, is doubtless the rule rather than the exception. 
 
 The arrangement of the small intestines suspended, or dangling 
 in festoons from the spine, by a very dehcate membrane is a con- 
 struction the advantages of which are not very apparent. It 
 appears to invite trouble. The long mesentery is considered to 
 favour volvulus, but no doubt the chief cause of this trouble is 
 tympany. If, after death, the bowels be artificially distended 
 with air, loops of them behave in such a way as would lead to 
 twist in the living animal. 
 
 Physical Characters of the Chyme. — The chyme having 
 passed into the bowel, its appearance at once changes, for the 
 acid albumin is precipitated by the alkaline secretion found there. 
 It is now observed that the material consists of clots floating or 
 suspended in a yellowish fluid, extremely slimy in nature and 
 
220 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 resembling in appearance, because of its precipitated albumin, 
 nasal mucus suspended in fluid. The proportion of mucin must 
 be considerable, judged from the ropiness of the fluid when poured 
 from one vessel to another, and this mucus is probably derived 
 largely from the stomach. Throughout the small intestines the 
 character of the chyme is as foUows — viz., a yellow, frothy, 
 precipitated, slimy fluid, the material from the anterior part of 
 the intestinal canal having a peculiar mawkish smell, while that 
 from the region of the ileum has a distinctly faecal odour; this 
 is due to indol and skatol, formed during, or subsequent to, pan- 
 creatic digestion. In the ileum the proportion of fluid material 
 is considerably reduced in amount, and what was previously 
 almost impossible, the character of the ingesta may now be 
 recognised. 
 
 Function of the Ileum. — As the flow of material into the small 
 intestines is controlled by a sphincter, so is the flow out of them. 
 The ileum is a remarkably thick and powerful bowel; it is always 
 found contracted and containing material which is dry compared 
 with that found in the anterior portion of the intestine. One of 
 the functions of the ileum is to control the passage of material 
 into the caecum. Colin describes the chyme in the horse as circu- 
 lating between the pylorus and ileum — that is to say, it is poured 
 backwards and forwards in order to expose it sufficiently to the 
 absorbent surface ; this necessitates a reversed peristaltic action. 
 He says that were it not tor this arrangement the material could 
 not be acted upon and the chyle absorbed, as the passage of fluid 
 through the small intestines is very rapid. It would have been 
 impossible to reason out that the fluid material of the small in- 
 testines was passed to and fro between the stomach and the 
 ileum, exposed, as Colin expresses it, twenty times over to the 
 absorbent surface of the bowels. This observation must have 
 been made as the result of his examination of the living animal, 
 and there can be no doubt of its correctness. 
 
 Experiment shows that water will pass from the stomach to 
 the caecum in from five to fifteen minutes. By applying the ear 
 over the duodenum of a drinking horse, the water as it passes 
 under the last rib on the right side may be heard rushing through 
 the intestines on its way to the caecum. One is always struck 
 by the fact that the small intestines are never found full— in fact, 
 are often practically empty — from which we judge either that 
 material passes very rapidly through them, or that only small 
 amounts of chyme are propelled into them at a time. The con- 
 tents are always in a liquid condition excepting at the ileum, 
 the fluid being derived from the secretions poured into and those 
 originating in the bowel. That active absorption goes on in the 
 intestines is proved by the differences in the physical characters 
 
DIGESTION 221 
 
 of the contents of their r-everal parts. The rate at which the 
 chyme passes through the small intestines varies with the nature 
 of the food, and the frequency with which the horse is fed . Ellen- 
 berger says it reaches the caecum six hours after feeding, but has 
 not entirely passed into this bowel for twelve or even twenty 
 hours; we have known it reach the caecum in four hours. 
 
 In the small intestines the chyme meets with the bile and 
 pancreatic juice ; the action of these will be described in the chapter 
 dealing with the liver and pancreas. The absorption of chyle, 
 and its elaboration before reaching the blood, are points which 
 must be reserved for the chapter on ' Absorption.' 
 
 Large Intestines. 
 
 There can be no doubt that in solipeds digestion in the large 
 intestines is a very important process; at least, we judge so 
 from their enormous development. In many respects they 
 present a considerable contrast to the small intestines ; they are 
 always found filled with ingesta, the contents are more solid, 
 
 / -^ • -- s 
 
 Fig. 73.— Cecum of the Horse in Position, its Inner Face being seen. 
 1, The first colon; 2, the ileum. 
 
 the material lies a considerable time in them, and no juices other 
 than the succus entericus are poured into the bowel. These are 
 conditions exactly the reverse of those found in the small intes- 
 tines. The bowels which are spoken of as the large intestines are 
 the caecum, double and single colon, and the rectum. 
 
222 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The Caecum has been described by Ellenberger as a second 
 stomach ; its enormous capacity and fantastic shape have always 
 rendered it an intestine of considerable interest (Fig. 73). To 
 our mind its most remarkable feature is that it is a bag, the 
 openings into and out of which are both found in the upper part, 
 close together; the exit, strange to say, is above the inlet, and the 
 contents have to work against gravity in order to obtain an entry 
 into the next intestine, the double colon. This is made possible 
 by the four muscular bands on the caecum (Fig. 74), which shorten 
 the bowel, forcing the contents upwards towards the ' crook.' 
 The ileum being closed, the only available outlet is into the colon 
 (Fig- 75)- 
 
 Fig. 74. — Schematic Arrangement of the Longitudinal Muscular Bands 
 op the Cecum. 
 
 Bands i.and 2 are one, and form a complete sling for the bowel; band 4 runs 
 from the caecum to the pelvic flexure of the colon. It is a remarkable band, 
 and doubtless intimately connected with the mechanism which brings 
 about the passage of material from ca?cum to colon. 3 is a single band be- 
 longing exclusively to the caecum. 
 
 On the nature of the communication between the large and 
 small intestines several questions suggest themselves. It would 
 seem certain that in order to get from the ileum into the colon 
 everything must pass into or, at any rate, through the caecum, 
 yet we feel sure that material does not remain there long. Is it 
 possible that the openings of the ileum and of the colon might be 
 brought together so that material may pass directly from one into 
 the other ? Nothing is returned into the ileum from the caecum; 
 there must, in consequence, be a sphincter keeping the ileum 
 closed, for when the caecum contracts material must cross the 
 opening of the ileum in'order to reach the colon (Fig. 75). This 
 sphincter is furnished by the thickened condition of the wall of 
 the ileum. We see no difficulty in believing that the rigid end 
 
DIGESTION 
 
 22.1 
 
 of this tube may pass almost all its solid contents direct into the 
 colon, and the slightly funnel-shaped arrangement of the latter 
 would readily admit the rigid nozzle of the ileum. 
 
 The contents of the caecum are always fluid, sometimes quite 
 watery, occasionally of the colour and consistence of pea-soup, 
 and when in this condition they are full of gas bubbles; when 
 watery, the fluid is generally brownish in colour, with particles 
 of ingesta floating about in it. The reaction of the contents is 
 always alkaline; all observers are agreed on this point.* 
 
 1--S 
 
 Fig. 75. — The Openings of the Ileum and Colon in the Caecum. 
 
 1, The ileum; 2, the colon. In the figure the openings are represented close 
 together, but even when stretched apart they are less than 4 inches distant. 
 
 The caecum is most admirably arranged as a receptacle for 
 fluids, and though absorption undoubtedly occurs from it, and 
 digestion of cellulose takes place in it, yet we believe its chief 
 function is the storing up of water for the wants of the body and 
 the digestive requirements, as it is absolutely certain that diges- 
 tion in the horse can be properly carried out only when the 
 contents are kept in a fairly fluid condition. We do not say that 
 the csEcnm produces no digestive changes in the food, for we have 
 stated that the contents are occasionally of the consistence of 
 pea-soup, exhibiting active fermentation, but we consider its 
 digestive function subordinate to its water-holding one. Ellen- 
 berger, on the other hand, views the caecum as a bowel for 
 the digestion of cellulose, where by churning, maceration, and 
 decomposition, this substance is dissolved and rendered fit for 
 * The writer only once found the caecum acid. 
 
224 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 absorption, and he likens it to the stomach of ruminants and the 
 crop of birds. He further considers that the caecum owes its 
 existence to the small size of the stomach, and the rapidity 
 with which the contents are sent along the small intestines. 
 His experiments demonstrated that the entire ' feed ' reaches 
 the caecum between twelve and twenty-four hours after entering 
 the stomach, that it remains there twenty-four hours, and that 
 during that time 10 to 30 per cent, of the cellulose disappears. 
 The digestion of cellulose is no doubt a very important matter, 
 especially as we know that the poorer the food the more cellulose 
 digested ; but we are not prepared to admit that food necessarily 
 remains in the caecum twenty-four hours, and we believe that 
 cellulose digestion occurs principally, though not entirely, in the 
 colon, and, further, that it is not absolutely necessary for the 
 material to remain in the caecum, but that it may pass on at once 
 to the colon. The writer's experiments on digestion have shown 
 that ingesta may reach the caecum three to four hours after enter- 
 ing the mouth, and we are quite clear on the point that oats may 
 travel some considerable distance along the colon in four hours 
 from the time of being consumed, though we regard this as ex- 
 ceptionally rapid. For example, a horse which had never had 
 maize and had not tasted oats for two or three years was fed 
 first with 2| pounds of maize, and seventeen hours later with 
 4 pounds of oats. The animal was destroyed four hours from 
 the time of commencing to eat the oats. Much maize and a few 
 oats were found in the pelvic flexure of the colon, and a certain 
 proportion of maize and a quantity of oats in the stomach. In 
 twenty-one hours the small ration of 2| pounds of maize was 
 distributed between the stomach and pelvic flexure of the colon, 
 which is a very large area. In four hours the oats reached the 
 same point in the bowel that the maize had arrived at ; this is 
 exceptionally rapid, but this experiment supports two points it 
 is desired to emphasise- — viz., the difficulty in getting the stomach 
 to empty itself completely, and the rapid transit of material 
 through the small intestines. 
 
 Colin believed that in the caecum starch can be converted into 
 sugar, fats emulsified, and that the active absorption of assimil- 
 able products occurs there. 
 
 The Colon. — The direction taken by the colon of the horse is 
 remarkable. It commences high up under the spine on the right 
 side, its origin being very narrow, but it immediately becomes of 
 immense size ; it descends towards the sternum, and, curving to 
 the left side, rests on the ensiform cartilage and inferior abdominal 
 wall. The colon now ascends towards the pelvis, and here makes 
 a curve, the bowel becoming very narrow in calibre ; the pelvic 
 flexure having been formed, the intestine retraces its course 
 
DIGESTION 
 
 225 
 
 towards its starting-point. Running on top of the portion pre- 
 viously described, it descends towards the diaphragm, growing 
 gradually larger in calibre, and then ascends towards the loin, 
 being here of immense volume — in fact, at its greatest diameter ; it 
 then suddenly contracts, and forms the single colon (see Figs. 76 
 
 Fig. 76,-The Double Colon looked at from Above (modified from 
 
 M'tiLLEE). 
 
 „ The fir st co,n the — ^ ^-^^^1'^ 
 sacculated portions of the bowels. 
 
 and 77). The object of the variations in the volume of the double 
 coloir appears to be the convenience of its accommodation m the 
 
 into four portions: the ingesta m the first and third .descend in 
 the second and fourth ascend. It is found that the physical 
 characters of the contents are not the same through out. I nthe 
 first colon the food is fairly firm, and the particles of corn, etc., 
 
226 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 can be readily recognised; in the second colon the material is 
 becoming more fluid, while at the pelvic flexure the contents are 
 invariably in a liquid, pea-soup-like condition, and the particles 
 of which they are composed are not readily recognised. In the 
 third colon the material becomes firmer, but only slightly so, and 
 bubbles of gas are being constantly given off from its surface; 
 in the fourth colon the entire ingesta are like thick soup, and the 
 
 Fig. 77. — Position of the Cecum and Double Colon on the Floor of the 
 Abdomen seen FROM Below. 
 
 The point of the cascum is directed towards the sternum. The abdomen has 
 spread open in front owing to the needful dissection. It should be egg- 
 shaped, the narrow end foremost. 
 
 material composing them is in a finely comminuted condition, 
 the surface being covered with gas bubbles. For the first foot 
 or so of the single colon this condition is maintained, when quite 
 suddenly the contents are found solid and formed into balls. 
 The remarkable suddenness of this change is invariable in a 
 state of health, and indicates either most active absorption, or 
 that the contents are subjected to great compression. The entire 
 contents of the colon are yellow in colour or yellowish-green, 
 
DIGESTION 
 
 227 
 
 becoming rapidly brown or olive-green on exposure to the air, 
 the colouring being due to the chlorophyll of the food. The con- 
 tents of the colon are normally alkaline throughout; we once, 
 however, found them acid. 
 
 Digestive Changes. — Large Intestines. 
 
 The changes food undergoes in the large intestine have never 
 excited the same interest as those effected in the small. The 
 absence of any secretion from the large bowel other than the 
 succus may help to account for this, and may also assist in ex- 
 plaining why the large bowels have been regarded as reservoirs 
 for ingesta, rather than as active centres of digestion. As a 
 matter of fact, the large intestines of the horse are actively em- 
 ployed in dealing with cellulose and its alhes, not by means of 
 any known enzyme peculiar to the body, but rather by the 
 process of bacterial disintegration, the result of decomposition. 
 It is known that organisms may hydrolyse cellulose, and render 
 it soluble. In the case of oats, we mentioned (p. 199) that they 
 probably furnish their own cellulose enzyme, but auto-provision 
 of enzymes has not been proved for all vegetable material. The 
 cellulose of hay, if utilised, can only be so after prolonged macera- 
 tion in the large intestines and the subsequent attack of bacteria. 
 By some it has been considered that the epithelial cells of the in- 
 testine are capable of dealing with cellulose, but on this point 
 no definite statement can be made. There is good reason for the 
 extensive preparations made for cellulose digestion in herbivora 
 — viz., that the cellulose encloses the protein, starch, and fat of 
 vegetable substances in a framework, and until this is broken 
 down these substances cannot be acted upon . We know that con- 
 siderable cellulose solution must occur before the material arrives 
 at the large intestines, otherwise neither in the stomach nor small 
 intestine could digestion be the important function it is. The 
 digestion of protein, fat, and sugar is largely, though not entirely, 
 effected in the stomach and small intestine, but there must be a 
 certain amount of these substances so firmly locked up in their 
 cellulose envelopes that they are not liberated until after pro- 
 longed maceration and digestion in the large intestines. We 
 may, therefore, safely assume that protein, fat, starch, and cellu- 
 lose are capable of being acted upon in the large intestines of 
 the horse. 
 
 As the final result of cellulose digestion carbonic acid and 
 marsh-gas are formed in equal volumes, and when produced in 
 excessive amounts cause acute disease and suffering.* We have 
 
 * The chapter dealing with ' Nutrition ' should be consulted for further 
 information on cellulose and its digestion. 
 
228 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 in our description of the large intestines drawn attention to the 
 appearance of the contents of the caecum and fourth portion of the 
 double colon. The contents, on the surface of which gas bubbles 
 are constantly breaking, were described as like pea-soup. It 
 may well be that these two places are the active seats of the final 
 transformation of cellulose, the caecum dealing with that which 
 has already been acted upon in the stomach and small intestines, 
 and the fourth colon being concerned with the more refractory 
 cellulose, which has required prolonged maceration in the large 
 intestines before becoming capable of solution. This view is 
 supported by the remarkably rapid change in the character of. 
 the contents in the single colon, the somewhat pea-soup likeness 
 giving way, in the space of a few inches, to the appearance pre- 
 sented by ordinary normal fasces. 
 
 The large intestines cannot exist entirely for the solution of 
 cellulose. There are other processes going on, chief of which is the 
 bacterial attack on the unabsorbed protein products of the small 
 intestines. The small intestine may be regarded as practically 
 free from putrefactive processes; in fact, it is only towards the 
 ileum that the unpleasant products of pancreatic digestion can be 
 detected. In the large intestine, on the other hand, putrefactive 
 processes are evident throughout ; the bacteria are here engaged, 
 among other things, in attacking the unabsorbed products of 
 protein digestion, and reducing them to simpler end-products, 
 such as proteoses, peptones, amino-acids, indol, skatol, phenol, 
 phenyl-propionic, phenyl-acetic and fatty acids, with the evolu- 
 tion of CO a , H 2 , H 2 S, and CH 4 . The aromatic end-products are 
 partly got rid of through the faeces, and partly absorbed into the 
 blood, taken to the kidneys, and, combining with sulphuric acid, 
 are got rid of through the urine ,- especially is this true of phenol, 
 indol, and skatol. 
 
 Metschnikoff made a special study of the organisms living 
 in the intestinal canal of man, and arrived at the conclusion 
 that in the myriad population of the intestinal tract there are 
 at least three anaerobic organisms which can produce very viru- 
 lent poisons. These poisons, which are of the aromatic series, 
 phenols and indols, are absorbed by the intestinal wall. Metsch- 
 nikoff regarded the colon with grave suspicion, and believed 
 that its defects are the cause of men not dying from old age. 
 Diseases of the colon certainly cut short the useful life of many 
 horses. 
 
 As the material moves towards the rectum it becomes drier 
 and drier, and more thoroughly formed into balls by the action 
 of the bowel-sacs, which squeeze the mass into a round or oval 
 shape. The contents of this portion are still alkaline, or slightly 
 so. As we approach the anus a distinctly acid reaction is ob- 
 
DIGESTION 229 
 
 tained on the surface of the faces, though at this time the interior 
 of the ball may be, and often is, alkaline ; the converse of this 
 may also be observed. In the rectum the single balls collect 
 in a mass, to be forced out of the body at the next evacuation. 
 The reaction of this mass is acid, and its colour depends on the 
 food, being, on an ordinary diet, of rather a reddish-yellow or 
 brownish tint, due to altered chlorophyll. 
 
 -Schematic Arrangement of the Intestines of the Ox. 
 
 1, The small intestines; 2, their termination in the caecum; 3, the caecum; 4, the 
 'spiral' colon; 5, the single colon. In the spiral colon it will be observed 
 that the ingesta travel apparently in opposite directions. 
 
 There is undoubtedly absorption from the single colon and 
 from the rectum. Animals may be killed by the rectal injection 
 of strychnine; narcosis can be produced by the rectal administra- 
 tion of ether, and life may be supported, at any rate for a short 
 time, by means of nutrient enemata. Absorption from the 
 single colon and rectum is rapid; the marked change in the 
 physical character of the faeces is evidence of this. 
 
 Intestinal Digestion in Ruminants. 
 
 Though intestinal digestion is so important in the horse, it 
 would appear in ruminants to occupy a position subordinate to 
 
230 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 stomach digestion. It is curious why in one animal the changes 
 should occur at the anterior, and in the other at the posterior 
 part of the digestive tract, but this difference in the arrangements 
 for digesting cellulose depends upon the one being capable of 
 rumination and the other not. The rumen of the ox corresponds 
 to the large intestines of the horse. The intestines of the ox are 
 of extreme length, but small in calibre ; they are half as long again 
 as those of the horse, and it would appear that their chief function 
 is that of absorption. Their arrangement, especially that of the 
 large intestine, is most singular. The small intestines are hung 
 in convolutions on a mesentery; they are narrow in diameter, 
 and about 120 feet in length. The large intestines are about 
 9 metres (30 feet) in length, also narrow and without muscular 
 bands or puckerings, as in the horse ; the colon is arranged in a 
 remarkable spiral manner between the folds of the mesentery 
 (see Fig. 78). 
 
 It is in this immense length of absorbent surface that the food 
 substances capable of being utilised are taken up. It is clear, 
 however, that certain digestive changes occur in the small 
 intestines, into which, as in other animals, the pancreatic and 
 biliary fluids are poured. Here the proteins which have escaped 
 the stomach, and the fats and starches, are changed and rendered 
 fit for assimilation ; the altered cellulose only finds its way here 
 when rendered fit for absorption by its digestion in the rumen, 
 that is, assuming any portion of it escapes being hydrolysed into 
 carbon dioxide and water, see p. 201. 
 
 Intestinal Digestion in other Animals. 
 
 In the pig intestinal digestion is said to be of short duration, 
 and absorption very rapid. In the dog the material passes out 
 of the stomach slowly, and only in small quantities, into the 
 small intestines, which are usually found collapsed. It is in the 
 small intestines of this animal that the chief digestion occurs, as 
 the large bowels are rudimentary. In the sheep, ox, pig, and 
 dog, the reaction of the contents of the small intestines is acid 
 anteriorly and alkaline towards the ileum ; probably in all animals 
 the contents of the large intestines, unless in the rectum, are 
 alkaline in reaction. 
 
 The following table gives some information regarding the 
 intestinal canal in the domestic animals, which helps to explain 
 some of the differences in their modes of digestion. 
 
 The intestinal capacity is, on an average :— 
 
 Horse 
 Ox 
 Pig 
 Dog 
 
 200 litres - 
 
 -44 gallons. 
 
 80 ,, 
 
 17-5 
 
 27 ., 
 
 5-9 - 
 
 8 „ 
 
 i-75 .- 
 
DIGESTION 231 
 
 Ratio of length of intestine to length of body : 
 
 Sheep - _ 2 6 times longer. 
 
 ° x 20 ,, * 
 
 Horse I2 
 
 D °S 5 " ." 
 
 Movements of the Intestines. 
 
 The movements of the intestines are brought about by the 
 involuntary muscle composing the wall. This muscle in the 
 small intestines is arranged in two layers, one circular, the other 
 longitudinal, while in the large intestines narrow bands of pale 
 muscle of considerable length take the place of the ordinary 
 longitudinal layer, and maybe found on all parts where the tubs 
 is sacculated. In fact, one function of the bands is to bring 
 about the sacculated condition of the canal, an important 
 arrangement whereby economy of space is effected with no loss 
 of surface. 
 
 Flexures : 
 Sternal Felvie Diaphragmatic 
 
 * \ To 
 
 Single 
 
 tecum \ s s l y"~coTo£ 
 
 > » : r — — * 1 ■* 
 
 1st Colon ; anaCJoion ] 3rd Colon ; 4th Colon 
 • 1 1 
 
 Fig. Jg. — Schematic Arrangement of the Muscular Bands on the 
 Double Colon. 
 
 The colon is supposed to be opened out into a straight tube. Bands 1, 2, and 3 
 run from the first colon to the pelvic flexure; one of the three actually comes 
 from the apex of the caacum. No. 4 is the only band running the whole 
 length of the bowel. Nos. 5 and 6 originate in the region of the third colon 
 and finally run to the single colon. 
 
 The sacculated condition of the double colon in the horse is con- 
 fined principally to the first, second, and fourth portions. The 
 third portion, especially at the pelvic flexure, is free from saccula- 
 tions, and the fourth portion is not so liberally puckered as the 
 first and second. On the first colon there are four bands, on the 
 second colon there are also four, three of which disappear at the 
 pelvic flexure ; on the third portion there is only one band, while 
 on the fourth colon there are three (see Figs. 74 and 79). In 
 the large intestines the longitudinal layer of fibres is confined 
 to the muscular bands, so that the greater portion of the wall con- 
 sists of circular muscle only. The longitudinal bands shorten 
 the bowel, but the work of pressing the contents along is per- 
 formed by the circular layer. The bands, in fact, are numerous 
 where the intestine is large, and reduced in number where the 
 
232 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 bowel becomes smaller. This arrangement suggests that they 
 may, under suitable conditions, produce an irregularity of pull, 
 and we can see no other explanation of displacement of the large 
 intestines of the horse (a matter dealt with more fully at the end 
 of this chapter) than such action of these muscular bands. 
 
 The muscular movements of the large intestine are slower than 
 those of the small bowels; possibly one reason for this may be 
 that the food has to remain a longer time in contact with the 
 absorbing surface — viz., for at least forty-eight hours, and 
 usually for as long as four days. The peristaltic movement of 
 the small intestines is quite distinct from that of the large ; the 
 one movement ends at the ileum, the other begins at the caecum. 
 
 The muscle of the intestinal wall causes the movement known 
 as peristalsis, which normally passes in the direction stomach to 
 rectum. Relatively quick in the small intestines, it becomes 
 slower and more deliberate in the large, but the wave has always 
 this one object in view — viz., to press the ingesta onward. A 
 wave of contraction passing the reverse way — viz., in the direc- 
 tion of rectum to stomach — is known as antiperistaltic: such a 
 movement is considered abnormal, but in the horse, according to 
 the observations of Colin, antiperistalsis. of the small intestines 
 is a na'ural condition. Some physiologists recognise anti- 
 pe istaltic movements of the large intestines as being normal 
 in certain animals, producing a to-and-fro movement of the 
 contents, but it is generally thought that in the small bowels 
 antiperistalsis is present only under abnormal circumstances. 
 If antiperistalsis be admitted for the large bowels, we see no 
 difficulty in its extending to the small, especially in view of Colin's 
 positive statement that it occurs. The peristaltic wave depends 
 upon a something peculiar to the bowel wall, for if a piece of 
 small intestine be experimentally reversed, so that the portion 
 orig'nally near the stomach is made to occupy the position 
 farther away from it, it is found that the peristaltic wave in the 
 reversed segment is still in the original direction instead of in the 
 new direction. The actual mechanism involved in a peristaltic 
 contraction, according to Starling and Bayliss, is as follows : 
 The circular muscle on the stomach side of the bolus contracts, 
 while that on the far side is relaxed for some distance, so that the 
 advancing wave drives the bolus into a relaxed portion of bowel. 
 If a solution of cocaine or nicotine be applied to the intestinal wall 
 these movements cease, from which it is argued that they are 
 probably due to local ganglia . 
 
 Rhythmical or Pendular movements of the small intestines 
 were first described by Starling and Bayliss. They consist of a 
 series of local contractions caused by the presence of food in the 
 canal, and occur in the dog at the rate of twelve, and in the cat 
 
DIGESTION 233 
 
 of thirty times a minute approximately. They have been studied 
 by means of the Rontgen rays and a bismuth diet. Pendular 
 movements are essentially connected with the division and sub- 
 division of food in the intestinal canal ; by means of the rays a 
 string of material may be seen to become suddenly segmented, 
 each segment again dividing, and each of these may in turn 
 be further subdivided in a perfectly definite manner. In this 
 condition each segmented portion is exposed to thorough mixing 
 with the secretions in the intestine, and to enable the finely divided 
 contents to be so acted upon the bowel is for a time free from 
 peristalsis. After this interval, a peristalsis sweeps together 
 all the scattered atoms, and forms them once more into a string 
 of material. This remarkable pendular movement is unaffected 
 by the action of cocaine or nicotine, which has been shown to 
 inhibit at once the ordinary peristaltic movements. 
 
 In the dog, in addition to peristaltic and pendular movements, 
 another movement has been described much slower, but also 
 rhythmical, which may be carried out for twenty or thirty minutes 
 at intervals of two hours, even when the canal is empty. 
 
 We have been told by Colin that digestion in the small intes- 
 tines of t e horse is carried on by peristalsis and antiperistalsis, 
 the fluid travelling from stomach to ileum, and from the ileum 
 towards the stomach. Pendular movements are of no value in 
 the small intestine of this animal, for as, until the ileum is 
 reached, the entire material is fluid, there are no strings of food 
 to be segmented. This may account for pendular movements 
 of the bowels not having been observed in the horse. 
 
 In the first and third portions of the colon of the horse the 
 ingesta travel by their own gravity; in the second and fourth 
 portions they travel against gravity, as in the caecum. As the 
 first and fourth and second and third portions of the colon are 
 united, the curious result follows that material is passing along 
 each section in two apparently opposite directions. 
 
 The frequency of intestinal affections in the horse causes the 
 canal to be of exceptional practical interest. When the caecum 
 is found completely inverted into the colon, as if a hand had 
 passed through the colo-caecal opening, laid hold of the apex of 
 the caecum, and drawn the entire bowel within the first portion of 
 the colon, it is then that the question of muscular movements so 
 strongly presents itself; or, again, when what is far commoner 
 and equally fatal occurs— viz., displacement or actual twist of 
 the large bowel, or a complete twist of the small intestine, leav- 
 ing the bowels in such indescribable complexity that the parts 
 cannot be unravelled, even when removed from the body; or, 
 finally, in a condition rare in the horse, probably in all animals, 
 but still well recognised, in which telescoping of the small bowels 
 
234 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 occurs, known as ' intussusception.' It is impossible to believe 
 that muscular action of the intestines is free from all blame in 
 the production of these conditions. It is easier to understand 
 a twist of the small intestine apart from muscular action than it 
 is to understand displacement or actual twist of the large intes- 
 tine. A loop or coil of small intestine may be so distended by gas 
 or by ingesta as to become twisted, but it is more difficult to 
 imagine either of these producing twist or displacement of the 
 large intestines, and it becomes a question, as we have previously 
 said, of how far the action of the muscular bands of the bowel may 
 have a contributing influence. That great force is necessary we 
 cannot doubt, bearing in mind the difficulty, if not impossibility, 
 of restoring the parts to their original positions post mortem, or 
 reproducing the lesions experimentally after death . These matters 
 will be referred to again. 
 
 Nervous Mechanism of the Intestinal Canal. — Two distinct 
 impulses are conveyed by the intestinal nerves — viz., to contrac- 
 tion and to inhibition. In the anterior part of the tract the 
 former function is mainly or entirely carried out by the vagus, 
 stimulation of which is found to cause active contraction of the 
 small intestines. Contraction of the large intestines is effected 
 through branches of nerves which issue from the sacral portion of 
 the cord, and pass with the nervi erigentes to the hypogastric 
 plexus. From this plexus fibres run to the large intestines, 
 which produce on stimulation much the same results as stimula- 
 tion of the vagus — viz., active contraction of both circular and 
 longitudinal coats. 
 
 Stimulation of certain branches of the sympathetic nerve stops 
 or inhibits the contractions produced by stimulation of the 
 vagus, hence the term ' inhibitory.' The inhibitory nerves of 
 the small intestine are derived from the dorso-lumbar portion of 
 the cord, pass by the rami communicantes (re, Fig. 80) to the 
 main sympathetic chain, Sy., and thence through the large 
 and small splanchnic nerves to the solar plexus, from which the 
 final distribution to the intestines is made. The inhibitory 
 fibres for the large intestines are derived mainly from the lum- 
 bar cord through re. and Sy. (Fig. 80), and pass through to the 
 inferior mesenteric ganglion. From this ganglion inhibitory 
 fibres are supplied to both longitudinal and circular coats. 
 The connections of the abdominal sympathetic ganglia of the 
 horse are shown in Fig. 81. 
 
 Contractions of the bowels and peristalsis can occur after all 
 nerves leading to the intestines have been divided; this points to 
 the existence of local ganglia, and such may yet be found in the 
 intestinal wall. The intestinal movements are automatic and 
 
DIGESTION 
 
 235 
 
 self-regulated, though they can be provoked by both chemical 
 and mechamcal stimuli. The normal stimulus to peristalsis is 
 
 R.V 
 
 . oe 
 
 VI Xh 
 
 v. 
 
 Fig. 80. — Diagram to illustrate the Nerves of the Alimentary Canal of 
 the Dog (Foster). 
 
 (The figure is very diagrammatic, and does not represent the anatomical relations.) 
 
 Oe. to Set., The alimentary canal from the oesophagus to the rectum. 
 
 LV., Left vagus nerve ending on the front of the stomach; r.l., recurrent laryngeal 
 supplying upper part of oesophagus.;' R.V., right vagus joining left vagus, 
 in the oesophageal plexus, Oe.pl., supplying the posterior part of the stomach 
 continued as R'V to join the solar plexus, Sol. pi., here represented by a 
 single ganglion, and connected through x with the inferior mesenteric gan- 
 glion (or plexus), a, a, a, branches from the solar plexus to stomach and 
 small intestines, and G.m.i. b, from the mesenteric ganglion to the large intes- 
 tines. 
 
 Spl., Large splanchnic nerve arising from the thoracic ganglia of the sympathetic, 
 Sy., and rami communicantes, r t c, of the dorsal nerves. 
 
 Spl.nii.^ Small splanchnic nerve. Both the large and small splanchnics join the 
 solar ple,xus, and thence make their way to the alimentary canal, supplying 
 the small intestine with inhibitory impulses. 
 
 G.m.i., Inferior mesenteric ganglion formed by nerves running from the dorsal 
 and lumbar cord. ,Frqm this ganglion inhibitory nerves are given off to the 
 large intestines. 
 
 i.e., Nervi erigentes arising from the sacral cord, and proceeding to the hypo- 
 gastric plexus, PI. hyp. From this plexus impulses of a motor kind' are 
 supplied to the large intestines. 
 
236 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the passage of ingesta along the canal. In the dog, even the sight 
 of food is said to promote peristalsis. Gases, such as C0 2 , H 2 S, 
 
 and CH 4 , and organic 
 acids, such as acetic, 
 propionic, caprylic, 
 etc., act as stimuli, 
 and promote contrac- 
 tion. This is a for- 
 tunate circumstance, 
 as they are normal to 
 the bowel in conse- 
 quence of bacterial 
 activity. Oxygen gas, 
 on the other hand, 
 inhibits movements, 
 and, as a matter of 
 fact, we know that 
 oxygen gas does not 
 normally exist, or 
 exists only in traces, 
 in the gaseous con- 
 tents of the bowels. 
 Cutting off the blood- 
 supply to the bowels 
 causes violent con- 
 tractions, which occur 
 again when the cir- 
 culation is re-estab- 
 lished ; the former 
 point is of interest in 
 those cases of twist 
 where the blood- 
 supply is wholly or 
 partly interfered with. 
 Under normal con- 
 ditions the mind is 
 not conscious of peri- 
 staltic movements, 
 but when these be- 
 come very energetic 
 pain arises Under the 
 influence of nervous 
 excitement rapid and 
 frequent evacuations 
 of the bowels may take place in both cattle and horses. So 
 rapid may the evacuations be that in the horse, in a short time, 
 
 Fig. 8i. — Abdominal Pre-Vertebral Sympa- 
 thetic System of the Horse (Diagram- 
 matic). 
 
 i, Anterior mesenteric ganglion; 2, posterior mes- 
 enteric ganglion; 3, sympathetic cord under 
 the arches of the ribs; 4, the splanchnic ; 5, the 
 vagus passing to the stomach ; 6, fibres from the 
 anterior mesenteric ganglion joining the vagus; 
 7, branches of the sympathetic passing to the 
 stomach; 8, the stomach; 9, the posterior 
 aorta; 10, the internal iliac artery; 11, the 
 external iliac; 13, pelvic branches of sympa- 
 thetic; 14, mesenteric branches of sympathetic; 
 15, the oesophagus; 16, rich plexus formed by 
 vagus and sympathetic nerves. 
 
DIGESTION 237 
 
 the whole of the rectum and single colon may be unloaded, 
 Ordinary exercise is always an important cause of peristalsis', 
 and a main factor in keeping the posterior bowels emptied. 
 
 As previously remarked, the normal stimulus to peristalsis 
 is the presence of ingesta in the canal. In the feeding of her- 
 bivora bulk is essential; they cannot live in a state of health on 
 concentrated food alone. Their intestines need bulky food, if 
 only in order to maintain peristalsis. Bunge has shown that if 
 cellulose be withheld from the diet of rabbits, they die from intes- 
 tinal obstruction . It is the ceUulose and lignin in the diet of herbi- 
 vora which largely provide the needful stimulus to peristalsis. 
 
 Regulation of the Digestive Processes. — Under normal con- 
 ditions, excepting that caused by hot fluids, there is no sensation 
 in the alimentary tract posterior to the fauces ; speaking generally, 
 once the food has passed the point mentioned, we are no longer 
 conscious of its existence. The intestines and stomach may be 
 handled without causing pain. Nevertheless, in disease, when 
 they are inflamed, tightly contracted, nipped, or immensely 
 distended, the pain produced is acute, and, judging from the 
 violence shown by horses, intolerable. 
 
 With the absence or apparent absence of an afferent nerve- 
 supply, it is remarkable in what way the gastro-intestinal tract 
 continues to discharge its supreme functions. There are no im- 
 pulses passing to the brain or to the medulla directing opera- 
 tions, and it can be shown, experimentally, that extirpation of 
 the abdominal sympathetic in the dog does not cause any inter- 
 ference with digestion, nor with the movements of the intestines. 
 In fact, as we have just seen, movements of the intestines occur 
 after all the nerves have been divided. It is more than likely 
 that the key to the difficult problem is that furnished by Bayliss 
 and Starling, and that the regulation of the gastro-intestinal 
 tract is a chemical regulation by means of hormones. The exist- 
 ence of these substances has already been mentioned, and more 
 wiU be said of their nature later, but it is well here to emphasise 
 the fact that they are capable of acting after all nervous connec- 
 tions have been severed. They act through the medium of the 
 blood. 
 
 Gases of the Intestines. — The largest amounts of gas found in 
 the intestinal canal are in the caecum and colon; the small intes- 
 tines naturally contain very little, frequently none, whatever 
 is formed there being probably rapidly passed into the large 
 bowels. In the large intestines marsh-gas commonly exists, 
 forming with carbonic acid the greater part of the gases present. 
 The pathological conditions arising in the large bowels of horses, 
 and in the rumen of cattle, as the result of fermentation — particu- 
 larly of green food — and the enormous size to which these organs 
 
238 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 may in consequence be distended, are matters of common clinical 
 experience. In both horse and ox the gas may generally be 
 ignited a short distance away from the cannula which has been 
 passed to give relief, the marsh-gas igniting readily on meeting 
 with the proper proportion of oxygen. The whole of the chemical 
 changes in the intestinal canal are carried out in the absence of 
 oxygen; the gases which are produced depend mainly on the 
 nature of the food, green material producing marsh-gas and 
 carbonic acid, leguminous material producing sulphuretted 
 hydrogen and hydrogen 
 
 For Intestinal Absorption, see p. 286. 
 
 The Faeces. — The fseces consist of that portion of the food 
 which is indigestible, together with that part which, though 
 digestible, has escaped absorption ; mixed with these are water, 
 colouring substances, mucin, organic matters in great variety, 
 inorganic salts, bile pigment, volatile fatty acids, remains of 
 digestive fluids, organisms, etc. 
 
 The composition of the fseces depends largely on the diet. 
 The following table from Gamgee* is intended to give only a 
 general idea of its composition : 
 
 Approximate Composition of the Faeces of the 
 Horse, Cow, Sheep, and Pig. 
 
 
 Horse. 
 
 Cow. 
 
 Sheep. 
 
 Pig. 
 
 Water 
 
 Organic matter 
 Mineral matter - 
 
 76' O 
 
 21'0 
 
 30 
 
 84-0 
 
 13-6 
 
 2'4 
 
 580 
 
 3 6-0 
 
 60 
 
 8o-o 
 
 17-0 
 
 3-0 
 
 IOO'O 
 
 1000 
 
 ioo - o 
 
 IOO'O 
 
 Considerable differences exist amongst animals in the con- 
 sistency of the faeces; they are moderately firm in the horse, 
 pultaceous in the ox, and hard in the sheep. These differences 
 depend upon the amount of fluid they contain. In the pig they 
 resemble human fseces and are very offensive ; in the dog they are 
 soft or hard, dark or light, according to the diet, the mineral 
 matter of bones producing the light-coloured excreta. It is 
 necessary to remember that the proportion of fluid in the fseces 
 does not depend upon the amount of water which is drunk, 
 but rather on the character of the food, the activity of intestinal 
 
 * ' Our Domestic Animals in Health and Disease,' p. 253. 
 
DIGESTION 239 
 
 peristalsis, and the energy with which absorption is carried on 
 in the digestive canal. Succulent green food in horses produces 
 a liquid or pultaceous motion ; other foods, such as hay and chaff, 
 have a constipating effect, the faeces being large and firm; excess 
 of nitrogenous matter in the food produces extreme foetor of the 
 dejecta, and frequently diarrhoea, probably owing to putre- 
 factive processes. Nervous excitement in horses frequently 
 induces a free action of the bowels, accompanied by liquid faeces. 
 
 Faeces always float in water so long as cohesion is maintained. 
 The colour of the faeces in the horse is yellowish or brownish-red, 
 in the ox greenish-brown; they rapidly become darker on ex- 
 posure to the air. When the animal is grass-fed the fasces are 
 green, and when a horse is fed wholly on corn they become very 
 yellow, and like wet bran in appearance. The colour of the 
 fasces of animals receiving hay or grass is due to altered chloro- 
 phyll. The faeces of the horse are moulded into balls in the single 
 colon. They are always acid in reaction, the acidity probably 
 depending upon the development of some acid from the carbo- 
 hydrates of the food. 
 
 Fasces contain, amongst the indigestible portion of the ingesta, 
 lignin, a proportion of cellulose, husks of grains, the downy hair 
 found on the kernel of oats, vegetable tubes and spirals, starch 
 and fat granules, gums, resins, chlorophyll, etc.; unabsorbed 
 protein, carbohydrate and fatty matters; products of digestive 
 fermentation, such as lactic, malic, butyric, succinic, acetic, and 
 formic acids ; leucine, tyrosine, indol, skatol, and phenol ; biliary 
 matters and altered bile pigment — stercobilin — which gives the 
 colour to the dejecta in the dog, but not in herbivora ; and, lastly, 
 mineral matter in varying proportions. In the dog, if fed on 
 flesh, portions of muscle fibre, fat-cells, tendinous and fibrous 
 tissue, are found in the fasces. 
 
 Of the inorganic matter, silica exists in herbivora in the largest 
 amounts, then potassium and phosphates; sodium, calcium, 
 magnesium, and sulphates, form a smaller but still important 
 proportion. The horse excretes but little phosphoric acid by the 
 kidneys, but considerable quantities pass with the fasces in the 
 form of ammonio-magnesium phosphate. This salt is derived 
 principally from the oats and bran of the food. In conjunction 
 with calcium carbonate it frequently forms calculi through col- 
 lecting in the colon around a pebble or nail as a nucleus. Other 
 intestinal calculi are formed from collections of the fine hairs 
 from the kernels of oats becoming encrusted with ammonio-mag- 
 nesium phosphate, and forming oat-hair calculi. In the Persian 
 wild goat and in certain antelopes intestinal concretions are found, 
 known as Bezoar stones, formerly much used in medicine, and as 
 antidotes to poison. There are two varieties of these calculi, one 
 
240 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 olive-green, the other blackish-green. The first melts when 
 heated, emits aromatic fumes, and consists chiefly of an acid 
 allied to cholalic acid. The chief constituent of the second variety 
 is an acid derivative of tannic acid, which indicates the origin 
 of these calculi from food substances. Stomach calculi have not 
 been unknown in the horse, while in cattle, ' hair balls,' formed 
 as the result of licking each other, are frequently found. 
 
 The following table by Roger gives the mineral composition of 
 the faeces in every 100 parts of the ash:* 
 
 
 Horse. 
 
 Ox. 
 
 Sheep. 
 
 Sodium chloride • 
 
 003 
 
 0-23 
 
 0-14 
 
 Potassium ... 
 
 11-30 
 
 2-91 
 
 8-32 
 
 Sodium - - 
 
 ro8 
 
 098 
 
 3-28 
 
 Lime - 
 
 4' 63 
 
 5 - 7i 
 
 18-15 
 
 Magnesium - 
 
 3-84 
 
 11-47 
 
 5" 45 
 
 Oxide of iron - 
 
 1-44 
 
 5-22 
 
 2- IO 
 
 Phosphoric acid 
 
 IO'22 
 
 8-47 
 
 g-IO 
 
 Sulphuric acid - 
 
 183 
 
 1-77 
 
 2-6g 
 
 Silica - 
 
 62' 40 
 
 62-54 
 
 50-II 
 
 Oxide of magnesium 
 
 2-13 
 
 
 
 The amount of faeces produced in twenty-four hours varies 
 with the quantity and nature of the food given. We have 
 observed that on a diet consisting of 6 kilogrammes (13-2 pounds) 
 of hay, 3 kilogrammes (6-6 pounds) of oats, and i\ kilogrammes 
 (3-3 pounds) of bran, the average amount of faeces passed by 
 fifteen horses during an experiment lasting seven days amounted 
 to 14 kilogrammes (30-8 pounds) in twenty-four hours, the faeces 
 being weighed in their natural condition— viz., containing 76 
 per cent, water; the dry material of this bulk of fasces is about 
 3-25 kilogrammes (7-5 pounds). More faeces are passed during 
 the night than during the day; in the above experiment, during 
 the twelve hours, 6 p.m. to 6 a.m., the average amount passed 
 per horse was 9 kilogrammes (19-8 pounds), while from 6 a.m. 
 to 6 p.m. the amount was 5-5 k'logrammes (12-1 pounds). The 
 largest amount of faeces we have known a horse produce was an 
 average for many days of 33-3 kilogrammes (73-2 pounds), 
 we-'ghed in their natural state, in twenty-four hours; the diet 
 consisted of 6 kilogrammes (13-2 pounds of oats, 1-5 kilogrammes 
 (3-3 pounds) of bran, and 12-8 kilogrammes (28-15 pounds) of 
 hay. In an experiment carried on for several months with dif- 
 ferent horses, each receiving 6 kilogrammes (13-2 pounds) of 
 hay, and varying proportions of bran and oats, the average 
 
 * Quoted by Ellenberger. 
 
DIGESTION 24 i 
 
 daily amount of faces was 12 kilogrammes (26-4 pounds). A 
 horse will evacuate the contents of the rectum about ten or 
 twelve times in the twenty-four hours, and the food he consumes 
 takes on an average four days to pass through the body. 
 
 In the ox the amount of faeces is between 32 kilogrammes 
 (70-4 pounds) and 37 kilogrammes (81-4 pounds) in the twenty- 
 four hours. In the sheep it varies from 1 to 3 kilogrammes (2 -2 to 
 6-6 pounds); in swine i\ to 3 kilogrammes (3-3 to 6-6 pounds), 
 depending on the nature of the diet. 
 
 The odour of faeces is distinctly unpleasant, owing to the 
 presence of indol and skatol; in disease it is often extremely 
 foetid, and occasionally horrible. 
 
 The act of defaecation is performed by a contraction of the 
 rectum, assisted by the abdominal muscles, the glottis being 
 closed. In the horse contraction of the rectum alone is suffi- 
 cient to expel its contents ; this is proved by the fact that this 
 animal can defalcate while trotting, showing there is no necessity 
 to fix the diaphragm and hold the breath, though when at rest 
 this does occur. It will thus be seen that the rectum of the horse 
 can exercise extraordinary power; the hand and arm may be 
 rendered almost numb by the pressure it can exert. The mass 
 driven backwards by this force causes the sphincters to dilate, 
 sometimes to an astonishing, degree, and as the last trace of 
 material is extruded, the contraction of the rectum is so great 
 that it forces to the exterior some of the mucous membrane, which 
 may be temporarily imprisoned by the contracting sphincters. 
 The muscle of the rectum receives both motor and inhibitory 
 fibres, as previously described. The extraordinary power of the 
 muscle in the rectum of the horse may partly be due to the hori- 
 zontal position of the body; there is no crouching of the body 
 during the act of defaecation, such as occurs, more or less, in all 
 other domesticated animals. The rectum has the whole work 
 to perform without aid, even, as we have just shown, without the 
 assistance of the diaphragm or abdominal muscles. 
 
 Two sphincters close the rectum in all animals — an external of 
 voluntary and an internal of involuntary muscle ; they are pre- 
 sided over by a centre in the cord. If this is destroyed, the 
 rectum remains uncontracted, unable to empty itself, and the 
 sphincter flabby ; in the dog the cord may be destroyed in the 
 lumbar region without interfering with the act of defecation, 
 which is then carried on by a reflex mechanism. 
 
 Meconium is the dark green material found in the intestines of 
 the foetus. It consists of biliary acids and pigments, fatty acids 
 and cholesterin, while salts of magnesium and calcium, phosphates 
 and sulphates, sodium chloride, soda, and potash are also found 
 in it. Meconium is the product of liver excretion. 
 
242 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Pathological. 
 
 The internal diseases of early life in the horse are mainly of the 
 chest, while those of the adult perioa are almost entirely confined to 
 the abdominal viscera, principally the intestines. The term ' colic ' 
 appears to be indissolubly associated with the horse, and it becomes 
 a question of the greatest practical and physiological interest to 
 ascertain the reason why digestive disturbances are so common and 
 so frequently fatal. There are certain obvious explanations of the 
 fact, but neither separately nor in combination are the accepted ideas 
 capable of explaining some of the mysteries surrounding the origin 
 of these diseases. 
 
 When muscular spasms of the intestines occur, the disease is 
 spoken of as colic. In many cases the pain which is exhibited is in 
 no respect due to muscular spasm, and is only a symptom. Still, 
 much the greater number of intestinal cases are of this kind — viz., 
 simple muscular spasms of some part of the digestive tract, but of 
 which part we are usually ignorant. It is obvious that the stomach, 
 the small or the large bowels, may be so affected, but there are no 
 definite symptoms which enable a positive diagnosis of location to be 
 established. It is important to bear in mind the possibility of 
 spasm of the muscular walls of the stomach, for there can be no doubt 
 this is generally overlooked, and the intestines almost universally 
 blamed. The evidence supporting the view we take of the liability 
 of the stomach to disorder, is afforded by the frequency of rupture 
 of this organ; not that the rupture is due to spasm of the walls, but 
 that the spasm is caused by stomach trouble, the rupture following 
 as a sequel, as detailed on p. 182. It is, however, admitted that 
 stomach spasm is far less common than spasm of the intestinal 
 portion of the tract. We would here emphasise the facts set forth 
 on p. 209 — viz., the general inability of the horse to vomit, and the 
 serious bar this proves to relief ; so much so, that it is hardly going too 
 far to say that if the animal could vomit, ruptured stomach would 
 be practically unknown, and stomach trouble generally a matter of 
 comparatively slight importance. The bursting strain of the stomach 
 is between four and five pounds on the square inch; when removed 
 from the body and distended, it may rupture when the pressure is 
 one-fourth or one-fifth of the above. 
 
 In connection with intestinal trouble, we are unable to say what 
 proportion the cases in which the small intestines are affected bear 
 to those in which the large are at fault. We cannot distinguish 
 colic of the one from colic of the other during life. Still, there are 
 good grounds for thinking that the large bowels are more frequently 
 affected than the small — viz : 
 
 1. Ingesta pass rapidly through the small intestines — so rapidly, 
 indeed, that, as mentioned at p. 220, these bowels are nearly always 
 found empty at ordinary post-mortem examinations, or the contents 
 in such a fluid condition that it is not reasonable to suppose, from 
 what we know of the behaviour of fluids generally in the anterior 
 part of the digestive tract, that they remain there long. 
 
 2. On the other hand, the large intestines always contain ingesta, 
 for the material passes along them very slowly, so that, of the three or 
 four days occupied in accomplishing the journey from mouth to 
 anus, all but a few hours are spent in the large intestines. It is 
 reasonable, therefore, to assume that in cases of purely uncompli- 
 
DIGESTION 243 
 
 cated disordered muscular action of the bowels, the large intestine in 
 the majority of cases is at fault. 
 
 Colic is not fatal, though Percival described a fatal case. The 
 writers experience leads him to believe that death from simple 
 spasm of the bowels is unknown, and he would emphasise the point 
 not only for the sake of accuracy, but as of value in prognosis. He 
 believes that m the case of any horse returned as dying from colic, 
 a more extensive search would have revealed some fatal lesion' 
 There is no reason for believing that the pain of colic per se is capable 
 of causing death. 
 
 If this be accepted— and it is fortunately capable of proof — it 
 considerably narrows the causes of death from intestinal affections, 
 and groups them mainly under two heads : (a) Inflammation of the 
 bowels, and (b) displacement of the bowels. 
 
 Enteritis, by which name inflammation of the bowels is known, 
 is spoken of as a common disease of the horse, but here again we join 
 issue with accepted doctrines, and urge that it is an uncommon 
 disease; further, that in the large majority of so-called cases of 
 enteritis, some displacement of the bowels with interference to the 
 circulation has occurred. That uncomplicated enteritis may exist 
 is not disputed, but we urge its relative infrequency, and press the 
 point that what looks like inflammation is more often strangulation. 
 When a deep purple, thickened coil of intestine is found on opening 
 the abdomen, such; a case is not enteritis. The colour indicates that 
 the blood-supply has been imprisoned as the result of strangulation, 
 and an identical appearance would have been obtained by ligaturing 
 the bowel. When half the double colon is found purple, thickened, 
 filled with blood-stained fluid ingesta, the wall of the bowel being 
 friable and its mucous membrane purple, then, however much we 
 may be tempted to speak of it as enteritis, it certainly is not this 
 disease, but strangulation. The term enteritis must be reserved 
 for that condition of bowel in which the mucous membrane alone is 
 inflamed. Such a bowel may give no external indication of trouble; 
 the general vascular supply is not interfered with; the full intensity 
 of the trouble falls on the mucous membrane, and such a condition 
 may be experimentally produced by the administration of an irritant 
 poison. It is probable that in the horse the majority, if not all 
 the cases, of pure enteritis met with, are due to a poison produced 
 during the process of digestive metabolism (see p. 228). That the 
 presence of an irritant without a poison has no such effect is abund- 
 antly proved by the pounds of sand, gravel, and calculi horses may 
 carry in their intestines for months, perhaps years, without producing 
 any apparent ill effect, certainly without producing enteritis. 
 Similarly, gastritis, excepting as the result of poison, is practically 
 unknown. 
 
 Our object in the above remarks is to focus attention on defects 
 in clinical observation, and to attempt a physiological analysis of 
 the most frequent, the most fatal, and by far the most acutely pain- 
 ful and distressing group of diseases that any animal is exposed to. 
 There is nothing in the whole range of comparative pathology, 
 including the diseases of man, which compares in violence, sudden- 
 ness, and mortality with the digestive diseases of the horse. We have 
 attempted to show how physiology is capable of enabling us to steer 
 a moderately accurate course, for it is certain that unless we are 
 agreed regarding the nature of the lesions found at post-mortem 
 examinations, we cannot reach that knowledge, the attaining of 
 
244 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 which is the object of our existence as a profession, and to which 
 physiology is only the guide. 
 
 What is the most common cause of death among horses from 
 intestinal affections, whether affecting the large or small bowels ? 
 There is only one answer to this, and time and careful inquiry will 
 prove its accuracy. The answer is Strangulation of the bowels, 
 partial or complete. This strangulation is capable of physiological 
 analysis. The most unobservant person cannot overlook a bunch of 
 small intestines so tied together as to defy all attempts at unravel- 
 ling, even when out of the body, but it takes no little careful observa- 
 tion to detect displacements of the large intestine. The size, weight, 
 and peculiar disposition of the double colon should secure it im- 
 munity from any form of displacement; when we lookat the bowels 
 in the abdomen, it appears impossible for any force, other than some 
 mysterious power, to be able to influence their position, yet we know 
 they are capable of being twisted as easily as if they were made of 
 cotton. We know also that one portion may be thrust into another, 
 in just the same way as a telescope collapses, constituting Intussuscep- 
 tion and it is even possible for a voluminous bowel like the caecum 
 to become completely inverted, and found within the colon, though 
 to get there it has to pass through an opening only an inch or two 
 in diameter. So remarkable, indeed, are these lesions that they can- 
 not always be imitated post mortem, and, as mentioned above, 
 it is impossible to untie many complicated knots in the small bowels, 
 even when the organs have been removed from the abdomen. 
 
 The actual mechanism which brings about twists of the large and 
 small intestines is disordered muscular action ; the factor responsible 
 for telescoping intestines is disordered muscular action, and dis- 
 ordered muscular action is the result of disordered nervous action. 
 For telescoping to occur, one portion of bowel must first contract 
 until it becomes but a mere shadow of its former self; the contracted 
 part must then be drawn within the dilated. A different cause is at 
 work to produce a twist of the small intestine; this, as we previously 
 indicated (p. 219) is tympany of the bowel, while in the case of the 
 large intestines the muscular action must be capable of causing the 
 bowel to perform a revolution more or less complete, and in this way 
 reverse its position. We cannot attempt to indicate the exact 
 disordered action which occurs; this problem would require to be 
 worked out in the living subject. The colon and caecum are most 
 liberally supplied with bands (Figs. 73, 74, 76, 77, and 79), and it 
 does not appear to us to be beyond the bounds of reasonable prob- 
 ability that these play a most important part in the production of 
 displacements of the large intestines. The cause of the disordered 
 nervous action which leads to this may, from its physiological in- 
 terest, be briefly dealt with. Apart from such obvious explanations 
 as errors in feeding (see, in this connection, pp. 186, 187, 198, 191, 
 26.5), the most common cause of derangement of the muscular action 
 of the digestive canal is work. It is this which accounts for the 
 majority of colic cases occurring towards the end of the day, for the 
 frequency with which the seizure occurs at or shortly after work, 
 especially that of an exhausting nature, and for the practical absence 
 of colic among non-working horses. We have even known a horse 
 in a ca\alry charge rupture the ileum as completely as if the parts 
 had been torn asunder by hand; and this, it will be remembered, 
 is the thickest and stoutest portion of the small intestine, and the 
 least likely to suffer laceration. The connection between such a 
 
DIGESTION 245 
 
 lesion and an exhausting gallop is at present not very apparent, 
 but the fact is undoubted. 
 
 The whole subject is of profound practical interest, and more has 
 been here said than is commonly considered to come within the 
 province of physiology; but the basis of exact clinical knowledge is 
 sound anatomy and physiology, and we feel that the physiological 
 aspect of digestive disorders has not yet received adequate attention. 
 It must, be borne in mind that the whole length of the digestive tract 
 is a chemical laboratory concerned in the analysis and synthesis of 
 food-stuffs, isolating and retaining those which are of use, getting 
 rid of those which are useless, and rendering harmless those substances 
 capable of acting injuriously. Not only is it a laboratory where the 
 above analytical operations are carried out, but it is also a factory 
 where the chemical reagents necessary for this process are prepared 
 beforehand. So thoroughly is the analysis performed, that the most 
 complex bodies are broken down into the simplest products. Can it 
 be wondered at that the chemical processes may sometimes fail, 
 and disorder result ? 
 
 We see a faithful reflex of the laboratory processes in the disorders 
 of the canal — the diarrhoea which is full of beneficence, impaction 
 which indicates a loss of muscular power and physical alteration 
 of the contents, acute tympany which announces active fermenta- 
 tion, rupture which indicates the strain on the walls of the appara- 
 tus; these, and other disorders too numerous to be dealt with, and 
 which no mere mention can explain, give some idea of the penalty 
 paid by horses for the doubtful privilege of domestication. The term 
 ' digestion of a horse ' has been framed in absolute ignorance of 
 the real facts. There is no animal in which these organs are more 
 readily disturbed, and none in which they are the subject of such 
 acutely painful and mortal lesions. On the other hand, the ass 
 appears to possess almost complete immunity, in spite of the coarse- 
 ness of his forage. If, as stated above, work is a factor in the pro- 
 duction of stomach and intestinal trouble, it may well be that the 
 slow, steady pace of the ass is his salvation. The mule, however, 
 is very liable to diseases of the digestive canal, though less so than 
 the horse. . 
 
 The ruminant, from the peculiarity of its physiological arrange- 
 ment, is far more liable to stomach than to intestinal trouble; 
 tympany, impaction, paralysis, and inflammation of one or more of 
 the compartments are common. In spite of the size of the oeso- 
 phagus, impaction is frequent — there is here a marked contrast to 
 the horse, in which the tube is narrow and impaction uncommon — 
 and calculi, the formation of which is a special feature in the intestine 
 of the horse, are found in the stomach of the ox, though arising from 
 very different causes. Strangulation of the bowels in the ox is not un- 
 known but limited to a special variety due to anatomical conditions. 
 
 Parasitic trouble is in all animals a prominent pathological feature, 
 the digestive canal from the mouth to the anus being liable to infec- 
 tion with numerous varieties of parasites, forming also the main 
 channel of parasitic entry for other parts of the body. Parasitic 
 aneurism of the mesenteric artery has been regarded as a_ cause of 
 colic in horses, though this does not accord with the writer s experi- 
 ence. A horse may have this vessel extensively diseased and have 
 never suffered from colic. The ass is practically never free from 
 parasitic invasion of the mesenteric arteries, yet he appears to possess 
 immunity to colic. 
 
CHAPTER VI 
 THE LIVER AND PANCREAS 
 
 Section i. 
 
 The Liver. 
 
 In considering the function of the liver it is necessary, to bear 
 in mind its peculiar blood-supply. Most glands of the body 
 which are called upon to produce a secretion are for that reason 
 furnished only with arterial blood, but the liver is an exception 
 to this rule ; the entire venous blood returning from the splanchnic 
 area — viz., the bowels, stomach, spleen, pancreas, etc.— consti- 
 tutes the material with which the liver is flooded. Blood de- 
 rived from so peculiar and considerable an area must be of very 
 mixed composition and charged with many products, some the 
 result of secretory activity, others the soluble constituents of 
 the elements of food, or, again, substances absorbed from the in- 
 testinal canal, which are by-products produced during the 
 gradual breaking-down of the food substances. It is with this 
 blood that the liver performs its various functions, and one of 
 the most evident — viz., the secretion of bile— will be dealt with 
 first. 
 
 Bile. 
 
 The bile is a fluid of an alkaline reaction, of bitter taste, of a 
 specific gravity in the ox of 1-022 to 1-025, in the sheep from 
 1-025 to 1-031, and in the horse 1-005. The colour is yellowish- 
 green or dark green in herbivora, reddish-brown in the pig, and 
 golden-red in carnivora. These differences in colour depend 
 upon the character of the pigment present. Bile taken from the 
 gall-bladder is viscid, due to admixture with nucleo-albumin 
 during its stay in the latter receptacle ; that taken direct from 
 the liver is relatively watery in consistence. The secretion con- 
 tains no protein, which is somewhat remarkable; biliary pig- 
 ments, bile acids, fats, soaps, lecithin, cholesterin, and inorganic 
 salts are found in varying quantities. By the secretion standing 
 
 246 
 
THE LIVER AND PANCREAS 
 
 247 
 
 in the gall-bladder the proportion of the solids is considerably 
 increased, owing to an absorption of part of the water of the bile. 
 The secretion in the horse contains no mucin, and, according 
 to Ellenberger, there is very little mucin in the bile of sheep; 
 what was believed to be mucin in ox bile, and what confers on the 
 latter its ropy character, is now known to be nucleo-albumin. 
 
 The dried alcoholic extract of bile contains in the ox 3-58 per 
 cent, of sulphur ; in the sheep, 571 per cent. ; and in the pig, 0-33 
 per cent. The gases found in bile are carbonic acid, and traces of 
 oxygen and nitrogen. The chief inorganic salts are sodium 
 chloride and phosphate, besides which are found salts of calcium, 
 of magnesium, of potassium, of iron, with phosphoric and 
 sulphuric acids; the sodium salts always exist in the largest 
 proportion. The iron, which is found as phosphate, is probably 
 derived from the haemoglobin of the blood during the formation 
 of the bile pigments. 
 
 The following table, showing' the percentage composition of 
 various biles, is compiled mainly from Ellenberger: 
 
 
 Horse Bile. 
 
 Ox Bile. 
 
 Dog Bile. 
 
 Pig Bile. 
 
 Water 
 
 95 
 
 go - 40 
 
 953 
 
 88-8 
 
 Solids 
 
 5 
 
 9' bo 
 
 4 - 7 
 
 I V2 
 
 Bile acids ■> 
 
 
 
 
 
 Bile pigments 1 
 Fat j " 
 
 — 
 
 8-30 
 
 4' 1 
 
 IO'I 
 
 Mucin / 
 
 
 
 
 
 Salts - 
 
 
 
 I- 30 
 
 o-6 
 
 I- 1 
 
 Percentage Composition of the Ash of Ox Bile. 
 
 Sodium chloride - - 27-70 
 
 Potassium - • 4' 80 
 
 Sodium - - 3 6 '7° 
 
 Calcium carbonate - i"40 
 
 Magnesium- - - 0-53 
 
 Iron oxide - - - 0-23 
 
 Manganese peroxide 
 Phosphoric acid 
 Sulphuric acid 
 Carbonic acid 
 Silica - 
 
 - I2 
 
 ,i°'45 
 
 ' 6-39 
 
 11-26 
 
 036 
 
 The differences found in the composition of bile probably 
 depend upon whether it be taken from the gall-bladder or from 
 a fistula, the fo mer being the more concentrated. 
 
 The Bile Pigments are two in number— bilirubin and bili- 
 verdin; the latter is produced by oxidation from the former. 
 Bilirubin is the colouring matter of human bile and of that of 
 carnivora, whilst biliverdin is the pigment of the bile of herbivora. 
 It is not uncommon to find both pigments in the same specimen of 
 bile. Though the bile of the dog contains exclusively bilirubin 
 as a pigment, yet the placenta of this animal is rich in biliverdin. 
 
248 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 In the ox and sheep a pigment is present in bile which shows 
 first a three banded, and later, after standing, a four-banded 
 spectrum. These bands are due to cholohaematin, which is not 
 a bile pigment proper. The pigments are insoluble in water, 
 but soluble in alkalies; in the bile they are held in solution by 
 the bile acids and alkalies. Bilirubin may be obtained from the 
 gall-stones of the ox in the form of an orange-coloured powder, 
 which can be made to crystallise in rhombic tablets and prisms. 
 If an alkaline solution of bilirubin be exposed to the air, it becomes 
 biliverdin by oxidation, and this latter pigment, by appropriate 
 treatment, may be obtained as a green powder. Both colouring 
 matters of the bile behave like acids, forming soluble compounds 
 with metals of the potassium group, insoluble ones with those of 
 the calcium group (Bunge) . 
 
 On the addition o| nitric acid (containing nitrous acid) to the 
 bile pigments a play of colour is observed ; this test is known as 
 Gmelin's test. In the case of bilirubin the colours pass from 
 yellowish-red to green, then to blue, violet, red, and yellow; 
 each of these colours is indicativeo f a different degree of oxidation 
 of the original bilirubin . Biliverdin gives the same play of colours, 
 except that the initial yellowish-red is absent. 
 
 Although bilirubin has not been obtained from haemoglobin, 
 there is no doubt that this is the source of the pigment, for if 
 haemoglobin be liberated in the blood and enter the plasma, 
 bile pigments appear in the urine ; further, haemoglobin may be 
 readily decomposed, yielding a protein and haematin; and if 
 this haematin be deprived of iron, the residue thus obtained is 
 not very dissimilar in cqmposition to bilirubin. We have pre- 
 viously mentioned (p. 15) that old blood-clots contain an iron- 
 free substance known as haematoidin, and this is practically 
 identical in composition with bilirubin. When red blood-cells 
 disintegrate in the ordinary course of wear and tear, the liberated 
 haemoglobin is brought to the liver, and under the influence 
 of the liver cells converted into the iron-free substance bilirubin 
 or biliverdin. Part of the iron so liberated escapes from the 
 body through the bile, but the greater part of it is retained, and 
 again used in the formation of haemoglobin by the organs which 
 discharge this function. 
 
 Though biliverdin is the colouring matter of the bile of her- 
 bivora, yet the gall-stones found in the ox consist very largely of 
 bilirubin combined with chalk ; in the pig the same combination 
 is observed. Bilirubin is said by Hammarsten to be constantly 
 present in the serum from horse's blood, though not in that from 
 blood of the ox, and Salkowski states that it is a normal consti- 
 tuent of the urine of the dog during the summer. In the large 
 intestines both bilirubin and biliverdin undergo reduction, re-' 
 
THE LIVER AND PANCREAS 249 
 
 suiting in the formation of stercobilin, the colouring matter of the 
 faeces in some animals. It is possible also that some of the pig- 
 ment is reabsorbed from the intestinal canal, carried to the liver, 
 and again eliminated. The value of this circulation of bile pig- 
 ment is unknown. 
 
 The Bile Acids are two in number — glycocholic and tauro- 
 cholic acids; they are formed in the liver by the union of cholalic 
 acid with glycine or taurine, and exist in the bile as salts, in 
 combination with soda. These salts are found in varying propor- 
 tions in different animals ; thus, glycocholate of soda is found 
 in large quantities in herbivora, taurocholate principaUy in 
 carnivora, while in the pig hyoglycocholic and hyotaurocholic 
 acids are found. Both salts are soluble in water, have a 
 markedly alkaline reaction, rotate the plane of polarised light 
 to the right, and may be obtained in a crystalline form as 
 highly deliquescent acicular needles. Glycocholic acid is 
 diminished by an animal and increased by a vegetable diet. 
 Taurocholic acid differs characteristically from glycocholic by 
 containing sulphur, by which it shows its protein origin.. 
 
 The origin of the bile acids is involved in obscurity. 
 Cholalic acid is a non-nitrogenous body, whereas both glycine 
 and taurine are nitrogenous. Glycine, also known as glycocoll, 
 is a member of the important amino-acid group and a constitu- 
 ent of protein. Taurine, on the other hand, though an amino- 
 acid, is not a constituent of protein ; it can, however, be readily 
 formed from cystine, which is a constituent of protein. It is 
 assumed that glycine and taurine are derived from the disin- 
 tegration of protein, but the precursors of cholalic acid are un- 
 known. Glycine cannot be traced in a free state in the body, 
 though, as we shall see later, it not only contributes to bile 
 formation, but in the kidney, in combination with benzoic acid, 
 produces hippuric acid. It is not known why glycine should 
 predominate in some animals and taurine in others. It appears 
 clear that the bile salts are formed in the liver cells. In the 
 intestines a portion of the bile salts is reabsorbed, carried to the 
 liver, and again excreted; or they may be split up in the intes- 
 tines into their constituents, the glycine arid taurine being carried 
 to the liver to be re-utilised, while the cholalic acid is excreted. 
 .This economical measure, the second of its kind noted in dealing 
 with the liver, has a two 'old advantage, for not only can the 
 glycine and taurine be used over and over again, but these bile 
 acids are the best of cholagogues, and stimulate the production 
 of bile. Pettenkofer's test for bile acids is performed as follows : 
 A drop of the fluid is placed on a white earthenware surface, and 
 to it is added a drop of a strong (10 to 20 per cent.) solution of 
 ca,ne sugar, and a similar quantity of strong sulphuric acid; a 
 
250 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 beautiful purple-red colour forms. The reaction is due to fur- 
 furol, which is produced by the action of the sulphuric acid on 
 the sugar ; the bile acids in the presence of furfurol give the 
 purple colour. 
 
 Cholesterin finds its way to the liver for the purpose of being 
 excreted. It is a substance found in many of the tissues of the 
 body, but especially in the white matter of the nervous system. 
 It is insoluble in water, but soluble in a solution of bile acids. 
 When cholesterin finds its way into the bile, it is eliminated with 
 the faeces. Cholesterin is found in very regular quantities in the 
 body, and forms one of the principal constituents of certain gall- 
 stones, and also, it may be added, of tumours in the lateral 
 ventricle of the brain of the horse. 
 
 Lecithin is another waste product brought to the liver for 
 excretion. It is of unknown physiological significance, but it 
 has been suggested that it may serve to activate the lipase of 
 the pancreatic secretion. 
 
 Secretion of Bile. — Bile is secreted under a very low pressure, 
 which is the reverse of what occurs in the secretion of saliva ; low 
 as the pressure is — 13 mm. (0-58 inch) of mercury — -it is higher 
 than that of the blood in the portal vein. If the pressure in the 
 bile-duct be raised, the bile is reabsorbed, being taken up by the 
 lymphatics of the liver, and so conveyed to the blood-stream. 
 It is probable that the majority of cases of jaundice are due to 
 obstructive causes, though exceptions to this rule occur. The 
 secretion of bile is continuous, whether the animal be in full diges- 
 tion or fasting ; the flow is not intermittent, as in the case of the 
 saliva. Though continuous, it is not uniform; it reaches its s 
 maximum in the dog between the second and fourth hours after 
 a meal ; this is followed by a fall, and again about the seventh 
 hour by a rise. A similar quantity curve is given by the pan- 
 creatic secreUon, which suggests how closely these two fluids 
 are co-operative in digestion, while it can be shown that a specific 
 substance, secretin (see p. 218), which stimulates the production" 
 of pancreatic juice, also hastens the secretion of bile. Acids 
 injected into the duodenum increase the flow of bile, even after 
 all nervous connections are severed. This is due to the acid 
 chyme converting the prosecretin into secretin, and this, as above 
 stated, has a specific action on the liver cells. 
 
 The secretion of bile is increased by any agent which destroys 
 the red blood-cells. A solution of haemoglobin injected into the 
 blood therefore produces an increase. The administration of bile 
 increases its production ; this is due to the bile acids having a 
 specific action on the liver cells, and acting as cholagogues. 
 
 In those animals possessing a gall-bladder this receptacle is 
 filled with bile during abstinence, but if it be empty, it is filled 
 
THE LIVER AND PANCREAS 251 
 
 even during digestion. The reflux of bile from the biliary duct 
 to the gall-bladder is caused by a sphincter-like contraction of 
 that part of the duct which penetrates the wall of the intestine; 
 in this way the bile is driven back along th e cystic duct to the 
 gall-bladder. The bile as formed is propelled along the bile- 
 ducts by a contraction of the muscular coat of the tubes, but 
 doubtless both the forcing onward of the bile and the circula- 
 tion through the liver are largely assisted by the respiratory 
 movements, during which the liver is compressed between the 
 abdominal viscera and the diaphragm. 
 
 By some it is considered that no bile enters the bowel while 
 the stomach is empty, but that the passage of acid chyme along 
 the duodenum causes a reflex contraction of the gall-bladder, 
 and an injection of bile into the intestine. 
 
 The amount of bile secreted varies, but is greater in herbivora 
 than in carnivora. Colin's experiments gave him the following 
 amounts as hourly secretions : 
 
 Horse 250 to 310 grammes (8 to 10 ounces) per hour. 
 
 Ox - 93 to 120 ,, (3 to 4 ounces) per hour. 
 
 Sheep - 8 to 150 ,, (Jounce to 5 ounces) per hour. 
 
 Pig - 62 to 150 ,, (2 to 5 ounces) per hour. 
 
 Dog 8 to 16 ,, (J- to \ ounce) per hour. 
 
 The Use of the Bile from a digestive point of view is dis- 
 appointing, inasmuch as it does not digest in the same sense that 
 pepsin and trypsin do. The bile and pancreatic fluid are in- 
 timately connected in function, for which reason the secretions 
 are poured out into the bowel, either close together, or, as in 
 some animals, by a duct practically common to the two glands. 
 As the horse possesses no gall-bladder, the secretion is poured 
 into the intestine as fast as it is prepared ; not so in the ox, sheep, 
 pig, and dog, where most of it is stored up in a capacious recep- 
 tacle until required. The explanation offered for the horse 
 having no gall-bladder is that as digestion, under ordinary cir- 
 cumstances, never ceases, the bile is poured into the bowel as 
 fast as it is secreted, but that in the case of other animals it is 
 poured out only when the contents of the stomach are passing out 
 into the intestine. This explanation, however, does not meet 
 all the difficulties of the case. The following animals, like the 
 horse, have no gall-bladder: the camel, elephant, rhinoceros, 
 tapir, and deer. . 
 
 The bile being alkaline, its first action on the chyme is to neu- 
 tralise the gastric juice, and to precipitate the albumoses and 
 peptones. One effect of this is probably to delay the progress 
 of the chyme along the bowel, by which means absorption is 
 assisted. 
 
 Bile has a solvent and emulsifying effect on fats, being in this 
 
i52 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 respect more active in the presence than in the absence of pan- 
 creatic juice. Bile cannot split up fats into fatty acids and 
 glycerol, but its presence increases threefold this action of. 
 the pancreatic fluid. Once the fats are split, the bile takes an 
 active share in digestion, for fatty acids, which are insoluble 
 in water, are soluble in bile salts and lecithin, the latter greatly 
 increasing the power of the bile salts as fat solvents. The fatty 
 acids form soaps with the alkali of the intestinal and pancreatic - 
 secretions, and these are also dissolved by the bile acids. The 
 solution of soaps so formed makes the emulsifying effect of the 
 bile permanent and the absorption of fat easier. In Voit's 
 experiments on dogs it was found that by cutting off the flow of 
 bile to the intestine the absorption of fat fell from 99 per cent, to 
 40 per cent. ; the solvent action of bile on fat is the chief digestive 
 function of. this fluid, but in its action on fat it works in con- 
 junction with the pancreatic secretion. We shall see later (p. 265) 
 how the presence of bile increases the energy of the pancreatic 
 fluid in the emulsification of fat. Bile has no action on proteins. 
 According to Hofmeister, the bile of the ox, sheep, and horse 
 converts starch into sugar, while the bile of the pig and dog pos- 
 sesses no such power, or only to a limited extent. It has been 
 said that bile has an antiseptic influence on the intestinal con- 
 tents, protecting them from putrefaction, for it has been found 
 that when it is prevented from entering the bowels, constipation 
 and extreme fcetor of the intestinal contents result. Bile, how- 
 ever, is not a true antiseptic. The clay-coloured fasces obtained 
 in jaundice are probably due to the presence of unacted-on fat; 
 the fat encloses the proteins which putrefy, hence the odour. 
 The bile acts as a natural purgative, and keeps up intestinal 
 peristalsis; by so doing it hurries the food residues out of the 
 system before they undergo excessive putrefactive decomposition. 
 
 Glycogen. 
 
 It is quite certain that the largest gland in the body must have 
 some other function than that of the secretion of a fluid of com- 
 paratively unimportant digestive power, and such is the case; 
 the liver manufactures and stores up in its cells a peculiar sub- 
 stance known as glycogen or animal starch. Glycogen is,spoken 
 of as a starch, though it differs from vegetable starch in many 
 important characteristics ; thus, it is soluble instead of insoluble 
 in cold water, and it is stained reddish-brown instead of blue by. 
 iodine. Though glycogen may be detected in the liver substance| 
 by the iodine test, it is now believed that it is not actually de-. :: 
 posited in the cells, but held there in weak chemical combination; 
 for it cannot readily be extracted from the liver by means of 
 
THE LIVER AND PANCREAS 253 
 
 cold water, whereas outside the body it is readily soluble in 
 water. 
 
 The literature of the formation and use of glycogen is extensive, 
 perhaps no substance has given rise to greater controversy; yet 
 the glycogen theory which is accepted to-day is the one originally 
 proposed by. Claude Bernard, who was the discoverer of this 
 singular substance. 
 
 The sugar in the food, together with that derived from starch- 
 conversion, finds its way by means of the intestinal vessels into 
 the portal vein, which may contain 0-4 per cent, after a heavy 
 carbohydrate meal or 0-2 per cent, during starvation; from here 
 it passes into the liver, where, under ordinary circumstances, it 
 is stored up as glycogen, being, in fact, reconverted into a kind of 
 starch, and when required gradually doled out by the hepatic 
 veins to the system as sugar. The liver regulates the amount of 
 sugar which should pass into the blood; so much, and no more 
 
 fasting. after food. 
 
 Fig. 82. — Liver Cells from the Dog during Fasting and after Food 
 (Waller, after Heidenhain). 
 
 During fasting the cells contain no glycogen; after receiving food they become 
 swollen with this substance. 
 
 is admitted to the circulating fluid, the amount varying between 
 0-05 and 0-15 per cent. The sugar in the blood in the ox was 
 estimated by C. Bernard at 0-17 per cent., in the calf at o-i per 
 cent., and in the horse at 0-09 per cent. There is consequently 
 a great difference between the sugar content of the portal and 
 that of the hepatic vein. When the liver fails to regulate the 
 amount of sugar in the blood diabetes is produced, and this 
 occurs when the amount of sugar rises to more than 0-2 per cent. 
 It is doubtful, however, whether this is a common form of 
 diabetes. 
 
 The glycogen which is stored up in /the liver for future use 
 may in two or more days be made to disappear by starving and 
 working the animal, the material in this way escaping from the 
 liver as sugar, and passing into the general circulation through 
 the hepatic veins. The administration of arsenic or of phos- 
 phorus, by the action of these substances on the liver cells, 
 also causes a marked diminution in the amount of glycogen, 
 
254 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 while strychnine in poisonous doses is most effective in this 
 respect, owing to the excessive muscular contractions produced. 
 
 The storing up of glycogen by the liver and its subsequent 
 utilisation are very closely allied to a similar process in the vege- 
 table kingdom ; the starch in the leaves of plants may pass down 
 the stem as sugar for the purpose of nourishment, and be again 
 formed into starch. Similarly in the animal the starch must be 
 first converted into sugar before the bloodvessels of the bowel can 
 take it up, then in the liver once more converted into a starch 
 substance, glycogen, and lastly, again into sugar before being 
 finally used by the tissues. The sugar formed from starch in the 
 bowel is maltose, while that formed in the liver from glycogen is 
 dextrose. This conversion of glycogen into dextrose is due to the 
 presence of a ferment in the liver cells. 
 
 The total amount of glycogen obtained from a given quantity 
 of food is not wholly stored in the liver ; the latter organ can only 
 hold a limited amount, which, on a rich carbohydrate diet, does 
 not exceed 17 per cent, in the dog, and in the rabbit 27 per cent., 
 of its weight, and in other animals is less. We know, as a fact, 
 that the liver, having taken up all the sugar it can from the portal 
 vessels and converted it into stored-up glycogen, allows the 
 balance to pass through the hepatic veins into the general 
 circulation as sugar, and that this balance is deposited in other 
 organs, principally the muscles, as glycogen for future use. The 
 muscles of well-fed animals contain in this way a considerable 
 quantity of glycogen ; even after nine days' starvation in the horse 
 from 1 to 2 -4 per cent, has been found in them. It may be stated 
 that the muscles hold ordinarily as much glycogen as the liver, 
 but it takes longer, by means of work and starvation, to free the 
 muscles from glycogen than to clear the liver. The presence of 
 glycogen in muscle is not essential to contraction, for there aTe 
 muscles in which no glycogen is found, and in which, nevertheless, 
 active contraction takes place. In the muscles of the embryo, 
 before striation has occurred, the amount of glycogen is some- 
 thing considerable; as much as 40 per cent, of the dry material 
 of the embryo muscle may consist of this substance. As stria- 
 tion appears the glycogen leaves the muscles to a great extent, 
 and the liver takes on the process of production. 
 
 The Use of Glycogen. — The muscles and liver are not the 
 only seats of glycogen deposits; traces may be found practi- 
 cally everywhere in the body, but none can be found in the blood- 
 plasma. The existence of glycogen in the embryonic muscle 
 points to its use in active nutrition and rapid growth; further, 
 it is found in the placenta, where it is used for the nourishment of 
 the foetus. In the adult the chief use of glycogen is to facilitate 
 the metabolic production of muscular energy and animal heat, 
 
THE LIVER AND PANCREAS 255 
 
 and this is effected by the oxidation of dextrose to carbon dioxide 
 and water. This oxidation does not occur in the blood; the 
 destruction of sugar (glycolysis) occurs in all active tissues, 
 especially in muscles and glands. It can be shown that there is 
 less sugar in the veins of an active muscle than in the arterial 
 blood supplying it. It was found experimentally that the muscles 
 of the upper lip of the horse used up in a state of activity 3-5 
 times more dextrose than during rest, and the actively secreting 
 parotid gland of the same animal used more sugar than the rest- 
 ing gland. 
 
 The sources of glycogen have been a fertile subject of dis- 
 cussion and an object of experimental inquiry. It was natural 
 to consider, as we have so far done, carbohydrate material as the 
 chief contributing agent ; it was less certain that protein con- 
 tributed, while the consensus of opinion was against fat taking 
 any share in the process. We must examine each of these points 
 in more detail. 
 
 We have learnt that starch is not absorbed as starch, but, 
 depending upon the nature of the diastatic ferment, is converted 
 into maltose, or maltose and some dextrin, and subsequently 
 into dextrose. These sugars are readily converted into gly- 
 cogen by the liver cells by the process of dehydration. Cane 
 sugar and milk sugar are not readily converted into glycogen, 
 but since these double sugars undergo inversion in the intestinal 
 canal before absorption — cane sugar into dextrose and levulose, 
 and milk sugar into dextrose and galactose — they may in these 
 forms be readily converted into glycogen. All carbohydrates, 
 then, which are capable of being changed into dextrose or levu- 
 lose, may be converted into glycogen, provided they pass through 
 the laboratory of the intestinal canal. For example, cane sugar, 
 if injected subcutaneously, passes unchanged into the urine; 
 in order to be converted into glycogen it must pass through the 
 intestine. 
 
 The effect of protein on glycogen formation is not so easily 
 explained. It is observed that in diabetes, though all carbo- 
 hydrate food be withheld yet sugar may appear in the urine on an 
 exclusively protein diet; the same thing is observed in the 
 experimental glycosuria which may be produced by the adminis- 
 tration of a substance known as ' phloridzin,'* and, furthermore, 
 sugar may be produced even when the animal is starved. The 
 conclusion appears irresistible that protein can produce sugar, 
 and this is explained by saying that certain proteins split into 
 a nitrogenous and a non-nitrogenous portion, the former being 
 converted into urea, while the non-nitrogenous residue is con- 
 verted into sugar, and may thus give rise to glycogen. It is now 
 
 * Obtained from the roots of the apple-tree. 
 
256 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 known, however, that proteins which do not contain a carbo- 
 hydrate group, such as casein, may take a share in the produc- 
 tion of glycogen ; this strengthens the belief that protein may give 
 rise to sugar. But the carbohydrate portion of the protein mole- 
 cule which yields glycogen is not the only protein source of sugar; 
 it is now known that sugar can be formed from some of the end- 
 products of the pancreatic digestion of proteins (p. 263) — viz., 
 the amino-bodies, of which glycine and alanine may be completely 
 converted into dextrose, and glutamic and aspartic acids partly so 
 converted. 
 
 There are a few observers who regard fat as a source of glycogen, 
 and there is some evidence to show that it may contribute, for it 
 has been said that glycerin acts as a sugar former. If this is so, 
 the conversion of fat into glycogen, through its splitting up in the 
 intestinal canal into fatty acid and glycerin, would not be a 
 difficult matter. On the other hand, experiment shows that 
 when an animal is fed solely on fat, the glycogen disappears from 
 the liver as quickly as it does in starvation. Nevertheless, it is 
 an interesting fact that during prolonged starvation, even forty 
 to ninety days, the amount of sugar in the blood is practically 
 constant, and its only sources at this time are the protein and 
 fat of the body. The question is, therefore, very far from being 
 settled. 
 
 The Liver Ferment. — When the liver is rapidly excised from 
 the body of a recently killed animal which has been appro- 
 priately fed, it contains a quantity of glycogen ; if it is allowed 
 to stand, the glycogen gradually becomes reduced in amount, 
 and sugar takes its place ; finally, all the g ycogen disappears. 
 This change is brought about by a diastatic ferment in the fiver 
 cells which changes the glycogen into sugar. If the liver on 
 removal from the body be rapidly minced and boiled, the ferment 
 is destroyed, and dextrose is not formed. 
 
 How the Supply of Sugar is Regulated. — We have seen that 
 in the tissues the glycogen in the form of dextrose is oxidised 
 into carbon dioxide and water, producing heat and energy. 
 After every meal a store of glycogen accumulates in the liver for 
 subsequent use, and in spite of changes in the amount of diet, 
 differences in the amount of daily work performed, or of heat 
 produced, yet the sugar in the hepatic veins maintains a per- 
 fectly regular proportion of from o -i to o -2 per cent. The means 
 which control the issue of sugar from the liver are very imper- 
 fectly known ; they are probably under the influence both of the 
 nervous system and of an internal secretion produced by the pan- 
 creas, and the subject will be again considered when the pancreas 
 is dealt with. One thing seems clear — that the liver itself is 
 unable to regulate the amount, and that whenever either the 
 
THE LIVER AND PANCREAS 25 j 
 
 nervous or the chemical factors fail, it allows sugar to pass into 
 the blood in a proportion largely over and above that which can 
 be oxidised [hyperglycemia), with the result that since the kidney 
 is permeable to dextrose present in blood to a greater extent 
 than 0-2 per cent., Sugar escapes with the urine [glycosuria), con- 
 stituting the disease known as diabetes. The sugar excreted 
 with the urine is, of course, lost to the system, and constitutes 
 a heavy drain on the body, which inconsequence rapidly wastes. 
 This condition may also bejexperirnentally produced in the dog 
 by removal of the pancreas. 
 
 We have studied two uses of the liver— viz., the formation of 
 bile and the storing up of glycogen — but there are other functions 
 of this gland to consider. 
 
 The Formation of Urea. — When the complex protein mole- 
 cule of the food is broken down into simpler end-products, one of 
 these, known as ' urea,' is excreted by means of the kidneys; this 
 substance, however, is not formed in these organs It is proved 
 conclusively that part, at least, of the urea in the body is formed 
 in the liver. During the process of protein disintegration certain 
 amino-acids, known as leucine and tyrosine, are produced, either 
 in the intestinal canal under the influence of pancreatic digestion, 
 or in the living cell as the result of the breaking down of protein. 
 Under any circumstances the leucine undergoes a series of 
 oxidative changes mainly in the liver, which result in the for- 
 mation of urea. The further facts regarding the formation of 
 urea are best dealt with in the section devoted to the kidneys. 
 
 As the result of protein decomposition in the intestinal canal 
 certain aromatic compounds are formed; these are united with 
 sulphuric acid, and got rid of by the kidneys as conjugated 
 sulphuric acids. In this combination the originally poisonous 
 protein products are converted into non-poisonous ones, and this 
 change is effected in the liver (Bunge) . In this we have a very 
 important function of the liver demonstrated — viz., as a neutral- 
 iser of poisons introduced into the blood by the intestines. It 
 is a noteworthy fact that many metallic poisons are also arrested 
 in the liver — for example, mercury and arsenic. 
 
 Summary. 
 
 The numerous and complicated changes produced by the 
 liver may thus be summarised : It forms bile, regulates the supply 
 of sugar to the system, and stores up as glycogen what is not 
 required. It guards the systemic circulation against the intro- 
 duction of certain nitrogenous poisons, such as ammonia, by 
 transforming them into urea, and against other poisons of pro- 
 tein origin by converting them into harmless products, by conju- 
 gation with alkaline sulphates. 
 
 17 
 
258 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 In animals, other than herbivora, the liver acts as an impor- 
 tant fat depot, obtaining its store of fat from adipose tissue. 
 The transport probably takes place in the form of fatty acids 
 and glycerol, the former being first desaturated and then broken 
 down into lower fatty acids. This transport of fatty acid to the 
 liver from other fat storehouses in the body takes place to a 
 great extent in various diseases, and after the action of certain 
 poisons such as phosphorus. 
 
Section 2. 
 The Pancreas. 
 
 The fluid secreted by the pancreas performs certain important 
 functions in digestion. It has been remarked that there is 
 scarcely any animal which does not possess a secretion allied to 
 the pancreatic; even those invertebrates without a peptic or a 
 biliary apparatus are in possession of a pancreatic one. From 
 the resemblance of the pancreas to the salivary glands, it has 
 been termed the ' abdominal salivary gland.' 
 
 The pancreatic fluid from herbivora can be obtained only with 
 extreme difficulty; to establish a pancreatic fistula in the horse 
 is a formidable operation, necessitating an incision from the 
 sternum to the pubis and the turning back of the bowels. Colin 
 established such fistulae both in the horse and in the ox, but the 
 profound impression on the nervous system produced by such 
 extensive interference must considerably affect the character of 
 the secretion and the amount manufactured. 
 
 Pancreatic fluid is ah alkaline fluid, clear, colourless, like water, 
 and though viscid in some animals, is not so in the horse or ox. 
 It has a saltish, unpleasant taste, and a specific gravity of about 
 i-oio; the viscid secretion of the dog has a specific gravity of 
 1-030. The following analysis of the fluid in the horse is given 
 by Hoppe-Seyler : 
 
 Water 98-25 
 
 (Organic matter o-88, containing o-86 of fer- 
 ments. 
 Salts o-86, containing much sodium 
 
 phosphate. 
 
 Schmidt" fpund the fluid of the dog to have the following 
 composition': 
 
 Water 90-00 
 
 (•Organic, matter 9-04, 
 Solids 9-92 J Salts - o-88, containing much sodium 
 
 [ chloride. 
 
 The salts present are sodium chloride in abundance, potassium 
 ehloride in traces, sodium carbonate and sodium phosphate, 
 calcium and magnesium phosphates in small quantities. To the 
 sodium carbonate is due the strong alkaline reaction . The organic 
 
 259 
 
260 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 solids are remarkable for the amount of protein present in them; 
 they vary in amount with different animals — for example, 9 per 
 cent, in the dog, and 0-9 per cent, in the horse. The pancreatic 
 secretion is remarkable for containing, in addition to the sub- 
 stances already named, three enzymes, which act on different 
 food substances. 
 
 Mechanism of Pancreatic Secretion. — The pancreatic secre- 
 tion is influenced by special secretory nerves ; stimulation of the 
 vagus or splanchnic may, after a long latent period, give rise to a 
 secretion, though it is not yet settled that these fibres produce 
 secretion during the act of digestion. The outpouring of the acid 
 chyme from the stomach into the duodenum at once causes a 
 secretion of pancreatic juice, and at one time it was supposed the 
 acid acted on the secretory nerves and produced a secretion 
 refiexly, for this action could be reproduced experimentally. 
 Bayliss and Starling, however, demonstrated the remarkable 
 fact that if an extract of the mucous membrane of the duodenum, 
 or of the jejunum, be made by scraping the bowel, and by then 
 acting on the scraping by weak hydrochloric acid, a substance 
 may be obtained which, when injected into the blood, produces a 
 profuse pancreatic secretion. To this internal secretion of the 
 intestinal cells they gave the name Secretin, the nature of which 
 has not been determined. Two facts are, however, clearly 
 established — first, that it is not a ferment, as it is not destroyed 
 by boiling ; and, secondly, that the action of acid is an essential 
 part of the process, for if the mucous membrane of the bowel 
 be extracted with either water or saline solution, secretin is not 
 obtained. It is the acid chyme, therefore, which, acting on the 
 mucous membrane of the intestine, produces secretin; this is 
 absorbed by the blood, and thus produces its specific action on 
 the pancreas. Within a minute or two of our introducing a 
 0-4 per cent, hydrochloric acid into the duodenum pancreatic 
 juice flows into the intestine. The acid produces the same effect 
 if introduced into the jejunum, but not if into the ileum. 
 
 Secretin is not a protein ; it is not destroyed, as stated above, 
 by boiling, and it is soluble in alcohol and in ether. Prosecretin 
 exists in the intestinal mucous membrane ; it may be extracted 
 with physiological salt solution, and though unable itself to 
 promote pancreatic secretion, it may be converted into an active 
 secretin by the action of acid or by boiling. Secretin from one 
 animal will increase the pancreatic flow in another, either of the 
 same or of a different species. Secretin is a hormone or chemical 
 messenger. 
 
 As a secretion can be obtained from the pancreas either by 
 stimulation of certain nerves or by introducing into the blood a 
 specific chemical substance, it would appear that under normal 
 
THE LIVER AND PANCREAS 2G1 
 
 conditions both processes may be operating in its production, 
 and that there may be, as in the case of the gastric juice, two parts 
 in the secretion. Evidence has been brought forward to show 
 that the secretions obtained by these two stimuli differ con- 
 siderably. The nervous secretion is thick, rich in ferments, 
 poor in alkali, and the trypsin it contains is active. The effect 
 of atropine is to suspend secretion, while that of pilocarpine is to 
 stimulate it. The chemically produced fluid is thin and watery, 
 contains but a small amount of ferment, and an abundance of 
 alkali. The secretion is unaffected by atropine, and its trypsin 
 is not in an active form when secreted. 
 
 Uses of the Secretion. — The pancreatic juice is poured into 
 the bowel in the horse and in the sheep by an opening common 
 to the pancreas and liver, while in the ox, pig, and dog, the ducts 
 of the liver and pancreas are separate, but open into the bowel 
 within a short distance of each other. 
 
 This juice is essentially a digestive fluid, and acts on the three 
 classes of food-stuffs — viz., proteins, fats, and carbohydrates. 
 To enable it to effect this, it contains three ferments or their 
 precursors — viz. : 
 
 A Proteolytic Enzyme which acts on proteins (Trypsin). 
 
 A Diastatic Enzyme which acts on carbohydrates (Amylopsin) . 
 
 A Lipolytic Enzyme which acts on fats (Lipase or Steapsin). 
 
 Earlier observations appeared to show that the proportion 
 of each of these ferments in the secretion depended on the char- 
 acter of the food; if, for example, the food was rich in fat, the 
 secretion would be rich in lipase. It was also held that the nature 
 of the food determined the predominance of each enzyme, 
 as well as the amount of fluid to be secreted. The specific action 
 of food-stuffs on the production of pancreatic enzymes is no 
 longer regarded as constant or characteristic. As a rule secre- 
 tion reached its maximum in the dog between the second and 
 fourth hour after taking food, corresponding to the greatest 
 activity of the liver. In dogs which have been starved active 
 secretion of bile, pancreatic juice, and intestinal fluid, associated 
 with gastro-intestinal movements (see p. 233), take place, it is 
 said, every two hours, and last for twenty minutes. The cause 
 of this is by no means clear. All the fluid thus poured out is re- 
 absorbed. 
 
 Trypsin.— Pancreatic juice taken direct from a fistula in the 
 duct has little or no action on the proteins of food, but if the same 
 fluid be allowed to become contaminated by the intestinal con- 
 tents it at once becomes active. Evidently the addition of a 
 something from the bowel has brought about a marked change 
 in the proteolytic character of the secretion . Investigation shows 
 
262 A MANUAL OP VETERINARY PHYSIOLOGY 
 
 that though the secretion taken direct from the pancreas con- 
 tains the precursor of trypsin---viz., trypsinogen — yet in this 
 form the ferment is unable to act on the protein of food until it 
 has itself been acted upon by another ferment. This ferment 
 is derived from the mucous membrane of the intestinal canal. 
 A ferment acting on a ferment has been described as a kinase or 
 activating ferment, and as this one is derived from the bowel 
 it is called enterokinase, a very small amount of which is capable 
 of converting inactive trypsinogen into active t 3'psin. It is 
 remarkable that of the three ferments secreted by the pancreas, 
 trypsin is the only one which is secreted in an inactive condition. 
 Pavlov considers this to be due to the fact that if trypsin were 
 active in the pancreatic juice, it would destroy its fellow- 
 ferments, but that in the bowel these ferments are protected. 
 
 The fact that extracts of pancreas, as usually obtained from 
 a slaughter-house, may be made more tryptically active by the 
 addition of a little dilute acetic acid, does not, according to the 
 modern theory, imply that the acid has converted the trypsino- 
 gen into trypsin, as has usually been supposed. The pancreas 
 used in the preparation of these extracts is already contaminated 
 with minute quantities of enterokinase, the activity of which is 
 greatly increased by neutralising the alkalinity of the extracts. 
 If a pancreas be obtained under conditions which insure the 
 absence of any admixture with even traces of enterokinase, 
 extracts of such a pancreas cannot be rendered more tryptically 
 active by the addition of dilute acid (Starling). It is now 
 believed that the conversion of inactive to active trypsin may be 
 effected by salts of calcium and of magnesium, as well as by 
 enterokinase. 
 
 The action of trypsin on proteins is most interesting. The 
 protein molecule is very complex; the use of trypsin is to split! 
 it up into simpler products, with the object of facilitating its 
 absorption. As we shall point out later, no food substance is 1 
 taken up excepting in its simpler form, and the proteins of oats,' 
 barley, hay, or flesh, have to be reconstructed in order to form 
 part of the tissues of the living animal . Trypsin acts on the large- 
 protein molecule, and breaks it down, producing a number of 
 simpler bodies of smaller molecular weight ; on these the tissue- 
 cells set to work, and by a process of synthesis construct the form 
 of protein needed by the body. It can.be easily shown that the 
 action of trypsin on protein is much more satisfactory and 
 thorough, if the protein has previously been acted upon by pepsin.' 
 Trypsin, like pepsin, produces albumoses and peptones ; but the 
 process does not stop at peptone ; no peptone can be found in the 
 blood, and none remains after a prolonged pancreatic digestion." 
 The effect of the action of trypsin on proteins is nearly as coni- 
 
THE LIVER AND PANCREAS 263 
 
 plete as that of boiling protein with acid. In each case the hydro- 
 lysis results in the production of a large number of simpler end- 
 products, chiefly of amino-bodies. Yet it would appear clear 
 that the products obtained by acid hydrolysis are not quite the 
 same as those produced by tryptic hydrolysis, for dogs may be 
 kept in health when fed on the split products of pancreatic diges- 
 tion, with a sufficiency of carbohydrates and fat, while the split 
 products of acid hydrolysis cannot be so utilised. This question 
 will be considered again in dealing with nutrition, but the sub- 
 ject is of interest here as showing that the laboratory of the body 
 reduces the complex protein molecule into many simpler sub- 
 stances, which can be reproduced outside the body by active 
 chemical means, and yet are not quite the same, for the material 
 artificially produced by means of boiling acid is of no subse- 
 quent use to the body, whereas that produced in the system can 
 be utilised. 
 
 The amino-bodies resulting from tryptic digestion are mainly 
 organic acids containing either one amino (NH 2 ) group, or two 
 such groups in union with carbon. Among the mon-amino- 
 bodies are leucine, tyrosine, glycine, aspartic acid, glutamic 
 acid, and tryptophan. Of the diamino-bodies, lysine, arginine, 
 and histidine are present. 
 
 It should be stated that another view regarding the breaking 
 down of protein into simpler substances exists — viz., that the 
 whole protein molecule is not split up by tryptic digestion, but 
 that a nucleus remains which in chemical character comes 
 between a peptone and amino-bodies. It is described as a.peptid, 
 or more generally as a polypeptid, as these bodies consist of 
 amino-acids which are linked together with the loss of one 
 molecule of water for each pair. The polypeptid may by acid 
 hydrolysis be converted into amino-bodies. It is suggested that 
 the polypeptid serves as a starting-point for synthesis, for the 
 protein substances taken in as food have to become converted into 
 the tissues of the living animal, reconstructed in part from the 
 amino-bodies, and the necessary synthesis, it is suggested, occurs 
 around, or is directed by, the polypeptids. Whether or not this is 
 correct does not in any way affect the important statement that 
 all protein received as food is foreign to the body, and that 
 before it can be built into the tissues of the living animal it has 
 first to be pulled to pieces, and then again constructed. 
 
 Should any protein or peptone have escaped the action of 
 pepsin and trypsin, it may be attacked by another enzyme found 
 in the intestinal mucous membrane, known as erepsin, which 
 also has the power of breaking down albamoses and peptones 
 into leucine and tyrosine. Erepsin is found in most of the 
 tissues of the body, and so is not specific to the intestine. 
 
264 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Under the influence of bacterial action numerous decompo- 
 sition products may b'e split off from protein; among others, 
 aromatic bodies are formed — phenol, indol, and skatol, the 
 latter accounting for the faecal odour of a pancreatic digestion 
 mixture. These substances are produced from tryptophan, 
 one of the end-products of the primary decomposition of proteins, 
 
 It is here desirable to draw attention to the fact that secretin, 
 enter okinase, and erepsin are all derived from the mucous mem- 
 brane of the intestinal canal. Care must be taken to avoid 
 confusing them. The second and last are ferments, secretin is 
 not. The function of secretin is to cause the production of 
 pancreatic juice, that of enterokinase is to endow one of the 
 ferments of the pancreatic juice with its remarkable proteolytic 
 properties, while erepsin breaks down albumose and peptones 
 into amino-bodies. 
 
 Amylopsin, the diastatic ferment, has an action on starchy food 
 similar to that of ptyalin, but more rapid and more active, for it 
 can deal with raw starch; the final products are maltose and 
 achroodextrin. The hydrolytic action of amylopsin stops at 
 maltose and achroodextrin, but these are in turn attacked by 
 the maltase of the succus entericus, and converted into dextrose 
 before absorption. All starchy food which has escaped conver- 
 sion in the mouth and stomach is also acted upon in the intestines 
 by maltase. 
 
 Lipase or Steapsinis the fat-splitting ferment of the pancreas; 
 it possesses the remarkable power of breaking up fats into fatty 
 acids and glycerol, and does this with the object of promoting 
 their absorption. We have learned a little of this action in 
 connection with the bile (p. 252), but must look at the matter 
 now in somewhat greater detail. The power of fats to form 
 emulsions is of great value in digestion. In the condition of 
 emulsion the fat is very finely divided; milk is a secretion in 
 which the fat is typically emulsified. Emulsion can also be 
 brought about by mixing fat with gum, egg-white, and soap 
 solution. The only fat emulsion received by animals as food is 
 when they are young, and it is said that the wall of the stomach 
 in young animals conta'ns a gastric lipase which deals with this. 
 If so, this section of the alimentary canal is of great importance 
 in those animals where the pancreas is functionless in early life, 
 a natural condition in man; in herbivora the pancreas is 
 probably in active work at a month old. The fat-splitting 
 power of pancreatic lipase is very marked, but, as mentioned at 
 p. 252, is greatly increased by admixture with bile. When fats 
 are split the fatty acids unite with the alkali of the intestinal 
 and pancreatic secretion, and form soaps. We have seen the 
 action of the combined bile and lecithin in dissolving fatty acids 
 
THE LIVER AND PANCREAS 265 
 
 and soaps, and the physiological importance of this, for until 
 solution is effected absorption is impossible. Without the assist- 
 ance of bile no solution of fatty acids and soaps would occur as 
 the result of the action of lipase. The removal of the soaps 
 by absorption permits the lipase to continue its action. 
 
 As free fatty acids, soaps, and glycerol, the fat enters the 
 villus and gains the chyle vessel. At one time it was believed 
 fat in fine emulsion passed between the epithelial cells of the 
 villus, but this is now held to be wrong. The oil globules seen 
 in the villus represent the newly reconstructed fat, for the 
 soluble products of fatty acids, soaps, and glycerol have no 
 sooner got into the villus than neutral fat is reconstructed. 
 Some think this is brought about by the reversible action of 
 lipase, others believe the effect is due to the living cells of the 
 villus, and is not the result of the action of a ferment. The fat 
 so formed enters the lacteals, passing by the thoracic duct 
 finally into the blood. Oil globules in the blood could not pass 
 along the capillaries, and plugging of the vessels would occur. 
 This is prevented by a change brought about in the blood, by 
 which the fat is rendered soluble, dialysable, and capable of 
 passing through the capillary waU. The method by which the 
 blood effects this change is unknown; it is said, however, that 
 lipase may be found in the blood, muscles, liver, and other glands, 
 and, if so, this will account for fat embolism not occurring in the 
 vessels, and also explain why the tissues are able to draw on their 
 body fat as required, for this must be brought into solution before 
 it can pass back into the blood. 
 
 Lipase is readily destroyed, so that, unless quite fresh, it does 
 not do its work in artificial digestions. It is believed that a 
 portion of the lipase is secreted in the inactive condition as 
 a zymogen, or pro-enzyme form, and that this is activated by the 
 action of the bile acids and lecithin. Whether this is so or not, 
 the dependence of the pancreatic secretion on the co-operation of 
 the bile is highly important. 
 
 On pp.194, 1 97 we have alluded to Pavlov's work on the quantity 
 and quality of the gastric juice being regulated by a specific 
 action on the part of the food itself. The same observer held 
 that the ferment contents of the pancreatic juice were adapted 
 to the character of the food ; for example, the lipase is increased 
 by a fat diet. A definite and unchanging diet leads to the forma- 
 tion of a pancreatic juice, which is unable to deal effectively with 
 a sudden change in food. If this were proved to be true, its 
 practical bearing on the feeding of animals would be far-reaching. 
 As a profession we have recognised for years the disastrous effects 
 of sudden changes in diet; modern science offers a possible 
 explanation of these effects. The whole matter is probably 
 regulated by an internal secretion. 
 
266 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The Changes occurring in the Cells of the pancreas corre- 
 spond very closely to those described for the salivary secretion. 
 
 When a pancreas or lobe of a pancreas has been at rest for 
 some time the cells forming it are rendered very indistinct; 
 the lumen of the alveolus is nearly obliterated by their swollen 
 condition, and the cells are seen crowded with granules; these are 
 so arranged that the margin of the cells presents a clear or fairly 
 clear zone, while within this there is an intensely granular zone 
 (Fig. 83, A) . The minute granules filling the cell are the mother- 
 substances of the secretion. When activity commences the 
 granules appear to pass centrally towards the alveolus, leaving 
 the cells comparatively clear excepting those portions immediately 
 abutting on the alveolus, which even in the exhausted condition 
 
 Fig. 83. — A Portion of the Pancreas of the Rabbit (KUhne and Sheridan 
 Lea). A, at Rest; B, in a State of Activity (Foster). 
 
 a, The inner granular zone in A is larger and more closely studded with fine 
 granules than in B, in which the granules are fewer and coarser, b, The outer 
 transparent zone is small in A, larger in B, and in the latter marked with 
 faint stria;, c, The lumen is very obvious in B, but indistinct in A. d, An 
 indentation of the junctions of the cells seen in the active but not in the 
 resting glands'. 
 
 remain granular. These changes result in the cells becoming 
 distinct and clearly defined, and, moreover, as they have emptied 
 their granular contents into the alveolus as pancreatic secretion, 
 they have consequently become much smaller. The narrow 
 clear zone seen in the resting gland has now become broad, the 
 previously choked alveolus is clearly defined, whilst the nucleus 
 of the cell, which was hidden in the charged condition, can easily 
 be seen in the exhausted gland (Fig. 83, B). These changes we're 
 worked out by observations on the pancreas of the living rabbit 
 by Kuhne and Sheridan Lea. 
 
 Amount of Secretion. — From the investigations of Colin and 
 others we know that in most animals the secretion of pancreatic 
 juice is continuous, though not uniform. In ruminants the 
 largest secretion is towards the end of rumination; in the dog 
 
THE LIVER AND PANCREAS 267 
 
 the maximum is reached between the second and fourth hours 
 after feeding, this maximum being followed by a fall, and about 
 the seventh hour by a rise. It will be remembered that the secre- 
 tion of the bile gives a similar curve. In the dog it is generally 
 considered there is no secretion during starvation, but that 
 immediately food begins to pass out of the stomach the pancreas 
 becomes active. In this connection, however, it is desirable to 
 remember that, according to some observers, a starved dog will 
 actively secrete pancreatic juice for twenty minutes every two 
 hours. The continuous secretion of the gland in herbivora is pro- 
 vided for by the lobes not all being active at the same time. In 
 the ox the amount of juice secreted is about 265 grammes 
 (9 ounces) per hour, in the horse it is much the same, in the sheep 
 7 to 8 grammes (about i ounce), in the pig about 5 to 15 grammes 
 (£ to J ounce) per hour, and in the dog still less, 2 to 3 grammes. 
 There is no necessary ratio between the size of the animal, the 
 weight of the gland, and the amount of pancreatic fluid secreted ; 
 carnivora secrete relatively more than herbivora. 
 
 The pressure under which the pancreatic juice is secreted is 
 low; it is said to be equal to 18 mm. (0-67 inch) of mercury, 
 which is very little greater than that of the bile. 
 
 Pancreatic Diabetes.* — If the pancreas of a dog be com- 
 pletely removed, there is a disappearance of aU glycogen from 
 the tissues, sugar appears in the urine within twenty-four hours, 
 and the animal dies in the course of a month or less from diabetes, 
 since the power of oxidising dextrose is lost. The dextrose 
 consequently accumulates in the blood, and is separated by the 
 kidneys. In addition to there being sugar in the urine, there is 
 also an increase in the amount of fluid produced and an excess of 
 urea ; consequently there is intense thirst, and this is associated 
 with a large appetite, in spite of which the animal wastes. If the 
 depancreated animal be placed on a purely protein diet, no dif- 
 ference occurs in the amount of sugar excreted; even if no food 
 be given sugar is still formed. If the removal of the gland is in- 
 complete, glycosuria may still occur, but its effects will vary in 
 intensity from fatal to transient, depending upon the amount of 
 pancreas left behind ; in fact, it is possible from experience to 
 leave behind just sufficient (one-fourth to one-fifth) of the gland 
 to prevent diabetes arising. In any case, fatal results may be 
 avoided by grafting portions of pancreas beneath the skin, 
 the presence of these preventing diabetes. This proves that the 
 prevention of glycosuria does not depend on the pancreatic juice. 
 
 Regulation of the Sugar Supply.— Throughout these remarks 
 on' the glycogen question, we have assumed that the view 
 
 * This matter should be read in conjunction with the remarks on 
 Glycogen, p. 252. 
 
268 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 originally put forward by Bernard is correct — namely, that the 
 sugar resulting from the conversion of carbohydrates in the 
 digestive canal is stored in the liver and muscles as glycogen, 
 while a definite proportion of sugar remains in circulation. The 
 chief opponent of this theory was Pavy, who believed that gly- 
 cogen is never converted back into sugar, but is built up into fat 
 and protein. Further, he denied the existence during life of a 
 liver ferment capable of converting glycogen into glucose. He 
 showed that egg-albumin was capable of being so treated, that it 
 yielded a reducing sugar which was obtained from the carbo- 
 hydrate group of the protein molecule, and that serum globuhn 
 behaves similarly ; mucin and nucleo-albumin are also now known 
 to contain the same substance. The importance of this discovery 
 was considerable, as the production of sugar from protein, though 
 suspected, had not been proved. The diet of the omnivora and 
 herbivora contains more carbohydrate than can be accounted for 
 in the muscles and liver as glycogen, and it is certain that all over 
 and above that required for the production of sugar must be con- 
 verted into fat, and some of it incorporated with the protein 
 tissues. Nevertheless the Bernard doctrine explains why the per- 
 centage of sugar in the circulating blood is constant — viz., by 
 t he gradual doling out of glycogen as sugar from the liver under 
 the influence of a ferment in that gland, and physiologists have, 
 with but few exceptions, accepted his teachings. 
 
 What we have now to consider is the manner in which this 
 regulation is effected, in order that the sugar content of the blood, 
 o-2 per cent., may remain at a normal level. The first to throw 
 some light on the process was Bernard, who showed that if the 
 floor of the fourth ventricle be punctured temporary diabetes 
 results, sugar appearing in the urine, while the liver uses up its 
 glycogen. This spot in the medulla is known as the diabetic 
 centre, and the puncture acts, not by destroying, but by stimu- 
 lating its activity. This is proved by the fact that if the animal 
 be starved before the puncture is made no sugar appears in the 
 urine, as no glycogen exists in the liver. The nerves passing to 
 this centre — the afferent nerves — are contained in most of the 
 sensory nerves of the body; if these are stimulated they act 
 reflexly on the diabetic centre, and sugar in the urine results. 
 The same occurs on stimulating the cerebral end of the divided 
 vagus. The impulses from the diabetic centre to the liver pass 
 down the spinal cord, and emerge in the anterior thoracic region 
 with the inferior roots of the spinal nerves, and are connected 
 with the inferior cervical and thoracic ganglia by the rami com- 
 municantes ; from these ganglia the impulses pass by means of the 
 splanchnic nerve to the liver. Whether the splanchnic contains 
 fibres which excite the secretion is unknown, but stimulation 
 
THE LIVER AND PANCREAS 269 
 
 of the inferior cervical and thoracic ganglia produces glycosuria 
 provided the splanchnic remains uncut. At one time it was con- 
 sidered that these results were due to vasomotor effect, and that 
 vascular dilatation, rather than secretory activity, was brought 
 about by stimulating the splanchnic, but it is now known that 
 glycosuria is produced when the central end of the depressor 
 nerve is stimulated, and the effect of stimulating this nerve is to 
 cause a fall in abdominal blood-pressure. The impulses passing 
 to the diabetic centre may, it has been suggested, originate in the 
 contracting skeletal muscles by the compression of the muscle 
 spindles. The heart muscle contains more glycogen than skeletal 
 muscle ; when the amount of glycogen in the latter has fallen to 
 one-tenth or even one-thirtieth of the normal the heart muscle 
 still maintains its due proportion. The fibres from the heart 
 to the diabetic centre are conveyed in the vagus, and it is easy 
 to see that on this theory the heart, which is the most active 
 muscle in the body, may regulate the production of the material 
 which furnishes it with energy. 
 
 Recent work by Mellanby locates a glycaemic centre in the 
 brain, somewhere between the cerebrum and spinal cord. This 
 centre is discharged by impulses passing to it by any afferent 
 nerve, and is inhibited by impulses from the cerebrum. 
 
 In distinction to the above, which may be termed the ' nervous 
 theory of sugar liberation,' we have another, the chemical, based 
 upon the knowledge which exists of the glycosuria which results 
 from depriving an animal of its pancreas. In some way or other 
 which is not known, the pancreatic tissue is intimately connected 
 with this sugar question, and this can be explained on the sup- 
 position of an internal secretion, which prevents the blood from 
 becoming overloaded with sugar, either by regulating the amount 
 which is liberated from the liver, or by stimulating the sugar- 
 splitting action of the tissue-cells. It would on this basis be 
 reasonable to suppose that the pancreatic extract should yield 
 a glycolytic substance, but no such has been found. That the 
 visible pancreatic secretion takes no share in the process is evident 
 from what has been previously stated — viz., that if only a frag- 
 ment of pancreas is left behind in the body of a depancreated 
 animal no glycosuria results. 
 
 An extract of pancreas, it has been stated above, has no 
 glycolytic power; further, an extract of muscle has no sugar- 
 destroying power, but if the extracts be mixed glycolysis results. 
 From this it has been supposed that the pancreatic extract 
 activates a sugar-destroying enzyme present in muscle, which 
 enables the latter, under physiological conditions, to oxidise 
 sugar in the body, and obtain from it heat and energy. The 
 activating substance furnished by the pancreatic extract is not 
 
270 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 an enzyme, for if the extract be boiled the substance is not 
 destroyed; it is therefore assumed that the internal secretion of 
 the pancreas is a hormone (see pp. 197, 260). There are others 
 who consider that the function of the internal secretion is not to 
 furnish the'tissues with the power to metabolise sugar, but rather 
 to regulate its output from the liver, and that when, this regula- 
 tion fails diabetes results, owing to the excess of sugar in the 
 blood. The balance of evidence, however, suggests that the 
 defect lies -in the inability of the tissues — especially the muscular 
 — to oxidise the sugar brought' to them. The question is still, 
 however, sub judice. 
 
 The pancreas in structure resembles the salivary glands in 
 being of compound tubular type, but in it may be seen with the 
 naked eye spherical or oval bodies, which are obviously not ordi- 
 nary pancreatic tissue; these are known as the Islands of Langer* 
 hans. The islands are composed of groups of cells, each group 
 surrounded by a rich capillary network of bloodvessels, and the 
 view has been advanced that they are the seat of the production 
 of the internal secretion. Ligature of the pancreatic duct causes 
 the ordinary gland tissue to atrophy, but does not affect the 
 islands, nor does ligature produce glycosuria. This is evidence 
 in favour of their furnishing the internal secretion. There are 
 other observers who consider the islands are concerned in the 
 production of the ordinary pancreatic secretion. This latter 
 view may now be regarded as disproved. 
 
 Diabetes. — In the forms of glycosuria hitherto described, its 
 production has been experimental. But there is a pathological 
 condition in which much the same symptoms are present; 
 though, so far as we know, the herbivora do not suffer from this 
 disease, it has been described in the dog, and under any circum- 
 stances its features in man are of interest to us. In diabetes the 
 sugar in the blood, instead of being o-i per cent., as it normally 
 is in man, may rise to 0-4, or even as high as 07 or 1 per cent. 
 In consequence of the tissues being unable to consume the dex- 
 trose brought to them, the sugar passes off by the urine, and the 
 body is starved of its source of heat and energy. As the disease- 
 advances, not only sugar, but products of deranged protein and 
 fat metabolism, appear in the urine, such as acetone, aceto- 
 acetic acid, and oxybutyric acids. These acids, by combining 
 with the alkali of the blood, produce the condition known as 
 acidosis, and reduce the carbon dioxide carrying capacity of the 
 fluid; in consequence, the carbon dioxide accumulates in the 
 tissues, and diabetic coma results. 
 
THE LIVER AND PANCREAS 271 
 
 Pathological. 
 
 The most common pathological condition of the liver is Jaundice, 
 and the majority of, if not all, cases of jaundice are obstructive — i.e., 
 there is some obstruction to the free pouring out of bile; in con- 
 sequence, there is a backward pressure, which, beirig greater than the 
 low blood-pressure under which bile is secreted, causes the bile to be 
 reabsorbed, and consequently stains the tissues yellow. There is 
 also a form of jaundice affecting the horse and the dog, due to a para- 
 site in the blood; in these cases the yellow tint is derived from the 
 destruction of red corpuscles stimulating the production of bile 
 (see pp. 8, 29). Biliary Calculi, consisting largely of the calcium salt 
 of bilirubin, are not uncommon in ruminants and swine, but rare 
 in the horse. Fatty Liver is common in all animals overfed and 
 underworked. In the horse it may lead to Rupture of the liver 
 during work. Enlargement of the liver is very common as the result 
 of vascular disturbance elsewhere; it is not uncommon as a sequel 
 to pneumonia, strangles, and other prolonged febrile changes. 
 Abscess of the liver is rare, but not unknown. Parasitic disease of 
 the liver is one of the epizootic diseases of sheep, and is common in 
 the ox, but rare in the horse. The parasite occupies the bile-ducts, 
 which become practically occluded. 
 
 In India, Calcareous Degeneration of the liver is one of the most 
 common affections of this organ, and throughout the tropics gener- 
 ally liver disorders are very frequent. In South Africa Hepatitis 
 occurs in horses as the result of eating " ragwort " (Senecio Jacobaea) . 
 The pancreas is seldom the seat of pathological disturbances; it 
 may be affected with abscess in strangles or in septic diseases, but 
 such conditions are unrecognisable during life. 
 
 Sugar in urine is described as occurring in dogs ; we have never 
 met with it in horses, though polyuria is a common affection. On 
 the other hand, many physiologists have described hyperglycaemia 
 and glycosuria in animals as occurring under the influence of certain 
 narcotics and anaesthetics. 
 
CHAPTER VII 
 ABSORPTION 
 
 Section i. 
 
 Lymph. 
 
 Lymph may be regarded as the material by which the tissues are 
 directly nourished, and by which effete material is collected from 
 them and taken back to the blood; there are certain non- vas- 
 cular structures, such as the cornea, cartilage, etc., where the 
 lymph circulation is the only means by which the part is supplied 
 with nourishment. Speaking generally, the lymphatic system 
 may be described as the drainage system of the body, in contra- 
 distinction to the blood or irrigating system, though the latter 
 is not exclusively devoted to irrigating, for it may also take up 
 material from the tissue spaces. 
 
 The Lymph Spaces. — The tissues are bathed in lymph, which 
 is contained in the lymphatic spaces existing between the capil- 
 lary bloodvessels and the capillary lymph- vessels. There is a 
 constant passage of material from the blood into the tissues, 
 from the tissues into the lymph, and likewise from the tissues 
 back to the blood. 
 
 The lymph spaces are irregular passages in the connective 
 tissue, the larger ones being lined by epithelioid plates of peculiar 
 irregular outline; these spaces exist outside the bloodvessels, 
 and the lymph finds its way from the bloodvessels into the lymph 
 spaces. From the lymph spaces the fluid reaches the lymph 
 capillaries, but the means by which it gets there is not clear, 
 for it appears certain that, excepting in a few cases, there is no 
 direct communication between the space and the capillary. In 
 the vessels of the brain a peculiar arrangement is present; the 
 lymphatic vessels surround the artery, and obtain their lymph 
 direct ; these are known as perivascular lymphatics. The lining 
 of the Lymph Capillary is composed of the same epithelioid 
 plates with irregular outline which are found in the spaces, and 
 
 272 
 
ABSORPTION 273 
 
 it is believed that at the junction of the plates, crevices or in- 
 tervals may exist through which fluid may find its way by the 
 simple process of transudation. From the lymph capillary 
 begins the Lymphatic Vessel, which, in addition to an epithe- 
 lioid lining, has also a muscular coat, more marked in the large 
 than in the small vessels, and also a connective-tissue covering. 
 In the interior of these vessels valves are found which are essen- 
 tially similar in structure, arrangement, and mode of action to 
 those in the veins. Immediately beyond each valve there is a 
 dilatation of the vessel which gives it a beaded appearance when 
 the lymphatic is distended. 
 
 The whole of the lymphatics of the body converge towards a 
 central vessel, the thoracic duct ; those from the left side of the 
 head and neck, the left fore-limb, the chest, abdominal cavity, 
 and hind-limbs, join the duct at different points, and this in turn 
 opens into the anterior vena cava ; from the right side of the 
 head and neck, and right fore-limb, the lymph vessels collect 
 and pour their contents by a separate duct into the same vein. 
 The thoracic duct is nothing more than a large lymphatic vessel, 
 possessing the same structure as the lymphatic vessels above 
 described, the muscular coat being especially well marked. The 
 thoracic duct receives the lymph, not only from the ordinary 
 tissues, but also from the intestinal canal. During starvation 
 the mesenteric lacteal vessels convey to the duct a fluid which is 
 essentially lymph, but during digestion this clear fluid is replaced 
 by a turbid white fluid known as chyle ; at this period the lacteal 
 vessels are carrying not only lymph, but also the products of 
 digestion, the milkiness of the chyle being due to the presence 
 of emulsified fats. 
 
 The Serous Cavities of the pleura, pericardium, and perito- 
 neum, have been looked upon as large lymphatic spaces, though 
 this is now considered doubtful. Yet the fluid they contain is 
 lymph, and they are in direct communication with lymphatic 
 vessels, especially those of the diaphragm. In the diaphragm 
 slits or stomata exist, and into these the lymph readily finds its 
 way, being aspirated into the vessels during the respiratory 
 movements of this organ ; so readily is this effected that the dia- 
 phragm may be injected in a recently dead subject, by placing 
 some milk on its surface and establishing artificial respiration. 
 
 The lymphatic vessels in their course pass through bodies 
 known as Lymphatic Glands, entering at one side and emerging 
 at the other. Experience shows that in its passage through 
 these glands the lymph has corpuscles added to it which ulti- 
 mately become white blood-corpuscles, and, moreover, it acquires 
 the property of clotting. The gland consists of a capsule, 
 within which is a mass of adenoid tissue divisible into a cortex 
 
 18 
 
274 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 and m:dulla. The capsule sends in bands of tissue (trabecules) 
 which divide the gland into compartments or alveoli, those in the 
 cortex being much larger than those in the medulla. The alveoli 
 contain a network of connective tissue, whose central part is 
 finely meshed (adenoid tissue), closely packed with lymph cor- 
 puscles, and constitutes the glandular substance. The adenoid 
 
 ad, 
 
 Fig. 84. — Diagrammatic Section of Lymphatic Gland. 
 Adenoid tissue containing lymph corpuscles. The region ad is normally 
 densely packed with lymph corpuscles, and constitutes the glandular 
 substance. The corpuscles are here drawn in scanty numbers, so as not to 
 obscure the central capillary v. In the adenoid, tissue may be seen a 
 capillary bloodvessel v. Outside the core of adenoid tissue is the lymph 
 sinus or space Is, across which run branched nucleated corpuscles which form 
 simply an open network of connective tissue. Surrounding the whole is 
 the trabecular framework t. 
 
 tissue does not occupy the entire alveolus, but fills up the centre, 
 and is maintained in position by branched, nucleated, connective 
 tissue corpuscles passing to the wall of the alveolus. In this way 
 a space or channel is formed between the central mass of adenoid 
 tissue and the wall of the alveolus ; this channel is known as a 
 lymph sinus (see Fig. 84). It is through the lymph sinuses of 
 
ABSORPTION t?5 
 
 the cortex that the gland is in direct communication with the 
 afferent lymphatic vessels. In the adenoid tissue of the alveolus 
 there is a network of bloodvessels; the tissue itself is filled with 
 corpuscles known as leucocytes, which are also found in the more 
 open network extending across the lymph sinus. The medulla 
 of the gland presents no essential difference in structure to that 
 of the cortex, excepting that the reticular network is more 
 complex, closer, and more extensive. The efferent lymphatic 
 vessels originate in the lymph sinuses of the medulla. 
 
 Lymph is a slightly yellow-coloured fluid, alkaline in reaction, 
 with a specific gravity of 1-012 to 1-022, and possessing the power 
 • of spontaneous clotting. The clot it yields is not so firm as that 
 of blood, and takes longer to form; moreover, the bulk' of 
 fibrin is much smaller. Lymph may be regarded essentially as 
 blood minus the red corpuscles; it contains, therefore, the pro- 
 teins of that fluid — viz., fibrinogen, paraglobulin, and serum 
 albumin, though in smaller amounts, also cells resembling the 
 white cells of the blood, extractives, salts, and gases. The fluid 
 in which these are contained is spoken of as lymph plasma. 
 The gases consist principally of carbon dioxide, the amount, of 
 which is greater than in arterial, but approximately the same as 
 in venous blood, a small quantity of nitrogen, and only traces of 
 oxygen. Amongst the extractives some observers have found 
 urea, a substance which exists more largely in lymph than in 
 blood, and which is said to be always present in the lymph of the 
 cow. The salts are distributed much as are those in blood — ■' 
 viz., potash in the corpuscles, and soda in the plasma. It is 
 evident that the composition of the lymph cannot be uniform, 
 but must depend, among other causes, upon the nature of the 
 food-supply and the source of the lymph. 
 
 The lymph-cells or leucocytes exhibit amoeboid movements, 
 and are identical with white blood-cells ; they are more numerous 
 in those vessels which have passed through lymphatic glands, 
 for it is in the gland that the leucocytes are manufactured and 
 added to the lymph. The cells consist of proteins, lecithin, 
 cholesterin, and fat, and their nuclei contain nuclein. The pro- 
 portion of lymph-cells to lymph is about the same as the pro- 
 portion of white corpuscles to blood. 
 
 i^The Quantity of Lymph in the Body is difficult to determine, 
 and varies considerably. From a lymphatic vessel in the neck of 
 the horse Colin obtained \ to 2 kilogrammes (17 to 70 ounces) 
 in twenty-four hours; but the variations were wide. Colin 
 noticed that the herbivora secrete more lymph than the carnivora, 
 and young animals more than adults. From the thoracic duct 
 of a cow this observer obtained the prodigious quantity of 91 
 kilogrammes (20 gallons) in twenty-four hours. This, of course, is 
 
276 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 no guide to the amount of lymph in the body, as the thoracic duct 
 contains a mixture of body lymph and chyle from the intestines. 
 In the observation mentioned on the cow the amount of material 
 collected from the duct was more than double the blood in the 
 body ; and if, as is usual, we regard two-thirds of the contents 
 of the thoracic duct flow to represent chyle and one-third lymph, 
 it still leaves the lymph flow at a very high figure' — in fact, nearly 
 equalling the entire blood-content of the body. As a matter of 
 fact, the total quantity of lymph in the body is unknown; it has 
 been supposed that it may be two, or even three, times greater 
 than that of the blood. 
 
 In the following table from Colin the amount of mixed chyle 
 and lymph flowing from the thoracic duct was measured; the 
 minimum represents a period of comparative digestive quies- 
 cence; the maximum amounts represent digestive activity. 
 
 Horse - 14 to 40 kilogrammes (24 to 70 pints) in 24 hours. 
 
 Ox 20 to 91 ,, (35 to 160 pints) in 24 hours. 
 
 Sheep - 3 to 9-5 ,, (5 to 16 pints) in 24 hours. 
 
 Dog 1-3 to 2-6 ,, (2-3 to 4-5 pints) in 24 hours. 
 
 Formation of Lymph. — The method by which lymph is 
 formed has been the subject of great difference of opinion and 
 of innumerable experiments: The question is still unsettled, 
 so that it is necessary to present both views of the case. The 
 difficulty in explaining the production of lymph lies in the fact 
 that the systems of bloodvessels and of lymph- vessels do not com- 
 municate ; both, in fact, are closed systems, while between them 
 lie the tissue spaces. What has to be explained is the passage of 
 fluid from one to the other across this space. If secretory nerves 
 for lymph production had -a definite existence, all difficulties 
 would disappear; but though the existence of these has been 
 suspected, and by some even described, the balance of 
 evidence is against their presence. But even quite apart from 
 the existence of a special nervous mechanism, there are other 
 grounds on which the theory of lymph formation could easily 
 be explained- — -viz., on a physical basis, as of filtration, or diffu- 
 sion; but, unfortunately, these processes frequently fail, under 
 experimental inquiry, to behave in the living body as they do 
 in the dead cell, though, in spite of this, physical processes have 
 been, and continue to be, invoked in explanation in those cases 
 where their action is not negatived by experimental results. 
 Finally, the explanation of the selective action of the living tissue- 
 cells has been urged as the'real explanation of the phenomenon. 
 As a matter of fact, it is no explanation, though it may, and prob-' 
 ably does, tell us something of what is occurring, or fixes on the 
 means by which it is taking place. Nevertheless, if the cell 
 could be proved to be the factor in lymph formation, this would 
 
ABSORPTION 277 
 
 make a substantial addition to our knowledge of the subject; 
 the difficulty of explaining the selective power of the cell which 
 secretes lymph is only the same difficulty which exists in consider- 
 ing the cell which secretes urine, or bile, or sweat. The nervous 
 mechanism, of which there is so little evidence, being put aside 
 entirely, the two chief theories of lymph formation resolve them- 
 selves into the physical and secretory, and each of these must be 
 examined separately. 
 
 The Physical Theory is based upon a knowledge of the laws 
 governing filtration, diffusion, and osmosis, and was first applied 
 by Ludwig. 
 
 Two liquids miscible, but utterly unlike, if brought into contact, 
 will gradually form a homogeneous mixture as the result of diffu- 
 sion. If they be separated by a membrane permeable to their 
 molecules, diffusion will occur through this, and a mixture of uni- 
 form composition result. Diffusion through a membrane is known 
 as osmosis. Substances which are diffusible are known as crystal- 
 loids, those which are non-diffusible are called colloids. Sugar or 
 salt are good examples of diffusible bodies, proteins and starch are 
 examples of colloids, the large size of the molecules of the latter pre- 
 venting their passage through an animal or other membrane. This 
 difference in the behaviour of these two classes of substance, as re- 
 gards their osmotic properties, affords a useful and ready means, 
 known as dialysis, of separating the crystalloids from the colloids. 
 
 If two masses of water be separated by a membrane, the diffusi- 
 bility of each being equal, as many molecules will pass into one 
 chamber as enter into the opposite, though to all appearances no 
 change in the fluid is taking place. If one chamber contains salt 
 solution and the other plain water, it will be found that much more 
 water passes into the salt solution than salt solution into the water, 
 the rate of transference of the salt depending upon the concentra- 
 tion of the salt solution ; the force which brings this about is known as 
 the osmotic pressure. It can be shown that the osmotic pressure is 
 proportional to the number of molecules of the crystalloid in solution. 
 
 Filtration is the passage of fluid through a membrane as 
 the result of pressure. If the pressure of blood in the capil- 
 laries could be shown to be higher than the pressure in the 
 lymph vessels or spaces, adequate ground would exist for regard- 
 ing nitration as an agent in lymph production. As a matter of 
 fact, the pressure in the lymph spaces is unknown, and the pres- 
 sure in the lymph capillaries is unknown ; but there is the best of 
 reasons for believing that the pressure falls from the blood capil- 
 laries to the lymph capillaries, and by increasing the pressure in 
 the former— say, by tying or compressing a vein— oedema results 
 in consequence of the filtration of fluid from blood capillary to 
 tissue space. The fluid in the tissue space may or may not be 
 lymph as we find it in a lymphatic vessel. The plasma has 
 passed through the wall of one vessel and become altered in 
 composition; in the tissue space it is certain that further change 
 
278 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 in its composition occurs in consequence of tissue activity, and 
 another vessel wall has to be negotiated before the lymph 
 stream is reached ; this passage may occasion further change in 
 its nature . The question of the actual changes in the composition 
 of the lymph in its passage from blood capillary to lymph capil- 
 lary does not immediately concern us here, but it helps to explain 
 why some physiologists have such difficulty in accepting purely 
 physical reasoning where living tissues are concerned, even when 
 the process concerned is as simple as that of filtration. Nor 
 must it be considered that the presence of an increase of pressure 
 in the capillaries necessarily results in the formation of more 
 lymph. In the horse it has been shown that the flow of lymph 
 from the parotid gland is not appreciably increased when the 
 gland passes from a condition of rest to that of secretory activity, 
 and yet we know that the capillary pressure at the time is greatly 
 increased. Other evidence will be quoted later showing that 
 serum may be absorbed from the bowel when the pressure in the 
 capillaries is greater than the pressure in the intestines; it may 
 be urged that the case of the bowel and bloodvessel is not com- 
 parable with that of the tissue space and bloodvessel, but our 
 object is rather to show that filtration may be non-existent at 
 the moment when the necessary physical conditions for its activity 
 are present". It has been shown, therefore, that there are features 
 in lymph formation^which cannot be entirely explained on the 
 theory of filtration. 
 
 T Diffusion and Osmosis. — For many years it has been known 
 that if two fluids- — one containing salt, and the other pure water 
 ■ — be separated by a membrane of parchment, one passes into 
 the other through the pores of the membrane, until the same 
 amount of salt exists on both sides of the diaphragm. Fluids 
 which thus pass through are termed crystalloids ; those which 
 refuse to pass through are known as colloids ; while the entire 
 process is termed osmosis or dialysis. We shall see presently 
 that physical chemistry of the present day gives a more restricted 
 definition to the term ' osmosis.' 
 
 It is known that perfectly pure water is not a conductor of elec- 
 tricity, and the same may be said of solutions of sugar, urea, albumin, 
 and other bodies. The explanation afforded by modern chemistry 
 is that the molecules of pure water and the molecules of sugar under- 
 go no dissociation into their constituent ions. Substances which dis- 
 solve in water and undergo dissociation are conductors of electricity. 
 For instance, sodium chloride is broken up in water into sodium ions 
 and chlorine ions, each group being charged with an opposite form 
 of electricity, the sodium ions being positively, the chlorine ions 
 negatively, charged. 
 
 Substances so capable of dissociation are termed electrolytes, and 
 their interest to us at the present time is in connection with osmosis; 
 
ABSORPTION 279 
 
 the act of dissociation liberates a number of ions, and by so doing 
 increases the number of particles moving in the fluid. It can be 
 shown that osmotic pressure — a term yet to be explained — is pro- 
 portional to the number of molecules of dissolved substances present, 
 and in the above example the ion behaves as a molecule in osmotic 
 pressure. Generally speaking, the greater the dilution, the larger 
 the number of ions dissociated. 
 
 Though not directly connected with the matter under considera- 
 tion, it is convenient in this place to look at the part played by ionic 
 action in the body. It is by a due adjustment of ions in their sur- 
 roundings that the contractile tissues which work in a saline artificial 
 circulation (p. 52), are capable of so doing; and the same may be said 
 of cilia, of amoeboid action, etc., each of which requires its own definite 
 proportion of ions. Loeb has classified the ions responsible for 
 rhythmic contractions, though such a classification can at present 
 be regarded only as provisional. So much importance does this 
 observer attach to ionic action- that he has brought forward evidence 
 to show that fertilisation of the ovum may be ionic in its nature, 
 the spermatozoa merely regulating the proportion of ions. Even a 
 nerve impulse he regards as depending on an electrolytic action. 
 
 Osmotic Pressure. — If a solution of an electrolyte such as common 
 salt be enclosed in a specially prepared semi-permeable cell, water 
 will pass in, but salt will not pass out, either by filtration or by diffu- 
 sion. This can be proved by placing the cell in distilled water, and 
 if a manometer be connected with the cell it will be found that as 
 the water passes in the pressure increases. This pressure is known 
 as osmotic pressure, and the term ' osmosis ' at the present day is 
 confined to the stream of water passing through a membrane, 
 while ' dialysis ' is restricted to the passage of the molecules dissolved 
 in water. In the above experiment it will be found that, if the 
 strength of the salt solution be doubled, the manometer will indicate 
 twice the pressure, so that the amount of osmotic pressure is always 
 proportional to the number of molecules of the dissolved substance 
 in a given volume of the solution. 
 
 The nature of osmotic pressure is unknown; it can be shown to 
 be independent of the nature of the substance in solution, and pro- 
 portional to the number of molecules of the dissolved substance. 
 It is governed by laws closely analogous to those governing gaseous 
 pressure, and, like gaseous pressure, it is affected by variations of 
 temperature and by the law of partial pressure. The latter, in the 
 case of osmotic pressure, is expressed by saying that the osmotic 
 pressure of a solution of different substances is equal to the sum of 
 the pressure which the individual substances would exert if they 
 were alone in the solution. Osmotic pressure is conveniently esti- 
 mated by ascertaining the freezing-point of a substance soluble in 
 water, which is always lower than that of the water itself; the lower- 
 ing of the freezing-point is proportional to the molecular concentra- 
 tion of the dissolved substance, and this molecular concentration 
 is proportional to osmotic pressure. Sugar, for instance, not being 
 an electrolyte, has a smaller number of particles moving m the solu- 
 tion than sodium chloride, which is capable of dissociation, so that 
 the osmotic pressure of salt is higher than that of sugar. A 1 per 
 cent, solution of sugar has an osmotic pressure of 473 mm. mercury, 
 while 0-9 per cent, of common salt, which has the same osmotic pres- 
 sure as blood-serum, exerts a pressure of 5,000 mm. mercury. 
 
 Blood-serum is taken as the standard in physiological inquiry; 
 
280 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 any solution which has the same molecular concentration as serum, 
 and consequently exerts the same osmotic pressure, is termed 
 isotonic ; if it has a greater osmotic pressure, it is termed hyper- 
 tonic; and if less, hypotonic. For example, on p. 8 the action of 
 salt solution on red corpuscles has been described; if the addition 
 of a solution causes no effect, it may be known that it is isotonic 
 to the material within the corpuscles; if it causes the corpuscles 
 to shrink and become crenated, it has a greater osmotic pressure 
 than the cells, and is hypertonic; if it causes them to swell and 
 discharge their pigment," it has a smaller osmotic pressure, and is 
 hypotonic. 
 
 It is not difficult to see how the physical factors of osmotic 
 pressure and diffusion may be turned to account in explaining 
 the formation of lymph, absorption from the intestines, secretion 
 of urine, and such-like processes. The danger, as has been pre- 
 viously indicated, lies in their uncompromising application, and 
 in the liability to forget that the living body is neither a parch- 
 ment membrane nor a vessel with a semi-permeable lining. 
 It has been urged that it is impossible to disregard the value of 
 osmotic currents in restoring equilibrium of composition between 
 the blood and the tissues or the tissues and the blood; for ex- 
 amp e, if a strong solution of common salt be injected into the 
 blood-stream, a current is created from the tissues into the blood, 
 by which the tissues may lose water ; but later on diffusion will 
 come into play, and the tissues will draw water from the blood. 
 It is suggested that constant and rapid osmotic changes are 
 occurring between the blood and the tissues; so rapid, indeed, 
 may these be that if the osmotic equilibrium be upset by in- 
 jecting a large dose of dextrose, within half a minute it is re- 
 adjusted. We have seen that the lymph in the spaces must 
 necessarily be undergoing constant change in its composition 
 and concentration, as the result of tissue activity, with its atten- 
 dant chemical changes. It is, therefore, readily conceivable 
 that osmotic or diffusion currents may be set up, water being 
 drawn from the blood to the tissue spaces and crystalloid bodies, 
 such as would result from the breaking up of the protein molecule, 
 passing by diffusion from tissue spaces to blood, and so being got 
 rid of by the excretory organs ; of such crystalloids the precursors 
 of urea are a good example. 
 
 Experimental inquiry, however, shows that it is not easy to 
 explain lymph formation by osmosis or by diffusion, and we have 
 previously seen how greatly filtration has failed. The injection ]',',', 
 into a vein of a strong solution of common salt, urea, or dextrose, 
 is followed by an immediate increase of lymph; whereas the 
 osmotic pressure exerted by sodium chloride, for example, should 
 at least primarily diminish the secretion by setting up osmotic 
 currents from tissues to bloodvessel, although, secondarily, the 
 
ABSORPTION 281 
 
 hydremic plethora should give rise to increased filtration from 
 the blood. Even as regards the subject of absorption, which 
 it is difficult to divorce from that of lymph formation, osmotic 
 pressure does not help as much as might be expected. Again, 
 it can be shown experimentaUy that there is no definite relation 
 between the rate at which the sugars are absorbed and the os- 
 motic pressure they exert. Serum, isotonic with blood-plasma, 
 may be absorbed from a loop of intestine, when the blood-pres- 
 sure in the capillaries of the intestine is greater than the pressure 
 within the bowel. Evidently in this case osmotic pressure cannot 
 have been exercised, while absorption by filtration is negatived 
 by the higher pressure in the capillaries. Salt solution, isotonic 
 with blood-plasma, may also be readily absorbed by the blood- 
 vessels in the peritoneal cavity. In all these cases something is 
 occurring which is opposed to what might have been expected 
 from a purely physical reasoning. But perhaps few things are 
 more remarkable in this respect, or more difficult to explain, 
 than the effect of the injection of dextrose, which causes a post- 
 mortem flow of lymph for as long as an hour after the circulation 
 has ceased. Whether the question be considered from its physi- 
 cal or secretory aspect, this post-mortem flow is inexplicable. 
 
 Indiffusible substances such as protein are believed to exert but 
 little osmotic pressure, and some consider none whatever. The 
 large size of the protein molecule, and the small number (6 or 
 7 per cent.) present in such concentration, as is represented by 
 blood-serum, explains why they exert little or no pressure. The 
 passage of protein through the capillary wall to the lymph 
 vessel cannot be satisfactorily explained by osmotic pressure ; a 
 filtration has consequently been assumed to be the agent at work, 
 as the blood in its capillaries is at a higher pressure than the 
 lymph in its capillaries. 
 
 Starling, whose name is so closely identified with the investiga- 
 tion of lymph production from its physical aspect, has laid stress 
 on the importance of the permeability of the capillary wall. 
 His observations show that the normal undamaged capillary of 
 the limbs and connective tissue offers a very considerable resist- 
 ance to the filtration of lymph, and keeps back a large portion of 
 the proteins of the blood- plasma ; on the other hand, the intestinal 
 capillaries, and especially the capillaries of the liver, are very 
 permeable; a very small capillary pressure in the latter suffices 
 to produce a large transudation of lymph containing as much 
 protein as the plasma itself. A normally impermeable capillary 
 of a limb may by injury be at once converted into a permeable 
 capillary, within which the slightest increase in pressure brings 
 about lymph production. 
 
 Starling records the remarkable fact that no lymph can be 
 
282 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 obtained from a restinglimb, though active or passive movements 
 of it at once cause a flow of lymph. The only part of the body 
 which produces a continuous flow of lymph during rest is the 
 alimentary canal. Though no lymph is yielded by a resting 
 limb, yet the chemical changes in the tissue are still occurring, 
 oxygen is being absorbed and carbonic acid and other waste pro- 
 ducts got rid of, but their channel of excretion is meanwhile 
 the bloodvessels. 
 
 The Secretory Theory of lymph formation is based on a know- 
 ledge of the secretory activity of epithelium in general. It was 
 natural to regard the endothelial lining of the capillary vessels 
 as the possible seat of secretory activity, as was known to be the 
 case in other tissues; and when Heidenhain was able to show 
 that by the injection of certain substances into the blood he was 
 able to increase the flow of lymph without increasing the arterial 
 pressure, it appeared that the solution of the vexed question of 
 lymph formation was at hand. 
 
 Heidenhain found that the injection into the blood of peptone, 
 extracts of leech, crayfish, mussel, egg-albumin, etc., increased 
 the rate of lymph flow, and also the total solids in the lymph 
 obtained from the thoracic duct. He called these substances 
 lymphagogues, and divided them into two classes, the first class 
 consisting of such substances as the above, while the second 
 consisted mainly of crystalloid bodies such as sugar and salt, 
 which, though increasing the total quantity of lymph produced, 
 rendered it more watery than usual. 
 
 As regards Class i, Heidenhain believed they contained a 
 specific substance which stimulated secretion. Starling showed 
 that the increased secretion was derived from the liver; he 
 believed the extracts acted pathologicaUy on the walls of the 
 blood capillaries of the gland, and rendered them more permeable; 
 hence the increased flow of lymph. The second class of lympha- 
 gogues was believed by Heidenhain to act by attracting water 
 from the tissues, and so increasing the bulk of lymph. Star- 
 ling, however, maintains that in this case they act by increasing 
 the osmotic pressure of the circulation, so that water from lymph 
 and tissues is attracted into the blood by osmosis. The excess 
 of fluid thus produced in the blood causes a rise in the capillary 
 pressure, especially in that of the abdominal area, followed by 
 increased transudation from the capillaries into the lymph, 
 vessels. 
 
 At present it is not possible to decide between the rival theories 
 of lymph formation ; it may be proved that under given condi- 
 tions both methods take a part in the process of secretion. It 
 seems impossible to exclude the living activity of the cell-body, 
 so strongly emphasised in the consideration of other secretions, 
 
A i SORPTION 283 
 
 while it is equally certain that there are other conditions which 
 are possible of explanation only on a physical basis. 
 
 The Movement of Lymph is effected largely by muscular 
 contractions in the neighbourhood of the vessels, by which 
 means the vessels are themselves compressed and their contents 
 forced onwards, since the valves which they contain prevent a 
 back flow. The obstruction caused by the lymphatics passing 
 through glands is not serious, while the involuntary muscle fibres 
 in the capsule of the gland more than compensate by their con- 
 traction for any resistance in the gland itself. The pressure of 
 the lymph in the lymph vessel is higher than that in the jugular 
 vein, so the flow of lymph from the tissues to the vein is assisted 
 by the fact that the fluid is passing from a region of higher to one 
 of lower pressure. The movements of the diaphragm, tendons, 
 and fasciae produce an aspirating effect on the lymph circulating 
 through them. In the diaphragm the lymphatic vessels drain 
 the two large lymphatic sacs the pleura and peritoneum; 
 owing to the direction taken by the fibres of the diaphragm, 
 compression is exerted on the lymph spaces during its contrac- 
 tion, while a sucking action is produced when it relaxes. This 
 pumping arrangement exists in tendons, fasciae of muscles, etc., 
 and is a valuable aid to lymph circulation . In the ordinary skele- 
 tal muscles during contraction the lymph is squeezed out of the 
 part by compression. During rest practically no lymph passes 
 by the lymph vessels of muscle, the exchange taking place, as 
 we have seen, by the bloodvessels. 
 
 FOnce the lymph from the abdominal viscera and hind-quarters 
 has found its way into the thoracic duct, its passage into the 
 general circulation is favoured y gravity, by the muscular con- 
 traction of the coats of the duct, and by movements of the 
 skeletal muscles, especially the abdominal. The lymph from the 
 right side- of the face neck, and right thorax has a duct of its 
 own, and this is so situated that gravity plays an important 
 part in moving the contents along. Both the thoracic duct and 
 the right lymphatic duct empty into the veins near the heart, 
 either into the anterior vena cava or into the jugular confluent. 
 The mode of entry varies; in fact, there are great variations m 
 animals of the same species, in the anatomical arrangements, 
 not only of the ducts, but also of their terminations; for example, 
 the thoracic duct is frequently duplicated in the ox. 
 !'"*' In whatever way connection with the venous system is made 
 a^valvular arrangement, looking towards the vein, and intended 
 ■to prevent a reflux of blood into the duct, exists between the 
 dilated termination of the lymph duct and the bloodvessel. This 
 valvular seal is a reliable one in the ruminant; in the camivora 
 it is less so; in the horse it is imperfect, so much so that blood 
 
284 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 may find its way even for some distance into the duct, and stain 
 the contents, as first pointed out by Colin. If a manometer 
 tube be placed in the thoracic duct of the ox at its termination 
 in the vein, the mixed contents of chyle and lymph will rise in 
 the tube, and in about five minutes reach a maximum. The 
 amount of this is variable ; Colin has registered during full diges- 
 tion a 3-foot pressure (say 75 mm. mercury) in the thoracic duct, 
 which is about one-third of the aortic pressure. This is very 
 high — indeed, is higher than that of the blood in the capillaries 
 or of the lymph in the lymph vessels. It would appear that this 
 marked degree of pressure can be due only to contraction of the 
 walls of the receptaculum chyli and contraction of skeletal muscles/ 
 the abdominal muscles doubtless taking the largest part therein. 
 The fluid in the tube rises and falls, and if the tube be removed 
 lymph issues from the vessel in jets. The rise and<fall of level 
 in the tube is associated with respiratory movements, but the 
 effect produced is not the same as, the general effect on blood- 
 pressure caused by inspiration and expiration. Inspiration does 
 not, for instance, draw the lymph from the duct towards the 
 vein- — its effect is the reverse ; while expiration, on the other hand, 
 raises the pressure in the duct, and discharges its contents. 
 Colin, who first described this singular fact, explains it by saying 
 that the intrathoracic pressure being reduced during inspiration, 
 the vessel dilates, and, in consequence, its capacity is increased. 
 During inspiration it fills up, while at expiration the pressure 
 exercised upon it, no doubt particularly by the abdominal 
 muscles, causes the now distended duct to discharge its con- 
 tents. Colin, "in fact, observed that the oscillations of the fluid 
 in the manometer tube are greater during laboured respiration. 
 In the horse the pressure n the thoracic duct must be much less 
 than in the duct of the ox ; in fact, for blood to pass into the duct , 
 the pressure must be at times below that of the pressure in the 
 anterior vena cava. There are great experimental difficulties in 
 getting at the duct in the horse, and exact information is wanting. 
 
 The lymph moves slowly in its vessels. Weiss has observed l\ 
 a rate of from 230 to 280 mm. (9 to 11 inches) per minute in a 
 large lymphatic in the neck of the horse, but the velocity in the 
 small vessels is very much less. Colin observed 120 mm. j 
 (4I inches) per minute. The flow from the thoracic duct of a 
 calf was found by Colin to be 1 metre (3 feet 3 inches) a minute; 5| 
 in a large mesenteric vessel the same observer found the velocity 
 to be 840 mm. (33 inches) a minute. 
 
 As fast as the lymph finds its way from the bloodvessels into 
 the spaces it is, normally passed on to the lymphatic capillaries, | 
 so that the rate of output is equivalentlto the rate of removal; 
 when, however, the output is greater than the rate of removal ; 
 
ABSORPTION 285 
 
 the lymph accumulates in the tissues, and (Edema results. It 
 is conceivable that the rate of removal need not necessarily 
 always be at fault, but that the rate of secretion may be so 
 greatly increased that the outgoing channels are unequal to the 
 demands made upon them. In such an increased secretion of 
 lymph the defect lies in the vascular system, and experience 
 shows that increased formation of lymph is a more common 
 cause of cedema than defective drainage. It is well known that 
 interference with the venous circulation is productive of cedema, 
 the explanation being that not only is there an increase of pres- 
 sure in the capillaries as the result of the venous obstruction, 
 but also the venous blood is kept in contact with the wall of the 
 capillary, and this induces changes in the epithelioid cells resulting 
 in increased lymph formation. The swollen legs, so common in 
 horses kept idle in the stable, are due to this cause. The venous 
 blood ascends the limbs against gravity, and exerts on the capil- 
 laries of the legs below the knees and hocks a pressure which is 
 nearly equivalent to the height of the vein ; as a result, the cells 
 of the capillary wall are the seat of an increased exudation, and 
 the legs accordingly ' fill,' a condition at once removable by 
 exercise. The pressure in a lymph vessel is low; in the neck 
 of the horse it was found to be from \ to finch of a weak solution 
 of soda ; in the dog the lateral pressure was half that found in 
 the horse. 
 
 Hsemolymph Glands. — These are found in the ox and sheep, 
 but are believed to be absent in the horse. They are of a deep 
 red colour; the cortical and medullary substances are not sharply 
 defined, and involuntary muscle is found in the trabecular These 
 glands occur among the gastric and mesenteric lymph glands, 
 along the course of the aorta, in the perineal fat, and externally 
 under the skin of the flank and under the trapezius muscle. 
 .Some possess afferent and efferent vessels, others do not. The 
 significance of these glands is unknown. (Sisson.) 
 
Section 2. 
 
 Chyle. 
 
 In the thoracic duct the lymph from the body meets with the 
 lymph coming from the intestines, termed ' chyle.' This chyle 
 is derived from the villi, and passes up the mesentery by many 
 vessels, which in the horse are said by Colin to number 1,200. 
 Each of these passes through a lymphatic gland before entering 
 the receptaculum chyli. Chyle is closely allied to lymph in its 
 chemical composition, but it differs from it in containing a 
 quantity of neutral fat, taken up during intestinal digestion, which 
 gives it a milky appearance. The amount of this fat in dogs. may 
 
 vary from 2 per cent, to 15 per 
 cent., or even more. The fat is 
 partly in the condition of measur- 
 ably large droplets, such as are 
 seen in milk, but for the most part 
 it exists as extraordinarily minute 
 particles ; hence the name ' mole- 
 cular basis,' which is applied 
 collectively to the fat particles in 
 chyle. 
 
 The Villi. — We have mentioned 
 that in the ordinary tissues the 
 radicles of the lymph vessels are 
 the lymph spaces, but in the wall 
 of the small intestines the origins 
 of the lymph vessels are highly, 
 differentiated structures, known 
 as villi and solitary glands. The 
 villi (Fig. 85) are innumerable 
 projections from the inner sur- 
 face of the mucous membrane 
 shaped like minute fingers; they 
 are found only in the small intestines, and have been calcu- 
 lated by Colin to amount to forty or fifty millions in the 
 horse and in the ox. In the interior and central part of the 
 villus is a vessel termed the lacteal ; it may be single or multiple, 
 straight or branched, and at the base of the villus it opens by a 
 valvular arrangement into the lymphatic system. Surrounding 
 the lacteal is a network of capillary bloodvessels, while filling up 
 the finger of the villus, where not otherwise occupied by vessels, is 
 
 286 
 
 Fig. 85. — Vertical Section of <i 
 Villus : Cat. x 300 (Stewart). 
 
 «., Layer of columnar epithelium 
 covering the villus — the outer 
 edge of the cells is striated; 
 6, central lacteal of villus; c, 
 unstriped muscular fibres; d, 
 mucin-forming goblet cells. 
 
ABSORPTION 
 
 287 
 
 a peculiar structure found especially in lymphatic glands, and 
 known as adenoid tissue (p. 275) ; this tissue is relatively larger 
 in amount in the villi of carnivora than of herbivora (Fig. 86). 
 Covering the adenoid tissue is a basement membrane on which is 
 set a layer of columnar cells, placed so that their narrowest end 
 is next the basement membrane, and their broadest at the sur- 
 face of the villus. The cells at their narrowest part are in touch 
 with the adenoid tissue of the villus. Each cell contains a nucleus, 
 and on that edge of the villus, next the interior of the bowel, is a 
 clear band bearing fine striations. Lying between the columnar 
 cells are others which from their shape are spoken of as ' goblet 
 cells ' (Fig. 86) ; by means of a pore they extrude their contents, 
 
 Epithelium. — 
 
 Fig. 86. — Transverse Section of Villi of Carnivorous and Herbivorous 
 Animals (Waller, after Heidenhain). 
 
 The large cells in the epithelial zone of the dog are the goblet cells. 
 
 consisting of a transparent material known as mucin, into the 
 intestine. Within the villus are bands of involuntary muscle 
 fibre arranged parallel to the axis of the villus, by the contraction 
 of which, combined with the peristaltic movements of the intes- 
 tine, the capacity of the lacteal vessel is altered in such a way 
 that it is alternately filled with lymph from the reticular adenoid 
 tissue, and emptied of lymph into the lymphatic vessel at the 
 base of the villus. This is known -as the pumping action of the 
 villus, and is an important factor in the conveyance of the chyle 
 (lymph) towards the thoracic duct. 
 
 The other lymph radicles found in the intestine are the Solitary 
 Follicles, which are found studding the whole of the mucous 
 membrane of the small intestines; these solitary follicles are, at 
 certain places in the ileum, found as masses, which are known as 
 Peyer's Patches. 
 
 The Solitary Follicle is essentially a lymphatic structure, and 
 
288 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 is not concerned like the villus in absorbing anything from the 
 food. It consists of a mass of adenoid tissue, the network of 
 which is, filled with leucocytes ; within the network are capillary 
 bloodvessels, and surrounding the whole is a space across which 
 branches of the adenoid network pass. This space is known as 
 a lymph space or sinus ; it is lined, like those previously described, 
 with epithelioid plates, and opens into a lymphatic vessel. As 
 the lymph passes through the adenoid tissue, some of the cor- 
 puscles found in the meshes of the network are added to it, and 
 become lymph corpuscles. 
 
 Chyle is a turbid fluid of alkaline reaction and a specific 
 gravity of 1-007 to 1-022. In starving animals it is transparent 
 owing to the absence of fat, and it is, in fact, at this time pure 
 lymph. Colin observed that the chyle of herbivora was yellowish 
 or yellowish-green ; it is possible that this colour may be due to 
 chlorophyll taken up from the food. In the horse, as collected 
 from the thoracic duct, it is often blood-stained, due, as we have 
 seen, to leakage from the vena cava. 
 
 In the small intestines of the horse it has been observed by 
 Colin that almost immediately after food has been given waves 
 of chyme are passed into the duodenum from the stomach; in 
 consequence, the lacteals in the mesentery in connection with 
 this portion of intestine become opaque, though previously they 
 were filled with a colourless fluid. As the chyme passes along 
 the bowel the other lacteals in their turn become opaque, until 
 at last the whole of them are filled with this milky fluid (Fig. 87). 
 Colin draws especial attention to this regular invasion of the 
 lacteals from the duodenum to the ileum. 
 
 The movement of chyle is due to the rhythmical muscular 
 contractions of the walls of the intestine, and to the muscular 
 contraction of the intestinal villi forcing it onwards, while the 
 valves in the lacteals prevent its return. Its subsequent passage 
 into the general circulation has already been traced. 
 
Section 3. 
 Absorption in General. 
 
 The activity of absorption, especially in the horse, has been 
 made known to us by the experiments of Colin. 
 
 Absorption from the Respiratory Passages is remarkably 
 rapid. Colin showed that potassium ferrocyanide could be 
 detected in the blood two minutes after being injected into the 
 trachea, and that it appeared in the blood before it was found in 
 the chyle ; the same salt was also found in the urine eight minutes 
 after being introduced into the trachea. A solution of nux 
 vomica injected into the trachea produced tetanic symptoms 
 in three minutes ; turpentine, alcohol, and ether were also rapidly 
 absorbed, but oil could not be taken up, and was rejected by the 
 nostrils. 
 
 Such drugs as morphia, pilocarpine, physostigmine, etc., are all 
 rapidly absorbed from the air passages,* and produce their 
 physiological effect in a shorter time than when simply injected 
 under the skin. The lungs also have the power of absorbing 
 certain poisons such as curare, which are not absorbed when intro- 
 duced into the digestive canal. The absorption of water from 
 the bronchial passages is very rapid. Colin introduced 6-8 litres 
 (6 quarts) of water per hour into the trachea of a horse; the 
 animal was destroyed at the end of three and a half hours, ,and 
 no fluid was found in the bronchi. He also poured into the 
 air passages 568, c.c. (ipint) of water at a time; repeating this 
 without intermission, he poured in 42 litres (74 pints) of water 
 before he caused death. So rapid is absorption from the bronchi, 
 that a horse may be placed under chloroform almost instanta- 
 neously by an intratracheal in j ection of the drug . f The rapidity 
 of absorption is therefore very great, but in spite of the facility 
 with which drugs are taken up, the lining membrane of the 
 bronchial tubes is remarkably tolerant of such irritating agents 
 as turpentine, strong liquid ammonia, acetic acid, etc., and 
 offers in a state of health an almost impassable barrier to putrid 
 
 * It is interesting to observe that the injection of liquids into the trachea 
 (either high up or as low as its bifurcation) excites the reflex act of swallow- 
 ing, probably due to stimulation of the sensory fibres of the inferior laryn- 
 
 t ItiTnot intended here to recommend the intratracheal administra- 
 tion of chloroform, which is not only dangerous, but produces the greatest 
 
 excitement in the patient. 
 
 289 19 
 
ago A MANUAL OF VETERINARY PHYSIOLOGY 
 
 organic infusions, or at any rate these do not appear to produce 
 any local irritation .when injected. 
 
 Absorption from the Cellular Tissue is very active, and both 
 bloodvessels and lymphatics take part in the process; ferrq- 
 cyanide of potassium injected into the face has been detected 
 in a carotid lymphatic in seven minutes. The rapidity of cellu- 
 lar tissue absorption is increased by muscular movement. 
 
 Absorption from the Conjunctiva is very pronounced for 
 some drugs, such as atropine and for certain organic poisons, 
 but there are other substances which are not absorbed so readily. 
 Curare is not absorbed through the conjunctiva, and Colin could 
 not infect horses with anthrax by placing anthrax blood and fluids 
 in the conjunctival sac. 
 
 Absorption by the Skin, if the surface be unbroken, is as a rule 
 slow even for those drugs which will pass through it, while there 
 are many organic and inorganic substances which refuse to pass 
 through the unbroken epidermis. Colin kept the lumbar region 
 of a horse wet for five hours with a solution of ferrocyanide of 
 potassium ; the salt was detected in the urine in four and a half 
 hours, although the skin was quite unbroken. A solution of 
 nicotine is readily absorbed, and in horses poisoning both by it 
 and arsenical preparations, unless properly diluted, may readily 
 occur. In the dog absorption from the skin of such drugs as 
 carbolic acid, even in a very diluted form, is rapid and frequently 
 fatal. From a wound or abraded surface, absorption will occur 
 rapidly with some agents, slowly with others. Colin placed a 
 horse's foot with a wound on the coronet in a solution of ferro- 
 cyanide of potassium; in twenty minutes he detected the salt 
 in a lymphatic of the thigh. In connection with absorption 
 from a wounded surface, he found that the poison was taken up 
 quite as readily by the lymphatics as by the bloodvessels. The 
 mucous membrane of the vagina was found by experiment to 
 absorb very slowly. 
 
 Experiments made on Absorption from the Pleural and Peri- 
 toneal Cavities showed that such drugs as strychnine rapidly 
 produce fatal symptoms when injected into these sacs; even in 
 such a short time as from three to seven minutes tetanic symp- 
 toms supervene. Potassium iodide injected into the peritoneal 
 cavity of a sheep may be detected in the thoracic duct five 
 to eight minutes after the operation. Starling and Tubby have 
 shown^ however, that the active agents in absorption from these 
 sacs are the bloodvessels, and that the share taken by the lym- 
 phatics is insignificant; for if methylene blue be injected into 
 the pleural cavity the dye appears in the urine long before any 
 trace of colour can be perceived in the lymph flowing from the 
 thoracic duct. 
 
ABSORPTION 
 
 291 
 
 Stomach Absorption, or, rather, its absence in herbivora, has 
 been dealt with at p. 207. Even in the dog it is now admitted 
 that absorption from the stomach is by no means so certain as 
 was at one time supposed. Water, for instance, passes through 
 the stomach and undergoes no absorption; salts are absorbed 
 only with difficulty ; sugars and peptones are taken up, but only 
 if in sufficient concentration ; ordinarily they are absorbed with 
 difficulty. 
 
 Intestinal Absorption. — The absence of stomach absorption 
 in the horse and ox points to intestinal absorption as being of 
 
 
 •«• T A 
 
 s&* 
 
 > 
 
 
 *5s>> 
 
 
 
 k 
 
 \ \ v. ' ' %■ 
 
 ■^/ 
 
 \ 
 
 
 Fig. 87. — Loop of Small Intestine of the Horse during Active Absorption, 
 with Distended Lacteals. 
 
 considerable importance in herbivora. That this absorption 
 is very rapid is proved by Colin's experiments. Hydrocyanic 
 acid injected into the small intestine of a horse caused death in 
 one to one and a half minutes, and potassium ferrocyanide 
 injected into the bowel, after the tying of the mesenteric lym- 
 phatics, was detected in the blood six minutes afterwards. 
 
 The Paths of Absorption. — The paths by which intestinal 
 absorption occurs are (1) through the villi into the lacteals, and 
 (2) through the bloodvessels into the venous system. This 
 points to the fact that some substances taken up from'the bowel 
 may at once pass into the blood via the thoracic duct (Fig. 87), 
 
292 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 while others must first proceed to the liver by the portal vessels 
 for further elaboration before entering the blood. 
 
 It will be remembered that the villi are found only in the 
 small intestines; in the large intestines there are no villi. It 
 must not, however, be supposed that absorption in the latter is 
 exclusively carried on by the bloodvessels, for the large chain 
 of glands, along the colon in particular, suggests as probable 
 that in greater or smaller quantity the material absorbed passes 
 through these glands, as through the glands of the mesentery, 
 before entering the circulation. There is, at any rate, a well- 
 developed lymphatic system in the walls of the large intestine, 
 and it is certain that material is taken up from this bowel both 
 by the bloodvessels and the lymphatics. The amount of this 
 must be considerable, in view of the size of these bowels and the 
 character of their contents. 
 
 Substances can be taken up with extreme rapidity from the 
 large bowels. Colin observed that eighteen minutes after in- 
 jecting a solution of nux vomica into the caecum of the horse 
 convulsions began, and eight minutes later the animal was dead. 
 Anaesthetics, such as ether, may also be administered per rectum 
 and produce narcosis. Finally, and from some points of view 
 most important of all, proteins may be absorbed from the rectum 
 and single colon, in spite of the fact that there is no proteolytic 
 ferment to render them soluble. , 
 
 Absorption of Fat. — If a cannula be placed in the thoracic 
 duct of a starving dog, the lymph which escapes is identical with 
 that from any other part of the body. If the animal be now fed 
 on a. diet rich in fat, the lymph becomes milky, and even the blood- 
 plasma becomes turbid from fat, if the contents of the duct are 
 permitted to enter the general circulation. It is evident that 
 the lymphatics are the chief path by which the fat enters the 
 body, for comparative analysis of the blood of the portal vein 
 and carotid artery shows that the amount of fat in the two is 
 the same. Nevertheless, the bloodvessels are not without some 
 action in the matter, as will presently be shown. 
 
 It has been stated (p. 264) that fat in the small intestine is 
 both saponified and emulsified, the former being a chemical, 
 the latter a physical change. These processes result from the 
 separate and combined actions of the pancreatic juice and bile, 
 and they give rise to two possible views of the mechanism of fat 
 absorption. Emulsification reduces the fat (and fatty acids) to 
 a state of subdivision into particles so minute that they might 
 conceivably be simply passed as such, through the epithelial 
 cells of the villi to the lacteals, by an activity of these cells com- 
 parable to the ingestive powers of a white blood-corpuscle. This 
 would readily account for the characteristic appearance of chyle 
 
ABSORPTION 
 
 293 
 
 (p. 286), the minuteness of the fat particles it contains being 
 probably intended to prevent embolism from plugging of the 
 capillaries. The view here indicated was the one formerly most 
 prevalent. On the other hand, bile has, in virtue of its bile salts, 
 an extremely active solvent action on both fatty acids and soaps ; 
 hence the possibility that fat is split up so as to give rise to 
 variable relative amounts of substances, which pass in solution 
 into the cells of the villi, as do the proteins and carbohydrates. 
 We shall see that the chemical view is the one now generally 
 adopted. 
 
 If the intestinal mucous membrane of an animal in full fat 
 absorption is stained with osmic acid, the epithelial cells are 
 found to be crowded with minute particles of varying size, whose 
 blackness shows them to be fat (Fig. 88). This fact provided 
 the chief support for the view that fat reaches the lacteals in a 
 state of minute mechanical sub- 
 division not necessarily involving 
 much chemical change. If this 
 were so we should expect to see 
 some of the fat particles in 
 transit through the striated bor- 
 der of the epithelial cells, and this 
 is never observed. 
 
 It is generally accepted that the 
 fat passes into the cell and not 
 between the cells, and the ques- 
 tion arises as to whether this 
 passage into the wall of the bowel 
 is a physical or a secretive pro- 
 cess. Leucocytes are not cred- 
 ited with absorption, though as 
 carriers of fat between the epithelial cells and the central 
 lacteal— viz., through the arterial network of the viUus— they 
 may assist. On the other hand, the absorption of anthrax 
 bacilli from the intestine, a process which is undoubted, must 
 occur from their being engulfed by cells. The wandering cells 
 of the body are, we know, capable of ingesting bacteria m other 
 places, and" there appears no reason why they should not be 
 so in the intestines. Nevertheless, there are good reasons for 
 believing that the fat is dissolved and absorbed by the epithelium 
 of the intestine, its solution into fatty acids and glycerol having 
 been effected by the combined action of the bile salts and pan- 
 creatic lipase. These split products are readily absorbed, 
 glycerin presenting no difficulties, while the fatty acids are taken 
 up either as such or as soaps. It has been stated that osmic 
 acid staining reveals the fact that fat as such exists in the epithe- 
 
 Fig. 88. — Mucous Membrane of 
 Frog's Intestine during Ab- 
 sorption of Fat (SchSfer). 
 
 ep, Epithelial cells; sir, striated 
 border; c, tymph corpuscles; 
 I, lacteal. 
 
294 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Hal cells. It is evident from this that the fats after solution are 
 reconstructed into neutral fats within the epithelial cells. The 
 synthesis of fats in the intestinal epithelium is effected by the 
 same ferment, lipase, which originally split it up ; it is not diffi- 
 cult to imagine a reverse action of lipase bringing this about. 
 If an animal be fed on fatty acids without glycerol, neutral fats 
 are found in the thoracic duct. Somewhere, therefore, the fatty 
 acids have picked up glycerol, and this is believed to be in the 
 intestinal wall, lipase effecting the reconstruction. The story is 
 interesting and instructive ; it emphasises the specific and selective 
 influence of the living cell, regarding which further evidence 
 will be given later. In the above process the valuable action of 
 the bile salts in dissolving the fatty acids, and so assisting the 
 lipase of the pancreatic secretion, is very evident. 
 
 When an animal. is receiving a known quantity of fat in the 
 food, and the whole of the chyle from the thoracic duct is collected 
 with the object of recovering it, a portion of the fat is found to 
 be missing. In these circumstances only 60 per cent, of the fat 
 can be recovered from the chyle, and that which is missing is 
 believed to be absorbed by the bloodvessels of the villi as fatty 
 acids and soaps, and taken to the liver before its entry into the 
 general blood-stream. This theory receives further support 
 from the fact that after ligature of the thoracic and right lym- 
 phatic duct, 30 to 40 per cent, of fat is absorbed from the 
 intestine. Even after excision of the pancreas, fat absorption 
 is not entirely abolished, and, strange to say, more may be 
 found in the intestinal canal than was given by the mouth. 
 This suggests that fat may even be excreted by the intestinal 
 wall. 
 
 All fats are not equally absorbed. In the dog olive oil is taken 
 up more completely (97 per cent.) than any other fat; next 
 comes mutton fat, of which 90 to 92 per cent, is absorbed; 
 while of spermaceti only 15 per cent, is taken up. A dog may 
 absorb up to 21 per cent, of fat in three or four hours, double 
 that in seven hours, and 86 per cent, in eighteen hours. Absorp- 
 tion of fat, in ruminants especially, is a most important question, 
 considering the large sums spent in adding fat to the diet of 
 animals intended for food. Experimental inquiry . shows that 
 90 per cent, of the fat in linseed cake, 79 per cent, in rape cake, 
 and 88 per cent, in decorticated cotton cake can be absorbed. 
 The digestive power of the horse for fat is lower than that of 
 ruminants; only 53 per cent, of the fat in linseed cake can be 
 utilised by this animal. 
 
 The following table shows the power possessed by the herbivora 
 of digesting fat from different food substances, and demonstrates 
 the relatively weak powers of the horse in this respect : 
 
ABSORPTION 
 
 295 
 
 
 Horse. 
 
 Ruminant. 
 
 Grass .... 
 
 Hay .... 
 
 Oats - 
 
 Barley - 
 
 Maize .... 
 
 Beans ... 
 
 Per Cent. 
 20 
 
 24 
 70 
 42 
 6l 
 13 
 
 Per Cent. 
 
 66 
 
 50 
 
 83 
 89 
 85 
 
 86 
 
 All the above foods in the matter of fat, and we shall see 
 presently in the matter of carbohydrates and protein, are richer 
 and more nourishing for the ruminant than the equine, owing to 
 the different powers of absorption in the two classes. 
 
 Food Absorption. — -At this point it is necessary to digress 
 somewhat, and to glance at the question of the absorption of the 
 various proximate principles of food — viz., protein, fat, and 
 carbohydrate. 
 
 The economic feeding of animals has led to the chemistry 
 of food absorption being inquired into by series of laborious 
 experiments which have been carried on for years. Each class 
 of animal has a different power of utilising the same food; for 
 example, grass and hay are more nourishing to the ox and the 
 sheep than to the horse, and the ox can utilise more than the 
 sheep. 
 
 If a food were entirely digested and absorbed, there would be 
 no faeces excepting the waste liquid secretion of the digestive 
 tract. There is, of course, no ordinary food substance capable 
 of complete absorption ; there are indigestible substances which 
 cannot be dealt with and are excreted ; and in the herbivora the 
 quantity of these must necessarily be very large. But entirely 
 apart from indigestible matter, there is only a proportion of each 
 of the proximate principles of a food which can be absorbed. The 
 protein, fat, carbohydrate, and cellulose of every food substance 
 have a distinct rate of absorption in each class of animal. Taking 
 hay as an example, the horse can, as a mean, digest : 
 
 Per Cent. 
 
 Protein - 57 
 
 Fat 24 
 
 Carbohydrates - 55 
 
 Cellulose - - 3& 
 
 There is no way of causing a larger percentage to be absorbed, 
 and so of rendering a given amount of hay more efficient . Absorp- 
 tion of the food principles is unaffected by rest or work. The 
 horse absorbs no more per cent, if the diet is increased; he can 
 extract no more per cent, if the amount be reduced. Feeding 
 experiments show that around 57 per cent, protein and 55 per 
 
296 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 cent, carbohydrates lies the digestive capacity of the horse for 
 these promixate principles in hay. 
 
 If we take the figures for the ox and the sheep, for the purpose 
 of comparison, we get the following table: 
 
 ! 
 
 Ox. 
 
 Sheep. 
 
 
 Per Cent. 
 
 Per Cent. 
 
 Protein - 
 
 57 
 
 57 
 
 Fat - 
 
 49 
 
 51 
 
 Carbohydrate - 
 
 62 
 
 62 
 
 Cellulose » 
 
 5% 
 
 56 
 
 
 nvjb t . 
 
 T^ — > 
 
 These facts are of extraordinary importance in the feeding of 
 animals. The practical conclusions to be drawn from them 
 belong to the realm of hygiene. They are mentioned here 
 owing to their physiological aspect, for though we cannot explain 
 the reason, there is no doubt that both ox and sheep in the 
 matter of food absorption are physiologically superior to the 
 horse, while no animal approaches the pig in the thoroughness 
 with which absorption and consequent utilisation of food sub- 
 stances occurs. 
 
 Absorption of Carbohydrates. — We have seen (p. 264) the 
 digestive changes undergone by starch in order to prepare it for 
 absorption, and we have learned that in the body only the mono- 
 saccharides, such as dextrose, levulose, and galactose, but 
 especially dextrose, are capable of being utilised, while the di-sac- 
 charides — lactose, cane sugar, and maltose — have all to be 
 feconstructed in order to fit them for absorption. The following 
 brief summary shows by what means the carbohydrates are 
 prepared for entry into the body : 
 
 Starches are converted by means of the saliva and the amylase 
 of pancreatic juice into maltose and dextrin, and then inverted 
 by the ferment maltase into dextrose. 
 
 Lactoserequires inversion by lactaseinto dextrose and galactose. 
 
 Cane sugar requires inversion by invertase into dextrose and 
 levulose. . 
 
 If an attempt bermade to cause the organism to use up such 
 sugars as cane sugar without passing it through the inverting 
 process, as, ior example, by injecting it subcutaneously, it is 
 excreted unchanged in the urine. 
 
 If very large amounts of cane sugar, or even of dextrose, be 
 given, they are taken up unaltered, probably by the lacteal 
 vessels, and excreted by the kidneys. This constitutes the 
 temporary glycosuria which sometimes follows a diet rich in 
 carbohydrate. The lacteal path of absorption of sugars is of 
 interest, for, as we shall see presently, physiologists have assigned 
 
ABSORPTION 
 
 297 
 
 the bloodvessels as the path by which sugar is taken up. Never- 
 theless, years ago Colin insisted that in the herbivora the chyle 
 vessels took up sugar. The chyle of a horse on an ordinary diet 
 of hay and oats was found by him to yield 13 to 16 per cent, sugar, 
 and several such observations are referred to by him in terms 
 which lead one to believe his experiments were adversely criti- 
 cised by Bernard. The physiologist of the present day regards 
 the bloodvessels of the villus as the path by which the sugar 
 mainly gains entrance to the body. From here it is carried to 
 the portal vein, and passes direct to the liver. We have seen 
 (p- 2 53) that the sugar percentage in the portal vessel may vary: 
 o - 4 per cent, has been found after a heavy carbohydrate meal in 
 the dog, and 0-2 per cent, in the same animal during starvation. 
 On the other hand, a definite percentage is maintained in the 
 other vessels of the body, and the cause of this has been fully 
 considered elsewhere (pp. 256, 269). 
 
 The absorption of carbohydrate by the animals of the farm 
 has been the subject of considerable investigation. The ruminant 
 in all cases is better in this respect than the horse, as may be 
 seen from the following table : 
 
 
 Horse. 
 
 1 
 Ox and Sheep. 
 
 
 Per Cent. 
 
 Per Cent. 
 
 Hay .... 
 
 24 
 
 5° 
 
 Clover hay 
 
 29 
 
 53 
 
 Oats - - - 
 
 70 
 
 83 
 
 Barley - 
 
 42 
 
 89 
 
 Maize - 
 
 6l 
 
 85 
 
 Beans - 
 
 13 
 
 86 
 
 The practical application of these facts is very evident. All 
 the above foods in a given quantity are richef for the ruminant 
 than for the horse. In dieting horses, according to the, market 
 value of grains, it is weU to ascertain what proportion of the 
 proximate principles of the grain chosen are digested, or it may 
 prove far from economical. 
 
 Absorption of Proteins. — Proteins, we have seen, are in- 
 capable of absorption as such. Pepsin, trypsin, and erepsin 
 convert them into peptones and proteoses, and in this form, or 
 when still further broken down by trypsin and erepsin into amino- 
 bodies, they are taken up by the bloodvessels of the intestine. 
 It is not known what extent of hydrolytic action is necessary 
 before absorption can occur; certainly it must be carried as far 
 as production of proteoses and peptones. If the thoracic duct 
 of a dog be ligatured, and a large protein meal given, it is perfectly 
 
298 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 absorbed, as shown by the increase in urea. Clearly, therefore, 
 the path of absorption for protein is the bloodvessels, the material 
 passing by the portal vein to the liver. From this it might be 
 supposed that proteose and peptone may readily be found in the 
 general circulation, but, as a matter of fact, there is no blood in 
 the body, including that of the portal area, which contains even 
 a trace of peptone or proteose; in fact, these substances in the 
 circulating blood are poisons, give rise to peptonuria, and are 
 excreted by the kidneys. 
 
 Some proteose and peptone in their passage through the 
 epithelial cells of the intestinal wall may be resynthesised. How 
 and in what way this is brought about is unknown. We saw that 
 the same thing occurred to the fats before they could pass into 
 the lacteals. Proteins administered by any other channel than 
 the digestive canal are excreted — egg-albumin, for instance, if 
 injected into the blood — and got rid of by the kidneys. Foreign 
 proteins are of no use until they have passed through the proper 
 laboratory — -viz., the intestinal -canal- — and that something more 
 than splitting the complex protein molecule occurs is evident 
 from the fact that some rearrangement of the molecule occurs 
 which enables it now to be built into and form part of the protein 
 tissues of the living body. 
 
 The absorption of protein by animals is an important question 
 in feeding. We saw that, in capacity for absorbing fat and 
 carbohydrate, the horse was distinctly inferior to ruminants. 
 In the matter of protein absorption the horse is sometimes 
 superior to the ruminant, and in all cases will bear comparison 
 with it, as may be seen from the following table : 
 
 
 
 
 
 Horse. 
 
 Ruminant. 
 
 
 Per Cent. 
 
 Per Cent. 
 
 Fresh grass 
 
 59 
 
 7° 
 
 Hay 
 
 57 
 
 57 
 
 Wheat straw- 
 
 19 
 
 17 
 
 Oats - 
 
 79 
 
 79 
 
 Barley- 
 
 8o 
 
 70 
 
 Maize ... 
 
 77 
 
 74 
 
 Beans 
 
 86 
 
 88 
 
 Absorption of Water and Salts. — By means of the b ood- 
 vessels water is readily absorbed, though all parts of the digestive 
 tract are not equally active in this respect. There is probably 1 
 no absorption of water from the stomach of any animal. There 
 :s very little absorption of this fluid from the small intestines, 
 whereas in all animals the large intestines are the chief seats of 
 
ABSORPTION 299 
 
 its absorption. In the horse the caecum, in other animals the 
 large colon, is the chief seat. 
 
 In the matter of the absorption of salts, a good object-lesson 
 of the peculiar selective powers of the epithelial cells is obtained. 
 Chlorides are readily taken up, but sulphates only with difficulty ; 
 iron is taken up, but its near ally, manganese, is absorbed with 
 difficulty. Though these facts suggest that the process is one of 
 secretion rather than of physical action, yet it is impossible to 
 neglect physical factors as playing a part. Finally, Colin's 
 experiments on herbivora showed that absorption of salts, such 
 as prussiate of potash, iodide of potassium, tartar emetic and 
 many other substances, and of colouring matters such as chloro- 
 phyll, occurred through the chyle vessels, though not necessarily 
 exclusively. 
 
 Much of the foregoing refers to absorption from the small 
 intestine, but it would be wrong if we failed to emphasise the 
 fact that in the herbivora especially, the large intestine is the 
 seat of vitally important digestive processes. An attempt has 
 been made (p. 227) to describe these in the light of our limited 
 knowledge, but the picture is a very imperfect one. The question 
 is only referred to here in order to emphasise the fact that the 
 large intestines, though they have no villi and no secretion other 
 than the succus entericus, are important seats of digestion and 
 absorption. A man may manage to get along without his large 
 intestines; the horse could manage with half his present length 
 of small intestine but he could not afford to spare an inch from 
 his large bowels. So imperfect is our knowledge, because of the 
 difficulties attached to experimental inquiry in the larger animals, 
 that we do not know the absorptive function of any portion of 
 the large intestine ; we know only the bare fact that it does absorb. 
 At p. 226 attention is drawn to the remarkable fact that a few 
 inches in the intestines of the horse separate fluid fasces from 
 solid fasces. Who can doubt the important changes which must 
 be taking place in those few inches ? In their anatomical 
 arrangement the ox and the sheep are very similar — almost 
 identical, in fact. Their diet is identical, perhaps their d'gestive 
 processes are identical, up to a certain point ; then they differ 
 widely : the faeces of the sheep are firm and dry, those of the ox 
 soft and unformed. There must be important differences in 
 the intestinal absorption of animals which bear such close resem- 
 blance to each other. v 
 
 Several matters referred to in this chapter will have to be dealt 
 with in greater detail in the chapter on Nutrition. 
 
CHAPTER VIII 
 
 THE ENDOCRINE GLANDS AND INTERNAL 
 SECRETIONS 
 
 The endocrine, originally known as the ductless glands, are 
 represented by the spleen, thyroids, thymus, adrenals, the 
 pituitary and pineal bodies. Until recent years the function of 
 these was entirely unknown, and those of small size, such as the 
 adrenals, pituitary, and pineal, were regarded as too insignificant 
 to merit attention. It is now known that these glands command 
 sometimes the life, and at all times the well-being, of the indi- 
 vidual, and that their removal in some cases means certain death. 
 Their function is to produce an internal secretion, a chemical 
 substance, or hormone, which is poured into the blood and 
 utilised elsewhere in the body. Ductless glands are not the only 
 bodies producing an internal secretion; the liver, pancreas, 
 testicles, ovaries, and prostate produce, in addition to the 
 evident secretion, a substance or substances which find then- 
 way into the circulation by the lymph or bloodvessels. The 
 chemical nature of the internal secretions is imperfectly known; 
 they are protein in character, and are not destroyed by boiling, 
 a characteristic which distinguishes them from ferments. Infun- 
 dibulin may be selected as an example; its active substance is 
 soluble in water, dialysable, insoluble in alcohol or ether, and 
 unaffected by repeated boiling, drying, or sterilisation. 
 
 All the light which recent inquiry has thrown on the function 
 of'the endocrine glands arises from the epoch-making discovery 
 of secretin (p. 260) by Starling and Bayliss. 
 
 The Spleen. — This is the largest ductless gland in the 
 body. It possesses neither an internal nor external secretion, 
 and in spite of the numerous observations to which it has been 
 subjected, is still a physiological enigma. Its vascular arrange- 
 ment is peculiar in that it is capable of holding a considerable 
 quantity of blood, and for this purpose readily lends itself to 
 change of size. Further, it is the only tissue in the body where 
 the cell elements are directly bathed in blood without the inter- 
 
 300 
 
THE ENDOCRINE GLANDS 391 
 
 vention of even a capillary wall. The spleen contains a consider- 
 able amount of ^involuntary muscular fibre, and is capable of 
 movement. These movements have been carefully studied, 
 and it is established that they are of two kinds. One is a slow 
 expansion due to unknown causes, which occurs after a meal; 
 it reaches its maximum about the fifth hour, and is followed by 
 contraction. The other is a rhythmical expansion and contrac- 
 tion occurring in certain animals, such as dogs and cats, at 
 intervals of about one minute. It is believed that the latter 
 movement is for the purpose of assisting the circulation through 
 the organ, to which the splenic pulp offers considerable resistance. 
 That the movement is brought about by the bands of involuntary 
 muscular fibre is undoubted; the spleen is liberaUy supplied 
 with motor nerves carried in the splanchnic, and stimulation of 
 these leads to a reduction in the volume of the organ. It is 
 also considered that there are nerves to the spleen, which on 
 stimulation produce dilatation. 
 
 The use of the gland is largely based on conjecture. By some it 
 has been considered the seat of formation of red blood-corpuscles, 
 and that this is the case during intra-uterine life and shortly 
 after birth is undoubted ; but there is no evidence of this function 
 in the adult. It has been claimed to be the seat of destruction 
 of the red cells and of phagocytosis, and as bases for this theory 
 there are some telling facts ; for instance, certain large amoeboid 
 cells found in the spleen are capable of ingesting and destroying 
 worn-out blood-cells and other solid matter such as micro- 
 organisms, while the richness of the splenic pulp in iron is re- 
 garded as due either to the haemoglobin of the destroyed red 
 blood-cells being stored up for future use, or to the preparation 
 of new haemoglobin. That the conservation of iron is one of 
 the functions of the spleen would appear from the fact that 
 removal of this organ in dogs causes a distinct daily loss of iron. 
 The theory is very plausible, though by no means definitely 
 proved; at the same time there is great difficulty in rejecting 
 the view that filtration is a function, since the spleen appears 
 in every way to be admirably suited to act the part of a blood- 
 filter. 
 
 The lymphoid tissue of the spleen, like lymphoid tissues in 
 general, is capable of forming a substance from which uric acid 
 may be readily produced, and the spleen has in consequence 
 been regarded by many as the seat of active metabolic changes 
 with the formation of uric acid. The evidence, however, is 
 not sufficiently conclusive to warrant uric acid being regarded 
 as a special product of the spleen. 
 
 Some physiologists have suggested that the spleen produces 
 an enzyme which converts trypsinog'en into trypsin. There 
 
» 3°2 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 is no reason why the spleen might not do so, but it by no means 
 follows that this is normally its function, nor would there appear 
 to be any necessity for this action, as this conversion is one of 
 the special duties of the intestinal juice. 
 
 In connection with all these theories it is well to remember that 
 the spleen may be removed completely, and no ill effects follow. 
 The Thyroid and Parathyroid Glands. — The relationship 
 between the thyroid gland and disease was first established in 
 connection with goitre, and, later, with a pathological condition 
 known as Graves' disease. In both of these affections the glands 
 are enlarged, and surgical attempts to remedy goitre led to the 
 production of another affection known as myxcedema. It was 
 evident, therefore, that either excessive secretion or defective 
 secretion by the thyroids was capable of producing serious 
 pathological conditions, and when these facts had been estab- 
 lished by clinical experience, experimental inquiry on the 
 thyroids of animals was undertaken. 
 
 This work led to very contradictory information being obtained; 
 when thyroidectomy was practised on the carnivora the majority 
 of the animals died, while of the herbivora so operated upon 
 one-half recovered. It was soon found that the thyroid consists 
 of two distinct portions, one part the thyroid proper, the 
 other the parathyroids, of which there are one or two to-each 
 gland, depending on the species of animal. The symptoms 
 following removal of the thyroid were ascertained to depend 
 upon whether the thyroid, parathyroids, or both, were in- 
 cluded in the surgical procedure. The disturbances produced 
 by removal, partial or complete, of the parathyroids are essen- 
 tially distinct from those characteristic of thyroidectomy. 
 In the former case the most striking disturbances are nervous 
 in type, and consist of clonic or tonic muscular contractions, 
 followed by convulsive movements, the so-called 'tetany' of 
 parathyroidectomy ; in the latter the disturbances mainly affect 
 general bodily metabolism, Occasioning that depression of tissue 
 activity as a whole so well exemplified in the lowered bodily 
 and mental vitality characteristic of the cretinous, goitrous, and 
 myxcedematous state's. If all parathyroids be removed death 
 usually results within a few weeks, the symptoms being most 
 acute in carnivora and in the young of herbivora. Young 
 animals are more affected by thyroidectomy than fully grown 
 ones, as has been clearly shown by Sutherland Simpson's 
 experiments on sheep and lambs. The convulsions attending 
 removal of the parathyroids are said to be abolished by injections 
 of extracts of the gland ; while the injection of calcium salts effects 
 a cure of all the symptoms produced by removal of these bodies. 
 A jelly-like colloid substance constitutes the internal secretion 
 
THE ENDOCRINE GLANDS 303 
 
 of the thyroid, which is the only endocrine gland which can store 
 its secretion, but forms no part of the secretion of the para- 
 thyroids. Histologically the thyroid gland consists of vesicles 
 lined by a single layer of cubical epithelium, while the para- 
 thyroid is composed of irregularly arranged columns of epithe- 
 lium-like cells. The gland contains a nucleo-protein in addition 
 to a colloid substance ; the latter is not a nucleo-protein, and is 
 remarkable for containing iodine in organic combination with 
 protein. The iodine-containing substance, abundant in her- 
 hivora, is termed iodothyrin; it is a brown amorphous material, 
 containing phosphorus and 10 per cent, of iodine.* The para- 
 thyroids contain no iodine. 
 
 The internal secretions of the thyroid and parathyroids are 
 not identical in function. That of the parathyroids depresses 
 the excitability of nerve cells, so that, when these minute bodies 
 are removed, the nervous symptoms, described above as tetany, 
 result. The thyroid secretion is not identical in function with 
 that described above. It lowers blood -pressure, leads to the 
 rapid excretion of lime salts, and is largely concerned in the 
 metabolism of the body. The removal of the glands is followed 
 by a reduction in the nitrogenous and fat metabolism, and 
 permits abnormal amounts of carbohydrates to be consumed 
 without glycosuria resulting. In the young subject a diminution 
 or absence of thyroid secretion prevents the physical and mental 
 development. The genital glands are undeveloped, and the 
 adult remains a child, as is witnessed in Cretins. 
 
 White rats fed upon fresh ox thyroid suffer from hypertrophy 
 of the adrenals, kidneys, pancreas, and heart, as the result of 
 greatly increased metabolism; if the feeding be continued it 
 causes death. There is also in young animals inhibition of the 
 growth of the uterus and of the pituitary body. It will be 
 observed that the adrenals are enlarged, while the pituitary is 
 diminished in size. This is evidence of the correlation which 
 exists between the various internal secretions, and to which 
 attention will later be directed. 
 
 The Thymus.— This body, composed of modified lymphoid 
 tissue, is of use mainly in foetal and very early life ; later on it 
 atrophies. Nothing is known of its function, though it is observed 
 that castration appears to have an effect on its disappearance. 
 The process of atrophy is much slower in the castrated as com- 
 pared with that in the uncastrated animal, while its early absorp- 
 tion or experimental removal has been observed to be associated 
 with a rapid growth of the testicles. 
 
 * Kendall has recently isolated a pure crystalline substance, an indole 
 compound of perfectly constant composition, called alpha-iodine. It 
 contains over 60 per cent, iodine. 
 
3P4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The Adrenals. — The adrenals consist of two distinct tissues, 
 a medulla, which can be shown to be derived during the process 
 of development from the sympathetic nervous system, while 
 the cortex is formed from the mesoblast. While very little is 
 known of the function of the cortex, the medulla is essential to 
 life, and yields under experimental inquiry some remarkable and 
 characteristic results. 
 
 The experimental removal of the adrenals in any animal is 
 rapidly followed by death preceded by symptoms of great 
 muscular prostration and diminution of vascular tone. In 
 Addison's disease in man these bodies are affected, and give rise 
 to much the same symptoms as above, and in addition bronzing 
 of the skin is present. 
 
 An extract of the medulla of the gland, when injected into the 
 blood, after section of the vagi, increases both the rate of rhythm 
 and the tone of the cardiac muscle, and causes a contraction of 
 the bloodvessels, which produces a remarkable increase in blood- 
 pressure.* This increase is of a purely temporary nature, which 
 indicates that the active principle is destroyed in the circulation. 
 Within a few minutes the heart-beats return to the normal and 
 the blood-pressure falls. When the vagi are intact, injections 
 give rise to a slowing of the heart and frequently to stoppage 
 of the auricular beats. The active principle known as adrenalin 
 can be detected in the veins leaving the gland, so that there can 
 be no doubt that it is poured into the blood, where it regulates 
 the rhythm and tone of the heart muscle and maintains blood- 
 pressure. In this important function the adrenals are probably 
 assisted by another internal secretion, to be considered presently 
 — namely, that from the pituitary body. 
 
 The action of adrenalin in causing constriction of the blood- 
 vessels is turned to account in minor surgery in controlling 
 haemorrhage. 
 
 It is believed that the adrenals possess secretory nerve-fibres 
 derived from the splanchnic, and that stimulation of these 
 increases the amount poured into the vein of the gland. Adrenalin 
 acts upon all plain muscle and gland-cells which receive sym- 
 pathetic fibres, and it is distinctly noteworthy that the effects are 
 identical with those produced by stimulation of the sympathetic 
 fibres (Langley), of which system the medulla of the gland is, as 
 pointed out above, merely an outgrowth. 
 
 It is probable that the function of the medulla of this gland 
 is concerned in the provision of a substance intimately con- 
 nected with muscular metabolism, and more particularly with 
 'tone,' possibly of the skeletal muscles, but especially of the 
 muscular fibres of the circulatory system. Adrenalin causes an 
 * Oliver, and Sharpey-Schafer. 
 
THE ENDOCRINE GLANDS 305 
 
 increased conversion of glycogen into sugar. There is considered 
 to be some connection between the cortex of the adrenals and 
 the sexual system. During the breeding season in some animals 
 the glands enlarge, and in rabbits the cortex of the gland 
 becomes twice the normal thickness during pregnancy. It is 
 believed that in man a connection exists between the adrenals 
 and the growth of the body, development of puberty, and sexual 
 maturity. 
 
 Under the influence of emotion, great muscular effort, fright, 
 or anger, the medulla of the adrenals pours out an excessive 
 secretion as the result of impulses received from the brain through 
 the sympathetic system. This secretion may be regarded in the 
 light of a defensive mechanism against the nervous explosion ; 
 the muscles are ' tuned up ' for an extra effort, an increased 
 quantity of glycogen is rendered available for their use, the 
 general blood-pressure is raised, and the thyroid co-operates by 
 furnishing a substance intended to restore the exhausted nerve 
 cells. Whether the storm results in fight, as is common in the 
 case of dogs, or flight, as in the case of horses, probably depends 
 almost entirely on the testicular secretion. In the absence of 
 these glands, it is easy to understand the gelt animal taking no 
 risks, and deciding to get away as early as possible from the 
 source of danger. The female in the defence of her progeny will 
 put up a fight under the influence of the internal secretion of the 
 ovary; and this applies to all animals from the elephant to the 
 barn-door fowl. 
 
 The Pituitary Body.— This gland consists of three portions, 
 an anterior lobe derived from the buccal ectoderm, a posterior 
 or nervous lobe separated from the anterior by a cleft, while the 
 nervous lobe is separated from the cleft itself by a narrow strip 
 of tissue, consisting of modified glandular tissue, known as the 
 intermediate portion.* This part is formed by the neck or stalk 
 of the pituitary. The nervous lobe is composed of neuroglia' 
 cells, and is remarkable in the ox for its vascularity. Bellf em- 
 phasises the importance of regarding the pituitary as one organ, 
 and not two. 
 
 The internal secretion of the pituitary has been known as 
 Pituitarin, but Bell has shown that the active substance is pre- 
 pared only from the posterior lobe, and it is accordingly called 
 by him Infundibulin. The effect of this on injection into the 
 body resembles that of adrenalin in its actions on the heart, 
 bloodvessels, and skeletal muscles. It causes the blood-pressure 
 to rise, and increases the tone of the heart muscle. Though it con- 
 stricts the arterioles generally, it causes dilatation of those of the 
 
 * Herring. . 
 
 f ' The Pituitary,' W. Blair Bell, 1919- 
 
306 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 kidney, accompanied by diuresis.* This diuresis, however, may 
 be due to a specific action on the cells of the. kidney. Infun- 
 dibulin may also slow the heart. The action of infundibulin on 
 smooth muscle differs from that of adrenalin by not occurring 
 through the sympathetic system, and further, contrary to what 
 would happen in the use of adrenalin, the pressor effects are not 
 obtained by a second injection within a short period of time. 
 An extract of the posterior lobe excites contraction of the intes- 
 tinal wall, and of the oestrus and pregnant uterus, but not of the 
 resting organ (Bell). Further, it possesses a substance containing 
 glycogenlytic properties, and one which acts as a galactagogue 
 on the secreting mammary gland. There is, however, good 
 reason for believing that the increased flow of milk is due to 
 contraction of the smooth muscle of the mammary alveoli. 
 Bell points out that one of the normal functions of the pituitary 
 is to retain the alkaline bases in the tissues ; in this respect, and 
 in its effect on blood-pressure it is antagonistic to thyroid secre- 
 tion. 
 
 Experimentally it has been proved by Bell that compression 
 of the infundibular stalk in the dog leads to atrophy of the 
 genital organs, uterus, ovaries, and mammae, together with im- 
 mense deposits of fat. The animal remains lethargic, and spends 
 most of its time in sleep, from which it has to be aroused for food. 
 Removal of the anterior lobe causes degenerative changes in the 
 ovaries, but no such result follows removarof the posterior lobe. 
 Removal of the entire body causes death. 
 
 In hibernating animals the pituitary undergoes very marked 
 changes during hibernation, and the large deposits of fat then 
 to be seen may be due to insufficiency of this gland. Enlarge- 
 ments of the pituitary glands are associated with a singular 
 disease in the human subject — acromegaly — characterised by an 
 overgrowth of the bones of the face and extremities, resulting 
 in gigantism. 
 
 . The Pineal Body remains a physiological enigma, though it 
 is believed to be concerned in the production of sexual maturity. 
 
 In forming a clear concept of the function of the endocrine, 
 glands it is all-important to regard them collectively and not 
 individually. Everything points to an inter-relation of functions; 
 these may be mutually stimulating or mutually restraining. 
 For example, infundibulin counteracts the low blood-pressure 
 occasioned by thyroid secretion and the rapid excretion of lime 
 salts produced by this gland. Eagerness to fight is decided by 
 the brain, but under the stimulus of the testicular secretion; and 
 
 * Sharpey-Schafer. 
 
THE ENDOCRINE GLANDS 3 o 7 
 
 simultaneously the pituitary and adrenals raise the blood-pressure, 
 while the voluntary muscles are furnished with an extra supply 
 of fighting material in the form of glycogen ; then the thyroid 
 co-operates by restoring nervous energy and lowering the blood- 
 pressure after the storm. 
 
 The extraordinary importance of the endocrine glands to the 
 genital system cannot fail to have been observed. Periodical 
 sexual excitement in both male and female; the growth and 
 development of the sexual organs; the sympathy between the 
 uterus and mammary glands; the fertilisation and implantation 
 of the ovum through the internal secretion of the corpus luteum; 
 the growth of the mammary gland through the production by 
 the foetus of a hormone, which finds its way into the maternal 
 blood; the alteration in the eating qualities of the flesh of cas- 
 trated animals ; the modification of sex characteristics, especially 
 in the male, resulting from this operation ; the absence of antlers 
 and in some animals (sheep) of horns in consequence of castration ; 
 the persistence of lactation after ovariotomy (cow) ; the influence 
 of the testicles and ovaries on psychical conditions, even the 
 storage of fat in the body, are all examples of the point under 
 consideration. Nor, as we have seen, are internal secretions con- 
 fined to ductless glands. The liver, with its external secretion of 
 bile, has an internal secretion of glycogen and urea ; the pancreas, 
 with its obvious secretion of pancreatic juice, has an internal 
 secretion which regulates the destruction of sugar ; and the kidney 
 has been described as also possessing an internal secretion. ' It 
 is believed by some that secretin is responsible not only for the. 
 production of pancreatic juice, but also of the bile and succus 
 entericus ; that gastric juice is the outcome of a hormone, gastrin, 
 secreted by the pyloric wall; and that intestinal peristalsis may 
 even be activated by a hormone furnished by the appendix of the 
 caecum. 
 
 Only the beginning of a study of the endocrine glands has at 
 present been made ; the future of this branch of physiology may 
 hold in its keeping some of the romances of science. Keith* has 
 recently propounded the theory that the differentiation of man- 
 kind into racial types is a function of the endocrine glands. He 
 suggests that the pituitary regulates growth, cast of feature, 
 texture of skin, and character of hair ; that the internal secretion 
 of the testicles determines the robust manifestations of the male, 
 which are not identical in Caucasian, Mongol, and Negro; that 
 the adrenals are probably concerned in race pigmentation, and 
 the thyroid in implanting racial characteristics. 
 
 * British Association foi the Advancement of Science, 1919. 
 
CHAPTER IX 
 
 THE SKIN 
 
 It is obvious that one important function the skin discharges 
 is that of affording cover to the delicate parts beneath; where- 
 ever the chance of injury is greatest, the skin is thickest, while 
 in those parts where sensibility is most required it is thinnest. 
 The skin of the back, quarters, and limbs affords a good illustra- 
 tion of the first kind ; on the back a protective covering is found 
 which, in some horses, is as much as | inch in thickness. The 
 skin of the face and muzzle is a good example of the latter variety, 
 the skin in some parts being as thin as paper. In those regions 
 not exposed to violence it is also thin, as on the inside of the 
 arms and thighs. In spite of the thinness of the skin its strength 
 is remarkable; a horse's body may be dragged along by the thin 
 skin of the head. 
 
 The skin as an organ of touch is of great importance. All 
 animals appear most sensitive to even slight skin irritation; 
 flies will cause horses considerable suffering, and the elephant, 
 despite its thick hide, is quite as intolerant of these tormentors 
 as is a well-bred horse. The sk'n is highly endowed with 
 sensory nerves, especially that part connected with the organs 
 of prehension; the long hairs, 'feelers,' growing from the 
 muzzle of the horse end in special tactile structures in the skin 
 (Fig. 89). 
 
 The skin is a bad conductor of heat, and in this retention of 
 heat is considerably assisted by the layers of fat found beneath 
 or at no great distance from it, as in the abdominal region ; it is 
 the subperitoneal fat which protects the viscera of animals living 
 in the open and lying in wet places. The epidermal covering of 
 the skin relieves the parts beneath from excessive sensitiveness; 
 through the sebaceous secretion it assists in preventing loss of 
 heat, while the greasy covering thus produced helps the hair to 
 throw off rain, prevents the penetration of water, and so saves 
 the epidermis from disintegration. Horn is skin which has 
 undergone a modification. 
 
 308 
 
THE SKIN 
 
 3°9 
 
 Hair.— Not all parts of the body arc covered by hair. There 
 is very little on the muzzle and lips, and it is very scanty on the 
 inside of the thighs, inside the cartilage of the ears, and on the 
 mammary gland and genitals. By means of the hair the heat 
 of the body is maintained and prevented from passing off too 
 rapidly. The thickness of the hairy covering varies consider- 
 ably with the class of horse ; the better bred the animal the finer 
 the coat. Draught horses yield between 3} to 3>- kilogrammes 
 (7 or 8 pounds) of mixed hair, dirt and dandruff by clipping; 
 in a well-bred horse this would be reduced to 283 grammes 
 (10 ounces), or even less; the 
 amount of hair of the mane 
 and tail is about § kilogramme 
 1 \ pounds). 
 
 It is a well-known fact that 
 the hair of every part of the 
 body, excepting that of the mane 
 and tail, has only a temporary 
 existence, and is changed twice 
 a year, once for a thick, and once 
 for a fine coat. During this period 
 horses are generally regarded as 
 not being at their best, and 
 changing the coat is always 
 urged as a cause of loss of con- 
 dition or stamina. The per- 
 manent hair is not entirely repre- 
 sented by that of the mane and 
 tail ; the eyelashes and the fetlock 
 hair are permanent, also the long 
 tactile hairs on the muzzle. The 
 temporary hairs on the horse are 
 of two kinds, which can be dis- 
 tinguished only by their rate of growth. If a part be clipped, or, 
 preferably, shaved, and the growth watched, in a short time it 
 will receive a scanty covering of long, rapidly growing hair, 
 followed by a slow growth of ordinary hair. There is no differ- 
 ence in the two hairs, excepting the length. The long, rapidly 
 growing hairs are known as ' cat hairs ' ; they are not numerous, 
 being about 4 per square centimetre (27 to the square inch), 
 while the ordinary hairs are nearly 700 per square centimetre 
 (4,300 to the square inch). It may yet be shown that 'cat 
 hairs ' are tactile in function. 
 
 The growth of the hair is regulated by the surrounding tem- 
 perature. If horses in the depth of winter arc placed in a 
 heated atmosphere, such as a horse deck on board ship, the 
 
 Fig. S9. — Section ofSkin of the 
 Horse's Lip, showing the 
 Nerve Endings in the Touch 
 Papillae. 
 
.U° 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 majority commence to shed the winter coat in a few days, 
 though the temperature of the outside air may be at freezing- 
 point; similarly, if horses are taken from a warm to a cold 
 locality, the hair responds by becoming longer. Speaking 
 generally, the above statements are correct, but there are excep- 
 tions and modifications. Some horses do not shed the coat 
 after passing into a warmer latitude; the mechanism which 
 regulates the periodical shedding of hair refuses to respond to the 
 changed condition of affairs, so that in passing from north to 
 south of the Equator, with its reversal of seasons, the animal 
 may grow a summer coat in winter and vice versa for at least a 
 year after entering the new latitude. 
 
 The permanent hair of the body — viz., the mane and tail- 
 may grow to almost any length, but the temporary hair of the 
 
 surface of the body 
 grows only to a de- 
 finite length. The full 
 length having been at- 
 tained, nothing will 
 make it grow longer, 
 yet if the horse be 
 clipped, hair at once 
 grows rapidly, but only 
 to its original length; 
 in other words, every- 
 thing required for the 
 needful growth to oc- 
 cur is present, but 
 there is a restraining 
 influence which deter- 
 mines the length of 
 hair according to the 
 season. 
 
 Hair Streams — 
 This term has been 
 very aptly applied to 
 the directions taken 
 by the hair. Very 
 little observation shows that, though the general trend of 
 the stream is obliquely downwards and backwards on the 
 neck and trunk, downwards on the face and limbs, and back- 
 wards on the cheeks, nevertheless, the stream alters its course 
 in an interesting manner at various parts of the large surface, 
 while whorls are frequent. The number and position of the 
 latter are variable; indeed, it has been considered that no two 
 horses are identically marked in this respect, and in Japan 
 
 Fig. 90— -Section of Horse's Skin, showing 
 the Casting Off of the Old Hair and 
 Growth of the New. 
 
 It will be observed that both are emerging from 
 the same follicle. 
 
THE SKIN 3II 
 
 these whorls are employed for the identification of horses 
 much the same way as the finger-print system is employed for 
 the identification of men. There are certain whorls seldom 
 absent: one, the largest, may generally be seen in the flank; the 
 hair over the loins distinctly divides as it approaches the crest 
 of the ilium, one current passing over the quarters, another down 
 the flank. This is met by a stream passing up the flank, which 
 divides the hair right and left; on the left (if the near side be 
 inspected) it passes forward and downwards over the flank, on 
 the right it curves outwards to pass down the quarters. Another 
 whorl may often be seen issuing from under the beUy, just in 
 front of the flank, and radiating upwards. The neck, below the 
 angle of the jaw, has also a whorl, and there is one, occasionally 
 more, on the forehead and face; there are many others not so 
 regular, though frequently seen— for instance, on the poll, on 
 either side of the neck, in the middle line of the neck, over the 
 pectoral muscles, the stifles, and in diffuse areas below the knees 
 and hocks. ' Feather ' was the very apt term applied a century ago 
 to the hair pattern of which we are speaking. In the present day 
 feather is employed to designate the hair on the legs of cart horses. 
 
 The direction taken by the hair in animals and man has been 
 made the subject of very critical study by Dr. A. Kidd,* who 
 noted that the direction taken by the hair in primitive hairy 
 mammals is simple and uniform, the slope being from head to 
 tail, thus affording the least possible resistance to the passage of 
 the animal through air, water, or brushwood. 
 
 There are three phenomena in the direction of the hair to 
 which Kidd draws attention. These are reversed areas, such as 
 may be seen on the arm of the dog due to resting on the part; 
 whorls, feathering, and crests, which are so well seen in the horse in 
 the convergence of hair streams ; and, finally, tufts, seldom found 
 in the horse, and, when existing, seen on the flank and over 
 the extensor muscles of the hind limb. A ' tuft ' resembles a 
 stack of corn standing in a field. 
 
 Fig. 91 shows the frontal whorl of the horse ; Fig. 92 the whorls, 
 featherings, and crests on the pectoral region ; and Fig. 93 the 
 whorls, featherings, and crest on the side of the body. Kidd 
 regards the phenomena of whorls, featherings, and crests as due 
 to underlying muscular action, and produced by mechanical 
 causes alone. He points to the primitive horse, zebra, and ass 
 as being free from whorls, or in this respect only poorly developed, 
 excepting for the universal frontal whorl, and he regards 
 domestication with its concomitant work as the cause of the 
 peculiar hair markings observed. 
 
 * 'The Direction of Hair in Animals and Man,' 1903. Dr. Kidd has 
 very kindly lent me his figures of the horse for reproduction. 
 
312 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The direction of the hair streams in cattle has been utilised 
 as an indication of milk production. On the buttocks the hair 
 grows in two different directions, upwards and downwards ; at 
 the line of junction a ' crest ' is produced which extends to the 
 udder, and in shape resembles a shield. The size of this shield- 
 like area is stated by Guenon of France to bear a relationship 
 
 Fig. 91. — Frontal Region of 
 the Horse (Kidd). 
 
 a, b, c, Universal whorl, feather- 
 ing, and crest ; d, e, f, occasional 
 supplementing whorls, feather- 
 ing, and crests. 
 
 Fig. 92. — Breast, showing Hair Streams, 
 Whorls, Featherings, and Crests 
 (Kidd). 
 
 A, B, C, Universal pectoral whorl; D, E, F, 
 rare central whorl ; G, occasional whorl. 
 
 The arrows and lines denote the direction 
 of the hair. 
 
 to the milking properties of the cow. The larger the "shield' 
 the greater chance there is of the animal being a good milker. 
 Guenon does not deny that a small '-shield ' may be present in 
 a good milker, but says that if the shield were larger she would 
 be a better milker. The forms and shapes of the shield are 
 
THE SKIN 
 
 3i3 
 
3H A MANUAL OF VETERINARY PHYSIOLOGY 
 
 various, and in this country no real importance has been 
 attached to the sign.. 
 
 Of the pigment in hair which gives colour to the coat, our 
 knowledge, until quite recently, has been of the scantiest. The 
 active investigation now being carried out of Mendel's theories 
 of heredity made it essential in the special case of heredity in 
 coat-colour to know more about the origin, nature, and behaviour 
 of the hair pigments, and so we now have some information 
 which is both interesting and promising.* Three different forms 
 of ' melanin ' — we use the name in its generic sense — are found 
 in hairs — black, chocolate, and yellow. Of these, the black is 
 extremely insoluble, and hence very difficult to deal with; so 
 also is the chocolate pigment, though to a less extent. The 
 yellow, on the other hand, dissolves readily in numerous solvents, 
 and may thus be easily obtained. In its reactions it differs 
 entirely from the black and chocolate pigments. In the case 
 of mice there is now no doubt that their varying colours are due 
 to the presence in their hairs of one or more of these three pig- 
 ments. The less numerous experiments so far made with horse- 
 hairs suggest no doubt as to the different colours of horses being 
 due to causes essentially the same as those which give the various 
 colours to mice. As to the origin of these pigments, it has 
 generally been presumed that .they must be derivatives of hae- 
 moglobin, but there are no pathological or purely chemical facts 
 in definite support of this view. On the other hand, it has been 
 shown - )" that an extract can be made from the skins of rats, rabbits, 
 and guinea-pigs, which acts on tyrosine in such a way as to give 
 rise to pigment substances. From the conditions under which 
 the conversion is most readily effected, and the fact that the 
 activity of the extract is at once destroyed by boiling, the active 
 agent is regarded as a ferment, and, in accordance with the 
 systematic nomenclature now used, is therefore known as 
 tyrosinase. A further fact of extreme interest is that the colour 
 of the pigment formed from tyrosine corresponds to the colour of 
 the animal from whose skin the active extract is made. Black 
 pigments are produced when animals are used whose skin contains 
 black pigment, and yellow substances are obtained when the skin 
 contains orange pigment. 
 
 Colours of Horses. — The body colour of animals is protective; 
 whatever the tint may have been in the ancestors of horses it 
 was intended to help in the struggle for existence. It is prob- p 
 able that the colour was dun. Far more protective colouring, 
 however, is afforded another species of equine in the form of 
 bars; the zebra under certain conditions of light is invisible. 
 With the exception of black and greys, which are liable to turn 
 
 * Florence M. Durham (Proc. Roy. Soc, vol. lxxiv., p. 310, 1904). *«; 
 t Loc. cit. 
 
 ' •■«.■ 
 
THE SKIN 315 
 
 grey or white, all other colours are practically permanent even 
 to old age. We do know, however, that injuries to the skin of 
 horses, even of a slight character, are commonly followed by a 
 growth of perfectly white hair, which never regains its pigment. 
 In these cases the skin also loses its pigment ; pigment granules 
 are not reproduced in injured skin. The connection between 
 body colour and constitution is a physiological problem to 
 which, unfortunately, no solution can be given. Colour and 
 temperament may go hand in hand. Chestnut horses are fre- 
 quently excitable, nervous, and irritable; all horses of light 
 colour, no matter what it may be, are, as a rule, wanting in 
 stamina and constitution. The ' mealy ' bay is typical of a bad 
 colour and almost invariably of a bad horse. Grey horses are 
 not popular, and a race-horse is rarely of this colour, yet in the 
 East such coloured horses are capable of great fatigue, and 
 possess remarkable stamina, provided that the skin is pigmented. 
 A grey horse with a pink skin is an albino, and is worthless for 
 hard work. For work and hard constitution a roan is difficult 
 to beat, especially a red roan, yet the colour is not popular. 
 The ' softest ' horse is the black, and on this point there appears 
 to be no difference of opinion. 
 
 It is seldom that the colour of a horse is not broken by the 
 introduction of white; white is common on the face and limbs, 
 and in days gone by a star on the forehead was so prized that 
 one was created if it happened to be absent. On the limbs the 
 amount of white is variable ; no exception is taken to the white 
 if limited, but four white legs are not only unsightly, but horses 
 so marked are frequently wanting in stamina. White legs are 
 liable to ' chap ' in winter, for white on the limbs of a coloured 
 horse is associated with a pink skin ; such a skin is readily 
 affected if washed and left wet. 
 
 Grey legs, contrary to white, are usually only found in grey 
 horses, but grey horses may have grey legs or white legs. In 
 horses of any colour white coronets mean colourless feet, and 
 grey coronets black feet, for the reason that the skin of grey horses 
 is pigmented. Black coronets are accompanied by a dark slate- 
 coloured horn commonly referred to as black. A coronet with 
 black-and-white markings gives a black-and-white striped foot 
 corresponding to the position of the markings on the coronet. 
 If the coronet is wholly white the hoof is without pigment ; it 
 is frequently spoken of as ' white,' but it is really yellow. White 
 hair, provided it does not arise from age, grows only from a non- 
 pigmented skin, and this condition constitutes albinism, so that 
 white coronets and yellow feet indicate local albinism. Colourless 
 feet are notoriously bad, and horses with white legs are, in con- 
 sequence, generally disliked. 
 
316 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Face and Limb Markings. — Special attention has in recent 
 years been drawn to the face and limb patterns produced by white 
 hair in animals.* The face pattern may be symmetrical or 
 one-sided, may cover the whole front and side of the face, and 
 even involve the orbit . In this case the iris, sclerotic, and choroid 
 partake of the non-pigmented condition, showing that this is not 
 a mere accidental surface marking. Names have been given to 
 face markings, but these need not concern us. Too much white 
 on the face is a serious blemish, and a white muzzle with a pink 
 skin is objectionable. In the paper referred to the writer stated 
 that the amount of white on the legs will be in proportion to the 
 amount of white on the face. If the ' star ' is not in the middle 
 line, he considered there would probably be a want of bilateral 
 symmetry in the markings of the limbs. He also pointed out 
 that the hind-legs are almost always more extensively marked 
 than the fore-legs, and that it is very rare to see only white 
 fore-legs. Hutchinson considered that, where there is want of 
 symmetry in the markings, as, for example, the face-patch to 
 one side, or two lateral legs white and their fellows coloured, it 
 may be suspected that the animal is not developed with perfect 
 bilateral symmetry in other respects . He may be a ' left-handed ' 
 horse, and, if so, even if strong and efficient, will move awkwardly 
 and be unpleasant to ride. We do not share this latter view, but 
 the whole matter is so interesting and the facts so readily collected, 
 that attention is drawn to our ignorance of the question, and the 
 necessity for observation. 
 
 Albinism. — This is the absence of pigment from the skin, and 
 may be general or local. Local albinism commonly affects the 
 face and legs. It may be extremely local, as in wall-eyed horses, 
 who possess no pigment in the iris of one or both eyes. It may 
 affect the whole surface of the body, as in white (cream-coloured) 
 horses with a pink skin; or large areas of the bodies of such 
 horses may be bay or black, and the remainder cream-coloured. 
 In cream-coloured horses the choroid and iris are without pig- 
 ment, the latter being bluish. A ' grey ' horse is not a ' white ' 
 horse, though he may become white with age. 
 
 Clipping. — Experience shows that the heavy winter coat grown 
 by horses is the cause of considerable sweating at work,, and the 
 general practice of clipping has therefore been introduced. Of 
 its value there can be no doubt ; it considerably reduces the risk 
 of cold and of chest diseases, for animals on coming in from work 
 may be readily dried and thus protected from chills. Horses 
 which sweat freely at work soon lose ' condition ' ; the writer's 
 observations have shown that this is due to the protein lost by 
 
 * ' On Paleogenetic Face-Pattern in Acroteric Piebalds.' Sir Jonathan 
 Hutchinson, F.R.C.S., F.R.S., British Medical Journal, June, 1910. 
 
THE SKIN 
 
 317 
 
 the skin, for, as we shall presently see, proteins are found regu- 
 larly in the sweat of the horse. Clipping largely prevents this 
 loss, though for this purpose it is not necessary to clip a horse 
 all over ; a half-clipped body suffices . The influence of clipping on 
 temperature is dealt with in the chapter devoted to Animal Heat. 
 
 Erection of Hairs. — In some animals, as for instance, the dog 
 and the cat, the hairs are rendered erect under excitement such 
 as anger or fear ; this is due to the involuntary muscle attached to 
 the hair follicle, and the process is under the influence of the 
 sympathetic nervous system. The fibres for the body-hair 
 emerge from the spinal cord by the inferior roots, pass to the 
 grey ramus of the sympathetic chain, and run to the skin by 
 the dorsal cutaneous nerves; the fibres for the head and neck 
 are in the cervical sympathetic. Under the influence of cold the 
 hairs on the horse's body may become erect, but there is no 
 indication of this under psychical excitement, as in the case of 
 the dog and cat. It is possible, even probable, that the pre- 
 science of a coming storm or change of weather exhibited by 
 cattle may be due to the highly hygroscopic properties of their 
 hair. Hair is one of the few organic substances which elongate 
 instead of shorten as they grow moist. The effect of movement 
 of every hair on the surface of the body may cause a mechanical 
 stimulation of the hair-follicle nerves, and so give rise to an un- 
 easiness which presages the coming change. 
 
 Effect of Rain. — It is astonishing the length of time horses 
 may be exposed to rain without the skin getting wet. That it 
 is not wet can be readily ascertained by lifting up and reversing 
 the hair. This only applies to animals with an undipped coat 
 or a skin containing a liberal amount of dandruff. Both of these 
 afford the greatest protection against rain, and this fact bears 
 out the old-fashioned view that horses living in the open should 
 not be too frequently groomed. 
 
 Rain causes the hairs to stand out from the surface of the body 
 and separate into groups, which run together at the point, re- 
 sembling a wet camel-hair brush. These form a series of miniature 
 water-sheds, the rain running off the upper to the lower groups. 
 The hair ' streams ' direct the course of the water in its passage 
 over the body, as isweU seen on the trunk, where the flanks remain 
 dry for a considerable time. The shape of the body, especially 
 that of the trunk, insures that the wet area is largely confined to 
 the upper part, behaving, indeed, as a barrel would if placed on 
 its side and exposed to rain. It takes a considerable amount of 
 rain to wet the lower half of the trunk. When ram is accom- 
 panied by wind the whole of the windward side gets wet, and 
 early penetration is more likely to occur. 
 
 It is not intended to suggest that horses appreciate ram; they 
 
318 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 dislike it intensely, and if given an opportunity always turn their 
 tails to it ; they shiver and shrink in size. The shivering is due to 
 the wet coat, and not to the wet skin ; the shrinkage to the fact 
 that in wet weather much of their food is lost or even not given 
 them. No harm arises from this exposure when animals are 
 in health, but ' war-worn ' horses may die by dozens during a 
 wet, cold night. 
 
 The impermeability to rain of the normal coat of animals has 
 proved a serious difficulty in dealing, by the process of ' dipping,' 
 with the repression of mange and the eradication of tick-borne 
 diseases. Colonel Watkins Pitchford found that, though the 
 entire body was immersed in a solution of arsenic, this did not 
 penetrate, owing to the protective action of the hair, and that 
 ' ticks ' consequently survived. He inhibited this by the 
 addition of paraffin emulsion to the dressing, and in this way 
 insured the complete wetting of the skin. 
 
 Sweat. — By means of glands in the skin a fluid termed ' sweat,' 
 and a fatty material known as 'sebum,' are secreted. Sweat, 
 or perspiration, is not found to occur over the general surface 
 of the body in any other hairy animal than the horse. There are 
 certain parts of the skin which sweat more readily than others: 
 the base of the ears is the first place in the horse where sweating 
 takes place; the neck, side of chest, and back follow; lastly, the 
 hind-quarters. No sweating takes place on the legs; the fluid 
 found there has run down from the general surface of the body. 
 Mules and donkeys sweat with difficulty, and then principally 
 at the base of the ears. The ox sweats freely on the muzzle, and 
 sweating even from the general surface of the body has occa- 
 sionally been observed. It has been said that sheep perspire, 
 while it is certain that both the dog and cat, especially the latter, 
 sweat freely on the foot-pads, as also on the muzzle, though not 
 in the form of sensible perspiration on the general surface of the 
 body. The sweating of the pig is confined to the snout. 
 
 The secretion of sweat is continuous. When excreted in small 
 amounts it evaporates as fast as it is formed, passing off as the 
 insensible vapour which is always rising from the surface of the 
 skin, and is known as ' insensible perspiration.' When the 
 secretion is rapid and copious or the surrounding atmospheric 
 conditions are unfavourable to its evaporation, it collects on the 
 skin as that visible fluid material which is ordinarily termed 
 ' sweat.' Colin gives various numerical statements respecting 
 the insensible perspiration, from which we gather that 6-4 kilo- 
 grammes (14 pounds) of water probably represents this loss in the 
 horse for twenty-four hours. Much depends upon the humidity 
 and temperature of the atmosphere ; the drier and hotter it is, 
 within certain limits, the greater the insensible perspiration. 
 
THE SKIN 319 
 
 The amount of sweat secreted daily can only be roughly 
 determined; there are many conditions which affect it, such as 
 'condition,' length of the coat, nature of the work, and pace. 
 Grandeau, by estimating the total water consumed in the food 
 and drink, and that voided in the urine and faeces, arrived at the 
 amount of vapour passing away in the breath and perspira- 
 tion. The mean amount of water evaporated daily by these 
 two channels, under different conditions of work, was as 
 follows: 
 
 At rest 2-9 kilogrammes ( 6-4 pounds 
 
 Walking exercise - 3-9 ,, (8-6 
 
 At work walking - 5-8 „ (12-7 ,', 
 
 Trotting - 6-o ,, (i3 - 4 
 
 At work trotting - - 9-4 ,, (20-6 
 
 The distances walked and trotted and the loads drawn were 
 the same. It is unfortunate that we have no means in the above 
 experiments of determining the proportion which the water of 
 respiration bears to that of perspiration. 
 
 Evaporation from the surface of the skin is a most important 
 source of loss of heat ; so marked is this loss in the horse that the 
 resulting fall in temperature may even carry it below the 
 normal if the sweating be very profuse or the wetted area 
 a large one. 
 
 ; )The compensating action existing between the kidneys and 
 skin observed in man exists also in the horse — viz., when the 
 skin is acting freely, less water passes by the kidney, and vice 
 versa. 
 
 Sweat obtained from the horse is always strongly alkaline; 
 after filtration it is the colour of sherry, which is probably 
 accidental, and due to contamination with dandruff. The latter 
 contains a pigment, chlorophyll. The sweat possesses a peculiar 
 horse-like odour, and has a specific gravity of 1-020. We found 
 it to have the following composition:* 
 
 Containing — 
 
 Water - 94 - 3 8 
 
 r Serum albumin 0-105 
 
 Organic matters 0-52 \ „ globulin 0-327 
 
 \ Fat , 0-002 
 (Consisting principally of potash 
 
 J and soda, chlorides, some 
 
 Ash 5' 10 1 maenesia, a little lime, and 
 
 I 
 
 magnesia, a 
 
 traces of phosphates. 
 
 The proteins are thus seen to be serum albumin and seium 
 globulin and their constant presence has been determined by a 
 number of observations ; the percentage of mineral matter is very 
 
 * ' The Sweat of the Horse,' Journal of Physiology, vol. xi., 1890. 
 
320 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 high, and consists principally of soda and potash, especially the 
 latter. It will be observed that the mineral matter greatly 
 exceeds the organic matter; in horses which have sweated freely 
 the dried, matted hair (albumin is the cause of the matting) is 
 often seen covered with saline material, looking like fine sand. 
 There appears to be some complemental action between the skin 
 and the kidneys in the elimination of soda and potash; during 
 rest the kidneys eliminate these salts, while during work they are 
 assisted by the skin. Urea is also probably present in sweat 
 (see p. 323). It is difficult to see why horses should excrete 
 albumin by the skin; the loss thus produced accounts for the 
 great reduction of vitality and strength in animals which sweat 
 freely at work. As we have already said, clipping is the only 
 preventive of sweating. 
 
 Nervous Mechanism of Sweating. — A skin may sweat under 
 quite opposite conditions — viz., both with a hot flushed surface 
 and with a bloodless cold one ; in other words, an animal may sweat 
 when it is hot or when it is cold. The former is a physiological 
 condition and regulates, as we shall see, the body temperature; 
 the latter is abnormal, but it occurs at death and disproves at 
 once any notion of sweating necessarily depending upon a con- 
 gested condition of the vessels of the skin. Experiments show 
 that most of the features of sweating can be accounted for through 
 the agency of the nervous system. The sudo-motor nerves, whose 
 function it is to determine the secretion of sweat, are quite 
 distinct from those which regulate the vascular supply, although 
 t ey reach the nerves of supply by the same sympathetic channels 
 as the vasomotor fibres. If the peripheral end of the divided 
 sciatic in the cat be stimulated, the foot-pads sweat ; apart from 
 the fact that stimulation of the sciatic causes a constriction of the 
 bloodvessels in the leg, the proof that this reaction is a specifically 
 nervous one is easy, for the sweating still occurs when the leg 
 has been cut off or the aorta tied, and it is absent under the 
 influence of atropine. The effect of atropine on the sweat glands 
 is very closely allied to its action on the salivary glands (p. 175); 
 it paralyses the secretory nerves which stimulate sweat secretion. 
 
 As with the salivary glands, so in the present case secretion 
 is not due to any increased supply of blood. It is true that in 
 normal sweating, as is so readily seen in man, the skin is flushed 
 when the increased secretion takes place, but the increased blood- 
 supply which the flushing indicates is merely the necessary 
 adjuvant, not the cause of the secretion ; it supplies the glands 
 with the extra material they now require, the secretory nerves 
 causing the gland-cells to utilise the increased supply. 
 
 The secretion of sweat may be induced in man, the cat, and 
 the dog, though not in the horse, by the injection of pilocarpine. 
 
THE SKIN 321 
 
 In this case the action is peripheral — that is to say, on the glands 
 themselves — since it occurs when the sciatic nerves are cut pre- 
 viously to the injection. 
 
 As we have seen, secretion is ordinarily brought about by 
 specific efferent nerves, and these originate in the central nervous 
 system, from which the necessary secretory impulses are directly 
 •supplied. But secretion may also be readily induced by the 
 stimulation of afferent nerves, as in the all-important case of 
 a rise in the surrounding temperature. These facts lead at once 
 to the belief that ' sweat centres ' must exist in the central 
 nervous system comparable to those of the respiratory and 
 vascular mechanisms, though they have not as yet been so 
 definitely localised. There seems to be no doubt that the spinal 
 cord contains sweat centres. The existence of a similar centre 
 in the medulla is less certain, though probable, since in some 
 men perspiration over the face and neck results from merely 
 smelling a pungent substance, such as curry-powder, and becomes 
 profuse if the latter is introduced into the mouth. 
 
 The sweat-nerve supply to the fore and hind limbs leaves the 
 spinal cord in company with the inferior roots of the spinal 
 nerves, and by means of the rami communicantes passes to the 
 sympathetic ganglia, and by post-ganglionic fibres reaches the 
 brachial and sciatic plexuses respectively. The sweat fibres 
 for the head and neck are in the cervical sympathetic; those 
 for the face in the horse, the muzzle in the ox, the snout in the 
 pig, run in branches of the fifth pair of nerves. Division of the 
 cervical sympathetic in the horse produces profuse sweating 
 of the head and neck, limited to the side operated upon; this 
 may be due to vasomotor paralysis, though a different inter- 
 pretation has been placed on it— viz., that the sympathetic 
 carries inhibitory impulses to the sweat glands of the head, so 
 that on division the secretory fibres act without opposition. 
 In the ox Arloing has shown that division of the cervical sym- 
 pathetic causes the muzzle on the same side to become dry; 
 stimulation of the cut end of the nerve is followed by secretion, 
 but this is not so when the nerve degenerates, though even then 
 the glands respond to pilocarpine. _ 
 
 As previously stated, a high temperature favours the activity 
 of the epithelium lining the sweat glands, for if the limb of a cat 
 be kept warm, a larger secretion of sweat is obtained on stimu- 
 lating the sciatic than from a limb kept cool; in the latter, 
 stimulation of the sciatic may produce no secretion whatever 
 Further, if a cat in which one sc-atic has been divided be placed 
 in a hot chamber, profuse secretion will occur on the foot-pads 
 of the limbs not subjected to interference, while on the side on 
 which the sciatic has been divided no sweating occurs This 
 
322 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 is a further proof of the existence of a reflex mechanism, to which 
 we have already drawn attention. It has been thought that the 
 sweating which takes place at death is due to a dyspnceic con- 
 dition of the blood, and in many cases this may be so ; perhaps 
 dyspnoea may also account fol the profuse cold sweating in 
 ruptures of such viscera as the stomach and intestines; but it 
 cannot explain the localised hot sweating which is often so well 
 marked in horses between the thighs immediately after they are 
 destroyed. Thrombosis of both iliac arteries may occur in the 
 horse, and a frequent symptom of this trouble is the peculiarity 
 in the accompanying sweating; the general surface of the body 
 may sweat freely, but not the hind-quarters. In man a similar 
 phenomenon has been met with in cases of spinal injury. The 
 cause of this peculiarity has not been worked out. 
 
 In comparing the sweat glands with the salivary, we must 
 be careful not to draw too close a parallel, for though in certain 
 features they agree, in others they are very diff erent ; f or instance, 
 in the horse pilocarpine produces, as in other animals, a profuse 
 salivary flow; but, contrary to its action on man, the dog, and 
 cat, it has no effect whatever in producing sweating. 
 
 The peculiar breaking out into sweats which occurs in horses 
 after work has no parallel in man ; some animals will break out 
 two and three times for hours afterwards, even after having been 
 rubbed quite dry. This may be connected with the necessity 
 for a discharge of body-heat, since the internal temperature rises 
 above the normal during work, in some cases, it is said, as much 
 as 2 -5° to 3° C. (4 to 5° Fahr.), and remains above normal for 
 some time afterwards. Another peculiarity in sweating of the 
 horse is the patchy perspiration observed occasionally, such as 
 a wet patch on the side or quarter, which dries slowly, or may 
 remain for days or weeks, even months, in a wet or damp con- 
 dition. This must be a paralytic secretion, but nothing is known 
 of its true nature. Finally, there is no drug, so far as we are 
 aware, which produces sweating in horses; this is perhaps an 
 explanation of the common use of nitre in veterinary practice, 
 the kidneys being made to do the work of the skin. 
 
 Sebaceous Secretion, or Sebum, is a fatty material formed in 
 the sebaceous glands of the skin, which in the horse are freely 
 distributed over the whole surface of the body. Though it is 
 spoken of as a secretion, yet the process involved is not secretory, 
 inasmuch as the cellular elements of the gland are not actively 
 employed pouring out material, but are themselves shed after 
 undergoing fatty metamorphosis. The greasy material thus 
 excreted saves the epithelium from the disintegrating influence 
 of wet, keeps the skin supple, and gives its gloss to the groomed 
 coat; from its greasy nature it assists in preventing the pene- 
 
THE SKIN 323 
 
 tration of rain, and thereby saves, to some extent, undue loss of 
 heat. 
 
 Dandruff. — The material removed from horses by grooming 
 consists of a white or grey powder which can readily be moulded 
 by pressure into a dough-like mass; it has a curious smell, 
 which can only be described as ' horse-like.' It consists of epi- 
 thelial scales, fat, largely in the form of lanolin, colouring matter, 
 salts, and a considerable amount of silica and dirt, the quantity 
 of the last two depending upon the cleanliness of the animal. 
 The amount of dandruff lost in an ordinary grooming varies from 
 1-25 to 375 grammes (20 to 60 grains) for clean horses, and 11 
 to 13 grammes (170 to 200 grains) for very dirty animals. An 
 analysis of dandruff from the horse gave the following composi- 
 tion :* 
 
 Water 17-96 
 
 Fat 12-40 
 
 Organic matter 56-22 containing 1-07 of urea. 
 
 Ash I3'4 2 .. 2-45 of silica. 
 
 The fatty matter in the skin proves to be lanolin, the same 
 substance as is found in the fleece of sheep. This explains why 
 horses living in the open should not be too freely groomed, and 
 the reason for the prejudice which has always existed against 
 this practice. It is evident that with free grooming the loss in fat 
 alone is something considerable, and the animal is exposed to 
 chill. The amount of fat lost depends upon the nature of the diet ; 
 on a purely hay diet there is very little fat in the dandruff, while 
 on oats there is a considerable amount. The urea shown in the 
 analysis is no doubt derived from the sweat. 
 
 Dandruff contains a colouring matter found by the writer to 
 be chlorophyll which has undergone modification by passing 
 from the digestive canal to the skin. The use of this pigment is 
 unknown; in fact, the horse is the only vertebrate in which 
 chlorophyll has so far been found as a constituent of any cuta- 
 neous excretion. 
 
 In certain places, as in the prepuce, considerable quantities 
 of sebum are found. The sebaceous secretion of the prepuce 
 of the horse consists of 50 per cent, fat, and also contains calcium 
 oxalate. The ear-wax and the eyelid secretions are also of a 
 sebaceous nature. In the sheep a considerable quantity of fatty 
 substance is found in the wool; it exists in two forms: (1) as a 
 fatty acid united to potash to form a soap, and (2) a fatty acid 
 combined with cholesterin instead of glycerin; the latter is 
 
 * ' Dandruff from the Horse, and its Pigment, ' Journal of Physiology, 
 vol. xv., 1893. 
 
324 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 known as lanolin, and is largely used as a basis for ointments. 
 It is also found in hair, horn, feathers, etc. The fatty substance 
 in the wool is known to shepherds and others as ' suint.' In 
 merino sheep it may amount to more than one-half the weight 
 of the unwashed fleece, but in ordinary sheep exposed to the 
 weather it may be 15 per cent, or less. The large amount of 
 potash in unwashed wool is very remarkable ; a fleece sometimes 
 contains more potash than the whole body of the shorn sheep 
 (Warrington) . 
 
 Respiratory Function of the Skin. — Certain vertebrates such 
 as the frog can respire by the skin in the entire absence of lungs; 
 in this way they absorb oxygen, and excrete carbonic acid. 
 Observations made on other animals and on men have demon- 
 strated that similar changes occur through the skin, but on a 
 very small scale. 
 
 Varnishing the skin causes death rapidly in rabbits, and more 
 slowly in horses. Death is due to loss of body-heat, and not 
 to the retention of poisonous products, as was at one time sup- 
 posed. Bouley* states that horses shiver when varnished, the 
 surface of the body and the expired air become colder, the visible 
 membranes respond by becoming violet ; n tint, and the animals 
 die after several days. According to Ellenberger, if only partly 
 varnished they do not die, but exhibit temporary loss of tem- 
 perature, and show signs of weakness. The effect of varnishing 
 the skin is to cause the capillaries to dilate, and so produce a 
 great discharge of heat. 
 
 For absorption from the skin, see ' Absorption,' p. 290. 
 
 The Chestnuts and Ergots are considered to be the remains of 
 hoofs belonging to digits long since lost by the horse. The 
 ergot grows from the back of the fetlock; the chestnut is found 
 on the inside of the arms and hocks, and is always larger in the 
 former position. In the heavy type of horse they may grow 
 to a considerable size. The horn of which they are composed is 
 tubular in structure, and produced by the papillae of the skin. 
 After attaining a certain size they drop or are pulled off. Both 
 ergots and chestnuts are found larger in horses wanting in quality 
 than in those better bred; in the donkey and the mule the 
 chestnut forms no outgrowth of horn, but a hairless black patch 
 represents its position. 
 
 Pathological. 
 
 The chief pathological conditions of the skin are those due to 
 parasitic invasions; these may produce widespread disease in all 
 animals. 
 
 * Colin's ' Physiologic Compar6e.' 
 
CHAPTER X 
 
 THE URINE 
 
 The urine is sometimes spoken of as a secretion, but this is not 
 strictly correct ; speaking broadly, it may be said that a secretion 
 is something which is formed in a part for the purpose of being 
 eventually utilised by the system. This does not apply to the 
 urine, the chief constituents of which are not even prepared in 
 the kidneys, but only separated by them ; moreover, the urine, 
 having once been formed, is of no further use to the body, and is 
 excreted. An excretion, therefore, is something removed from 
 the system as no longer required, and the retention of which 
 would be harmful. Such a removal is effected by the kidneys, 
 which may in a sense be regarded as the filters of the body, 
 regulating the composition of the blood by removing from it 
 waste and poisonous products, and maintaining in it, as will be 
 later explained, the proper degree of chemical neutrality. 
 
 The method adopted by the kidney for the secretion of urine 
 has been for many years one of the chief subjects for physiological 
 dispute. These very vascular glands are capable of separating 
 from the blood a fluid which is essentially different in composition 
 from the blood itself. The blood is faintly alkaline in reaction, 
 the urine acid or alkaline, depending on the class of animal ; the 
 blood is an albuminous fluid, the urine in a state of health is free 
 from protein ; the blood contains sugar, the urine contains none ; 
 the blood has one colouring matter, the urine another ; the blood 
 contains urea and salts in small quantities, the urine contains 
 them in relatively large amounts ; the blood maintains the whole 
 of its inorganic material in solution or in such a form as to be 
 readily soluble, the urine may be of such concentration or 
 reaction as to be unable to retain its substances in solution. 
 Nevertheless, the kidney only takes from the blood what is 
 brought to it, for, with the single exception of hippuric acid, 
 none of the urinary constituents are formed in the gland. There 
 is no other body secretion which exhibits such striking differ- 
 ences, and, further, there is no other gland the structure of which 
 resembles the peculiar histological structure of the kidney. 
 
 325 
 
326 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The vascular arrangements of the kidney are intimately 
 connected with the function of the organ. The renal artery is 
 short, it comes off close to the posterior aorta, and the pressure 
 within it is practically the same as in that vessel; the pressure 
 in the renal vein, on the other hand, is low — nearly as low as that 
 in the posterior vena cava. It will be observed that the same 
 amount of blood-pressure as is required to fill the vessels of the 
 lumbar region and hind-limbs is expended on driving the blood 
 through the kidneys. At every increase in the amount of blood 
 in the kidney the organ swells, at every decrease it contracts. 
 These movements on the part of the kidney have been carefully 
 studied by means of Roy's oncometer or similar plethysmographs. 
 An oncometer (Fig. 94) is a metallic capsule in which the living 
 
 kidney is enclosed, 
 so arranged that 
 the expansion and 
 collapse of the 
 organ can readily 
 be recorded. A 
 tracing made by 
 this instrument 
 shows that the 
 volume of the kid- 
 ney is affected by 
 every beat of the 
 heart, and even by 
 the respiratory un- 
 dulations in the 
 blood-pressure. 
 
 Structure of the 
 Kidney. — The kid- 
 ney consists of a 
 central part, the 
 medulla, surrounded by an external part, the cortex; the 
 boundary of the two is easily visible in a sliced kidney. The 
 branches of the renal artery break up at the boundary of the 
 cortical and medullary portions. The cortex of the kidney is 
 the essential secreting region, and it is here that the Malpighian 
 tufts, or capsules are found. These consist of small clusters of 
 capillaries, • the glomeruli, derived from the renal artery. The 
 afferent arteriole entering the glomerulus is larger than the 
 efferent or outgoing one, and as in addition the blood passing 
 from the efferent vessel has to pass through a second set of capil- 
 laries supplying the uriniferous tubules, a high blood-pressure is 
 maintained within the glomerular capillaries. The glomerular 
 tuft invaginates the blind end of the uriniferous tube, the inner 
 
 Fig. 94. — Diagram of Oncometer. 
 B, Metal box in two halves, opening on the hinge H; 
 M, thin membrane; A, space filled with oil; O, 
 organ enclosed in oncometer; V, vessels of organ; 
 t, tube for filling instrument with oil; T, tube 
 connected with D, which opens into cylinder C; 
 P, piston attached by E to writing lever. 
 
THE URINE 
 
 327 
 
 layer of the dilated end being separated from the outer layer 
 (Bowman's capsule) by a space into which the urinary filtrate 
 passes. Dilatation of the capil- 
 laries diminishes this space, 
 while it is enlarged by their 
 collapse. Fig. 95 shows the 
 general arrangement of the 
 Malpighian bodies, and Fig. 96 
 that of the vessels in a tuft. 
 The minute vein or efferent 
 
 F IG . 95 , — Diagram of Bloodvessels 
 of Kidney (Klein, after Ludwig). 
 
 ai, Interlobular artery; vi, interlobular 
 vein; g, glomerulus with its afferent 
 and efferent vessels, the latter break- 
 ing up into a plexus around the renal 
 tubules; vb, vena? recta?; ar, artena? 
 recta?; vp, apex of papilla; vs, vena 
 stellata (Stewart) 
 
 Fig. 96. — Diagram showing the Re- 
 lation of the Malpighian Body 
 to the uriniferous tubules and 
 Bloodvessels (Kirke, after Bow- 
 man). 
 
 a, An interlobular artery; a', branch 
 of artery passing into the glomeru- 
 lus; c, capsule of the Malpighian 
 body forming the commencement of, 
 and continuous with t, the urinifer- 
 ous tube; e'e'e', vessels leaving the 
 tuft forming a plexus, p, around the 
 tube, and finally terminating in e, a 
 branch of the renal vein. 
 
 vessel leaving the tuft breaks 
 up into capillaries around the 
 uriniferous tubule; thus the 
 blood in the plexus of capil- 
 laries around the tubule is 
 derived from two sources— viz., 
 from the tuft, and directly 
 from the renal artery. The 
 
 capsule of Bowman, which 
 surrounds the tuft, is lined with cells resembling the epithelioid 
 P latr S 2n in capillaries; they are flat polygonal cells contain- 
 
328 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 ing a nucleus. The capsule is practically the dilated beginning 
 of a uriniferous tubule, and the latter is continued from the 
 capsule, taking a course of extraordinary complexity to the 
 pelvis of the kidney; but the cells found in the tubule are no 
 longer the flat polygonal cells of the capsule; they are special 
 to the tubule, and even to different parts of it (Fig. 97). 
 
 If the course of a uriniferous tubule is briefly followed (Fig. 98), 
 it is foundl'that on leaving the capsule it becomes twisted in the 
 cortex, forming the convoluted tube ; it then forms a spiral tube, 
 
 Fig. 97. — From a Vertical Section of the Dog's Kidney, to show the 
 Structure of the Different Portions of the Renal Tubule (Klein). 
 
 a, Bowman's capsule enclosing glomerulus; , ( , neck of capsule; c c, convoluted 
 tubes cut. in various directions; 6, from zigzag tubule; d, from collecting 
 tubule; e e, from spiral tubules; /, narrow part of Henle's loop tubule. In b, 
 <:, and e, ' rodded ' epithelium is seen (Stewart). 
 
 and leaving the cortex, runs straight into the medulla, forming 
 the descending limb of Henle ; it now makes a sharp turn, the 
 loop of Henle, and travels back to the cortex, in the same way 
 that it left, by the ascending limb of Henle. The descending 
 limb is straight and narrow, the ascending limb is wavy in char- 
 acter and larger. Having reached the cortex, the ascending 
 limb becomes distinctly wider and twisted, forming the zigzag 
 or irregular tubule ; from this a tubule is continued which re- 
 sembles in its contortions the first convoluted portion; it is 
 termed the second convoluted tubule. This now leaves the cortex 
 and enters the medulla as a straight tube, known as the collecting 
 
THt. UtiLNt. 
 
 3 2 9 
 
 it runs towards the apex of the pyramid, and joins other 
 collecting tubes ; by so doing it becomes larger, and on reaching 
 the apex is known as a discharging tube or duct of Bellini. The 
 epithelial cells lining the tubules are not of the same character 
 
 A, Cortex of kidney; 
 «., subcapsular layer 
 not containing glome- 
 ruli; a', inner struc- 
 ture of cortex also 
 without glomeruli; B, 
 boundary layer of me- 
 dulla; C, papillary 
 part of the medullary; 
 i, Bowman's capsule 
 of the glomerulus; 2, 
 neck of capsule; 3, 
 proximal convoluted 
 tube; 4, spiral tube; 
 5, descending limb of 
 Henle; 6, loop of 
 Henle; 7, thick part 
 of ascending limb; 
 8, spiral part of as- 
 cending limb; 0, nar- 
 row ascending limb in 
 the medullary ray; 
 ro, the irregular tu- 
 bule; ir, distal con- 
 voluted tube; 12, 
 curved collecting tube ; 
 13, straight collecting 
 tube; 14, collecting 
 tube of boundary 
 layer; 15, large col- 
 lecting or discharging 
 tubule of_. papillary 
 layer. 
 
 Fig. oS.-Diagram of the Couese of the Uriniferous Tubules (Klein and 
 
 Noble Smith). 
 
 throughout; broadly, they may be divided into striated or 
 ' rodded ' cells, staining readily, and clear transparent cells, 
 staining with difficulty. The ' rodded ' epithelium is suggestive 
 of secreting cells, and is found in the two convoluted tubules 
 
33° A MANUAL OF VETERINARY PHYSIOLOGY 
 
 and in the spiral and zigzag tubules (Fig. 97) ; the clear cell, on 
 the other hand, partakes more of the characteristics of the 
 epithelial lining of ducts. 
 
 Vascular Mechanism.- — The vascular arrangements of the 
 kidney are under the control of a rich supply of vaso-constrictor 
 nerves, and dilator nerves are also known to exist. If the 
 general blood-pressure be constant, dilatation of the renal vessels 
 means an increased secretion of urine, while constriction of the 
 vessels means a reduced secretion. An increase in the general 
 blood-pressure produces an increase in the amount of blood in 
 the kidney, and this is rendered evident by the swelling of the 
 organ in the oncometer and an increased production of urine. 
 If the increased general blood-pressure is accompanied by a 
 constriction instead of a dilatation of the small arteries of the 
 kidney, as, for instance, when the vaso-constrictor nerves are 
 stimulated, then the increased blood-pressure cannot lead to 
 increased secretion, but, on the contrary, the amount of urine 
 becomes less and the kidney shrinks. A fall in general blood- 
 pressure, such as is caused by dividing the spinal cord, brings 
 about a reduction in the flow through the kidney, and the blood- 
 pressure becomes so low that the secretion of urine is entirely 
 suspended. It is thus evident that the vasomotor influence over 
 the kidney is of the greatest importance, and largely regulates the 
 amount of urine excreted. If the renal vein be obstructed, the 
 pressure of blood in the kidney rises, but no urine is secreted; 
 evidently, therefore, a continuous flow of blood through the 
 kidney is as essential to secretion as is increased blood-pressure. 
 
 The Theories of Urinary Secretion are at least three in 
 number. One put forward by Bowman regarded the epithelium 
 of the glomerulus as the seat of secretion of the water and in- 
 organic salts of the urine, while the epithelium in the convo- 
 luted tubules was considered to secrete the urea and other 
 organic substances. The view is essentially secretory, and is 
 opposed to the theory formulated by Ludwig, which is wholly 
 mechanical in character. Ludwig regarded the glomerulus as 
 forming the entire urinary constituents under the influence of 
 pressure brought about by the peculiar structural arrangements 
 of the part. In the glomerulus, according to this view, a highly 
 diluted fluid was formed which, in its passage to the pelvis of the 
 kidney, was exposed to absorption by the epithelial cells, by 
 which means it became concentrated. The majority of physio- 
 logists, at least until recently, accepted the Bowman theory. 
 
 If the whole process of urinary secretion were a question of 
 pressure in the glomerulus, then ligature of the renal vein should 
 result immediately, though not continuously, in a greater secre- 
 tion of urine. As a matter of fact, we know the secretion ceases. 
 
THE URINE 331 
 
 This experiment seems a very crucial one. The fact of closing 
 the venous outlet must greatly raise the capillary pressure, and 
 favour in a remarkable manner the passage of fluid by filtration, 
 yet none is filtered off. Ludwig explained the diminution in 
 secretion after constriction of the renal vein as being due to the 
 distended veins compressing the tubes in the medulla, and so 
 preventing the passage of the fluid to the ureter. Even if this 
 were not the explanation of the anuria, such constriction of the 
 vascular path from the stoppage of the arterial blood-supply 
 would produce asphyxial conditions which must affect the con- 
 dition of the filtering membrane, as is evidenced by the appear- 
 ance of albumin and casts in the urine. It is now recognised 
 that the experiment does not disprove the role of the glomerulus 
 in nitration. Many experiments have been made to prove the 
 secretory activity of the cells of the convoluted tubes ; the one by 
 Heidenhain was at one time regarded as conclusive. 
 
 If sulphindigotate of soda be injected into the blood of the 
 dog, within a short time the urine acquires an intensely blue 
 colour, though the blood may be only slightly affected. If 
 the kidney be removed and examined, all parts excepting the 
 Malpighian bodies are found stained blue. In order to determine 
 what portion of the tubule excretes the dye, it is necessary to 
 stop the secretion in the glomeruli, otherwise the colouring 
 matter gets carried through the whole length of the tubule. 
 In order to stop glomerular secretion, the blood-pressure is 
 lowered by dividing the spinal cord in the neck, and the blue 
 colouring matter is then injected, and the kidney subsequently 
 examined. The blue is now found in the cortex only, and within 
 the striated or rodded epithelial cells of the first and second 
 convoluted tubes, in which the pigment may be seen lying in 
 granules. From this experiment it is clear that the cortical 
 tubules elected to turn out the pigment from the blood, while the 
 medullary tubules were unable to effect this. It is therefore 
 fair to assume that a specific secretory activity of the cells of 
 the convoluted tubes is shown for indigo, and it is assumed that 
 a similar function may be exercised towards other bodies — for 
 instance, urea and the other constituents of the urine. Some 
 investigators hold that the pigment seen in the tubes is in process 
 of being absorbed, not secreted. 
 
 Stated briefly from the point of view of secretion, these facts 
 amount to this, that in the glomeruli the water of the urine— and 
 perhaps the salts — are passed' out chiefly as the result of varying 
 glomerular blood-pressure, while the organic matter is excreted 
 in the tubules as the result of the distinctly secretory activity 
 of their cells. These substances are carried along by the fluid 
 which trickles down the tubules into the pelvis of the kidney, 
 
332 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 and so becomes urine. Secretion of protein in the tuft, and its 
 reabsorption in the tubule, was at one time believed to take 
 place, but inasmuch as no protein is found in the normal urine 
 of any animal, it is safe to assume that in an undamaged state 
 the epithelial cells of the glomerulus allow none to pass. Under 
 pathological conditions the glomeruli permit the exit of both 
 albumin and sugar. There are no secretory nerves to the kidney ; 
 the influence of the nervous system is here confined to its action 
 on the bloodvessels, and thus to regulating the flow of blood 
 through the kidney. 
 
 Brodie regarded the glomerular activity as secretory,* and 
 considered that the high blood-pressure in the glomerulus was 
 the propelling force which drove the urine secreted by the tubules 
 through their very long and narrow passages. He proposed to 
 call the glomerulus a ' propulsor, ' and pointed out that the kidney 
 during activity was tense, hard, and distended its capsule to the 
 utmost. The importance to the kidney of a firm inextensible 
 capsule had not previously been insisted upon. 
 
 The ' Modern ' Theory. — Recently (1917) attention has been 
 again directed to this subject by Cushny.f The position which 
 this writer takes up is a somewhat different one from that adopted 
 by Heidenhain on the one hand, and Ludwig on the other. 
 According to this theory, there filters through the glomerular 
 capsule simply deproteinised plasma, the nitrate therefore con- 
 taining all substances other than proteins in the concentrations 
 in which they exist in the plasma. Such a filtrate must not only 
 be much more dilute than urine, but must also contain substances 
 which are not normally present in the urine. The theory, there- 
 fore, necessitates the absorption of a large quantity of fluid by 
 the tubes, and also of many of the dissolved constituents of the 
 filtrate, to such an extent as to bring the final fluid as it enters 
 the ureters to the, normal composition of urine. 
 
 The absorbed fluid, for example, would require to contain all 
 the glucose which had filtered through the glomerulus, in the 
 concentration in which this body occurs in the plasma. In the 
 case of an acid urine the excess alkali which had passed through, 
 would also require to be absorbed as well as chloride, sodium, 
 and potassium, approximately in the proportions in which these 
 last three exist in the plasma. 
 
 Such a fluid would be similar in some respects to Locke's fluid 
 as used in perfusion experiments. The following table from 
 Cushny's monograph explains clearly the nature of the process 
 concerned in the excretion of 1 litre of urine : 
 
 * ' Glomerular Activity,' Proceedings of the Royal Society, June 15, 
 1911. Dr. T. G. Brodie, F.R.S. 
 t ' The Secretion of Urine,' 1917. Professor A. R. Cushny, F.R.S. 
 
THE URINE 
 
 333 
 
 
 67 Litres Plasma 
 contain — 
 
 62 Litres 
 
 Filtrate 
 
 containing 
 
 in all — 
 
 61 Litres Reabsorbed 
 Fluid contain — 
 
 1 Lit 
 con 
 
 Per 
 Cent. 
 
 re Urine 
 tains — 
 
 Per 
 Cent. 
 
 Total. 
 
 Per 
 Cent. 
 
 Total. 
 
 Total. 
 
 Water 
 
 Colloids 
 
 Dextrose 
 
 Uric acid 
 
 Sodium 
 
 Potassium 
 
 Chloride 
 
 Urea 
 
 Sulphate 
 
 92 
 
 62 1. 
 
 62 1. 
 
 67 gr. 
 
 1-3 gr- 
 
 200 gr. 
 
 13-3 gr- 
 248 gr. 
 
 20 gr. 
 1-8 gr. 
 
 — 
 
 61 1. 
 
 95 95° ex. 
 
 1 8 
 
 5.360 gr.| 
 
 o-i 
 
 0-O02 
 
 0-3 
 
 O-02 
 
 0-37 
 0-03 
 0-003 
 
 67 gr. 
 
 1-3 gr- 
 200 gr. 
 
 13-3 gr- 
 248 gr. 
 
 20 gr. 
 i-8 gr. 
 
 O-Il 
 
 0-OOI3 
 
 0-32 
 
 0-019 
 
 0-40 
 
 67 gr. 
 0-8 gr. 
 196 gr. 
 n-8gr. 
 242 gr. 
 
 0-05 
 
 o-35 
 0-15 
 o-6 
 
 2-0 
 
 0-18 
 
 o-5 gr- 
 3-5 gr- 
 i-5 gr- 
 6-ogr. 
 20 gr. 
 i-8 gr. 
 
 — 
 
 — 
 
 
 From 'The Secretion of Urine,' p. 48, by A. R. Cushny. 
 
 The action of diuretics has been studied in connection with 
 the question of urinary secretion, and the general outcome of 
 the work is that these effect their purpose either by increasing 
 the flow of blood to the kidneys, or by directly stimulating the 
 secretory activity of the cells. 
 
 The function of the cells of the tubules does not end with the 
 removal from the blood of the substances presented to them; 
 they are also capable of forming new material. Thus the union 
 of glycine with benzoic acid, resulting in the formation of hip- 
 puric acid, takes place in the cells of the tubules, and observations 
 have shown that, providing the benzoic acid be presented to it, 
 the kidney is capable of furnishing the needful glycine. It can 
 hardly be doubted that what is true of glycine and benzoic acid 
 may also be true of other substances, and that in the cells trans- 
 formations may occur which lead to the production of colouring 
 matters, etc., and of which our knowledge is at present obscure. 
 Mucin, so prominent a feature in the urine of the horse, must also 
 be prepared by the cells of the tubules, or those of the pelvis of 
 the kidney, as being a colloid it cannot escape through the 
 glomerulus by filtration. 
 
 The Composition of the Urine.— The urine contains nearly 
 the whole of the waste nitrogenous, some of the non-nitrogenous, 
 and the greater part of the saline products of the body, the 
 difference being made up by the skin, especially in the horse, 
 where, we have already seen, protein, urea, and salts are to be 
 found 'in the sweat. We have yet to study the complex subject 
 of tissue disintegration; nevertheless, to enable a clear perception 
 to be formed of the physiology of urinary secretion, it is necessary 
 
334 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 to very briefly state some of the essential facts in order that the 
 functions of the kidney may be appreciated.* 
 
 The blood passing through the kidneys carries with it the 
 disintegrated products of the tissues and of those substances 
 of the food which have been absorbed. It is not possible to 
 state precisely what proportion of the urinary products are due 
 to changes in the tissues, and what proportion to the breaking 
 down of food substances which have formed no part of the tissues. 
 Whatever their source, by the time they are excreted in the 
 urine they are waste products and incapable of further use by 
 the system. Speaking broadly, the kidneys form the path by 
 which the end products of the disintegration of protein food and 
 inorganic salts obtain exit from the body, while those of non- 
 protein food are eliminated mainly by the lungs, a small pro- 
 portion being dealt with by the kidney in herbCvora. It is, 
 therefore, the all-important protein which mainly concerns us 
 in connection with the urine; the exceedingly complex protein 
 molecule, whether it be that which has formed part of the tissues, 
 or that immediately derived from the food and not yet built into 
 the body structure, is broken down step by step into simpler 
 bodies, which when the end products are reached are elimin- 
 ated from the blood through the kidneys. The simplicity of 
 these end products may be judged from the fact that, though 
 protein has never yet been obtained in a pure condition, and its 
 molecular structure is therefore unknown, it is certain that its 
 molecular weight is very high. It is believed, for example, that 
 the molecular weight of a globulin known as edestin, obtained 
 from hemp-seed, is approximately 14,500, while its end products 
 in the urine, after the system has been chemically tearing it to 
 pieces, would contain only 8 atoms if eliminated as urea, or 
 22 atoms if eliminated as hippuric acid. 
 
 The composition of the urine varies with the species of animal. 
 In all herbivora the secretion, with certain minor differences, is 
 much the same ; omnivora and carnivora each possess a distinc- 
 tive urine; in all cases the differences are explained by the 
 character of the food. When herbivora live on their own tissues, 
 as during starvation, or under pathological conditions, they 
 become carnivora, and their urine alters in character, corre- 
 sponding now to that of flesh-feeders. Similarly, the young of 
 herbivora, while still sucking, have a urine possessing much the 
 same characteristics as that of carnivora. 
 
 In amplification of these general statements it is necessary to 
 point out that in animals of the same class the composition of 
 the urine may vary within very wide limits in accordance with 
 several causes, of which diet is the most important. 
 
 * The student is advised to read the chapter on Nutrition in conjunc- 
 tion with that on the Urine, as the subjects are largely complemental. 
 
THE URINE 
 
 Urine consists of — 
 Water. 
 
 Organic matter 
 
 Salts 
 
 335 
 
 al- 
 
 ( Nitrogenous end-products: urea, uric acid, 
 
 lantoine, hippuric acid, creatinine. 
 Aromatic compounds : benzoic acid, ethereal 
 
 sulphates of phenol, cresol, etc. 
 Non-nitrogenous substances (herbivora) of un- 
 known origin. 
 ^Colouring matter and mucus. 
 /"Sulphates, phosphates, chlorides, and carbon- 
 | ates of sodium, potassium, calcium, and mag- 
 (_ nesium. 
 
 Nitrogenous End Products. — The total nitrogen of the urine 
 
 consists of — 
 
 i. Urea nitrogen. 
 
 2. Uric acid nitrogen. 
 
 3. Ammonia nitrogen. 
 
 4. Creatinine nitrogen. 
 
 Urea, or carbamide, is the substance in urine in the form 
 of which the greater part of the nitrogen is got rid of : 1 gramme 
 of protein yields J gramme 
 of urea. It is a substance 
 very soluble in water and 
 crystallises readily, the cry- 
 stals melting on heating and 
 giving off ammonia. In the 
 urine it exists in a free con- 
 dition, but is capable of 
 forming salts with acids. 
 These yield typical crystal- 
 line formations — e.g., octa- 
 hedra, with nitric acid (see 
 Fig. 99) — by which they can 
 be identified microscopi- 
 cally. Under the influence 
 of an enzyme, urease, liber- 
 ated from the Micrococcus 
 
 urece, urine, on standing, becomes ammoniacal, owing to 
 the breaking up of the urea into ammonium carbonate. This 
 sometimes occurs in the bladder under pathological condi- 
 tions. Urea is found in blood in proportions which vary 
 between 4 and 6 per 10,000 ; in the urine it exists in the proportion 
 of 300 parts to 10,000. It is only because of the continuous 
 flushing of the kidneys with blood that this amount of urea is 
 separated, and the size of the stream may be judged by the fact 
 that a dog weighing 35 kilogrammes (77 pounds) has 300 kilo- 
 grammes (660 pounds) of blood streaming through the kidneys 
 
 Fig. 99. — Crystals of Nitrate of Urea. 
 
336 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 every twenty-four hours. The urea is separated in the glomeru- 
 lus by filtration, and the peculiar selective power of the cells of 
 the tubules cannot be better exemplified than in their rejection 
 of urea, as it streams along towards the pelvis of the kidney, and 
 their absorption of sugar. 
 
 It is evident that the disintegration of protein material which 
 leads to the formation of urea may be occurring in the actual 
 protein of the tissues (endogenous production), or it may be taking 
 place in the protein substances absorbed from the intestinal 
 canal which are not part of the tissues. This is known as ex- 
 ogenous production. It is not possible to say exactly how 
 much is contributed from each source, but it would seem that 
 the major portion is furnished by the protein food substances 
 not yet built into the protein of the tissues. 
 
 The history of urea is still very incomplete, but it is now gen- 
 erally accepted that it is formed, though perhaps not exclusively, 
 in the liver, and that the kidneys are only its path of excretion. 
 The form in which the antecedents of urea arrive at the liver is 
 not definitely known, nor, indeed, are the various stages existing 
 between protein and urea agreed upon. We have seen (pp. 195, 
 262-264, 334) that the protein molecule, under the influence of 
 pepsin, trypsin, and erepsin, is gradually reduced from complexity 
 to simplicity, and in this process its nitrogen appears as ammonia, 
 monamino-acids, and diamino-acids. There is no reason to 
 doubt that what occurs in the intestinal area in the breaking up 
 of the protein molecule may also occur in the tissues. Muscles, 
 for example, contain ammonia in large quantities. Ammonia is 
 formed in the body tissues by intracellular ferments, and is 
 carried by the blood to the liver. The form in which it is carried 
 is not agreed upon, but as the ammonia in the tissues unites 
 with carbon dioxide, it is probably carried either as ammonium 
 carbamate or carbonate. In the liver the conversion of this 
 into urea by the loss of water is easy to understand. The 
 latest view is that ammonium salts reach the liver in the form of 
 ammonium lactate. Ammonium lactate is formed by the 
 sarcolactic acid of muscle metabolism uniting with the am- 
 monium of the protein, and this in the liver is oxidised into am- 
 monium carbonate, the ammonium carbonate being converted 
 into urea. If the blood of the portal vein be experimentally com- 
 pelled to pass into the posterior vena cava without circulating 
 through the liver, the output of urea is decreased, while the 
 ammonia compounds in the blood increase and cause poisoning. 
 The blood of the portal ve:'n is found to contain three or four 
 times more ammonia compounds than arterial blood, and there 
 can be no doubt that these highly poisonous compounds are 
 converted in the liver into the less poisonous urea. It is well 
 
THE URINE 337 
 
 known that monamino-acids, resulting from protein disintegra- 
 tion, such as leucine, glycine, and aspartic acids, may be converted 
 into urea, and it is most likely that this change is effected during 
 their passage through the liver. The diamino-acids, represented 
 in the body by histidine, lysine, and arginine, are also capable of 
 conversion into urea. Arginine, for example, is acted upon by 
 the ferment arginase, which is found in the liver, kidneys, and 
 other organs, and converted into urea and ornithine (diamino- 
 valerianic acid) . No arginase is found in muscle. 
 
 It is believed that the larger part of the urea excreted is not 
 produced by the metabolism of tissue protein, but is derived 
 from the products of food absorption, and these are mainly 
 represented by the ammonia compounds taken up from the in- 
 testinal area. The urea contributed by the tissues may also be 
 represented by ammonia compounds and monamino-acids, and 
 perhaps by the splitting up of arginine. It has been found that 
 when the liver is removed urea does not disappear from the urine, 
 and it is supposed that the origin of this portion is in the tissues, 
 though this urea may not necessarily be formed in the same way 
 as the liver urea, nor from the same material. Uric acid has 
 by some physiologists been considered a possible source of urea, 
 for it has been observed in man that about one-half of the uric 
 acid produced is not excreted as such, but is got rid of as urea ; 
 in dogs, however, only one-twentieth undergoes this conversion. 
 This change is effected by a uricolytic ferment found in the liver 
 and other organs. Uric acid given by the mouth is excreted as 
 urea, and outside the body it is readily converted into urea by 
 oxidation; nevertheless, this is not very strong evidence in 
 favour of the existence of uric acid conversion in the living body. 
 Creatinine was at one time regarded as a source of urea, but with 
 a better knowledge of this substance and of creatine, the con- 
 version of creatinine into urea, which offers no difficulty as a 
 laboratory process, is regarded in the living body as improbable. 
 
 The proportion of urea in urine varies according to the nature 
 of the diet . In man and the dog the larger the amount of nitrogen 
 in the food the more urea excreted, but in herbivora it has been 
 observed that on a diet consisting principally of hay more urea 
 is excreted than on one of oats and hay. The amount of urea 
 excreted was at one time considered to be a measure of the 
 amount of work performed by the animal body, but this view 
 has long been known to be wrong, though there can be no doubt 
 that under the influence of work rather more urea may be ex- 
 creted than during rest. 
 
 Apart from these marked causes of variation there are others, 
 which certainly in the case of the horse lead to great fluctuations 
 in the amount of the daily excretion, even under identical condi- 
 
338 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 tions, of work, food, and rest. We shall see also that this is so 
 in the dog, an animal in which occur the widest variations in the 
 urea. In the horse, about ioo grammes (3| ounces) daily is a 
 mean excretion. The percentage of any substance in urine 
 conveys no information unless the total amount of excretion for 
 the twenty-four hours has been ascertained. Nevertheless, it is 
 usual to speak of the percentage of urea and other substances, 
 and it is a useful expression if the other data be known. In all 
 animals the urea varies from 2-5 to 4 per cent. 
 
 Creatine and Creatinine.- — Creatine is found in muscular 
 tissue, and the amount, generally under 5 per cent., may be 
 doubled by severe exercise or prolonged starvation. It is not 
 regarded as a normal constituent of the urine excepting in the bird. 
 If administered to dogs by the mouth, it appears in the urine as 
 creatinine; if given to rabbits, as representing herbivora, only a 
 minute proportion is recoverable as creatinine, while the re- 
 mainder cannot be recovered in any form. Indeed, there are 
 some who consider that the creatine of muscle is not converted into 
 creatinine in the urine, but that creatinine is formed in the liver 
 as the result of protein metabolism, then passed to the muscles, 
 where it is stored up as creatine, the excess being got rid of by 
 the urine as creatinine. In flesh-feeders a portion of the urinary 
 creatinine is derived from the food, though it continues to be 
 excreted on a meat-free diet; if a diet be given in which the 
 proteins are reduced in amount, the creatinine is increased. The 
 source of creatinine is unknown, though Thompson found that 
 when arginine, a body chemically related to creatine, and present 
 in most articles of diet, was injected subcutaneously into ducks 
 it gave rise to an increased formation of creatine. The elimina- 
 tion of creatine in diabetic subjects has been attributed to a 
 deficiency of carbohydrate in the body, but it is said that pigs 
 fed on maize or oats, though containing adequate carbohydrate, 
 excrete large quantities of creatine, and the urine becomes acid. 
 It is believed by F. P. Underhill, to whom these observations 
 are due, that there is an inter-relationship between acidosis and 
 creatine elimination, and that if the acid state of the urine be 
 abolished the excretion of creatine ceases. J. B. Orr urges that 
 creatine is formed in relatively large quantities by ruminants, that 
 it is utilised in the tissues, and that its utilisation is intimately 
 connected with the metabolism of carbohydrate, the creatine 
 formed being in inverse proportion to the amount of carbohy- 
 drate in the diet. 
 
 Creatinine occurs in the urine of all mammals. In man the 
 amount varies with different people, yet it is so constant for the 
 individual that the total quantity of urine secreted may be cal- 
 culated from the creatinine found. Creatinine may be found in 
 
THE URINE 
 
 339 
 
 muscle, probably as the result of post-mortem conversion of 
 creatine; it also is a regular constituent of the cereals. 
 
 Uric Acid. — Thisis the chief end-product of protein metabolism 
 in birds and reptiles. It is manufactured in the liver by synthesis 
 from ammonia and lactic acid, and excreted as acid ammonium 
 urate. In contrast to urea, it is a substance very insoluble in 
 water, but soluble in alkaline solutions and in alkaline phos- 
 phates. With the latter an acid urate of the alkali is formed, 
 and acid sodium phosphate results, which gives the reaction to the 
 urine of the carnivora. In the mammal it is known that the chief 
 source of uric acid is not synthesis, as in the bird, but the splitting 
 up and oxidation of a special 
 group of protein sub- 
 stances known as nucleo- 
 proteins . These bodies may 
 be derived from the food, 
 or from the tissues, as we 
 have seen is the case with 
 urea. In man food rich in 
 nuclein, such as thymus, 
 pancreas, and herring roe, 
 and in purin bases as meat 
 extract, increases the 
 amount of uric acid pro- 
 duced. Uric acid (Fig. ioo) 
 does not exist in the urine 
 in a free state, but as urates 
 of soda or potash. Two 
 
 classes of urates are formed— neutral and acid. The acid 
 sodium urate is the chief constituent of the reddish deposit 
 of urates which occurs in acid urines. 
 
 We have referred to purin bases as forerunners of uric acid, 
 and a word in explanation is necessary. Purin is the name given 
 by E. Fischer to the nucleus common to the uric acid group of 
 substances; from this nucleus, by transformations, several mem- 
 bers of the group may easily be obtained. They are found m 
 both animals and plants. 
 The purin group cpnsists of the following bodies: 
 
 ( Hypoxanthine . 
 Xanthine. 
 
 Fig. ioo. — Crystals of Uric Acid (Funke 
 
 Purin Bodies. 
 
 Purin Bases. 
 
 Uric Acid. 
 
 \ Adenine. 
 I Guanine. 
 
 From the purin bases uric acid is probably formed by the 
 following process: Under the influence of ferments the nuclein is 
 split off from the protein and acted upon by a tissue enzyme 
 
34Q A MANUAL OF VETERINARY PHYSIOLOGY , 
 
 known as nuclease. This splits the nuclein, and results in the 
 formation of the above-named purin bases. Adenine and 
 guanine, under the influence of tissue enzymes, are converted 
 into hypoxanthine and xanthine, and these hpdies, on being 
 acted upon by an oxidase ferment, are oxidised to uric acid. 
 Of the uric acid so formed, a portion under the influence of a 
 uricolytic ferment is, as previously noted, converted into urea. 
 
 The uric acid problems have been worked out mainly on the 
 dog, apart from the work done on birds. In the case of the non- 
 suckling herbivora, it is known that in a condition of health uric 
 acid is not found, or only to a trifling extent. It is present in 
 the suckling, also in the adult, immediately the animal begins 
 to live on its own tissues, as in disease. It appears clear that a 
 flesh diet, especially of cellular organs such as the various glands, 
 favours its production, while a vegetable one either inhibits its 
 formation or else destroys it, perhaps in the liver, under the 
 influence of a body ferment, producing urea in its stead. This, 
 however, is pure conjecture. In dogs it is said that allantoine 
 takes the place of uric acid. 
 
 Allantoine is an oxidation product of uric acid, and was first 
 found in the allantoic fluid of the cow. It has been found in the 
 urine of several of the herbivora — cattle, sheep, and rabbits — and 
 in carnivora, as represented by the dog. Whenever found in 
 urine it is derived from uric acid. 
 
 Hippuric and Benzoic AcidsVare conveniently considered 
 together, though benzoic acid belongs to the aromatic series and 
 contains no nitrogen. These acids are characteristic' of the 
 urine of herbivora ; in man and carnivora they are found only in 
 small amounts, in birds they are absent. Hippuric acid is 
 formed by the union of glycine with benzoic acid, glycine being 
 derived from the breaking down of the protein of food. The 
 seat of the synthesis is only imperfectly known. It has 
 been ascertained by experiments on the dog that it may be 
 effected in the kidney, so that hippuric acid is one of the 
 few substances capable of being formed by that gland; in the 
 rabbit, and probably other herbivora, the synthesis may occur 
 in the liver, as the rabbit can form hippuric acid after the removal 
 of the kidneys. The synthesis of benzoic acid and glycine may 
 readily be brought about outside the body by using ground-up 
 kidney tissue mixed with blood and kept at the body tempera- 
 ture. If the kidney be perfused with a mixture of blood, glycine, 
 and benzoic acid, its epithelial cells must be undamaged in order 
 to obtain hippuric acid. This suggests that the active agent in 
 the synthesis is an enzyme. The source of the necessary benzoic 
 acid in not completely known. Hay, grass, and grain contain in' 
 the cuticular covering aromatic substances which yield hip- 
 
THE URINE 
 
 341 
 
 puric acid in the body. It is possible to remove this cuticular 
 covering by a laboratory process, and food so fed yields no hip- 
 pur ic acid. 
 
 When no benzoic acid exists in food, hippuric acid may be 
 formed from aromatic compounds containing the benzoyl radicle 
 derived from protein putrefaction in the intestines (Armsby).* 
 This authority, however, draws attention to the fact that the 
 food of herbivora does not contain sufficient of the aromatic 
 series which yield benzoic acid to account for the amount of 
 hippuric acid produced, especially in cattle. He states that 
 search has been made among the non-nitrogenous constituents 
 
 Fig. ioi. — Crystals of Purified 
 Hippuric Acid (Funke). 
 
 Fig. 102. — Crystals of Impure 
 Hippuric Acid. 
 
 for an explanation of the riddle, and that investigations suggest 
 the pentose carbohydrates^ of the food as having some connection 
 with the formation of benzoic acid. If cherry gum be added to 
 the food of sheep, a marked increase in hippuric acid occurs, 
 though this is less evident in the case of the horse. 
 
 Neither hippuric nor benzoic acids exist free in the urine, but 
 combined with bases, especially potash . There is in consequence, 
 especially in ruminants, a heavy drain on the bases of the 
 body. 
 
 Liebig, many years ago, stated that benzoic acid was found 
 in the urine of working horses, and hippuric acid in the urine 
 of those at rest. Our observations show that hippuric acid is 
 generally found in the urine of working horses, and seldom 
 
 * ' The Principles of Animal Nutrition, '191 4. H.P. Armsby, Ph.D., LL.D., 
 United States Department of Agriculture. 
 
 f These are ribose, obtained from the nucleic acid of plants; arabinose, 
 from gum arabic, cherry gum, etc. ; xylose, from wood gum, straw, and 
 cellulose. 
 
342 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 found in the urine of horses at rest — in fact, the reverse of Liebig's 
 view. Owing to its easy and rapid fermentative decomposition, 
 hippuric acid is rarely to be found in urine twenty-four hours 
 old; out of fifty-four specimens examined, we found it in only 
 eight. It, however, always appears to be present in the urine 
 of cattle. Diet plays an all-important part in the production of 
 hippuric acid ; it is increased in quantity by grass, hay, and straw 
 feeding, and decreased by the use of clover, peas, wheat, and oats. 
 The amount of hippuric acid excreted by horses depends, there- 
 fore, upon several factors : 
 
 On an exclusive grass diet a horse will excrete, on an average, 
 
 120 grammes daily (4-2 ozs.). 
 On an exclusive hay diet, 50 grammes daily. 
 On a diet of hay, oats, and straw, 85 grammes daily. 
 On a mixed diet the amount may be 60, 140, or 165 grammes 
 
 daily. 
 
 In sheep, urea stands to hippuric acid in the proportion of 
 2 to 3, whereas in cattle on the same diet the proportion is 2 of 
 urea to 1-1 of hippuric acid. The food most productive of hip- 
 puric acid in the horse is old meadow hay, whilst new meadow 
 hay has this effect on sheep. 
 
 With the usual caution regarding urinary percentages- — viz., 
 that they are worthless unless the total amount of secretion is 
 known — it may be said that hippuric acid in the horse exists in 
 the proportion of from f to 2 per cent. 
 
 The crystalline forms of hippuric and benzoic acids are shown 
 in Figs. 101, 102, and 103. 
 
 The Ammonium Salts present in urine are an index to the 
 neutralisation of acids in the body. Acid substances are pro- 
 duced in carnivora and omnivora as the result of metabolism — 
 for example, carbonic, phosphoric, and sulphuric acids — and 
 these would prove highly poisonous were it not that ammonia, 
 which neutralises them, is simultaneously formed. This defen- 
 sive mechanism against acidosis depends on smaller amounts of 
 ammonia being converted into urea. When, as occurs in her- 
 bivora, there is already an excess of alkali in the diet, a sufficiency 
 of bases is present to neutralise the acid formed during metabo- 
 lism, so that the normal conversion of ammonia into urea con- 
 tinues. The evidence which bears on this is afforded by the 
 injection of acids into the circulation. This, in a dog, does 
 not alter the reaction of the blood, for ammonia is produced in 
 sufficient quantities to neutralise the acid. In herbivora, on the 
 other hand, the ready supply of ammonia may not be available; 
 the store of vegetable alkaline salts is soon used up, for we have 
 already seen the heavy draft on these which hippuric acid alone 
 makes, so that the animal as the result of the injection of acid 
 
THE URINE 
 
 34! 
 
 suffers more acutely and is more readily killed by acidosis than 
 is the case with carnivora. Doubt, however, has been thrown on 
 this difference between carnivora and herbivora in the matter of 
 acidosis, and Armsby draws attention to the fact that if the 
 amount of protein in the diet is low, as it frequently is, there is 
 relatively less ammonia available than is the case with car- 
 nivorous animals. 
 
 It is rather a difficult matter to increase the acidity of an acid 
 urine. Mineral acids effect it, but if the acid be pushed too far 
 ammonia is split off from the protein in order to prevent toxic 
 results. On the other hand, it is very easy to increase the alka- 
 linity of an alkaline urine. It may be well to explain that the 
 poisoning which occurs from acidosis is due to the reduced 
 carrying power of the blood for carbon dioxide. In consequence, 
 
 Fig. 103. — Crystals of Benzoic Acid. 
 
 this substance is retained in the tissues with fatal results. The 
 administration of ammonium carbonate by the mouth will 
 not prevent this condition, since it appears in the urine 
 as urea. 
 
 Free ammonia exists in the urine of the horse. It may be 
 owing to ammoniacal fermentation in the bladder, but it is quite 
 certain that perfectly fresh urine may give marked evidence 
 of the presence of free ammonia. On standing a short time 
 outside the body, especially in summer weather, the urea decom- 
 poses, as previously mentioned, and ammonium carbonate is 
 largely formed. Recent work by Beccari in Italy confirms the 
 presence of free ammonia in the urine, not only of horses, but of 
 cattle. 
 
 Sulphuric Acid. — The sulphates in the urine, especially in 
 that of herbivora, and more particularly in the urine of the horse, 
 
344 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 are an excretion of considerable importance. In all animals 
 three varieties of sulphur-containing substances are found — 
 
 Inorganic sulphates. 
 Neutral sulphur. 
 Ethereal sulphates. 
 
 From this it might be supposed that the sulphates taken into 
 the body with the food are considerable. As a matter of fact, 
 the sulphates in food exist only in traces, and most of the sulphur- 
 containing substances in urine are really derived from the oxida- 
 tion of the amino-acid, cystine, which contains the organic sulphur 
 of protein. 
 
 The Inorganic Sulphates are believed to be derived from the 
 katabolism of proteins in the intestinal canal, or, at any rate, 
 from the destruction of protein substances not yet forming part 
 of the body; hence they are spoken of as the exogenous portion. 
 The quantity of sulphates excreted in this form varies consider- 
 ably, as it depends upon the amount of protein in the diet. They 
 pass from the body as salts of sodium and potassium. 
 
 The Neutral Sulphur is furnished from the actual proteins of 
 the tissues; it is not liable to great variations in quantity, and 
 the amount for the individual may even be constant. Included 
 with it is the sulphur of the pigment urochrome, cystine, thio- 
 cyanates, and minor sulphur compounds, such as ethyl sulphide, 
 found in the urine of the dog, and sulphocyanic acid said to be 
 a constant constituent of the urine of herbivora. 
 
 Coming between the above is a third variety of sulphur com- 
 pounds — the Ethereal Sulphates, which are furnished by the tissue 
 proteins, but also by non-protein substances; they are a very 
 characteristic group in the urine of the herbivora, especially in 
 that of the horse. The sulphur forms compounds with such 
 poisonous substances as indol, phenol, skatol, kresol; these are 
 produced in the intestinal canal of the animal in consequence 
 of the disintegration of protein substances, but also from certain 
 aromatic constituents in vegetable food. These bodies are either 
 excreted unabsorbed from the intestinal canal, or if absorbed, 
 unite in the blood-stream with an alkaline sulphate to form con- 
 jugated or ethereal sulphates, and in this way are rendered in- 
 nocuous. 
 
 The compounds formed by sulphuric acid with the aromatic 
 bodies are potassium phenyl, kresyl, indoxyl, and skatoxyl sul- 
 phates. 
 
 To the compound potassium indoxyl sulphate the name 
 indican has been given. This substance on oxidation yields 
 indigo blue, and is comparatively abundant in the urine of 
 the horse. Phenyl-sulphuric a,cid in the presence of air under- 
 
THE URINE 345 
 
 goes oxidation, and yields pyrocatechin, which gives the brown 
 colour to the stale urine of the horse. In the dog, glycuronic 
 acid may be united to some of the skatol and phenol, and this 
 substance exercises a reducing action on salts of copper, which 
 might be mistaken for the action of dextrose. 
 
 In man and the dog the ethereal sulphates are regarded as 
 a measure of protein disintegration, but in the horse this is not 
 so, for we have already seen that ethereal sulphates in this animal 
 may be derived from aromatic constituents in vegetable food. 
 
 Cystine, which, as we have seen, is the chief source of sulphur 
 in the body, is readily oxidised to sulphuric acid, and the neutra- 
 lisation of this is another source of drain on the bases of the body. 
 Where the protein in a diet is in excess, as, for instance, in the 
 case of a horse fed wholly on oats, the amount of acid, both sul- 
 phuric and phosphoric, is in excess of the available bases, and the 
 urine becomes acid. A pound of protein may contain sufficient 
 sulphur to yield on complete oxidation an ounce by weight of 
 concentrated sulphuric acid. In a properly balanced diet the 
 herbivora obtain a surplus of bases which more than neutralise 
 the acid salts contained in concentrated foods. 
 
 Non-Nitrogenous Organic Substances. — The amount of these 
 in the urine of herbivora is relatively considerable (Armsby). 
 Their origin is imperfectly known, excepting that they are derived 
 from the food, especially those constituents described in analyses 
 as ' crude fibre,' ' bodies related to carbohydrates,' and ' nitrogen 
 free extract.' These substances do not undergo complete oxida- 
 tion to carbon dioxide and water, and in consequence are excreted 
 in the urine as imperfectly oxidised substances. As might be 
 expected, a diet of hay or straw causes a larger excretion of these 
 substances, but precisely how they originate is unknown. 
 
 The colouring matter of the urine is a question not fully settled. 
 There are known to be several pigments, the chief one being 
 urockrome, which is probably an oxidation product of urobilin. 
 Urobilin is formed from stercobilin, which is absorbed from the 
 intestinal canal. The mother-substance of urobilin is a chro- 
 mogen, which yields urobilin on oxidation. 
 
 The Inorganic Substances found in the urine are calcium, 
 magnesium, sodium, and potassium, existing in the form of 
 chlorides, sulphates, phosphates, and carbonates. The origin 
 of these salts is, first, the food taken into the body; secondly, the 
 metabolic processes occurring in the tissues. The nature and 
 amount of the salts varies with the class of animal and the 
 character of the food. In the urine of the horse calcium and 
 potassium salts predominate, sodium and magnesium are present 
 in small amounts, phosphates are practically absent, while 
 sulphates and chlorides are in considerable quantity. It has 
 
346 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 been found that in ruminants the calcium salts are mostly 
 excreted with the faeces, whereas in the horse they principally pass 
 through the kidneys. It is certain that phosphates, so important 
 a feature in the urine of carnivora and omnivora, are in the horse 
 almost wholly excreted by the intestines. 
 
 Calcium.— -More lime exists in the urine of the horse than is 
 soluble in an alkaline fluid, so that it is found both in suspension 
 and in solution ; the amount of the former increases with the age 
 of the urine, owing to the development of ammonia, until nearly 
 the whole of the lime is precipitated. The lime exists in com- 
 bination with oxalic, carbonic, hippuric, and sulphuric acids; all 
 these combinations do not necessarily exist in one specimen of 
 urine, the salts formed depending on the varying relative amounts 
 of the acids formed in metabolism. The amount of lime in the 
 
 Fig. 104. — Crystals of Oxalate 
 of Lime (Funke). 
 
 Fig. 105. — Crystals of Carbonate 
 of Lime (Funke). 
 
 food does not influence the elimination through the kidneys; 
 more lime, however, is found in the urine of horses at work than 
 in that of those at rest. Oxalate and carbonate of lime crystals 
 are common microscopic deposits in the urine of the horse 
 (Figs. 104 and 105). The oxalic acid is derived from the oxalates 
 contained in the vegetable substances of the food. 
 
 The turbidity of the healthy urine of the horse is due to sus- 
 pended calcium carbonate. This may be proved by the addition 
 of an acid, which causes profuse evolution of gas, when a clear, 
 transparent urine results. Simple as the point is to understand, it 
 would appear that in practice turbid urines in the horse are 
 regarded as pathological, and that the marked deposit which 
 follows every evacuation of urine is considered incompatible with 
 a normal condition. The fact is, that no healthy horse passes 
 
THE URINE 347 
 
 anything but a turbid urine, sometimes with a considerable 
 deposit, but always with a deposit. This may be seen in the 
 stable, or in the road, after the urine has either soaked in or 
 partly dried, as a fine, yellow, sandy -looking deposit. The ' sand ' 
 is calcium carbonate; the yeUow colour is the pigment of the 
 urine. It is most remarkable that vesical calculus in the horse 
 is so rare, considering the fact that from the pelvis of the kidney 
 onward the urine has far more mineral matter in it than it can 
 hold in solution; fortunately it rarely coheres. 
 
 Magnesium. — There is but little magnesium in the urine of the 
 herbivora, its chief excretory path being the bowels. The sheep 
 is an exception to the general rule that in the urine magnesium 
 is excreted in smaller amounts than calcium. Magnesium exists 
 in the urine partly in solution and partly in suspension, the latter 
 being principally due to ammoniacal fermentation. 
 
 Potassium, derived from the potash of the food, exists largely 
 in the urine of herbivora ; it forms numerous combinations, the 
 one with carbonic acid being the cause of the fixed alkalinity of 
 the urine in the horse. There is more potash found in the urine 
 of horses at rest than in that of those at work ; this is explained 
 by the considerable amount of potassium excreted with the 
 sweat. Sodium exists in the urine of herbivora only in relatively 
 small quantities, on account of the fact that very little sodium 
 is found in vegetable food. For the relationship between potas- 
 sium and sodium in the body, see p. 382. 
 
 Chlorine is supplied by the chlorides of the food. The pro- 
 portion of chlorides in the food of herbivora is not very high; 
 the amount excreted by horses, combined with sodium, was 
 found by us to equal a daily excretion of 5-5 grammes (85 1 grains) 
 of common salt. Salkowski places it much higher — viz., about 
 27 grammes (f ounce) daily. 
 
 Phosphoric Acid, though existing in large quantities in food 
 such as oats, passes off almost wholly by the alimentary canal. 
 Sometimes only traces are to be found in the urine of herbivora ; 
 at others the amount is appreciable, but never considerable. 
 Work does not influence its production. In the urine of car- 
 nivora the phosphates are an important constituent. They exist 
 in the urine in two forms— viz., alkaline phosphates, such as 
 phosphate of sodium or potassium, and earthy phosphates, such 
 as phosphates of calcium and magnesium ; these triple phosphates 
 are common as a microscopical object in the decomposing urine 
 of the horse, though trifling in actual amount (Fig. 106). The 
 phosphates are derived from the food and tissues. According 
 to Munk, if there is an abundance of lime salts in the diet, as m 
 vegetable food, the phosphates are not eliminated to any extent 
 by the kidneys, for the reason that they combine in the intestinal 
 
348 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 canal with lime and magnesia, and pass off by that channel; if, 
 on the other hand, there is but little lime and magnesia in the 
 intestines, the phosphates are united to soda and potash, pass into 
 the blood, and are eliminated by the urine. In febrile conditions 
 in the herbivora phosphates appear in the urine in marked amounts. 
 The Reaction of the urine in carnivora is acid, due to acid 
 sodium phosphate ; in omnivora the reaction is acid or alkaline, 
 depending upon the nature of the diet, an animal diet producing 
 an acid, a vegetable diet an alkaline, urine. In herbivora the 
 reaction is alkaline in health and under a suitable system of diet. 
 Where protein is supplied in excessive amounts, especially in the 
 absence of hay, the urine becomes acid. Vegetable food may 
 result in acid or alkaline salts predominating in the urine. All 
 the cereals, with the exception of bran, possess an excess of acid 
 
 salts ; all grasses, hay, straw, 
 and roots possess an excess 
 of alkaline salts. It is the 
 excess of acid salts in oats- 
 which causes the urine of 
 the horse to possess an acid 
 reaction when fed on this 
 grain without hay. Acid 
 salts are also formed from 
 fermentable material, such 
 as grass and silage ; most of 
 these acids are neutralised, 
 though others, such as tar- 
 taric, malic, and oxalic, are 
 oxidised with difficulty. 
 The organic acids appear 
 in the urine as carbonates 
 in combination with lime 
 and potash. The greater part of the alkalinity of the urine in the 
 horse is due to carbonates of potash and lime ; there is a further 
 alkalinity outside the body, the result of the development of 
 ammonia. 
 
 No free acid exists in the urine of any animal. 
 The amount of water secreted with the urine differs consider- 
 ably in the various classes of animals. It is generally considered 
 that in the horse, of the total intake of water, not more than 
 one-third leaves the body by the kidneys. The writer obtained 
 as the mean of many observations, 4-8 litres (8£ pints) of urine 
 in twenty -four hours. This agrees with the observations of 
 Munk,* who regarded 3 to 4 litres as the mean excretion in the 
 horse. In the ox a larger amount of urine is passed — 6 or 10 to 
 
 * Quoted by McKendricl?. 
 
 Fig. 
 
 106. — Crystals of Triple 
 Phosphate (Funke). 
 
THE URINE 349 , 
 
 25 litres (10, 17, 44 pints) daily; in sheep, 0-3 to 0-9 litre (| pint to 
 i? pints) ; pigs, 1-5 to 8 litres (2-6 to 14 pints) ; dogs, 0-5 to 1 litre 
 (o-8 pint to i-6 pints). Tereg,* who gives this table, states that 
 the excretion of water by the kidneys and other channels in the 
 various classes of animals is as follows: 
 
 Herbivora: 20 per cent, excreted by the urine; 80 per cent, by 
 
 lungs, skin, and bowels. 
 Omnivora: 60 per cent, excreted by the urine; 40 per cent. 
 
 by lungs, skin, and bowels. 
 Carnivora: 85 per cent, excreted by the urine; 15 per cent. 
 
 by lungs, skin, and bowels. 
 
 Food rich in nitrogen causes a greater secretion of urine. 
 
 Horses at work excrete less urine than those at rest, no doubt 
 owing to the loss by the skin. In winter, in consequence of the 
 lessened action of the skin, more urine is excreted than during 
 summer. Lieut. -Colonel Head, C.M.G., R.A.V.C, noted the 
 amount of urinary excretion in horses affected with mange, before 
 and after dressing the body with an oily application. He found 
 that those undressed furnished from 8 to 10 pints of urine a day, 
 but after dressing the amount secreted rose to 14 or even 18 pints. 
 On washing off the dressing the quantity of urine returned to 
 normal. (Communicated.) 
 
 The Urine of the Horse. 
 
 Specific Gravity. — This varies within wide limits dependently 
 on the diet and the amount of dilution. The mean of a large 
 number of observations was 1-036, the highest registered was 
 1-050, and the lowest 1-014. 
 
 The Quantity of urine is liable to very considerable variation, 
 depending on the season and the diet; the more nitrogen the 
 food contains the larger the amount of water consumed, and the 
 greater the bulk of urine excreted. The mean of many obser- 
 vations was 4-8 litres (8J pints) in twenty-four hours, the diet 
 being moderately nitrogenous, but in individual instances very 
 much more than this may be obtained — viz., 6 to 11 litres (12, 
 15, or even 20 pints). 
 
 The Odour of urine is said to be due to certain aromatic sub- 
 stances of the phenol group. Perfectly fresh urine has commonly 
 a most distinct, though faint, smell of ammonia. This may be 
 due to fermentative changes occurring in the urea before the 
 urine is evacuated. The normal fluid is always turbid, some 
 specimens more so than others ; very rarely is it clear, and then 
 only for a short time. The turbidity is due to the amount of 
 suspended carbonates of lime and magnesia; on. standing, par- 
 ticularly if it is undergoing ammoniacal fermentation, the 
 turbidity becomes intense. 
 
 * Ellenberger, op. cit. 
 
35° A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The Consistence of the fluid depends upon sex, and perhaps 
 on the season. It is certain that some mares excrete a glairy, 
 tenacious fluid which, owing to the amount of mucin it contains, 
 can be drawn out in strings. It is very common to find the 
 urine as thick as linseed oil, and very rare to find it fluid and 
 watery. During cestrum the urine is of the consistence of oil. 
 On a diet of oats and no hay, the urine may be so mucinous as 
 to pour like white of egg. The mucin of horse's urine is a gluco- 
 protein, containing 12 per cent, nitrogen and nearly 2 per cent, 
 sulphur. The carbohydrate radicle, calculated as glucosamine, 
 amounts to 30 per cent. (A. V. Sahlstedt.) 
 
 The Colour of fresh urine is yellow or yellowish-red, due to 
 the pigment urochrome. On standing, the fluid rapidly turns 
 brown, the dark tint commencing on the surface, and gradually 
 travelling into its depth. The cause of the colour on standing 
 is due to the oxidation of pyrocatechin (see p. 345)- 
 
 The Total Solids consist of organic and inorganic matter, of 
 which, on a mixed diet, 142 grammes (5 ounces) are organic, 
 and 83 grammes (3 ounces) inorganic; these quantities are liable 
 to great variation, sometimes being found greatly in excess of 
 those quoted. The total solids are considerably affected by the 
 diet ; E. Wolff* found that, when he reduced the hay and increased 
 the corn ration, the solids in the urine decreased, whereas on a 
 diet consisting principally of hay and but little corn the solids 
 increased; for example — ■ 
 
 On a diet of 8 kilogrammes of hay and 2 kilogrammes of oats 
 
 there were 566 grammes of total solids. 
 On a diet of 4 kilogrammes of hay and 6 kilogrammes of oats 
 
 there were 460 grammes of total solids. 
 
 The percentage composition of the mineral solids is given in the 
 following table by Wolff, the diet being hay, oats, and straw: 
 
 Potassium 36-85 per cent. 
 
 Sodium 3-71 „ 
 
 Calcium - 21-92 ,, 
 
 Magnesium 4-41 
 
 Phosphoric acid — 
 
 Sulphuric acid -. 17-16 ,, 
 
 Chlorine 15-36 ,, 
 
 Silicic acid 0-32 ,, 
 
 In the table on p. 352 are given the results obtained by the 
 writer in examination of the twenty-four hours' urine of horses 
 at rest and at work.f 
 
 SalkowskiJ examined the urine of the horse, and gives the 
 following as the composition of one specimen : 
 
 * Ellenberger. 
 
 f ' Chemistry of the Urine of the Horse,' Proceedings of the Royal 
 Society, vol. xlvi., 1889. J EUenberger's ' Physiologie." 
 
THE URINE 35 r 
 
 Water - I -8 litres 3-50 pints. 
 
 Urganic solids 198-0 grammes 6-25 ounces. 
 
 • ™ h 5o - o ,, i-6o 
 
 Urea 92-0 ,, 3*25 
 
 Ammonia ■ 0-357 gramme 5-53 grains. 
 
 Hippuric acid 15-60 grammes 0-49 ounce. 
 
 Phenol 2-44 ,, 37-89 grains. 
 
 Organic sulphur 13-4° ,, 208-69 
 
 Inorganic sulphur 5-33 >( 85-77 ','• 
 
 Phosphoric acid 0-22 gramme 3-40 "„ 
 
 Lime - 5-70 grammes 88-50 ,, 
 
 Sodium chloride 27-12 ,, 0-87 ounce. 
 
 In the following summary of the urine of animals other than 
 the horse, the main facts are those given by Tereg.* 
 
 The Urine of the Ox. 
 The urine of the ox is much the same as that of the horse, 
 excepting that it is secreted in larger quantities, 57 to 22-8 litres 
 (10 to 40 pints) per diem. The fluid is clear, yellowish, and of an 
 aromatic odour ; it is of a lower specific gravity than that of the 
 horse, 1020 to 1030 (in milch cows, according to Munk, roo6 to 
 1-015), owing to the larger amount of water secreted. The nitrog- 
 enous matter found in the urine is represented mainly by urea 
 and hippuric acid; in contrast to the urine of the horse, hippuric 
 acid is always present. On a diet of wheat straw, clover hay, 
 beans, starch, and oil, the amount of urea may be 4 per cent., 
 while on one of oat straw and beans, it may fall to less than 
 1 per cent. When the percentage of urea is high, that of the 
 hippuric acid is low, and vice versa. The largest amount of 
 hippuric acid is produced by feeding on the straw of cereals, the 
 smallest is furnished by feeding orv leguminous straw, whilst a 
 medium amount is produced by feeding on hay. It was stated 
 many years ago by Faas that the urine of the working ox is 
 turbid, and contains so much hippuric acid that a pound of this 
 substance could be obtained by precipitation with hydrochloric 
 acid. The amount of urine from which it was extracted is not 
 given. The urine of ruminants contains less aromatic sulphur 
 compounds than that of the horse, and more inorganic sulphur ; 
 as in the horse, the phosphates are either absent or occur only 
 in small amounts. 
 
 On a diet of 8-45 kilogrammes of meadow hay, the urine had 
 the following composition : 
 
 Quantity - 7-57 kilogrammes. 
 
 Specific gravity 1-042 
 
 Hippuric acid 9 8 '° grammes. 
 
 Urea - 131-0 
 
 Ash 350-0 
 
 * Ellenberger's ' Physiologie.' 
 
-?5 2 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 o 
 
 K 
 
 f» 
 o 
 
 m 
 g 
 
 2 
 
 &. 
 o 
 
 o 
 
 o 
 o 
 
 <D 
 
 »1 o 
 
 -H Z 
 
 .a 
 
 Ph^ o 
 
 00 «"> Ov 
 00 O ■ M 
 
 ro 00 
 
 O o 
 
 K M M IT) N CO O O 
 
 a 
 a 
 
 O 
 
 O O t(- p _"*■ 
 
 w b ob co *o 
 
 me * n n>£i oi "I'D 
 
 lOO lOM t>- 0> O 
 
 bbbbbbbbb 
 
 VO t-^ co *0 00 N "O O IN 
 H H N N 
 
 (ft •£> 
 
 do 
 
 o 
 
 a s 
 
 3 
 O 
 
 o 
 
 + Nin C^ CO 
 
 Oi + -t O « 
 
 o 
 
 00 io M fO O O 
 
 O 
 
 b - 
 
 ro "3" 00 C» 
 
 
 
 <D 
 
 <1> 
 
 
 4) 
 
 fl) 
 
 
 
 <J 
 
 
 
 O 
 
 O 
 
 * * 
 
 * 
 
 a 
 
 c 
 
 ; 
 
 CI 
 
 a 
 
 
 
 3 
 
 3 
 
 
 3 
 
 s 
 
 
 
 O 
 
 O 
 
 
 o 
 
 O 
 
 •+ t- 
 
 vr> 
 
 N 
 
 N 
 
 o 
 
 Ov 
 
 Oi 
 
 o m 
 
 pj 
 
 H 
 
 H 
 
 M 
 
 N 
 
 u 
 
 b b 
 
 o 
 
 H 
 
 
 
 o 
 
 H 
 
 o 
 
 01 
 
 t 
 
 1 
 
 1 
 
 
 
 
 o 
 
 
 
 
 
 
 
 a 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 b 
 
 
 
 
 
 
 
 O O Tf O "O JO 
 
 *b ro oo <o n *b 
 
 in to t> •*• oo in m 
 b in h ro « 'ro N 
 
 ■t; 
 
 £ o g 
 
 +J ss — 
 
 3 o cd rt »-i 
 
 a en H O tS P < 
 
THE URINE 353 
 
 Calves still suckling excrete an acid urine containing uric 
 acid, allantoine, and creatinine. It is poor in urea, and, according 
 to Moeller, contains hardly i per cent, of solids. 
 
 The Urine of the Sheep. 
 This has an alkaline reaction, a specific gravity of i-oo6 to 
 1-015, and the amount excreted in twenty-four hours varies 
 from 0-3 to 0-9 litre (0-5 pint to 1-5 pints). Tereg gives the 
 following percentage composition of a sample : 
 
 Water - . 86-48 
 
 Organic matter 7-96 
 
 Inorganic matter 5-56 
 
 The organic matter contained — 
 Urea 2-21 
 
 Hippuric acid 3-24 
 
 Ammonia - 0-02 
 
 Other organic substances 2-07 
 Carbonic acid - 0-42 
 
 7-96 
 
 The inorganic matter contained — 
 
 Chlorine - 1-05 
 
 Potassium chloride 1-84 
 
 Potassium 2-08 
 
 Calcium 0-07 
 
 Magnesium - 0-20 
 
 Phosphoric acid o-oi 
 
 Sulphuric acid 0-24 
 
 Silica - 0-07 
 
 I 5-56 
 
 It will be observed from the table how rich the urine of the 
 
 sheep is in hippuric acid. In the urine of sheep there is very 
 
 much more magnesia than lime, consequently the reverse obtains 
 
 in the faeces of this animal. 
 
 The Urine of the Pig. 
 
 This resembles that of carnivora, but its composition depends 
 on the character of the food. The specific gravity is roo3 to 
 1025. It is either acid or alkaline ; it contains uric acid, hippuric 
 acid, xanthine, guanine, and much urea; the amount excreted 
 varies between 1-5 to 8 litres (2! to 14 pints). 
 
 In the following analysis of the urine the diet consisted of 
 peas, potatoes, and sour milk: 
 
 Total urine 4-1 kilogrammes (7 pints). 
 
 Specific gravity 1-018 . . ^ 
 
 Dry substance 2-768 per cent. 
 
 Total nitrogen 0-604 
 
 Ammonia 0-024 
 
 Ash- - r - 234 
 
 The ash consists largely of phosphates and potassium salts, 
 a moderate amount of magnesium, and very little sodium or 
 
 The Urine of the Dog. 
 
 It is impossible to give the composition of the urine of the 
 
 dog, as the amounts of constituents secreted vary considerably 
 
 according to the nature of the diet. 
 
 23 
 
354 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The urine is acid in reaction on a flesh diet, the acidity being 
 due to acid phosphate of soda; on a vegetable diet it may be 
 alkaline. The amount excreted is from 0-5 to i litre (f pint to 
 if pints) daily, but varies with the size of the animal and the 
 nature of the diet ; the specific gravity is from 1016 to ro6o, 
 dependent on the diet ; the colour is pale yellow to straw-yellow ; 
 the urea varies from 4 per cent, to 6 or 10 per cent. On an 
 animal diet uric acid (allantoine; see p. 340) is excreted, but dis- 
 appears on vegetable food being given. Certain breeds of dogs 
 • — Dalmatians- — are stated to have a high uric acid elimination; 
 hippuric acid in small quantities appears with fair regularity; 
 sulpho-cyanic acid is present, especially after a diet rich in 
 purins; indican and phosphoric acid are well-marked constit- 
 uents. Glycuronic acid may be found; it is an oxidation 
 product of glucose, and exists in traces, but after the adminis- 
 tration of camphor or chloral it is obtained in well-marked 
 quantities. It is a point of practical importance to avoid 
 regarding a urine which reduces salts of copper as necessarily 
 containing sugar, for glycuronic acid is a reducing substance. 
 The presence of bilirubin in the urine of the dog has been noted 
 by Salkowski. 
 
 As an illustration of the variation of the dog's urine, depen- 
 dently on the nature of the diet, we may take an example from 
 a long series of experiments by Bischoff and Voit. 
 
 On a diet consisting of meat 0-57 pound, starch 071 pound, 
 salts 77-5 grains, a specimen of urine gave the following compo- 
 sition : 
 
 Amount 252 c.c. 0-44 pint. 
 
 Specific gravity — 1-049 
 
 Urea 21-0 grammes 326-6 grains. 
 
 Salts - 5-53 ,, 85-6 „ 
 
 On a diet consisting of meat 275 pounds and fat 0-55 pound, 
 the following was the composition : 
 
 Amount 702 c.c. 1-23 pints. 
 
 Specific gravity ■ — 1*054 
 
 Urea 80-7 grammes 1,351 grains. 
 
 Salts I2-2I ,, 189 
 
 The Discharge of Urine. — The urine is constantly being 
 secreted, and it either trickles or is propelled by rhythmic mus- 
 cular contractions down the ureters to the bladder. It is quite 
 likely that both movements take place, depending upon the 
 condition of bladder distension ; whereas ' trickling ' is suitable 
 for an empty bladder, some muscular effort on the part of the 
 ureters will be required when the bladder is full. Either drop 
 by drop or by ' spirts ' the urine enters the bladder, which 
 gradually advances in the pelvis, and rises in the direction of the 
 
THE URINE 335 
 
 sacrum. All reflux of urine into the ureters is prevented by the 
 oblique manner in which the coats of the bladder are pierced, so 
 that the greater the internal pressure the tighter are the ureters 
 closed. If circumstances prevent the evacuation of the bladder 
 contents, the organ gradually advances to the brim of the pelvis, 
 and then impinges on the abdominal cavity ; in a state of extreme 
 distension it may project for some distance into the cavity, the 
 weight of the fluid having a tendency to cause the organ to incline 
 towards the floor of the abdomen. In the chapter on the Muscular 
 System attention will be directed to the fact that the muscle of 
 the bladder may alter its length, so as to admit of increased dis- 
 tension of the organ without any increase in the tension of its 
 walls (see p. 437). 
 
 The entrance to the urethra is controlled by a circular layer 
 of unstriped muscle, part of the bladder muscle, but outside 
 this is a band of voluntary muscle, which must be regarded as 
 part of the urethra. Physiologists are not agreed as to the 
 mechanism involved in the act of micturition. Ordinarily it is 
 a voluntary act, but the dog, its spinal cord having been divided 
 in the lumbar region, will carry out the process perfectly, though 
 there can be no question of consciousness involved. In this case 
 the afferent impulses conducted to the cord by the second and 
 third sacral nerves stimulate a vesical centre in the grey matter 
 of the sacral cord. There can be no doubt that, under ordinary 
 circumstances, the act is a voluntary one. The efferent nerves 
 supplying the bladder are derived from two sources — viz., 
 directly from the sacral spinal nerves, and, secondly, through 
 the sympathetic. The sacral nerves furnish the nervus erigens, 
 which is connected with the hypogastric plexus, while the sym- 
 pathetic supply is furnished by the second to the sixth lumbar 
 ganglia ; these pass to the inferior mesenteric ganglion, and issue 
 from it as the hypogastric nerves. Stimulation of the nervus 
 erigens causes relaxation of the sphincter and contraction of the 
 wall of the bladder, while the hypogastric nerves, though relaxing 
 the wall of the bladder, contract the sphincter. The latter action 
 is antagonistic to that of the spinal nerve supply. It is almost 
 certain that the most important supply to the bladder is the 
 nervus erigens. 
 
 At the moment the bladder wall begins to contract, it is 
 assisted by the abdominal muscles and a fixed diaphragm. The 
 flow is never so powsrful in the female as in the male, the final 
 expulsion of the last drops from the urethra of the latter being 
 effected by the rhythmical contraction of the perineal muscles 
 and by the accelerator urince. During the act both the horse and 
 mare stand with the hind-legs extended and apart, resting on the 
 toes of both hind feet, thereby sinking the posterior part of the 
 
356 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 body. The male animal also often advances the fore-legs in 
 order to avoid their being splashed; in this position the penis 
 is protruded, and the tail raised and quivering. The streams 
 which flow from the two sexes are very different in size, depending 
 on the relative diameters of the urethral canal. The mare after 
 urinating spasmodically erects the clitoris, the cause of which is 
 difficult to see; it may be due to the passage of a hot alkaline 
 fluid over a remarkably sensitive surface. The horse can, under 
 ordinary circumstances, pass urine only when standing still, 
 though both sexes can defalcate while trotting ; but in a con- 
 dition of cestrum the mare can empty her bladder while cantering. 
 In the ox the urine simply dribbles away, owing to the curved 
 character of the urethral canal, and is directed towards the 
 ground by the tuft of hair found on the extremity of the sheath. 
 The ox can pass his urine while walking. , The cow arches her 
 back to urinate, but instead of extending her hind-limbs, as does 
 the mare, she brings them under her. body, at the same time 
 raising the tail. 
 
 The upright position is essential to micturition; no horse of 
 either sex can evacuate the bladder while lying down — a point 
 of extreme importance in practice.* Further, it will be remem- 
 bered that the fundus of an over-distended bladder hangs into 
 the abdominal, cavity, and is thus on a lower level than the 
 urethra; this contributes to the difficulty of emptying an over- 
 distended organ. As a horse cannot micturate at work, it is 
 obvious that opportunity for this should be regularly afforded, 
 or much suffering must result. 
 
 Pathological. 
 
 There is scarcely any organ of the horse's body so free from disease 
 as the kidneys. The material in the pelvis of that organ, which looks 
 like pus, and is frequently so described, is really the natural mucus 
 of the urine, mixed with insoluble lime salts. We have never found 
 sugar in the horse's urine; it is, however, stated that sugar is present 
 in the urine of animals after chloroform or ether anaesthesia. Protein 
 is not uncommon, but only as the result of inflammatory affection 
 of the lungs and pleura. 
 
 Vesical calculus would be one of the most common diseases of 
 herbivora were it not that they excrete the insoluble salts at 
 each evacuation of the bladder and that calcium carbonate, their 
 invariable component, has but little tendency to cohere. Never- 
 theless, calculus of the kidney and bladder, and "the so-called sabulous 
 deposits in the bladder, are by no means unknown both in the horse 
 and ox. In sheep the sabulous matter may obstruct the vermiform 
 appendix of the penis. In the dog uric acid is the most common con- 
 stituent of vesical calculus. 
 
 * At the moment of violent death from destruction of the brain, the 
 urine dribbles away while the horse is lying on its side. 
 
CHAPTER XI 
 
 NUTRITION 
 
 The nutrition of the domesticated animals, especially of those 
 intended for food, has, owing to its economic bearing, been 
 studied with the greatest care for many years past. There can 
 be no doubt that far more is known of their nutrition than of 
 that of man, the reason for which is obvious. Investigations of 
 this character are extremely laborious, and require the united 
 labours of the chemist, physiologist, and stock-feeder; in the 
 first instance they were made with the object of ascertaining the 
 nutrition of animals intended for the purpose of food, but in 
 more recent years they have included inquiries into the nutrition 
 of working animals. The subject is still far from exhausted; 
 many problems await solution, especially those dealing with the 
 chemical changes by which the elements of food are built up into 
 the body, and later pulled down for its utilisation. In many 
 cases only the beginning and the end of these processes are 
 known with anything approaching exactitude, and it is probable 
 that we shall never know how the dead food substances become 
 living tissue. 
 
 Composition of the Body. — -The animal body consists of 
 proteins, fats, salts, and water, together with a small proportion 
 of carbohydrates. The proteins are nitrogenous substances of 
 extraordinary complexity and variety, and it is only within 
 recent years that any light has been thrown on the nature of the 
 basic substance, protein. Its molecular weight affords a good 
 idea of its complexity. The molecular weight of egg albumin 
 is given as 5,378, of the globin of haemoglobin at 5,000 to 8,000, 
 while that of the globulin, edestin, from hemp seed, has been 
 stated to be 14,500. The feature which distinguishes the proteins 
 is that they all contain nitrogen in addition to carbon, hydrogen, 
 and oxygen. The fats contain carbon, hydrogen, and oxygen 
 only; the carbohydrates, represented in the vegetable world by 
 starch, sugar, etc., contain the same elements as fat, but in 
 different proportions; the salts of the body are in all the tissues 
 
 357 
 
358 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 and secretions, and the water is everywhere. The water in the 
 body predominates in respect of weight over the other consti- 
 tuents, the weight of the salts being the least, if the small and 
 variable amount of carbohydrate be excluded. The proportion 
 of fat is always greater than that of protein, though the condition 
 of the animal decides what this amounts to. 
 
 The following table from Lawes and Gilbert shows the per- 
 centage proportions of these various tissues in oxen, sheep, and 
 pigs in lean and fat condition. The figures given are those 
 recalculated by Armsby* on the basis of ' empty weight 'f— 
 i.e., the live weight minus the contents of the digestive canal. 
 
 
 Oxen. 
 
 Sheep. 
 
 Pigs. 
 
 Half Fat. 
 
 5 6-i 
 1 8-4 
 
 20-8 
 
 5-° 
 
 Fat. 
 
 Lean. 
 
 6i-o 
 
 i 5 -8 
 
 19-9 
 
 3-4 
 
 Fat. 
 
 Fat. 
 
 Lean. 
 
 Water 
 Protein- 
 Fat 
 Ash 
 
 4 8- 4 
 
 15-4 
 32-0 
 
 4-2 
 
 46-2 
 
 I3'0 
 
 37-8 
 
 3-0 
 
 58-1 
 
 14-6 
 
 24-6 
 
 2-8 
 
 43-0 
 u-4 
 
 43-9 
 17 
 
 Half of the water and half the protein found in the body are 
 contained in the muscles, which represent about 45 or 50 per 
 Cent, of the body weight. 
 
 Income and Expenditure. — The Income of the body consists 
 of carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, 
 salts, and water. These are contained in the protein, fat, 
 carbohydrate, and salts of the food ; the oxygen is mainly supplied 
 by the air taken in at the lungs. 
 
 The Expenditure of the body made up of the same elements, 
 consists of what is removed by the lungs, urine, faeces, and skin. 
 The Nitrogen is excreted almost wholly by the urine in the form 
 of urea, hippuric acid, and other nitrogen-containing bodies. 
 It is usual to regard the urinary nitrogen as a measure of the 
 protein changes in the body, though it is obvious that the sweat, 
 hair, and hoofs are also channels through which nitrogen is lost. 
 The Carbon is excreted as carbonic acid by the lungs and in the 
 various carbon compounds in the urine . The Hydrogen is removed 
 as water by the lungs, skin, and urine, and in the herbivora a 
 portion, as marsh and other gases, escapes by the respiratory 
 
 * ' The Nutrition of Farm Animals.' H. P. Armsby, Ph.D., LL.D., 
 New York, 1917. To this excellent work I am greatly indebted in the 
 preparation of this chapter. It contains an account of the classical 
 experiments in animal nutrition, and of all recent advances. 
 
 f The empty weight in fat cattle is 91 per cent, of the live weight; in 
 half-fat cattle, 89 per cent, to 90 per cent.; and in thin cattle, 87 per cent, 
 to 89 per cent. 
 
NUTRITION 
 
 359 
 
 passage and bowels. The Salts are excreted by the kidneys, skin, 
 and glandular secretions. Among these channels of excretion 
 we have not referred to the digestive tract. Broadly speaking, 
 this may be regarded as being outside the body, and the 
 material it contains as the unutilised residue which has formed 
 no part of the system. This statement, however, is not strictly 
 accurate, as there are excretions from the system into the 
 digestive tract ; especially is this true in the case of the salts. 
 
 Tables have been prepared showing the income and expendi- 
 ture of the body for both animals and man. They represent 
 laborious experiments; everything passing into and out of the 
 body has to be weighed and submitted to analysis to ascertain 
 the nature and amount of the chemical elements concerned. 
 
 Fig. 107. — Steer in Digestion Stall (Bailey's ' Cyclopasdia of 
 American Agriculture'). 
 
 The earliest method by which these data were arrived at was the 
 simple one of collecting the urine and faeces of an animal living 
 on a known diet, and determining the differences between the 
 amounts of the elements in the food and in the excreta. This 
 method was imperfect, for it neglected the excretions of the 
 respiratory track and of the skin, but, nevertheless, it was a 
 valuable means of inquiry, and is still employed in determining 
 the digestibility of foods. The apparatus now used is shown 
 in Fig. 107. In order to determine the income and expenditure 
 
360 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 by the respiratory passage, as well as the loss by the digestive 
 canal and other sources, a Respiration Chamber is employed in 
 which the animal under observation lives. Of these there are 
 two types— in one the chamber is sealed, the consumed oxygen 
 being replaced as required; in the other the chamber is open, the 
 entering air being measured. In both types the gases produced 
 are analysed and the constitution of them ascertained. The 
 composition of the food is known, and the excreta are analysed 
 in order to ascertain the chemical composition. In this way 
 the amounts of the income and expenditure of the body are 
 ascertained. There is probably no class of experiment so tedious 
 as this. The work is continuous for twenty-four or forty-eight 
 hours, and a very small error in the technique renders the whole 
 observation useless. In the latest form of respiration chamber 
 appliances have been introduced to enable a definite amount of 
 work to be performed, and in this way the metabolism of the 
 body at work as well as at rest, can be ascertained (see Fig. 108). 
 The most advanced method of inquiry is by means of the Respira- 
 tion Calorimeter, in which not only the above facts can be ascer- 
 tained, but also the heat given out by the body and the use the 
 body makes of the material supplied as food. Such an appliance 
 for large animals has been erected at the Institute of Animal 
 Nutrition of the Pennsylvania State College (see Figs. 109, no). 
 It is extremely costly, and so elaborate that it takes seven men 
 to work it, exclusive of those in attendance on the animal. 
 
 Metabolism. — By this term are understood the changes 
 occurring in living tissues. It is evident, from what has been 
 said, that constant breaking down and building up is taking 
 place in the body. Every muscular contraction, every respira- 
 tion, the beating of the heart, and the movements of the bowels, 
 all mean wear and tear, and as a part is destroyed it must be 
 replaced. The process of construction is known as anabolism, 
 and of destruction as katabolism. In a perfect state of health 
 these should be in equilibrium. Both repair and destruction 
 are dependent upon definite chemical changes occurring in the 
 system, of some of which we have a fair knowledge, while others 
 are obscure. 
 
 The metabolism of the tissues is apparently under the influ- 
 ence of the nervous system. We have previously studied a good 
 example of this in the secretory nerves of the submaxillary 
 gland, and it is probable, though our information on the point 
 is very defective, that the nutrition of the body is largely directed 
 by the nervous system. In some forms of lameness and injury 
 in the horse we constantly observe muscular wasting, which is 
 out of all proportion to the atrophy a part suffers by being 
 simply thrown out of use, and which can only be explained by 
 
NUTRITION 
 
 361 
 
NUTRITION 
 
 363 
 
 o ^ 
 
 < -3 
 
364 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 injury to the trophic nerves which regulate the nutrition of the 
 part. Even a better example is afforded by the peculiar changes 
 which sometimes follow direct injury to trophic nerves, as in 
 plantar neurectomy of the horse. The sloughing of the entire 
 foot, or gelatinous degeneration of the phalanx, is due to injury 
 of the trophic nerves. Injuries to the fifth pair of nerves have 
 been followed by sloughing of the cornea, and pneumonia has 
 followed division of the vagi, the sequelae in both cases being 
 possibly due to the loss of trophic influence. 
 
 Disordered nutrition of a tissue may show itself without any 
 obvious injury to trophic nerves — as, for example, in the phe- 
 nomenon known as inflammation, or in the well-known sympathy 
 existing between the digestive system of the horse and the 
 laminse of the feet. Further evidence of nervous action is 
 afforded by nutrition which is normal in character, such as the 
 change of the coat with the season of the year. The influence 
 of light on metabolism is also probably conveyed through the 
 nervous system. In making these statements we must guard 
 against the error of considering that no growth, repair, or repro- 
 duction can take place excepting under the influence of the 
 nervous system. The trophic influence exercised by nerves 
 appears to be directed to maintaining in equilibrium the processes 
 of building up and breaking down which are occurring in all 
 tissues. Though the metabolism of the body is largely regulated 
 by the nervous system, yet the process cannot be carried out 
 without food; metabolism goes on during .starvation, for even 
 then food is being supplied, inasmuch as the animal is living on 
 its own tissues. The influence of light on metabolism, alluded 
 to above, is well known in the fattening of cattle. The explana- 
 tion is that more carbon dioxide is produced in the light than 
 in darkness, and this indicates increased metabolism. By re- 
 ducing metabolism fat is stored up. Dark stables for horses 
 favour the putting on of fat, which is the antithesis of physical 
 fitness. 
 
 Food must contain the substances required by the body — 
 viz., water, protein, fat (or carbohydrate), and salts in the car- 
 nivora, and the same principles together with carbohydrate inthe 
 herbivora. In 'both ca ; ses an essential constituent of a normal 
 diet are the Viiamines. • Each of these proximate principles must 
 be in a proper proportion, neither below nor in excess of the 
 animal's requirements. Each must be present; nothing can 
 replace protein nor salts;. a. carbohydrate-free diet in the her-. j 
 bivora would be fatal ; a water-free diet in all animals sustains life 
 for a shorter period than does water when there is an entire 
 absence of food ; and no diet, no matter how nutritious or diges- 
 tible, sustains life if vitamines be absent. We have, therefore, to 
 
NUTRITION 365 
 
 inquire why it is each of these substances is essential in every 
 diet, and how each behaves in the system. 
 
 Nitrogenous Equilibrium. — The storing up of protein is an 
 expression which has repeatedly occurred in the previous chapters 
 and the impression may have been created that of the total 
 protein in a diet some is daily stored up, so that less leaves the 
 body than entered it. There can be'no doubt that some of the 
 protein of the daily diet is stored up, but the greater part is not. 
 There is no more familiar fact than that the body may continue 
 from month to month at much the same weight, and that could 
 never be the case if storing up occurred daily. What occurs is 
 this, that whatever total amount of nitrogen is received daily 
 in the food, a similar amount is recovered daily from the excreta. 
 Whatever portion of the protein of the food is converted into 
 tissue protein, an equivalent amount of the latter is displaced, 
 and appears in the urine as urea, hippuric acid, and other nitrog- 
 enous products; thus the total excretion of nitrogen equals the 
 total receipts. A body in this condition is in nitrogenous equi- 
 librium. It does not follow from this that it is not increasing in 
 weight; it may be, owing to excess of non-nitrogenous food; on 
 the contrary, it may be losing weight, due to a deficiency of non- 
 nitrogenous material. The matter is of no moment at present, 
 the main point being that in the urine an amount of nitrogen 
 is excreted equivalent to that absorbed into the system from the 
 food, and that such an animal is in nitrogenous equilibrium . 
 
 It is obvious that nitrogenous equilibrium cannot exist in young 
 growing animals, nor in those in impoverished condition. In 
 both these cases nitrogen is being stored up, as the tissue re- 
 quirements are considerable, and therefore under these circum- 
 stances any attempt to establish nitrogenous equilibrium ex- 
 perimentally would fail. On the other hand, an adult animal 
 in good condition gives off daily by the excreta as much nitrogen 
 as it receives in the diet. If the ingoing nitrogen is increased, 
 the outcoming nitrogen is increased; if the ingoing nitrogen is 
 decreased, the outcoming nitrogen is decreased, and this is true 
 within physiological limits. 
 
 It is evident, therefore, that in the same animal there may be 
 different levels of nitrogenous equilibrium, and that these are 
 dependent entirely on the income. The more the body receives, 
 the more it spends; the less it receives, the less it spends, the 
 important point being that the system is determined to make 
 its expenditure of nitrogen equal its income. This being the 
 case, how does the body behave when it receives no nitrogen ? 
 The previously improvident expenditure of nitrogen is now 
 rectified. None is being received by the mouth, and the store 
 in the tissues is therefore most economically expended. 
 
366 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 If an animal experimentally subjected to fasting has previously 
 been living on a rich nitrogenous diet, the fall in nitrogen excre- 
 tion is not at first very obvious, but when this excess has been 
 eliminated the output of nitrogen suddenly falls and remains at 
 a minimum, there being a temporary rise as death approaches. 
 During this experiment the animal has not been living entirely 
 on its own protein tissues, but at first on the carbohydrate, 
 which is rapidly exhausted, and then on the fat stored up. 
 When the fat has been exhausted a heavier draft is made on the 
 protein, which explains the temporary rise in the nitrogen output 
 before death occurs. 
 
 The minimum nitrogen excreted during fasting is regarded 
 as that which is essential to maintain the vital processes of the 
 body, such as respiration, circulation, digestion, and, in the case 
 of horses, the effort of standing. 
 
 A starving animal may be kept alive a longer time by being 
 given some carbohydrate and fat. It will die because it is not 
 receiving nitrogen, but the mixed carbohydrate and fat will effect 
 economies in the store of body protein, and cause it to last longer. 
 In this respect the carbohydrate is of much more value than 
 the fat. 
 
 It was reasonable to suppose, as Liebig did, that the protein 
 substances of the food, described by him as ' flesh formers, ' carried 
 out the mechanical work of the body. We now know better, for 
 we have learned from the facts connected with nitrogenous 
 equilibrium that the body continues to work satisfactorily though 
 the equilibrium is established at different levels. Nevertheless, 
 nitrogenous equilibrium, which we have seen possesses charac- 
 teristic peculiarities, is affected by excessive work in man and 
 animals, though it is correct to say that within normal limits 
 it is unaffected by work. 
 
 The views laid down by Liebig were never seriously disputed 
 until Lawes and Gilbert had completed their classical work, 
 extending over twenty years, on animal nutrition at Rotham- 
 stead. During the years 1849-1859 these observers gave their 
 results to the world, but it was a long time before their views 
 were accepted. Though working at the question of the fattening 
 of animals, and not at the question of energy, they were the first 
 to announce that the value of a food for feeding purposes should 
 not be calculated on the basis of the so-called flesh-forming ' 
 substances it contained, but of the non-nitrogenous elements. 
 We can now understand why it is that nitrogenous equilibrium 
 experimentally established is produced on a smaller amount 
 of protein when non-protein substances enter into the diet. No 
 animals, not even the carnivora, live entirely on protein. The 
 effect of the non-protein portion is to reduce the amount of 
 
NUTRITION 
 
 367 
 
 protein destroyed, and to establish equilibrium at a lower level. 
 Fat, starch, and sugar are protein economisers, while, conversely, 
 a large protein diet is a fat obliterator, and this fact was turned 
 to account by Banting in his treatment of obesity. 
 
 If a fasting animal be brought on to a protein diet equivalent 
 to the amount of nitrogen being daily excreted.it would be reason- 
 able to suppose that nitrogenous equilibrium would at once be 
 established. Experimental inquiry shows this is not so, but 
 that the effect of giving more nitrogen is to cause a greater 
 excretion of this substance. It is not, in fact, until an amount 
 of protein is given equivalent to at least three. times the daily 
 excretion during fasting that equilibrium is established. The 
 explanation of this will now be given. 
 
 The Non-Nitrogenous Residue. — When the whole of the 
 nitrogen has been split off from protein in the manner above 
 described, there remains a residue which is formed in the following 
 manner : 
 
 
 Carbon. 
 
 Hydrogen. 
 
 Oxygen. 
 
 Nitrogen. 
 
 100 grammes protein con- 
 tain 
 
 34-29 grammes urea (from 
 100 grammes protein) 
 contain 
 
 53''>o 
 6-86 
 
 7'00 
 2-29 
 
 24-00 
 9-14 
 
 16 
 16 
 
 Difference 
 
 46-14 
 
 4-71 
 
 14-86 
 
 — 
 
 This residue contains more than half of the hydrogen and 
 oxygen of the protein, and the greater part of the carbon which 
 has not been employed in the building up of urea, and it is from 
 this portion of the protein molecule that glycogen, and perhaps 
 fat, may be formed in the body. This explains the difficulty in 
 obtaining nitrogenous equilibrium on a purely protein diet, and 
 why more and more protein is required for the purpose. The 
 animal is living on the non-nitrogenous residue, and the reason 
 why it goes on consuming more and more protein is in order to 
 obtain the necessary amount of carbon. This fact explains the 
 extravagant nature of purely protein diets. It enables us to 
 understand the loss in body weight which may be occurring even 
 when nitrogenous equilibrium has been established, and to grasp 
 a fundamental fact in feeding, that carbohydrate or fat added to 
 such a diet enables equilibrium to be established on a lower plane 
 in consequence of the economical effect it exercises on protein. 
 The economising effect of fat or carbohydrate on protein is one 
 of the well-established facts in dietetics. 
 
 Protein is not a single chemical substance, but a compound of 
 
368 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 several amino-acids, each of which has its function in the body. 
 Gelatin, on which an animal will die, is deficient in tyrosine and 
 tryptophan ; the zein of maize lacks lysine, glycine, and trypto- 
 phan, and maize is therefore worthless for growth or as a main- 
 tenance diet when employed alone ; the gliadin of wheat lacks 
 lysine, and while wheat can maintain life it cannot promote 
 growth. Yet both maize and wheat are food substances of the 
 highest value, for the reason that they contain several proteins, 
 and not a single protein, the deficiencies in one being balanced 
 by excess in another. Recent researches, however, show that 
 the absence of certain vitamines may be of far more importance 
 than the absence of particular amino-acids. 
 
 Nitrogenous substances other than protein exist in both 
 animal and vegetable foods; in some as much as one-third or 
 one-half of the total nitrogen is not protein; for example, the 
 nitrogenous glucosides, alkaloids, amino-acids, amids, nitrates, 
 and ammonium salts of plants are obviously not protein. It is 
 believed that in ruminants these bodies can be utilised in 
 the first stomach in nourishing the micro-organisms which act on 
 carbohydrates, and which by so doing economise the true protein 
 of the food. It has even been thought that the protein of the 
 micro-organisms may be utilised by the system. 
 
 The Metabolism of Protein. — When describing the digestion 
 of protein substances in the digestive canal, sufficient was said 
 to indicate the big gaps which exist in our knowledge of this 
 question. Still greater ignorance exists of the subject now to be 
 considered — viz., the behaviour and disposal of the protein 
 matters after they have entered the system through the portal 
 vein. The protein substances, once in the blood, are utilised in 
 the repair and restoration of the tissues, but a study of nitrog- 
 enous metabolism has shown that under ordinary circumstances 
 the extent of this repair is probably not considerable, and it 
 certainly has proved that whatever portion of the protein is so 
 used must turn out a similar amount of degraded material from 
 the tissues, or the outgo of nitrogen would be less than the income. 
 What is the probable behaviour of protein in the body until the 
 stage of urea is reached ? 
 
 Pfluger taught that the whole of the absorbed material must 
 first be converted into protein before any destruction of it can 
 occur ; in other words, that there is no route to urea excepting 
 through the disintegration of the living cell. Voit held that the 
 protein, when absorbed, is divided into two portions' — one, the 
 smaller, repairs wear and tear in the body, and he spoke of it as 
 tissue protein ; the other circulates with the blood and lymph, 
 and bathes the body cells, but does not form part of them. This 
 protein is destroyed by the tissues with the liberation of heat and 
 
NUTRITION 369 
 
 the formation of nitrogenous end-products, which escape by the 
 urine. This portion Voit described as the circulating protein. 
 
 No positive statements can be made regarding the fate of 
 protein in the body, as only the beginning and end of the process 
 are known with anything like exactitude, but it is not unlikely 
 that the story is as follows : The complex protein molecule in the 
 digestive canal is hydrolysed by the digestive ferments, and split 
 up into the simple amino-acids of lower molecular weight than 
 the protein from which they have been derived. These amino- 
 acids, of which there may be a large number in each protein, are 
 what have been described as the ' building stones ' of the body. 
 They represent the material to be used in the repair of the tissues, 
 and they gain access to the blood through the vessels in the intes- 
 tinal walls. Until recently it was thought that before they could 
 enter the blood they had to be reconverted to protein by the 
 epithelium of the intestine, as no amino-acids could be traced 
 in the blood. Recent inquiry has shown that amino-acids can 
 be found in the blood, so that we may assume the protein 
 material reaches its destination in the form of amino-acids, or as 
 a combination of amino-acids known as peptids. In the tissues 
 the process of reconstruction begins; the amino-acids are now 
 synthesised under the influence of tissue enzymes, and by a 
 reverse process to that which took place in the digestive canal 
 they pass from simple bodies to the complex compound known 
 as protein. Be it an animal or vegetable protein thus synthe- 
 sised, its original likeness is lost in the tissues, and it becomes an 
 animal protein of the specific pattern required by the particular 
 part of the body into which it is built. The part played by 
 intracellular enzymes in the production of this synthesis is no 
 doubt of capital importance. It is possible that, by a reversed 
 action, such as some enzymes have been proved to possess, 
 the intracellular enzyme which builds up the cell is the same 
 which carries out its katabolism. Urea may have its origin 
 either from the destroyed cell or from the material sent to repair 
 it; the latter view is generally favoured. All of the absorbed 
 amino-acids are not synthesised, some are deaminised without 
 playing a part in protein building, the ammonia being con- 
 verted into urea and the non-nitrogenous portion used for 
 energy production. 
 
 The above refers to the changes undergone by the simple pro- 
 teins in the body, but there is another group represented in both 
 animal arid vegetable tissues, known as the conjugated or nucleo- 
 proteins, where, among the end-products of digestion, are purin 
 and pyrimidine bases, pentoses and phosphoric acid; these pass 
 into the blood. The interest lies in the bases; purin yields uric 
 acid, or allantoine, depending on the species of animal (see p. 339), 
 
 24 
 
370 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 while the end-products of pyrimidine are unknown. Neither 
 base is wholly katabolised, for a portion of each appears in the 
 urine. Urea is likewise formed from nucleo-proteins, a portion 
 of the molecule, the NH 2 group, being split off as ammonia, as 
 previously described. 
 
 It is believed that the building up of body tissues containing 
 nucleic acid does not necessarily take place from the nucleo- 
 proteins of the food, for it has been found experimentally that 
 if purin bases be administered they are excreted. Further, the 
 nucleo-proteins of the body may be repaired in the entire 
 absence of this substance from the food. Evidently, then, the 
 body cell can produce nucleo-proteins by synthesis, and in con- 
 nection with this subj ect it may be added that it has recently been 
 discovered that amino-acids may be formed in the body experi- 
 mentally from ketonic or hydroxy-acids and ammonium salts. 
 
 The fates of the sulphur of proteins and of phosphorus in the 
 nucleo-proteins have yet to be traced, but are best considered in 
 connection with the general question of the metabolism of the 
 salts of the body. 
 
 The proteins are most expensive food-stuffs, and dietary 
 economists have repeatedly drawn attention to the unneces- 
 sarily nitrogenous, and therefore wasteful, nature of many diets 
 for animals. Especially is this the case in the diets given to 
 animals being fattened for the purposes of show. Only a small 
 proportion of the nitrogen in these diets is stored up; the greater 
 part passes away with the excreta, and finds its way on to the 
 land. Precise experiments on men have shown that they can 
 be kept in health for months on a diet far poorer in protein 
 than that generally accepted as necessary, and the same finding, 
 within limits, holds good for animals. On the question of food 
 nitrogen many physiologists come into conflict with practical 
 experience. 
 
 Theory says the quantity of nitrogen required is largely inde- 
 pendent of the amount of muscular work performed; practice 
 says the harder the machine is worked the more nitrogen must 
 be given. Theory says proteins are not the source of muscular 
 energy, the source being in the non- nitrogenous food; practice 
 replies that may be so, but experience shows that the harder the 
 work performed by an animal, the more richly nitrogenous must 
 the diet be, while the amount of the food given is only to be 
 limited by the appetite. In this matter our personal experience 
 places us entirely on the side of practice and in opposition to 
 theory. Why the hard-worked horse needs more nitrogen we 
 are not prepared to explain. The suggestion that the machine 
 works more easily and smoothly on a liberal nitrogenous diet, 
 which stimulates metabolism, and so leads to increased oxida- 
 
NUTRITION 371 
 
 tion, does not bring us much nearer to a solution of the problem. 
 The fact remains that, whatever may be the energy obtainable 
 from starch and fat, this energy is in some unknown way directed 
 by protein. All nitrogen over and above that required for repair 
 must not be considered a wasteful or luxur> consumption. On 
 the other hand, that a wasteful consumption of protein occurs 
 where horses are not fed in accordance with the work they are 
 performing is undoubted. Under these circumstances the excess 
 of nitrogenous material produces clinical disturbances; we are 
 able to recognise these in the liver disorders and diarrhoea of 
 tropical climates, and in the lymphangitis, azoturia, and diarrhoea 
 of temperate latitudes. 
 
 The storing-up of protein occurs in young animals and in 
 working animals so long as the muscles are increasing in bulk. 
 It is necessary to remember that this statement does not exclude 
 the daily repair of the tissues. Whatever the extent of this may 
 be — and it appears in the adult, as we have already seen, to be 
 small, the embodiment of newly arrived protein into the body 
 tissues is associated with an equal output of worn-out material 
 containing the same proportion of nitrogen, so that, though the 
 nitrogen does not vary, the tissues do. 
 
 All true proteins are capable of being used as food, but when 
 albuminoids, such as gelatin, are given, tho\igh they produce the 
 same amount of urea as an assimilable protein, the animal loses 
 flesh. Gelatin was held of high nutritive value in diet, mainly 
 owing to its chemical analysis and its close relationship to true 
 protein. It took years to find out that it was worthless as a tissue 
 builder, and the explanation of this is the absence from the 
 molecule of tyrosine and tryptophan (see p. 368). 
 
 Function of the Nitrogenous Foods. — What we have learnt 
 regarding nitrogenous food may now be summarised : 
 
 1. The body requires nitrogen; no diet is complete, nor can 
 life.be supported, in its absence. 
 
 2. The body having obtained its nitrogen, stores up the small 
 amount required to replace wear and tear, and excretes the whole 
 of the remainder, principally in the urine. 
 
 3. The body requires less nitrogen when fat or carbohydrate 
 forms part of the diet. 
 
 4. Neither herbivora nor omnivora can live on a purely protein 
 diet; carnivora can so live, their supply of carbohydrate being 
 furnished by the non-nitrogenous residue. 
 
 5. The proteins are not the source of muscular energy, yet 
 increased muscular effort must be met by an increased nitrog- 
 enous diet. In some unknown way nitrogen directs the pro- 
 duction of energy in the muscle machine. 
 
 6. Provided the food supply be ample, variations in the amount 
 
37* A MANUAL OF VETERINARY PHYSIOLOGY 
 
 of work performed do not affect the excretion of nitrogen. If, 
 on the other hand, the work be excessive and the body loses 
 weight, the nitrogen in the urine is then increased. 
 
 7. Many animal dietaries are too highly nitrogenous, rarely 
 for horses, commonly for cattle. 
 
 Carbon Equilibrium. — This condition implies that the total 
 carbon leaving the body by the lungs, urine, and skin must equal 
 the total carbon received in the food. If less is excreted, the 
 animal gains weight; if more is excreted, it loses weight. In 
 order to ascertain the amount of this gain or loss the subject is 
 placed in a respiration chamber. The amount of carbon in the 
 food, and the quantity leaving the body by the lungs and excreta, 
 are determined. The carbon in the body is contained in the 
 protein tissues, fat, and glycogen. As the last, under ordinary 
 conditions, remains pretty constant, a gain or loss of carbon is 
 attributable to a gain or loss of protein or of fat. Should it 
 be the protein tissues the fact will be revealed by the amount 
 of nitrogen excreted; if this fails to indicate a change, then the 
 body fat is the source of the gain or loss. 
 
 Non-Nitrogenous Food. 
 
 Metabolism of the Carbohydrates. — Starches, dextrins, gums, 
 and cellulose constitute the carbohydrate food of animals, and 
 of these the most important are the starches and cellulose. The 
 method by which these complex substances are rendered avail- 
 able for the body has been studied in the chapter dealing with 
 Digestion. The carbohydrates are broken down into simpler 
 bodies until the ' building stones ' stage is reached. The build- 
 ing stones which the body can utilise are then selected, and by 
 a process of synthesis they are reconstructed, the molecule pass- 
 ing from the simple to the complex. One of the most character- 
 istic of these reconstructions is glycogen, which is stored up in 
 the liver and muscles. It is obtained from dextrose, the com- 
 paratively simple dextrose being built up into the more complex 
 glycogen. When required by the body the pieces are pulled 
 apart, the glycogen reverting to the simple dextrose, the pro- 
 cesses being enzymic in nature. It is not essential that the 
 liver should alone be concerned in this reconstruction and pull- 
 ing down of glycogen; the same process takes place independ- 
 ently in the muscles. Nor is it essential that the sugar from 
 which glycogen is formed should be derived from starch, as the 
 carnivorous animal has an abundance of glycogen, which in this 
 case is manufactured from the carbohydrate residue of protein, 
 or even, perhaps, from fat. It has been calculated that the 
 total amount of glycogen which the herbivora can store up in the 
 
NUTRITION 373 
 
 liver and muscles is about 4! pounds ; all dextrose assimilated 
 over and above the glycogen storage capacity is disposed of 
 by conversion into fat. This fat is stored up in various depots 
 of the body for future use. 
 
 The final stages of the katabolism of dextrose are carbonic 
 acid, Water and organic acids (lactic and acetic). The inter- 
 mediate stages are unknown. It is thought that the energy 
 yielded by dextrose may be due to the oxidation of the organic 
 acids in the tissues. 
 
 The katabolism of the other carbohydrate substances in food, 
 cellulose and its allies, is effected by destructive fermentation. 
 We have seen that no enzyme is known which can render this 
 substance soluble, but it is attacked by the micro-organisms in 
 the digestive canal. This is a prolonged operation resulting in the 
 ceU wall, but not the tough lignin, being rendered soluble. This 
 fermentation is not of the nature of hydrolysis and cleavage, such 
 as we have seen occurring to protein and dextrose ; it results in 
 the formation of a considerable amount of gas, carbon dioxide, 
 methane, and a little hydrogen, together with organic acids, prin- 
 cipally acetic and butyric. These constitute the sole contribu- 
 tion which cellulose makes to the body (Armsby), but the same 
 observer pertinently asks whether these organic acids may not 
 be absorbed from the digestive canal, and by oxidation yield 
 energy, as happens in the case of dextrose. Such absorption may 
 be effected by the acids combining with the alkalies of the 
 digestive fluids, and the salts so formed being oxidised into car- 
 bonates, which appear in the urine. It is evident that if cellulose 
 is completely destroyed in the digestive canal there can be no 
 absorption, and consequently it is of no value to the tissues. 
 Nevertheless, its solution permits of the escape of the cell 
 contents, and so liberates the food-stuffs. 
 
 There are carbohydrate substances in food, belonging to the 
 gum, pectin, and crude fibre groups, which would appear to 
 escape complete katabolism . They appear in the urine, especially 
 in that of ruminants, and evidently originate from coarse forage, 
 hay, and straw, for their proportions may be increased by adding 
 these articles of diet . The nature of these substances is unknown . 
 The amount in the urine of sheep has been found as high as 
 30 per cent, of the total organic matter present, while in oxen 
 as much as 67 per cent, of the total carbon of the urine has been 
 found to be of non-nitrogenous origin (see p. 345)- 
 
 The carbohydrates are readily oxidised, as their oxygen and 
 hydrogen exist in the proportion to form water. One gramme 
 (151 grains) of carbohydrate requires 0-832 litre (50-8 cub. in.) of 
 oxygen for the oxidation of its carbon, and produces an equiva- 
 lent amount of carbon dioxide. 
 
374 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The function of carbohydrate food is the liberation of energy 
 on oxidation. In the food of the herbivora it is the chief source 
 of energy, as usually so little fat exists in vegetable bodies . Stored 
 up in the muscles and liver, the animal carbohydrate (glycogen) 
 is fired off by the muscles; it is replaced, as exhausted, from the 
 dextrose of the circulating blood, and from the fat reserves of 
 the body. During its oxidation it generates heat; i gramme of 
 dextrose yields 4,100 calories, or 4-1 large calories of heat.* 
 Oxidations are constantly going on throughout the life of the 
 animal, for the body is never at rest. The heart muscle never 
 ceases working, nor the muscles of respiration, nor those which 
 carry along the contents of the digestive canal; glandular and 
 cellular activity is always present; heat, also, is always being 
 lost, and has to be replaced. 
 
 An undue proportion of carbohydrates in a diet ' depresses ' 
 the digestibility of the proteins, especially in ruminants. Starch 
 or sugar added to a diet in excess, as frequently occurs in the 
 feeding of horses with sugar or its by-products, causes less protein 
 to be absorbed. An excess of cellulose, as in hay, straw, tubers, 
 and roots, is believed to have the same effect. It is thought 
 that the depression of digestibility may be more apparent than 
 real, and explained by the carbohydrates increasing the nitro- 
 genous excretory products in the faces, as the result of the extra 
 metabolism they impose on the digestive organs. Under normal 
 conditions the excretory products in the faeces resulting from the 
 digestion of every 100 parts of the dry matter of food, whether 
 the diet be nitrogenous or non-nitrogenous, contain 0-4 part of 
 nitrogen. Whether, therefore, there be less protein absorbed 
 or more used up in metabolism does not affect the general state- 
 ment that a loss of protein occurs when an excess of carbohj'drate 
 is present in a diet. 
 
 An excess of carbohydrates in a diet also depresses the 
 digestibility of the non-nitrogenous ingredients, especially of 
 cellulose. This is very marked in ruminants, and is explained 
 by saying that the micro-organisms responsible for cellulose 
 fermentation turn their attention to the more readily fermentable 
 starch and sugar. This depression of cellulose digestion is con- 
 trolled by the addition of more protein to the diet, the explanation 
 being that in all probability the protein stimulates the multipli- 
 cation of the micro-organisms, and that in consequence a suffi- 
 ciency is present not only for the starch and sugar, but also for 
 the cellulose. In connection with this question, see the remarks 
 on non-protein nitrogen (p. 368). 
 
 * A large calorie is the amount of heat necessary to raise 1 kilogramme 
 (2-2 pounds) of water 1° C. (i-8° F.), and is sometimes named a kilo- 
 calorie. 
 
NUTRITION 375 
 
 Metabolism of the Fats. — -We have seen that in most articles 
 of vegetable food the amount of fat is small, though in some 
 seeds — -cotton, linseed, and such like — -it exists in large amounts. 
 Some of the material existing in vegetable food described in 
 analyses as ' ether extract ' is not necessarily all fat ; waxy and 
 resinous matters are frequently included which are not of the 
 nature of fat. Both in animal and vegetable bodies Lecithins 
 are found; these are substances related to fat, but differ from it 
 in containing nitrogen and phosphorus, though they yield fatty 
 acids on hydrolysis as true fats do. 
 
 The digestion of fat has already been studied; broken up in 
 the intestines into fatty acids and glycerol, it is resynthesised 
 in the intestinal wall under the influence of the ferment lipase, 
 and in a state of emulsion enters the lacteals. On reaching the 
 blood, however, the condition of emulsion ends, and the fat then 
 being soluble, easily reversible, nltrable, and dialysable, in this 
 condition is transported to the tissues. Here it returns to its 
 condition of emulsion by a process not understood, and a body 
 fat, peculiar to each animal, is stored up in the fat depots. The 
 fat of horses is soft, that of sheep hard, the fat of cattle occu- 
 pying a midway position. A dog fed on hard fat converts it into 
 canine fat, which is soft; cattle fed on fluid fats convert them 
 into semi-hard body fats. Experiments, however, show that 
 foreign fats used for feeding may be detected in the tissues. 
 Oil cake and linseed oil may produce an oily milk, and dogs 
 constantly fed on mutton fat may accumulate this type of fat 
 in their tissues ; pigs receiving too large a proportion of fluid fat 
 in the diet accumulate a soft fat, which boils away on cooking. 
 It is generally believed that green food, hay, and carbohydrates 
 produce a hard body fat, while grain foods, such as oats, conduce 
 to a soft fat. It is curious that two animals so closely alike in 
 their digestive arrangements and diet as the ox and sheep should 
 each produce a different fat; the hard, white fat of the sheep is 
 very different from the yellow and softer fat of cattle. The 
 yellow colour is due to carotin, a pigment found in leaves. 
 
 The food-fat is not the only or principal source of fat in the 
 body ; great stores of fat exist in the bodies of animals receiving 
 a trifling amount of fat in their food. The fat is derived from the 
 carbohydrates, and this explains why a cow produces more fat 
 in her milk than she receives in her food. It was Gilbert and 
 Lawes who first demonstrated the production of fat from carbo- 
 hydrate; it is a remarkable physiological synthesis, the seat of 
 which is unknown. 
 
 Fats contain energy in a concentrated form, yielding more 
 per unit than any other food substance; they are oxidised with 
 difficulty, as they possess in their constitution only one-eighth 
 
376 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 of the total oxygen required for the oxidation of their hydrogen 
 to water. One gramme of fat requires 2-8875 litres (176 cub. in.) 
 of oxygen, and produces 1-434 litres (87-5 cub. in.) of carbon 
 dioxide. By its oxidation heat is produced, 1 gramme of fat 
 yielding 9-3 large calories. 
 
 The way in which the fat is taken up from its body reserves 
 for utilisation is unknown. Presumably there is a reversal of the 
 process by which it was deposited in the tissues, and when the 
 stage of fatty acids and glycerol is reached the fatty acids are 
 oxidised step by step, with the production of carbon dioxide 
 and water. In the decomposition of the fatty acid, oxidation 
 takes place at the second carbon atom from the carboxyl group 
 — i.e., the /3-position. 
 
 The Function of the Non-Nitrogenous Foods may be sum- 
 marised as follows : 
 
 1. The fats supply a considerable amount of energy in a 
 tmall bulk, and in the forms of cholesterin, lecithin, phospha- 
 tides, etc., they are essential constituents of the body proto- 
 plasm. 
 
 2. The carbohydrates are also carriers of energy, and as more 
 carbohydrate than fat can be consumed they furnish the greater 
 part of the energy of the body. 
 
 3. Too large a proportion of starch or sugar in a diet de- 
 presses the digestibility of protein and cellulose. 
 
 4. The fats and carbohydrates of the food are worthless, as 
 energy producers, without the presence of protein ; it is this which 
 enables the muscle-machine to convert the chemical energy of 
 the non-nitrogenous substances into mechanical work. 
 
 Growth.— The rate of growth of the fcetus is considerable; 
 from birth to adult age the rate diminishes at first rapidly, then 
 slowly. The metabolism of growth is only imperfectly known. 
 The gain in weight of the very young animal is principally 
 water, and in a much less degree protein; as maturity approaches 
 the percentages of water, protein, and ash stored up decrease, 
 while that of fat increases. 
 
 We have already seen (p. 368) that there are certain amino- 
 acids necessary for growth, and that in their absence normal 
 growth is not attained. Lysine, for example, appears indispens- 
 able, its absence in maize rendering the grain unsuitable for 
 young animals ; tryptophan, too, in which maize is also deficient, 
 is of considerable importance in growth. The deficiency of maize 
 in these two amino-acids is further associated with a deficiency 
 in salts and in vita-mines, especially in the vitamine soluble in fat, 
 which is known to be essential to growth. 
 
 The protein requirements of the young animal are of two kinds : 
 for maintenance and for growth. The rapidly growing tissues 
 
NUTRITION 
 
 377 
 
 make a considerable demand on protein, and the protein so 
 utilised does not rind its way out of the body in the form 
 of waste nitrogenous products. As growth diminishes and 
 maturity approaches the demand for protein becomes less, and 
 in consequence a larger amount of its degraded products appears 
 in the urine. Nevertheless, experiment shows that only a 
 certain proportion of protein can be utilised for maintenance and 
 growth, and that all over and above that amount is metabolised 
 and finds its way out by the urine. Experimental inquiry also 
 indicates that liberal feeding does not materially promote growth, 
 though it may lead to the deposition of fat, and suggests the 
 conclusion that the amount of protein hitherto given has been 
 unduly high. But, precisely as in the case of the protein supply 
 
 
 ISPIll 
 
 *«fl " l 
 
 
 
 
 
 
 
 'fa 
 
 
 
 i 
 
 ■ ilftfciS 
 
 
 
 ™ 
 
 M#»^pppy ; 
 
 
 1 
 
 vWM' : ' 
 
 
 JHafy-': '■■■■:■■ 
 
 
 JJL, 
 
 -i 
 
 t- 
 
 lira ,*% 
 
 
 
 'I ' ■ * ■ 
 
 
 
 
 Fig. in. — Effect of Food Supply on Development (F. B. Mumford).* 
 
 These animals were born on the same day. The one on the left was fed generously 
 from birth, and weighed i,6io pounds when two years old. The animal on 
 the right was limited in the amount of food supplied, and at two years old 
 weighed 361 pounds. 
 
 for adult animals referred to at p. 370, practice is here opposed 
 to science. It is the general experience of graziers and breeders 
 of stock that an insufficient food supply stunts growth, and that 
 a liberal ration increases it (see Fig. in). In the light of 
 modern discovery, it may be urged that a ration deficient in 
 quantity is also deficient in vitamines, and that the liberal 
 ration occasions growth because, quite apart from the protein 
 content of the diet, the vitamine factors for growth exist in 
 it in larger quantities. It would be unwise to dogmatise as 
 to the precise agents at work, but this can, at least, be definitely 
 
 * ' The Breeding of Animals.' New York, 1919. 
 
37« A MANUAL OF VETERINARY PHYSIOLOGY 
 
 stated, as the result of the impressions of a lifetime, that a 
 stunted race of animals could be produced at will by under- 
 feeding the young stock. During the period of intra-uterine life 
 the result of underfeeding the mother is to affect seriously the 
 full development of the progeny. Underfeeding the young 
 animal does not completely arrest growth, for the skeleton slowly 
 increases in size at the expense of the other tissues. The influence 
 of underfeeding on growth has been submitted to experimental 
 inquiry in the U.S.A.* It was found that it reduced the capacity 
 to grow, but that after a period of partial starvation, lasting a 
 year, an abundant food supply permitted active growth to take 
 place, though as the result of the previous hard life the size of 
 the adult animal was permanently diminished. 
 
 A proportion of fat in the diet of growing animals is rega:ded 
 as indispensable, but it is not unlikely that the vitamine soluble 
 in fat is principally concerned in the growth of the animal, though 
 it does not exist in vegetable oils. It is, however, abundantly 
 present in the leaves of grass, clover, and almost certainly 
 in hay. 
 
 Fattening of Animals. — -There are certain fat-reserve depots 
 natural to the animal, on which, under ordinary circumstances, 
 little or no drain occurs ; such are found beneath the skin, in the 
 peritoneum, around the kidneys, in the mesh of the omenta, and 
 surrounding the base of the heart. The chief means to induce the 
 laying on of fat is a liberal diet and freedom from exercise and 
 work. The farmer feeding for beef or mutton understands the 
 value of keeping the animals as quiet as possible, and recognises 
 also that there are certain breeds which have a distinct predis- 
 position to store up fat. Further, he learns how necessary it is to 
 introduce animals gradually to a fattening diet until toleration is ' 
 established, and he knows from practical experience that he will 
 not succeed in fattening within a reasonable time unless to the 
 diet of carbohydrate and fat he also adds proteins liberally. The 
 measure of the diet is the animals' appetite ; they can never eat 
 enough to please the feeder, who cheerfully accepts the heavy 
 initial outlay, as he knows the subsequent saving in time effected. 
 It is at this point that science and practice again come into 
 conflict. It is urged that most of the diets given to fattening 
 animals are wastefully nitrogenous. This is certainly true l ; 
 where ' show ' condition is aimed at; the obesity then produced | J : 
 in cattle, sheep, and pigs is a pathological condition repugnant 
 to common sense, and the outcome of a barbarous fashion. All j j 
 modern experimental inquiry directed to ascertain the most "i : 
 suitable protein content of a fattening diet shows that animals 
 
 * T. F. Trowbridge in F. B. Mumford's ' Breeding of Animals.' New 
 York, 1919. \s 
 
NUTRITION 
 
 379 
 
 may be fattened on a diet containing far less protein than that 
 advocated some years ago. 
 
 _ In the investigations of Lawes and Gilbert, tables of which are 
 given below, the animals were permitted to make their own choice 
 of food substances, and the amount of protein and non-nitro- 
 genous material was quite unrestricted. 
 
 The consensus of opinion is in favour of castration as facilitating 
 fattening. It is quite certain, however, that geldings have no 
 greater disposition to accumulate fat than mares; if castration 
 favoured fattening among horses, there would be no need for that 
 constant striving after fatness instead of ' fitness ' which is so 
 characteristic of all who have charge of them. There are, of 
 course, some animals which have a tendency to store up fat and 
 others which never do any credit to their ' keep,' but this is an 
 individual peculiarity not explained by castration. It has been 
 suggested that the accumulation of fat observed in castrated 
 animals is due, not to an increased production of fat, but to a 
 failure to utilise it. It is thought that the liver prepares the 
 fat for utilisation under the influence of a hormone secreted by 
 the testicle ; if this is the case, the liver in castrated animals should 
 be reduced in size, and this has been found to be the case in the 
 cock. Observations on other animals are wanting. 
 
 Reference has been made at p. 258 to the fact that in animals 
 other than herbivora the liver acts as an important fat depot. 
 This is the case in man and carnivora; it is well known to occur 
 in the goose; the rabbit also, during late pregnancy and early 
 lactation, may show the same condition. In these cases the 
 liver obtains its store of fat from the adipose tissues, the trans- 
 port probably taking place in the form of fatty acids and glycerol ; 
 the higher fatty acids are desaturated in the liver and then broken 
 down into lower fatty acids; these in turn are- transported to the 
 active tissues for combustion. 
 
 Storage of Tissue. — Every diet for fattening must contain 
 the food principles we have been considering — viz. : 
 
 Protein. 1 
 
 Carbohydrate. 
 
 Fat. 
 
 Vitamines. 
 
 Salts. 
 
 It is interesting to learn in what proportion these are stored 
 up in animals being fattened, also the amount of food required 
 for a definite increase in weight, and the rate at which that 
 increase occurs. The following tables are compiled from the 
 classical experiments of Lawes and Gilbert, which extended over 
 a period of twenty years : 
 
3 So 
 
 A MANUAL OF VETERINARY FHYi>iui.uvi 
 
 TABLE I. 
 
 Metabolism occurring while the Body is increasing its 
 Weight by ioo Pounds. 
 
 
 Oxen. 
 
 Sheep. 
 
 Pigs. 
 
 Remarks. 
 
 
 lbs. 
 
 lbs. 
 
 lbs. 
 
 
 Water-free 
 
 I.iog 
 
 912 
 
 420 
 
 Oxen consume more food and 
 
 food required 
 
 
 
 
 produce considerably more excreta 
 
 Utilised in in- 
 
 636 
 
 548-5 
 
 276-2 
 
 than sheep, and sheep very much 
 
 ternal work 
 
 
 
 
 more than pigs, during the period 
 
 Excreta pro- 
 
 404-4 
 
 291 
 
 70 
 
 occupied in adding 100 pounds to 
 
 duced 
 
 
 
 
 the body weight. This period for 
 
 Stored up, 
 
 68-6 
 
 72-5 
 
 73-8 
 
 oxen is five or six, and for sheep 
 
 water-free 
 
 
 
 
 three or four times as long as for 
 pigs (see Table IV.) . The nature of 
 the stored-up matter is shown in 
 Table II. 
 
 The maintenance and internal 
 work was highest in pigs, rather 
 
 
 
 
 
 less in oxen, and still less in sheep. 
 
 TABLE II, 
 
 Proportion of Food Principles stored up for every 
 100 Pounds increase of Body Weight. 
 
 Oxen 
 Sheep 
 Pigs 
 
 
 
 Total 
 
 
 Pro- 
 tein. 
 
 Fat. | Salts. 
 
 Water-free 
 Substance 
 
 Remarks. 
 
 
 ! 
 
 stored up. 
 
 ' 
 
 lbs. 
 
 lbs. , lbs. 
 
 lbs. 
 
 
 9-0 
 
 58 J 1-6 
 
 68-6 
 
 Oxen store up the most protein, 
 
 
 
 
 
 pigs the most fat, and sheep the 
 
 7"5 
 
 &3 
 
 2-0 
 
 72-5 
 
 most salts. Oxen .store up ,V of 
 their food, sheep rather more than 
 
 7-0 
 
 66 o-8 
 
 73-8 
 
 T V. an d pigs nearly £. The chief 
 
 
 
 
 
 increase in body weight is fat. 
 
 
 
 
 
 Sheep and oxen develop bony 
 
 
 
 
 
 structure during the fattening pro- 
 
 
 
 
 
 cess more than pigs. 
 
 TABLE III. 
 Metabolism of 100 Pounds of Water-Free Food. 
 
 
 Oxen. 
 
 Sheep. 
 
 Pigs. 
 
 Remarks. 
 
 Utilised in in- 
 ternal work 
 
 Excreta pro- 
 duced 
 
 Stored up : 
 Protein 
 Fat and car- 
 bohydrate 
 Salts 
 
 lbs. 
 
 57-3 
 
 36-5 
 
 o-8 
 5-2 
 
 0-2 
 
 lbs. 
 6o-i 
 
 31-9 
 
 o-8 
 7-0 
 
 0-2 
 
 lbs. 
 65-7 
 
 16-7 
 
 1-7 
 
 i5'7 
 
 0-2 
 
 Oxen produce more excreta than 
 sheep, and sheep very much more 
 than pigs, while metabolising 100 
 pounds of water-free food. 
 
 During this process the internal 
 work is highest in pigs and lowest 
 in oxen, for the reason that pigs 
 assimilate between three and four 
 times more from a given quantity 
 of food than oxen and sheep. 
 
 1 00-0 
 
 100-0 
 
 100-0 
 
NUTRITION 
 
 381 
 
 TABLE IV. 
 Metabolism, calculated on 100 Pounds of Body Weight 
 
 occurs Week by Week during im. 
 100 Pounds of Food Substances. 
 
 Process 
 
 which 
 of storing up 
 
 Water -free 
 food con- 
 sumed per 
 100 pounds 
 body weight 
 
 Utilised in in- 
 ternal work 
 
 Excreta pro- 
 duced 
 
 Increase in 
 body weight 
 
 Oxen. 
 
 lbs. 
 12-5 
 
 7-16 
 4-56 
 1-13 
 
 Sheep. 
 
 lbs. 
 
 i6 - o 
 
 9-62 
 5-10 
 1-76 
 
 Pigs. 
 
 lbs. 
 27-0 
 
 17-74 
 4-51 
 6-43 
 
 Remarks. 
 
 In every 100 pounds of body 
 weight oxen and sheep consume 
 less food than pigs, and in conse- 
 quence use up less on internal 
 work. 
 
 The excreta produced per 100 
 pounds of body weight is almost 
 identical in the three animals. 
 
 The percentage of increase in 
 body weight is markedly different ; 
 consequently, it takes oxen five or 
 six, and sheep three or four times 
 as long as pigs to add 100 pounds 
 to the body weight. 
 
 TABLE V. 
 
 Amount assimilated for every 100 Parts of Each Substance of 
 Food consumed. 
 
 
 Oxen. 
 
 Sheep, 
 lbs. 
 
 Pigs. 
 
 lbs. 
 
 Remarks. 
 
 1 
 
 lbs. 
 
 
 For every 100 
 
 4'i 
 
 4-2 
 
 13-5 
 
 This is a digestion experi- 
 
 pounds of protein 
 
 
 
 
 ment. The food of the pig is 
 
 For every 100 
 
 7 -2 
 
 9'4 
 
 18-s 
 
 much more concentrated than 
 
 pounds of fat and 
 
 
 
 
 that of the ox and sheep, but 
 
 carbohydrate 
 
 
 
 
 the two herbivora, fed on 
 
 For every 100 
 
 1-9 
 
 3-1 
 
 7-3 
 
 identical foods, show a marked 
 
 pounds of salts 
 
 
 
 
 
 
 difference in the powers of 
 assimilation. 
 
 
 
 13-2 
 
 16-7 
 
 39-3 
 
 
 TABLE VI. 
 
 Amount of Food required to produce One Pound increase in 
 Body Weight. 
 
 
 Oxen. 
 
 Sheep. 
 
 Pigs. 
 
 Remarks. 
 
 Linseed cake - 
 Clover hay 
 Swedes ■ 
 Barley meal 
 
 lbs. 
 2 
 6 
 
 35 
 
 lbs. 
 2-5 
 
 3.0 
 40-0 
 
 lbs. 
 5 
 
 The 43 pounds for oxen weighed 
 1 1 pounds in a water-free condition. 
 
 The 45 pounds for sheep weighed 
 9 pounds in a water-free condition. 
 
 The 5 pounds for pigs weighed 
 4-2 pounds in a water-free condi- 
 tion. 
 
 43 | 45-5 
 
 5 
 
382 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 11 Metabolism of the Salts. — The salts of the body direct its 
 metabolism; they are connected with assimilation, secretion, 
 excretion, and the building up of the skeleton; moreover, they 
 maintain the body fluids at their normal reactions — i.e., the 
 blood and lymph at neutrality, the saliva alkaline, the gastric 
 juice acid, the intestinal fluid alkaline, and the urine alkaline or 
 acid, depending on the species of animal. The salts also regulate 
 the water-flow from blood to tissues, and vice versa ; they play 
 an essential part in blood-clotting, rhythmical contraction of the 
 heart, irritability of muscle and nerve, milk curdling, and growth. 
 Salts are always being excreted, and must be replaced. This is 
 done through the medium of the food, which, if salt-free, though 
 suitable in every other respect, would fail to support life; in fact, 
 animals fed with salt-free food die earlier than those receiving no 
 food. An abundance of salts is found in vegetable food. The plant 
 is unable to grow without potassium or calcium ; phosphorus and 
 sulphur are needed by it for the transport of its protein, iron for 
 the chlorophyll, magnesium for the seed. 
 
 The distribution of the salts throughout the body structure is 
 remarkably regular, sodium being found in the blood-plasma, 
 potassium and iron in the red cells, sulphur in hair and horn, 
 potassium in muscle and sweat, sulphur in protein, calcium 
 and phosphorus in bones, protein and some fats. When a 
 deficiency in salts occurs, the body for some time apparently 
 draws on its own store, and then nutritive changes follow. These 
 are more likely to show themselves earlier in young than in adult 
 animals, but the influence of inorganic food on the nutrition of 
 the skeleton is little understood. 
 
 The chief salts used by the herbivora are those of potassium, 
 while salts of sodium are used by the carnivora. The ratio of 
 potassium to sodium salts in the diet is a question of importance; 
 potassium salts, when in excess, cause a withdrawal of sodium 
 salts from the body, and nutritional diseases of the skeleton in 
 horses have been associated with an excess of potassium salts 
 in hay. This excess has involved a loss of sodium, while the 
 loss in sodium has in turn produced losses in calcium and phos- 
 phoric acid.' The alkalinity of the urine of herbivora is due to 
 the presence in the food of an excess of alkaline ash ingredients ; 
 over acid ash ingredients. By errors in diet, as, for instance, 
 feeding a horse on oats only and no hay, the urine may be 
 rendered acid. The protein of the oats, containing sulphur 
 and phosphorus, produces acids in the body which have to be 
 neutralised. The salts in the hay furnish the necessary alkali, ' 
 and in its absence or insufficiency the urine becomes acid. 
 Maize is a food very poor in calcium, and would be quite un- 
 fitted for young growing animals but for the requisite calcium 
 
NUTRITION 383 
 
 salts being furnished by hay or grass. It has been found ex- 
 perimentally in the feeding of pigs on maize alone that these 
 animals failed to grow, while the addition of salts to the maize 
 enabled nearly normal growth to be made. For instance, 
 5 grammes of calcium carbonate added to 1 kilogramme of maize 
 has changed a loss of calcium and of phosphorus into a gain 
 (Armsby). 
 
 It is generally considered that the herbivora require the 
 addition of sodium chloride to the food; this has been scien- 
 tifically explained by saying that, as potassium salts normally 
 withdraw sodium from the body, the latter is replaced by the 
 addition of common salt. The wild herbivora are reported to 
 travel considerable distances to obtain the necessary sodium 
 from 'salt-licks.' This may be true, but the fact remains that 
 horses may remain fit and well for years without common salt 
 being added to their diet, the little already in the food being 
 ample ; the same statement has recently been made in connection 
 with dairy stock. 
 
 Potassium and sodium salts are excreted by the urine, and in 
 the horse by the skin. The amount of potassium in sweat is so 
 considerable that the hair feels sandy when the coat has dried. 
 In the ruminant the loss of potassium by sweat is trifling, but the 
 considerable amount of hippuric acid furnished by this species 
 makes a heavy draft on the alkali of the body, as this acid is 
 mainly excreted in the form of hippurate of potassium. 
 
 Calcium and Magnesium, especially the former, are required 
 for the nutrition of the skeleton. Calcium abounds in the body, 
 magnesium exists only in small proportions. The precise 
 manner in which calcium performs its functions is unknown. 
 It is excreted in the milk — cows may even excrete more than 
 they receive — and by the urine and faeces; in the dog it has 
 been shown to be absorbed in the anterior part of the digestive 
 canal, and excreted in the posterior portion . Clover hay contains 
 more than five times as much calcium as timothy hay, hay 
 contains more than oats, but straw contains more than hay. Food 
 poor in calcium leads to deficiencies in growth and changes in the 
 skeleton. Rapidly growing animals, such as pigs, fed on grain 
 rations are especially liable to such changes, but in the herbivora 
 on ordinary food no such changes are likely to occur. There is 
 far more calcium in the food than horses require; the surplus is 
 unabsorbed, and is excreted with the fasces. Occasionally it 
 fails to be excreted, and forms calculi, a foreign body, nail or 
 pebble, being an invariable nucleus. Other intestinal calculi 
 are due to ammonio -magnesium phosphate and lime being en- 
 tangled in the fine insoluble hairs on the kernel of oats, and 
 forming a felt-like mass of remarkable lightness. 
 
384 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The calcium carbonate excreted in the urine of the horse is 
 always in excess of what can be kept in solution . The precipita- 
 tion occurs in the kidneys, and the solid material is excreted in 
 suspension; in cattle the calcium in the urine remains in 
 solution . 
 
 Sulphur enters the body principally as cystin in the protein. 
 Cystin yields taurin, which is necessary for the bile. Both of 
 these substances are readily oxidised in the system, and yield 
 sulphuric acid, which, either in combination with phenol, derived 
 from putrefactive processes in the intestine, or as salts, finds its 
 way out of the body by means of the urine. An excess of pro- 
 tein may lead to an acid reaction of the urine, as mentioned 
 above and in the chapter on the urine. A pound of protein, 
 having the composition of serum albumin, if fully oxidised in the 
 body, would yield nearly 1 ounce by weight of concentrated 
 sulphuric acid (Armsby). There can be no doubt that much 
 sulphur escapes unacted upon from the body by means of the 
 faeces, and does not contribute to the production of acid in the 
 body. The origin of the neutral sulphur of the body is unknown. 
 
 Phosphorus is introduced into the body in organic combi- 
 nation with phospho-protein, nucleo-protein (lecithin), as phos- 
 phatides in the phosphorised fats, and in inorganic combination 
 as phosphates. An important source is the organic phosphorus; 
 it is believed that most of this enters the blood as phosphoric acid, 
 the acid radicle being split off under the influence of the enzymes 
 of the digestive juices. Experimental inquiry shows, however, 
 that the tissues are capable of using inorganic phosphates in the 
 production of the complex phosphorus-containing compounds' 
 found in the body. The skeleton is the chief seat of the depo- 
 sition of phosphorus. Far more phosphates are taken in with 
 cereals (oats, bran) than are required for body purposes ;■ they 
 are accordingly excreted with the fasces, and in the horse, as just 
 mentioned, are a cause of intestinal calculi. Feeding-stuffs con- 
 taining an excess of phosphorus either lead to the production of 
 acid in the blood, which necessitates neutralisation, or else they 
 produce an acid ash in the urine. The neutralisation in the blood 
 is effected by sodium bicarbonate. In carnivora the urinary 
 ash is acid, the acidity of the fluid being due to the acid phos- 
 phate of soda. In herbivora the proportion of phosphates in 
 the urine is small; in all probability, after having done their 
 work, they are excreted as phosphates into the intestinal canal, 
 such as has been observed in the case of calcium in the dog, 
 though in dairy stock there is a large outlet through the 
 secretion of milk. 
 
 Iron and Iodine enter the body with the food, mainly in organic 
 combination; they are required, the one for the blood, the other 
 
NUTRITION 
 
 385 
 In- 
 
 for the thyroid gland, but their metabolism is unknown, 
 organic iron and iodides are capable of replacing them . 
 
 The storage of salts by the growing animal is a question of 
 considerable importance; the experiments of Lawes and Gilbert 
 have revealed the amount of ash ingredients retained by animals 
 during growth, as will be seen from the following table computed 
 by Armsby : 
 
 Average Retention by Growing Animals of Ash 
 Ingredients per Day. 
 
 
 Yearling Cattle. 
 
 Yearling Sheep. 
 
 Six Months' Old 
 Pig- 
 
 Potassium 
 
 Grammes. 
 1-64 
 
 Grammes. 
 0-20 
 
 Grammes. 
 
 °'45 
 
 Sodium - 
 
 1-08 
 
 0-I2 
 
 0-22 
 
 Calcium 
 
 !5-37 
 
 1-29 
 
 2-II 
 
 Magnesium 
 Phosphorus 
 Chlorine 
 
 0-50 
 
 8-14 
 
 o-59 
 
 0-05 
 0-71 
 O-IO 
 
 0-09 
 1-27 
 
 0-16 
 
 The percentage of salts retained in the body decreases from 
 birth to maturity; with young animals 53 per cent. oLthe total 
 ash of milk may be assimilated, while of individual salts absorbed 
 those of calcium may stand as high as 97 per cent., and of 
 phosphorus 72 per cent. Both the organic and inorganic phos- 
 phorus of food is assimilated; even in the coarser foods, such as 
 hay and straw, 50 per cent, of the phosphorus may be taken up. 
 
 The whole question of the metabolism of salts in young and 
 adult animals, under all conditions of growth, food production, 
 and work, requires investigation. What is very clearly known 
 is the constant production of acid in the body, and the necessity 
 for its neutralisation if the blood and lymph are to remain 
 normal. We have seen how the acid produced by the kata- 
 bolism of protein is neutralised by means of the ammonia pro- 
 duced from the amino-acids, ammonium salts being excreted by 
 the urine. 
 
 We have also seen how the loss of salts from the body is in- 
 fluenced by the composition of the ash of the food, an excess of 
 acid or basic elements in these affecting the gain or loss of sulphur, 
 phosphorus, and calcium. The alkali ratio of the salts of the food 
 is another example of this, an excess of potassium salts leading 
 to a loss of those of sodium, calcium, and phosphoric acid. 
 
 25 
 
386 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Summary of Ash Metabolism. 
 
 A general account of the metabolism of the salts in the body is 
 somewhat as follows: The source of the salts is the food. In 
 green or fresh vegetable bodies these are held in simple solution 
 in the form of chlorides, sulphates, carbonates, phosphates, etc. 
 Certain elements, such as iron, iodine, phosphorus, and sulphur, 
 may also be held in organic combination. If the food be dry 
 the salts may exist in crystalline form; the grinding in the mouth 
 exposes them to the solvent action of the saliva, and this process 
 of solution is continued in the stomach or in the rumen of rumi- 
 nants. What escapes the saliva is dealt with by the gastric 
 juice; especially is this true of the calcium salts, since the saliva 
 is already saturated with these (p. 168). 
 
 In the digestive canal as much saline matter is taken up as 
 the system requires. It is by no means the full amount contained 
 in the food, for much is excreted with the feces unacted upon. 
 This is especially the case in the horse in respect of calcium and 
 phosphorus. The salts are taken up into the system by the 
 process of diffusion, and are carried in the blood both by the 
 plasma and cellular elements. Each tissue selects with unerring 
 accuracy the salts which it requires either for building up or 
 for secretory purposes. It is reasonable to suppose that some 
 of the older salts in the tissues are displaced; perhaps some are 
 got rid of by being utilised for glandular secretions, those not 
 so required passing into the blood and being excreted through 
 the kidneys. There is a great deal of evidence - to show that a 
 storage of salts occurs in both the hard and soft tissues of the 
 body, and that it is this store which provides the salts in those 
 cases where there may be a temporary shortage in the food. 
 
 The various secretions constitute an important source of loss; 
 the salts contained, for instance, in milk and sweat, leave the body 
 for good, while others — those, for example, in the digestive fluids- 
 are reabsorbed and eventually got rid of by the kidneys, or if 
 not reabsorbed gain exit by the faeces. 
 
 The elements which are held in organic combination, as, for 
 example, iron in haemoglobin, iodine in the thyroid gland, phos- 
 phorus in nuclein and lecithin, and magnesium in chlorophyll, 
 are gotMd of by the various secretions. Thus the worn out red 
 cells get rid of their iron in the spleen (?), the thyroid secretion 
 uses up iodine, and the iron and magnesium contained in chloro- 
 phyll are got rid of through the intestinal canal and, in the horse, 
 partly by the skin. It is now ascertained that the inorganic 
 salts of the food may furnish the elements required by the 
 organic combinations; that phosphates serve as a source of 
 
NUTRITION 387 
 
 organically bound phosphorus; that iron salts can supply what 
 is required by haemoglobin, and iodides supply the iodine for 
 the thyroid. 
 
 It is easy to understand that the salts required day by day are 
 readily available from the food, as there is, as we have seen, 
 alreadjr an excess present. Those required for an unexpected 
 secretion — such, for example, as a profuse sweat after a gallop — ■ 
 are no doubt taken up from the store in the tissues, the muscles, 
 for instance, furnishing the large amount of potassium contained 
 in sweat, in the same way that solid tissues may furnish the salts 
 required for milk. 
 
 The saline matters for excretion, having reached the kidney, 
 pass through the glomerular filter in solution. It does not follow 
 that the various salts in the uriniferous tabes are all excreted 
 with the urine. According to Cushny's theory, some would be 
 reabsorbed by the cells of the tubules in order that the blood may 
 retain its normal saline concentration, and be finally excreted 
 when they can be spared, but the larger part would pass out of 
 the body with the urine. It will be remembered that the fluid 
 as filtered by the glomerulus is extremely dilute, and that in the 
 tubules water is reabsorbed. In the case of the horse this con- 
 centration throws out of solution a great deal of calcium carbonate, 
 which accordingly, as we have seen, is deposited in the pelvis 
 of the kidney and produces a turbid urine. Under pathological 
 conditions other salts may be precipitated in the tubules or pelvis 
 of the kidney — for example, those of uric acid. 
 
 Organic acids enter the body by means of the food — silage, for 
 example, which contains acetic, butyric, and lactic acids. Such 
 are neutralised by the digestive fluids, and after their absorption 
 and oxidation to carbon dioxide and water the bases are 
 returned to the body. Some organic acids — tartaric, malic, and 
 oxalic — are not readily oxidised, and carry off the bases or even 
 calcium from the body. Adult ruminants are capable of dealing 
 with the acids of their food by fermentation in the rumen, and 
 consequently the loss of bases observed in horses, young rumi- 
 nants, and swine is not so evident in adult ruminants. Organic 
 acids are also produced in the body as the result of the fermen- 
 tation of cellulose. It is believed that these may be oxidised, 
 and so utilised by the system, but before this can occur they 
 withdraw bases from the body, and so tend towards the produc- 
 tion of acidosis. 
 
A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Accessory Food Substances.* 
 
 Vitamines. — These bodies — there are probably several — have 
 been identified only within the last few years. They are chemical 
 substances contained in fresh food in extremely minute propor- 
 tions, and their function would appear to be, in some way or 
 other, to direct metabolism. They are absolutely essential in 
 a diet, and to their absence are due the diseases in man known as 
 beri-beri, pellagra, scurvy, and rickets. If a man be fed on a diet 
 of pure protein, purified starch, pure sugar, and a vegetable fat, 
 he will die, though the diet is wholesome and digested. If to 
 the diet be added a small quantity of a watery extract made 
 from wheat grain, or from the germ of wheat, or from the ' polish- 
 ings ' (husk) of rice, and if an animal fat be substituted for a 
 vegetable fat, growth and vitality are maintained. 
 
 The diseases due to the absence of vitamines from the diet 
 are classed as deficiency diseases. 
 
 At the present time there are only three vitamines which have 
 been isolated. One is soluble in water, and is provisionally 
 known as water soluble ' B '; a second is contained in animal 
 fats (lard excepted), such as milk fat and cod-liver oil, and in the 
 green portions of plants; it is known as fat soluble 'A.' The 
 third vitamine is found in fresh vegetables and in the juice of 
 fruits, especially oranges and lemons; it possesses antiscorbutic 
 properties. All the vitamines must be present in a diet, and all 
 are primarily derived from the vegetable kingdom directly or 
 indirectly. In plants they are produced by synthesis. 
 
 Water soluble ' B,' or antineuritic factor, was the first vitamine 
 discovered. _K is found i n all natu ral food-stuffs, but its prin- 
 cipal sources anrtrre ~secds or^arits~anTl~trce-^ge>lk ofeggs ; this 
 suggests its importanceTSTconnec'tion with fhi growth- of the 
 embryo. The cereals contain this vitamine in tEF^eripHefal 
 -layer oi the seed and in the seed envelope, but especially in the 
 germ. When grain is decorticated by milling, the vitamine is 
 lost, which explains why beri-beri is due to feeding on milled 
 rice and can be cured by the administration of water soluble 
 ' B ' or by the use of unhusked rice. The pulses contain it 
 throughout the seed, but only in moderate amounts. Yeast 
 contains it in abundance; flesh contains very little. 
 
 There is no important reserve store of this vitamine in the 
 
 * In the compilation of this section the valuable Report of the Com- 
 mittee on " The Present State of Knowledge concerning Accessory Food 
 Factors (Vitamines)," 1919, has been largely drawn upon. I am indebted 
 to its chairman, Professor F. Gowland Hopkins, D.Sc, F.R.S., and to the 
 Controller, His Majesty's Stationery Office, for permission to reproduce J 
 Figs. 112 and 113. ' 
 
 ' ,-> i 
 
NUTRITION 389 
 
 body, so that its absence from a diet is felt very early, especially 
 by young growing animals, which cease to grow and lose body 
 weight. The body weight of adults also suffers, and muscular 
 in co-ordination appears. Fig. 112 is a photograph of a young 
 rat suffering from a deficiency of water soluble ' Q ' ; the hind 
 legs are paralysed and stretched out behind. Nevertheless, this 
 animal recovered under the employment of yeast extract, and 
 in twenty-four hours was able to walk about. Pigeons paralysed 
 in the legs and wings as the result of deprivation of this vitamine 
 recover in a few hours after its administration by the mouth. 
 
 Fig. 112. — A Young Rat suffering from a Deficiency of Water Soluble ' B.' 
 
 The hind limbs are paralysed and stretched out behind. This animal walked 
 about with ease twenty-four hours after a dose of yeast extract had been 
 given by the mouth. (Committee Report on Vitamines.) 
 
 H. H. Green* is of opinion that water soluble ' B ' vitamine 
 is chiefly concerned in the metabolism of the food. The larger 
 the quantity of food consumed, the more vitamine is required to 
 metabolise it; if the diet is vitamine-poor, the greater the con- 
 sumption the earlier do deficiency symptoms appear. He states 
 that the vitamine requirements should be estimated, not so 
 much from the point of view of a daily physiological minimum, 
 as from that of a minimum quantity per unit of food metabolised, 
 a fixed quantity of digested food requiring a fixed minimum of 
 wafer soluble ' B ' for its combustion. 
 
 Green regards the requirements of men and pigeons as very 
 similar, and much higher than those of cattle. He fed cattle 
 
 * Department of Agriculture, South Africa, Fifth and Sixth Reports of 
 the Director of Veterinary Research, 1919. 
 
390 
 
 MANUAL OF VETERINARY PHYSIOLOGY 
 
 almost exclusively on ' polished ' rice for over a year without 
 producing deficiency disease. 
 
 Fat soluble 'A,' or antirachitic factor. This vitamine is necessary 
 not only for growth in young animals, but for the maintenance 
 of health in adults. It is contained in fats of animal origin, in- 
 cluding fish oils, but not lard; it is practically absent from vege- 
 table oils. Green leaf vegetables contain it, but it is deficient 
 in root vegetables. Except, in the case of the pig this_yitamine 
 is stored in the body, so that there is a reserve in the fat depots 
 on which the animal can draw for some time. 
 
 Fig. 113. — Experimentally produced Rickets in a Dog. 
 
 This animal received a diet of 175 c.c. whole milk, bread ad lib., and 10 c.c. linseed 
 oil per diem. Time of experiment, five and a half months. Increase in weight 
 during that period, 2,670 grammes. (Committee Report on Vitamines.) 
 
 E. Mellanby's researches on rickets in dogs show that the 
 disease may be prevented bj? suitable additions to the diet, 
 especially by giving cod-liver oil, which contains fat soluble ' A ' 
 in large quantities. Mcllanbv has shown that a puppy taken from 
 its mother at six weeks old will develop rickets within six weeks if 
 placed on a diet of separated milk, white bread, yeast, orange 
 juice, and linseed oil. Fig. 113 shows rickets in the dog experi- 
 mentally produced. In this case whole milk was substituted for 
 separated, which explains the delay in the onset of the disease. 
 
 Noel Paton and his fellow-workers hold that evidence of 
 rickets being clue to dietary deficiency alone is still lacking, 
 and the possibility that it may be primarily an infective disease 
 must be borne in mind. 
 
 The antiscorbutic factor is contained in fresh vegetables, and 
 
NUTRITION 391 
 
 to an extent in animal tissues. It isr ichest in ca hbage, swedes, 
 turmpSjJeituce, oranges, and lemons ; potatoes, carrots, mangolds, 
 and lime-juice omy^cohfain inferior amounts, while in milk and 
 meat it is of low value. It is deficient or absent in all dried 
 vegetables, cereals, and pulses. Scurvy may be produced ex- 
 perimentally in guinea-pigs by withholding the vitamine; the 
 symptoms appear about the twentieth day, and death occurs 
 ten or twenty days later. The account of the post-mortem 
 appearances of animals so affected reminds one of those pre- 
 sented by cattle, sheep, goats, and occasionally by horses in 
 South Africa in a disease described by D. Hutcheon* as osteo- 
 malacia. He regarded this affection as being due to the absence 
 of calcium phosphate from the food. In these cases there was 
 also a train of nervous symptoms associated with paralysis of 
 the hind extremities. 
 
 It is stated! that guinea-pigs fed on whole oats, hay, and 
 water, even with the addition of cod-liver oil, develop scurvy, but 
 that on the hay being replaced by straw the disease is cured. We 
 do not know with precision the vitamines in ordinary feeding- 
 stuffs used for animals. A diet of oats and bran does not permit 
 of the growth of guinea-pigs. | In fact, the seeds of any plant 
 — -wheat, oats, maize, beans, peas, etc. — do not furnish the 
 growth or fat soluble ' A ' factor, but if the whole plant be given 
 there is a compensation, for it has been shown§ that the leaf 
 may make good the deficiency in the seed. This occurs in the 
 case of clover, the leaves of which contain an abundance of fat 
 soluble ' A.' Clover also contains water soluble ' B.' Timothy 
 in the green state contains both factors. It is almost certain that 
 fat and water soluble factors are not destroyed in hay, but exact 
 information is not yet available. 
 
 Nothing definite is known of the position of osteoporosis of 
 the- horse in its relationship to deficiency disease. 
 
 The chemical nature of the vitamines is unknown; they are 
 distinguished by their solubility in water and other agents, and 
 by the effect of heat on them; for instance, the antis corbut ic 
 vita mine is de stroyed by heat and by drying, which _ explains whjl 
 it - is absent ifmrrpreserved vegetables . 
 
 Water.— The amount of water found in the same class of 
 tissues is very constant. The muscles of creatures as far re- 
 moved from each other as the pig and the snail, the ox and 
 the lobster, contain 78 to 79 per cent., and other tissues are 
 equally uniform. 
 
 * Report of the Colonial Veterinary Surgeon, Cape of Good Hope, 1903. 
 
 t H. F. Hess and L. J. Unger. 
 
 } H. Chick, E. M. Hume, and R. F. Skelton. 
 
 § E. V. McCollum. 
 
392 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Under the conditions of rest and work varying quantities of 
 water are lost, and in hot weather the loss is still further increased. 
 It has been calculated that a man may lose water at the rate of 
 5 per cent, of his body weight on a hot day, and that muscular 
 work in hot weather may increase the output of water as much 
 as six times; but we are not aware of any exact experiment on 
 this question on animals, though we know that in the case of 
 horses the loss of water is considerable. Of the total water 
 received in the food or consumed, the bulk passes away by 
 the kidneys; during work a considerable amount is lost by 
 the skin and lungs, and less in consequence passes by the 
 kidneys. 
 
 The very constant proportion of water in the tissues shows 
 that the consumption of excessive amounts of fluid does not lead 
 to storage. Adjustments are readily effected, and the excess of 
 fluid in the blood is rapidly got rid of. All animals withstand 
 a deficiency of water badly; the horse is probably the weakest 
 in this respect, and shortage of water is far more immediately 
 serious for any horse than shortage of rations. A thirsting 
 animal dies when it has lost 10 per cent, of its body weight in 
 water, though 50 per cent, of its protein and the whole of its 
 fat will disappear before death from starvation ensues. A man 
 may avoid putting on weight by keeping himself short of fluid, 
 and horses will rapidly lose condition by having their water 
 supply limited. Without sufficient water neither rumination nor 
 intestinal digestion in any of the herbivora can go on ; the contents 
 of the rumen of the ox and of the colon and caecum of the horse 
 must be kept fluid, and much of the water consumed is devoted 
 to the purposes of digestion. Further, the blood must be kept 
 fluid and concentration avoided; a concentrated blood draws 
 on the tissues for fluid, but later on this source dries up, and 
 unless dilution of the blood be effected, death is only a matter 
 of time, and in horses undergoing severe exertion only a very 
 short time elapses before complete collapse occurs. 
 
 Starvation. — 'When an animal is starved it lives on its own 
 tissues; in the herbivora the urine becomes acid, hippuric is 
 replaced by uric acid, and the secretion becomes transparent. 
 We have seen that the elimination of nitrogen by the starving 
 animal at first falls rapidly, then gradually. During starvation 
 the carbon dioxide excreted falls in amount, and the oxygen 
 absorbed becomes reduced, though not in proportion to the fall 
 of carbon dioxide. If water be given, life is considerably pro- 
 longed; a horse receiving water may live thirty days without 
 food. It is notorious that herbivora, though they lose less 
 protein during starvation than carnivora, do not withstand 
 starvation so well; nor need we go so far as a starvation experi- 
 
NUTRITION 393 
 
 ment to ascertain this fact. When men and horses are being 
 hard worked, the loss in condition amongst the horses sets in 
 early, and is extremely marked for some time before the men 
 show any appreciable muscular waste. 
 
 Horses have been known to live without food or water for as 
 long as three and dogs for four weeks ; but it is said that if horses 
 have suffered fifteen days' starvation, the administration of food 
 after this time will not save them.* Colin records an experiment 
 where a horse weighing 892 pounds died after thirty days' star- 
 vation, being allowed only 2 J pints of water per diem. The 
 animal was nourished on its own tissues, the daily loss in weight 
 being 5-9 pounds, which must be considered as exceptionally 
 small. Dewarf records two remarkable instances of the length 
 of time sheep will withstand starvation ; in one instance eighteen 
 sheep were buried in the snow for six weeks, and only one died. 
 In the second case seven sheep were buried for eight weeks 
 and five days, and all were recovered alive, and eventually 
 did well. 
 
 In some very accurate experiments on a starving cat, it was 
 shown that the principal loss occurred in the fat, 97 per cent, of 
 which disappeared in thirteen days. The following table shows 
 the percentage of dry solid matter lost by the tissues : 
 
 Fat 97-0 per cent. 
 
 Spleen 63-1 ,, 
 
 Liver 56-6 ,, 
 
 Muscles - 30-2 
 
 Blood 17-6 
 
 The loss in the glandular organs was very heavy; next 
 followed that in the muscles, and then that in the blood. 
 The heart-muscle and central nervous system suffered no loss; 
 evidently their nutrition was kept up at the expense of other 
 tissues of less importance. Old animals bear starvation much 
 better than young growing ones, as their requirements are 
 smaller. 
 
 Maintenance and Production Rations. — It is evident that ■ 
 the food required for the internal work of the body must be 
 considered apart from that needed for production, and a large 
 number of experiments have been made to ascertain the amount 
 required for the former purpose. This internal work we have 
 seen consists in respiration, the action of the heart, mastication, 
 peristalsis, glandular secretions, hydrolysis and fermentation 
 of food-stuffs, warming of ingesta, cleavage and synthesis during 
 metabolism, and the production of animal heat. All of these 
 
 * If the writer's memory serves him, this period also applied to man 
 during the Great Famine in India many years ago. 
 •f Veterinarian, May, 1895. 
 
394 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 have to be provided for before any production can occur, be 
 it growth, meat, milk, or work. The food for internal work 
 is described as the maintenance ration ;* it is the minimum on 
 which the resting animal can be supported without losing 
 body weight and without yielding anything. The amounts of 
 carbon, hydrogen, oxygen, nitrogen, water, and salts entering 
 the body have to be counterbalanced by the amounts of these 
 leaving the body. The production ration is that required for 
 growth, or work, or the storing up of flesh, fat, or wool, or the 
 production of milk. 
 
 It is impossible here to give an idea of the enormous amount-of 
 work which has been carried out on animals in an endeavour 
 to solve the many problems connected with these questions. 
 Investigations of the chemical composition of food substances, 
 and of the proportion of each digested under various conditions, 
 represent the bulk of the earlier labours extending over half a 
 century. In this way was ascertained the percentage of each 
 food principle digested by various animals, and these discoveries 
 led to a more economical standard of feeding being adopted, but 
 this method of inquiry did not explain the cause of the differ- 
 ences observed. The system adopted in the present day en- 
 deavours to get at the root of matters ; it utilises the old method 
 of food analyses and the observed digestibility of each food, and 
 then by converting the results into terms of heat units — i.e., 
 energy values — endeavours to .show what these are capable of 
 contributing to the maintenance of the body or the building up 
 of tissues. The method aims at showing the actual effect of the 
 food on the body, and its content of matter and energy. For 
 instance, under the old system the study of a food, say maize, 
 terminated when its digestibility under various conditions had 
 been ascertained; but by the system of energy values it is possible 
 to go further, and to show with a degree of precision the amount 
 of energy which escapes from the body unused, the amount used 
 up in the work of digestion, the amount used by horses during 
 work, and the amount stored up as fat and protein by the 
 tissues. 
 
 Some of the modern appliances used in nutrition experiments 
 on large animals' are figured at pp. 359, 361, 363. 
 
 The Energy yielded by Foods is ascertained by burning 
 the various food substances in a specially constructed calori- 
 meter in the presence of compressed oxygen, and measuring the 
 amount of heat given off. Every gramme (15-432 grains) of 
 water in the calorimeter raised 1° C. (i-8° F.) in temperature 
 constitutes a heat unit or calorie. In this way it was shown 
 that: 
 
 * Also known as the basal metabolic figure. 
 
NUTRITION 395 
 
 i gramme of average protein evolved approximately 5,770 
 
 calories, or 57 large calories.* 
 1 gramme of fat evolved approximately 9,300 calories, or 9-3 
 
 large calories. 
 1 gramme of carbohydrate evolved approximately 4,100 calories, 
 
 or 4 1 large calories. 
 
 These constitute physical heat values, and also physiological 
 heat values for fat and carbohydrate; for what is true of the 
 calorimeter is equally true of the body, provided the food is fully 
 metabolised and the end products of carbon dioxide and water 
 are the results of the oxidation. 
 
 The heat evolved by protein in the calorimeter is not the heat 
 given off to the body; protein is not fully oxidised, being got rid 
 of as urea and other nitrogenous products, so that not more than 
 two-thirds of its energy is available in the body. One gramme 
 of average protein, according to Riibner, yields only 4-1 large 
 calories in the animal body. It is not the heat evolved which 
 does the work, but the chemical energy contained in the food 
 substances ; by the time that the stage of heat has been reached 
 the food has done its work. 
 
 It by no means follows that because a food substance has a 
 high chemical energy it is necessarily the most nutritive in the 
 body; this depends on its degree of digestibility. It will be 
 remembered that in the herbivora no food or food principle is 
 wholly digested; there is a residue in the digestive canal which 
 cannot be assimilated, and in consequence passes away as a 
 waste product with the faeces. The amount so lost varies with 
 the particular food employed, and tables of digestibility (for a 
 brief abstract see p. 295 et seq.) have been drawn up showing 
 not only the total amount of organic matter assimilable, but 
 also the amount of protein, fat, and carbohydrate in each food 
 substance which is capable of being digested. 
 
 The theoretical energy of food is not realised in dietetics, 
 owing not only to the above serious sources of loss, but also to 
 losses due to fermentation, such as that of cellulose, which by 
 becoming converted into marsh-gas and hydrogen is lost to the 
 body. Then there are losses arising from the chemical energy 
 used up in mastication, peristalsis, rumination, cleavage and 
 synthesis of the food substances, none of which results in energy 
 to the body, though all result in heat. There is also a loss as the 
 result of the muscular effort involved in standing. If horses are 
 induced to lie during the day, not only do they rest their limbs 
 and feet, but more of their food is capable of being used for 
 the body requirements; the recumbent position assumed by 
 
 * See footnote p. 374. 
 
396 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 cattle, sheep, and pigs is of evident value. There are sources 
 of loss resulting from the warming of the ingesta, especially of 
 the water consumed. That this loss of heat is considerable, 
 especially in the winter, may be seen by the shivering from which 
 horses suffer after ' watering ' — the result of some gallons of 
 cold water having to be raised to the temperature of the 
 body. 
 
 If the tables compiled from Lawes and Gilbert be referred to 
 (pp. 380, 381), we shall obtain illustrations of the following 
 important facts: 
 
 1. No food is wholly digested. Taking Table I., and selecting 
 oxen as an example, 1,109 pounds of water-free food produced 
 404-4 pounds of water-free excreta (36-5 per cent.), leaving only 
 704-6 pounds (63-53 P er cent.) metabolised. 
 
 2. The whole of the food metabolised is not stored up. The 
 cattle in Table I. metabolised 704-6 pounds (63-53 P er cent.), 
 but only stored up 68-6 pounds (6-2 per cent.) in the form of 
 dry substance; there was accordingly a deficit of 636 pounds, or 
 57-33 per cent. 
 
 3. The deficit represents the internal work — -i.e., circulation, 
 respiration, mastication, rumination, glandular activity, fermen- 
 tation, movements of the digestive canal, and heat production. 
 This amounts in the case of cattle to more than half of the 
 total food substances supplied — to be precise, 57-33 per cent. 
 It is a remarkable fact that the herbivora need at least half 
 of their food exclusively for vital functions and the work of 
 digestion. 
 
 4. The various food nutrients do not undergo equal digestion. 
 Table V. shows that a little more than 4 per cent, of the protein 
 is digested by cattle and sheep, and from 7-2 to 9-4 per cent, 
 of the fat and carbohydrate matters ; the pig obtains much more 
 from his food, but even he can only assimilate 18-5 per cent, of 
 the fat and carbohydrate matter. 
 
 We learn, therefore, that there is a great difference between 
 {a) the amount of food metabolised, and (&) the amount which 
 subsequently finds its way into the tissues or is capable of being 
 utilised by them. The first is described in modern terms as the 
 metabolised energy, or that portion of the gross energy of the 
 food which is not lost in the excreta ; while the second is called 
 by Armsby the net energy, or that part which is capable, after all 
 the previous losses have been deducted, of producing something, 
 be it work, milk, muscle, fat, or wool. 
 
 The gross energy of a food is ascertained by burning it in a 
 calorimeter (p. 395) ; the effect on the system is ascertained by 
 means of the respiration calorimeter (p. 360), which enables food 
 to be traced in the body, and a precise expression of its value in 
 
NUTRITION 397 
 
 terms of calories to be obtained. Examples of these will be 
 given. 
 
 One hundred pounds of the dry matter of timothy hay contain: 
 Gross energy, as ascertained by the 
 
 calorimeter 204,940 large calories. 
 
 Energy lost by the excreta 120,840 
 
 Metabolisable energy 84,100 ,, 
 
 Metabolisable energy 84,100 
 
 Energy lost in the preparation of the 
 
 food for digestion, etc. 35»47° 
 
 Net, or productive, energy in 100 pounds 
 
 of hay- - 48,630 ,, 
 
 The net energy is only 23-7 per cent, of that originally existing in 
 the food, and but 57-8 per cent, of the metabolisable energy. The 
 results were obtained with cattle ; the figures for the horse are still 
 lower, only 35 per cent, of the metabolisable energy in timothy hay 
 being available. 
 
 An example of a metabolisable energy experiment may be quoted 
 from Armsby. The animal was a steer. 
 
 Total calories in the ration 29,538 
 
 Energy recovered in the excreta, including the 
 
 gases of fermentation 17,349 
 
 Metabolisable energy 12,189 
 
 Finally, the method adopted of tracing the nutrients in the body 
 may be stated : 
 
 Total calories in the above ration - 29,538 
 
 In the excreta, including the gases of 
 fermentation and brushings from the 
 sk in " " ' - 1 7,437 \ g 
 
 Heat lost by the body 11,493/ ' y ° 
 
 Calories stored up 608 
 
 The difference in the amount between the nitrogen and carbon 
 contained in the food and those recovered in the excreta showed 
 that the body had gained ii-i grammes nitrogen and 46-6 grammes 
 carbon; this was equivalent to 66-6 grammes of protein and 
 15-2 grammes of fat. 
 
 The 66 grammes protein contained 380 calories. 
 
 The 15-2 grammes fat contained 144 
 
 Total stored up 524 
 
 It happens that in this experiment there are 84 calories missing 
 as compared with the calorimeter test, but the principle is unaffected. 
 
398 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 These examples give some notion of the laborious nature of 
 nutrition experiments which have been made, or are in course 
 of being made, on all classes of animals and food-stuffs in order 
 to determine with exactitude what becomes of the food in the 
 process of its metabolism. The results form the modern tables 
 employed for calculating the number of calories yielded by food- 
 stuffs to the body. 
 
 Tables of food requirements were in the past prepared to show 
 the amount needed during rest, work, or the production of flesh 
 and milk. The physiologist of the present day, while not 
 rejecting these tables, points out that they led people to attach 
 undue importance to the standards, as it appeared obvious that 
 they were being treated as if rigidly exact, though only issued 
 as a guide. 
 
 The calories for maintenance given below are extracted from 
 Armsby's valuable tables. They embody the work of many 
 observers, and include his own contributions to the subject. He, 
 however, issues a word of warning that the figures should be 
 regarded as the economic rather than as the exact physiological 
 requirements for energy. 
 
 Maintenance Requirements for Animals. 
 
 
 
 Energy for 
 
 Additional 
 
 Total 
 
 
 Digestible 
 Protein. 
 
 Internal 
 
 required 
 
 Maintenance 
 
 
 Work. 
 
 for Heat. 
 
 Energy. 
 
 
 Large 
 
 Large 
 
 Large 
 
 
 
 Calories. 
 
 Calories. 
 
 Calories. 
 
 Horses, body weight per 
 
 
 
 
 
 1,000 lbs. 
 
 0-50 lbs. 
 
 4,100 
 
 7,800 
 
 11,900 
 
 Cattle (unfattened), body 
 
 
 
 
 
 weight per 1,000 lbs. - 
 
 O-50 ,, 
 
 6,OCO 
 
 4.47° 
 
 10,470 
 
 Sheep, body weight per 
 
 
 
 
 
 100 lbs. 
 
 0-055 .. 
 
 790 
 
 580 
 
 i.37° 
 
 Pig's, body weight per 
 
 
 
 
 
 100 lbs. 
 
 0-048 ,, 
 
 1,250 
 
 280 
 
 1.530 
 
 It will be observed that cattle, sheep, and pigs require more 
 energy for internal work than the horse, while horses require more 
 for body heat. 
 
 In the above table the 11,900 calories for a horse at rest and 
 not increasing in weight would be furnished by 15 pounds of 
 timothy hay. The researches of Grandeau and Leclerc, carried 
 out on different horses for several months, showed that animals 
 of about 1,000 pounds body weight, doing no .work, but receiving 
 half-an-hour's exercise daily, require for their maintenance about 
 17I pounds of meadow hay daily, the metabolisable energy of 
 
NUTRITION 399 
 
 which is equal to 13,000 calories. The total digestible dry 
 organic matter needed was between 7 and 8 pounds daily, irre- 
 spective of the. composition of the diet, provided the due amount 
 of protein' — not less than 0-5 pound daily — was present. The 
 writer kept riding-horses on 12 pounds of hay daily without loss 
 of body weight, the animals doing no work, but the period was 
 much shorter than that employed by Grandeau and Leclerc, whose 
 figures may be adopted for horses in a condition of idleness, though 
 it is to be noted that their animals gained slightly in weight. 
 
 Cattle, under the old tables, were allowed for maintenance 
 -5 pounds protein and 7 to 8 pounds of non-nitrogenous matter 
 reckoned as starch for 1,000 pounds body weight. 
 
 The calories required for production — i.e., for work, flesh, milk, 
 or wool — cannot be dealt with here, as the subject is far too large 
 and falls to the province of hygiene. Nevertheless, a brief state- 
 ment may be made. 
 
 Requirements for Work Production by the Horse 
 (Armsby). 
 
 Full work (draught) of 8 hours per day- 
 Half work (draught) of 4 hours per day 
 One-fourth work (draught) of 2 hours per day 
 
 These calories cover both maintenance and production. When 
 compared with the maintenance table the figures may appear 
 small, but during work no food is required to be set apart for 
 heat production, as more than sufficient arises from muscular 
 contraction. As a matter of fact, the measurement of work is 
 an extremely difficult matter, and for riding-horses performing 
 natural work at fast paces there seems no probability that even 
 approximately correct figures will ever be available. The 
 revolving platform is valuable for experiments on draught and 
 for the slower paces, but nothing more. 
 
 Requirements for Fattening (including Maintenance). 
 Cattle per 1,250 pounds weight (average requirements during 
 
 the whole period), 14,000 calories daily. 
 Sheep per 100 pounds weight (average requirements during the 
 who.le period), 4,000 calories daily. 
 
 Pigs per 100 pounds weight (average requirements during the 
 whole period), 4,500 calories daily. 
 
 Requirements for Milk Production 
 (including Maintenance). 
 
 Cows per 1,000 pounds body weight, the milk containing 4 per 
 cent, fat, and the quantity being 20 pounds daily, 11,300 
 calories. 
 
 Digestible 
 Protein. 
 
 Net Energy- 
 Large Calories. 
 
 2-0 lbs. 
 
 l8,200 
 
 I- 4 " 
 
 11,100 
 
 I'O ,, 
 
 7,600 
 
400 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Net or Productive Energy Values per ioo Pounds of Some 
 
 Foods. 
 
 Horses : 
 
 
 
 
 Timothy hay 
 
 26,640 large calories 
 
 Oats 
 
 93,440 „' 
 
 Maize 
 
 112,840 ,, „ 
 
 Beans 
 
 109,400 ,, „ 
 
 Pigs : 
 
 
 Barley - 
 
 106,080 
 
 Maize 
 
 118,820 
 
 , 
 
 
 Bran 
 
 74.95° 
 
 , 
 
 „ 
 
 Potatoes 
 
 24,690 
 
 
 
 Skimmed milk 
 
 14,740 
 
 , 
 
 , 
 
 Cattle and Sheep : 
 
 
 Timothy hay 
 
 32,250 „ 
 
 Red clover hay 
 
 43,020 
 
 , 
 
 
 Oat straw- 
 
 34,810 
 
 
 
 Wheat straw 
 
 7,220 
 
 
 
 Mangels 
 
 5,680 
 
 
 \ 
 
 Maize 
 
 85,500 
 
 , 
 
 , 
 
 Oats 
 
 67,560 
 
 , 
 
 , 
 
 Brewers' grains (wet) 
 
 14.530 
 
 , 
 
 
 Bran 
 
 - 53.000 
 
 
 ■ 
 
 Pathological. 
 
 
 
 Disorders of nutrition, though much more apparent in some dis- 
 eases than in others, occur with every departure from the normal 
 condition. 
 
 Fever. — The tissues are readily broken down in supplying fuel for 
 the increased metabolism which is giving rise to the abnormally 
 great production and loss of heat; both the fats and proteins suffer, 
 and in some disorders it is remarkable how rapidly wasting occurs 
 the moment the reserve is used up. In acute lung cases this is very 
 obvious — in a fortnight the -patient may be a wreck. The increased 
 nitrogenous metabolism which is here indicated suggests an increased 
 excretion of urea, but exact work in this direction is still much 
 needed. During fever there is an increased production of carbon 
 dioxide, and absorption of oxygen; uric acid is formed by the her- 
 bivora, and the urine becomes acid. 
 
 Marked muscular waste may occur in the absence of fever; any- 
 thing which causes a drain on the system, such as internal parasites, 
 tuberculosis, internal growths, etc., may reduce the animal to little 
 more than a skeleton. Starving, underfeeding, or overworking of 
 animals are obvious causes of metabolic change ; defective teeth in 
 horses are also a frequent cause of malnutrition. 
 
 The food supply may be deficient in proteins or carbohydrates, 
 or both, or there may be an excess. Disorders from the latter cause 
 are very evident in the horse. Lymphangitis and haemoglobinuria 
 are diseases of the horse intimately associated with overfeeding and 
 idleness, and have no parallel in any other animal. An excess of 
 salts in the food may be productive of considerable trouble. One 
 
NUTRITION 4 oi 
 
 form of intestinal calculus in the horse is due to the amount of 
 ammonio-magnesium phosphate existing in the bowel through feed- 
 ing too largely on bran. 
 
 Deficiency Diseases have been dealt with in the text. 
 
 Broken Wind is referred to at p. 156 as having its origin in errors 
 of dieting and management, such as a bulky and frequently innutri- 
 tious diet combined with heavy work on a distended stomach and 
 intestines. In view of the new facts stated at p. 156, defective nutri- 
 tion occupies a less important position than has generally been 
 supposed, and it would appear that under good management, 
 especially during heavy or fast work, the disease should be unknown. 
 
 Local loss of nutrition, such as occurs in lameness, is referred to 
 at p. 360. 
 
 26 
 
CHAPTER XII 
 
 ANIMAL HEAT 
 
 In the chapter on nutrition and elsewhere, the expressions 
 oxidation and combustion, in reference to the food substances 
 and tissues, have been of frequent occurrence. One is tempted 
 to visualise them in likeness of a fire or lamp, and to regard 
 combustion within the body as something very closely akin to, 
 if not identical with, combustions outside the body. A very 
 brief reflection will show that this cannot be the case, for it is 
 obvious that the temperature at which combustion occurs outside 
 the body could not be tolerated by the living organism. Oxida- 
 tions within the body occur at a relatively low temperature, and, 
 further, the substances contained in the cell are not dry, but are 
 three-fourths water. Again, if oxidation within the body were 
 carried out as oxidations are outside the body, it would be pos- 
 sible to increase tissue oxidation by furnishing the tissues with 
 an increased supply of oxygen. Yet this result is not obtained; 
 the rate of oxidation in the body is not affected either by breath- 
 ing pure oxygen or by taking in a larger supply of air. Never- 
 theless, this does not alter the essential facts that the food sub- 
 stances, and in a less degree the body tissues, undergo oxidation, 
 that the source of the oxygen is the air taken into the body, 
 and that this process is continually occurring throughout the 
 whole period of life. It is, therefore, necessary that we should 
 attempt to understand something regarding the processes of 
 tissue oxidation and of how they occur within the living body. 
 It is unfortunate that no positive statement on this question can 
 at present be made, the whole subject being extremely obscure 
 and imperfectly worked out. 
 
 Oxidations. — This fundamental fact has already been learned, 
 that the oxidations occurring in the body do not take place in 
 the blood, but in the tissues. By means of oxidations energy 
 is produced, the substances oxidised being, protein, fat, and 
 carbohydrate. One form of energy yielded by these on oxidation, 
 and the only one which concerns us in this chapter, is heat. 
 
 402 
 
ANIMAL HEAT 403 
 
 It may be well, before going further, to observe that there are 
 various forms of energy, but the energy of motion, heat energy, and 
 chemical energy, which are the only sources of supply of energy to 
 the body, are those of most interest in physiology. All forms of 
 energy are convertible into one another, and no loss occurs as the 
 result of conversion ; for example, a nail may be hammered to a red 
 heat on the anvil, the energy of motion (kinetic energy) being in 
 this instance converted into heat. Or heat may be a source of 
 kinetic energy, as in the steam-engine ; chemical energy may be con- 
 verted into heat and motion, as in the cylinder of the motor-car, and 
 so on. The heat or work obtained when a unit of one energy is 
 converted into another is always the same; in spite of continual 
 changes in form, energy is neither created nor destroyed. 
 
 This great law of the conservation of energy applies to the animal 
 body; the potential or stored up energy contained in food is con- 
 verted into motion and heat energy in the body, and it is possible, 
 as we have seen in the chapter on nutrition, to trace how much 
 escapes as work, how much as heat, and to account with the greatest 
 precision for the total amount received. It has been stated above 
 that all forms of energy are convertible into one another; the second 
 law governing energy deals with the fact that, though other forms of 
 energy may be converted into heat, heat cannot be completely 
 transformed into, say, work, for a part remains as heat. Owing to 
 this incomplete transformation, the losses in the body arising from 
 the production of heat derived from food substances which have 
 done no mechanical work, have received detailed consideration in the 
 chapter on nutrition (pp. 393-399); the subject, which concerns the 
 herbivora almost exclusively, is one of extreme importance. 
 
 It has been stated above that the process of tissue oxidations 
 is obscure; what is definitely known of the subject is that the 
 free oxygen of the air, when taken into the body, must in some 
 way be changed so as to increase its powers of oxidation. This 
 change, or rather series of changes, is greatly promoted by the 
 presence of ferment-like bodies. If glucose or certain phos- 
 phorised fats be exposed to ordinary free oxygen, the oxidation 
 occurring in glucose is so small and so exceedingly slow as to be 
 valueless for the purposes of the body. On the other hand, 
 phosphorised fats and some other fatty substances are acted upon 
 by free oxygen and undergo a process of autoxidation, during 
 which process energy is liberated. This energy calls into exist- 
 ence a peroxide — namely, an oxide containing an extra atom of 
 oxygen, with consequently a higher power of oxidation than that 
 contained in ordinary oxygen. Peroxide having been formed, 
 the process of oxidation is greatly strengthened, but even the 
 peroxide needs the presence of a body which will increase the 
 rate of chemical reactions. Such a body is known as a catalyst, 
 and catalysts act as ferments do, by bringing about changes and 
 yet suffering no loss themselves. Peroxide, plus a catalyst, will 
 oxidise such bodies as glucose and lactic acid into carbon dioxide 
 and water, the necessary catalyst in this connection being the 
 presence of a ferrous salt. Enzymes which furnish catalysts are 
 
( 4°4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 known as caMases. Their function is to liberate gaseous oxygen 
 by decomposing peroxide; they cannot raise the activity of the 
 oxygen. Blood, milk, and most animal and vegetable tissues 
 contain catalases. In living cells, especially in those in the 
 tissues of plants, ferments are found which play the same part 
 as the iron salt above mentioned ; they enable peroxide to yield 
 ' active ' oxygen, and to oxidise, among other bodies, glucose 
 and lactic acid. These ferments are called peroxidases ; they 
 contain iron and manganese, which composition appears essential 
 to their working. They are distinguished from catalyses by 
 their ability to raise the oxidising power of peroxide. Peroxidases 
 are found in blood, milk, and other animal tissues. Under the 
 term oxydases are included substances found in many tissues, 
 which are a mixture of peroxides and peroxidases; the best 
 known are those acting on phenols (phenol- oxidase), on tryosine 
 (tyrosinase), xanthanase, and uricase. 
 
 In the present state of knowledge it is believed that oxida- 
 tions in the tissues occur through ordinary oxygen having its 
 potential raised to active oxygen, and this is effected by the 
 production of an autoxidisable substance from which is produced 
 a peroxide, which in turn is further activated by means of per- 
 oxidase. 
 
 The oxidations in the body occur in the cellular elements, and 
 the question arises whether it is the actual living structure of the 
 cells or their fluctuating, and at present unorganised, contents 
 which undergo combustion. It seems quite clear that under the 
 process of normal nutrition the actual wear and tear of the body 
 tissues is relatively small, so that we must look to the floating 
 rather than to the fixed elements as being mainly the seat of 
 oxidations. The protein, fat, and carbohydrate, which, as we 
 have seen, are capable of yielding heat on combustion outside the 
 body, yield an equivalent amount of heat on combustion within 
 the body, excepting that protein, by not being oxidised lower 
 than urea, suffers a loss, one-third of its heat energy being carried 
 off by the urine. We must now inquire as to the behaviour of 
 the heat which is formed in the body. 
 
 The Body Temperature. — One important division of the 
 animal kingdom is into warm-blooded and cold-blooded animals. 
 A poikilothermal, cold-blooded, or animal of changeable tem- 
 perature is one in which the body temperature depends upon the 
 external surroundings. When these are cold the bodies of such 
 animals are cold, being about a degree or so higher than the 
 medium in which they are- living; when the outside temperature 
 rises, the body temperature correspondingly rises. Such a con- 
 dition exists in reptiles, fish, etc. A homoiothermal, warm-blooded, 
 or animal o^ practically constant temperature, is one in which 
 
ANIMAL HEAT 4 o 5 
 
 the body temperature is independent within wide limits of the 
 temperature of the medium in which it is living; whether this be 
 high or low makes practically no difference. Between these two 
 comes a class partaking of the characteristics of each, hibernating 
 animals, which during the summer are homoiothermal, and during 
 the long winter sleep are poikilothermal. 
 
 The temperature of the body is not uniform; the interior 
 is warmer than the exterior, and the blood in the interior veins 
 is warmer than that in the corresponding arteries. The blood 
 in the veins leading from a gland in a state of activity has a 
 higher temperature than the blood entering the gland. In the 
 animal body the hottest blood is found in the hepatic veins, 
 while the blood in the posterior vena cava is hotter than that 
 in the anterior. There is also a difference in the temperature 
 of the blood in the right and left hearts; it is generally con- 
 sidered that the blood in the right heart is the warmer, though 
 Colin found that in the horse the blood of the left side was gener- 
 ally the hotter. The brain has also a high temperature. The 
 practical aspect of this question is that the interior of the body 
 is hotter than the exterior. A surface temperature does not 
 indicate the temperature of the body, which for clinical pur- 
 poses should be taken in the rectum. With the air at freezing- 
 point there may be as much as 3-0° C. (5-4° F.) difference in 
 temperature between the readings in the rectum and on the thin 
 skin of the breast in the horse, while at the same external tem- 
 perature the limbs of this animal, which are naturally cold, in con- 
 sequence of the underlying tissues having very little vascularity, 
 may indicate 25-4° C. (44 F.) difference between the tempera- 
 ture of the pasterns and that of the rectum. 
 
 The Normal Temperature of Animals.* — Wide differences 
 exist in the normal temperatures of the domestic animals, and 
 marked physiological variation occurs among animals of the 
 same species. In the following statement the temperatures are 
 all rectal or vaginal, and as far as practicable of animals at rest. 
 
 Horse: The temperature varies between ioo° and 100-5° F. 
 The average obtained by Hobday from a large number of obser- 
 vations was 100-3° F. A temperature of ioi° F. is compatible 
 with perfect health, but is exceptional. 
 
 Cattle: Robertson found among the fat stock at Smithfield 
 Show an average temperature of 10 1 -6° F. ; the temperature of 
 dairy stock was found by Wooldridgef to average 101-4° F., 
 
 * These records are very largely based on ' Notes on Physiological 
 Temperatures,' by Professor F. Hobday, Journal of Comparative Pathology, 
 vol. ix., No. 4, 1896. 
 
 f 'The Temperature of Healthy Dairy Cattle,' Professor WooJ- 
 dridge, Proceedings Royal Dublin Society, vol. x., part iii,, 1905. 
 
406 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 with variations between 100-4° and 102-8° F. Hobday places 
 the temperature of adult cattle at 101-3° F., with a physiological 
 range from 100° to 102° F. Calves have a rather higher tempera- 
 ture than adults, but, in fact, this applies to young animals of 
 all species. 
 
 Sheep : Wide variations compatible with health exist in this 
 animal. Hobday gives the temperature as ranging from 103-4° 
 to 1044° F., and thinks that 104° F. might be regarded as an 
 average temperature. 
 
 Swine : According to Hobday, the temperature of adults ranges 
 from ioo° to 102° F., of young pigs from 101-5° to I0 5'5° F- 
 
 Dog and' Cat : The average normal temperature is 101-5° F- 
 (Hobday), but the temperature may vary between 101-3° and 
 102-3° F- 
 
 A cause of variation in sheep and swine is the securing of the 
 animal and the overcoming of its resistance to the introduction 
 of the thermometer, but the chief cause of variation in all animals 
 but the horse is the limited power of the skin to get rid rapidly 
 of excesses of heat by sweating. Local temperatures, especially 
 of the limbs and feet in the horse, are frequently ascertained in 
 the routine of practice. Normally, these parts are ' cold ' to 
 the sense of touch, but in some forms of lameness they are hot; 
 even warm legs, and especially feet, are pathological. The.4egs 
 of the horse, it has been said, are cold, owing to the relative non- 
 vascularity of the underlying tissues; on the other hand, the car- 
 tilages of the ear have very little vascularity, yet cold ears are 
 pathological. Horn is a bad conductor of heat; the base of the 
 horns in cattle is frequently examined in judging of the presence 
 or absence of fever. The nose of the dog and muzzle of bovines 
 are ' cold ' in a state of health, due to evaporation of sweat from 
 these surfaces. 
 
 Variations in Body Temperature. — A rise or fall in body 
 temperature does not necessarily imply an increase or dimi- 
 nution in the production of heat. To prove increased heat pro- 
 duction it is necessary to show that the metabolism is increased, 
 that more oxygen is absorbed, and more carbonic acid produced. 
 In all animals there is a daily variation in temperature, the lowest 
 records being obtained in the early morning, 2 to 4 a.m., the 
 highest in the evening, 6 to 8 p.m., after which the temperature 
 falls during the night. These variations are due to metabolism; 
 during rest the metabolism sinks, while during activity it rises. 
 The temperature of the young animal is higher than that of the 
 adult, while the temperature of animals in temperate climates 
 living in the open is lower than that of those under cover; in the 
 case of the horse as much as i° F. difference in temperature has 
 been registered under this condition. In the tropics the tempera- 
 
ANIMAL HEAT 407 
 
 ture of animals exposed in the open, during the day, is higher than 
 that of animals under cover. Other causes of variation in tem- 
 perature will be considered presently. The thermometer does not 
 tell us the amount of heat formed in the body; it only indicates 
 the resulting difference between the heat produced and the heat 
 lost. These important points must now be studied. 
 
 Heat Production. — The oxidations occurring in tissues, and 
 leading to the production of heat, have previously engaged our 
 attention; these changes occur mainly in the skeletal muscles, 
 in which four-fifths of the daily heat are generated, and in active 
 glands such as the liver. The heat furnished by glandular activity 
 is amply demonstrated in the case of the liver, though certainly 
 not in all secreting glands. The temperature of the blood in 
 the hepatic veins is higher than in the portal, higher even than 
 in the aorta. It was shown by Bernard that in the dog, while 
 the portal vein was registering 39-6° C. (103-5° F-)» the blood 
 in the hepatic veins was at 41-2° C. (106-3° F.). Heat is formed 
 during muscular contraction. Experiments carried out on the 
 external masseter muscle of the horse showed that during con- 
 tractions the thermometer registered 2-8° C. (5-o 3 F.) higher 
 than in the same muscle at rest. 
 
 The muscles of the skeleton are not always actively contract- 
 ing, yet heat is always being furnished by them. The heat pro- 
 duced in muscle not actively contracting isNformed as the result 
 of muscle tonus — i.e., the stretched condition inherent inmuscle, 
 which explains the gaping that occurs when it is cut across. 
 This state of tonus is essential to posture. The simple act of 
 standing demands work from the muscles and evolves heat. 
 The part played by muscle tonus in contributing to the heat of 
 the body is more marked in horses than in cattle and sheep, which 
 lie for long periods during the day. As tonus leads to the absorp- 
 tion of oxygen and excretion of carbon dioxide, the loss of energy 
 due to the standing position may be ascertained experimentally; 
 it has been observed in cattle that the energy generated by muscle 
 tonus in the resting position may be 60 or 70 per cent, of that 
 so generated in the standing position. 
 
 As the blood streams out of the muscles its temperature is 
 higher than in the corresponding arteries, and in this way the 
 whole mass of blood would be raised in temperature were it 
 not for mechanisms by which the heat is dissipated. But the 
 excessive production of heat is not always compensated by a 
 sufficiently rapid loss, and a high temperature may in consequence 
 be produced. This is a most important point in connection with 
 working horses. In man a rise of i° to 1-5° C. in body tempera- 
 ture may occur as the result of work. In the horse half^ an 
 hour's trotting may raise the temperature from 0-4° to 1-4° C. 
 
4 o8 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 (07° to 27° F.) above the normal; the amount of rise is largely 
 a question of ' condition ' ; temperatures of 40 to 41° C. (104 to 
 106-5° F-) after hard work, especially in a hot sun, are not un- 
 common (Blenkinsop and Hobday). With rest the temperature 
 falls in the course of three or more hours, the mechanism for 
 getting rid of heat being able to act; as heavy a fall as 4° F. in 
 two hours has been recorded (Willis). In animals unfit for work 
 owing to want of condition the temperature may take longer to 
 fall, or even remain above the normal sufficiently long to be desig- 
 nated febrile; ' fatigue fever' is not unknown in man. If the 
 handling of animals produces excitement or struggling, the tem- 
 perature will rise. 
 
 The act of feeding, which involves increased muscular activity, 
 not only immediately, but subsequently in the muscles of the 
 whole alimentary canal, slightly raises the temperature of the 
 body. In the horse, according to Siedamgrotzky, the tempera- 
 ture as the result of feeding may rise 0-2° to o-8° C. (0-4° to 1-4° F.); 
 but, according to this observer, there is no similar rise in the ox; 
 on the other hand, Wooldridge found a rise of 0-3° F. in dairy 
 cattle. That heat is formed during the masticatory processes 
 we have already seen from the observations on the masseter 
 muscles of the horse ; but the mechanisms for regulating heat in 
 the body are such that a rise of anything like 1-4° F. as the result 
 of feeding must be regarded as exceptional. 
 
 We shall see presently that in the herbivora a large amount . 
 of heat is generated in the body in consequence of the ' work of 
 digestion,' but this is used up in cold weather in warming the 
 body, and in warm weather is an excretion. The intake of food 
 increases the heat production in all animals, protein by stimulat- 
 ing metabolism being particularly effective in this respect; but 
 in the herbivora there is, in addition, a heat production due to the 
 fermentative character of the food. This source of heat is 
 negligible in carnivora, more important in omnivora, but most 
 important in herbivora. An increased production of heat does 
 not necessarily entail a higher body temperature; if more' is 
 formed, more is got rid of. 
 
 Influence of the Nervous System on Heat Production. — 
 Heat production in muscles is under the control of the nervous 
 system ; impulses transmitted through the motor nerves, and thus 
 giving rise to chemical changes in the muscles, result in the forma- 
 tion of heat. These impulses mainly originate in the skin, and 
 by their means heat is produced according to requirements. If 
 an animal be poisoned with curare, the motor end-plates are 
 paralysed; in consequence, less heat is formed in the muscles, and 
 the temperature sinks; in fact, the animal becomes for the time 
 being practically cold-blooded, the body temperature rising and 
 
ANIMAL HEAT 
 
 409 
 
 falling with the surrounding temperature. The same condition 
 may be produced by dividing the spinal cord in the neck, by 
 which means the motor nerves are cut off from the muscles, and 
 the animal becomes practically cold-blooded. In chloroform 
 narcosis heat production is also greatly disturbed ; in prolonged 
 operations this should be borne in mind, and the loss of heat 
 guarded against. Shivering is a physiological process associated 
 with the production of heat to compensate for a loss. A hiber- 
 nating animal, on waking up, shivers, and thus rapidly raises its 
 temperature. The shivering which occurs in horses after being 
 ' watered ' is caused by the consumed water abstracting heat, from 
 the tissues in order' that its temperature may be raised to that 
 of the body. The ' freshness ' of a horse on a winter's morning 
 is the outcome of nervous impulses instinctively started with the 
 object of generating more heat in the body. 
 
 Heat Centres. — It is evident that the mechanism for the 
 production and loss of heat must be under the influence of some 
 nerve centre. 'A heat centre has been located in the corpus 
 striatum, and this is activated by the temperature of the blood 
 circulating through it. If the blood be raised in temperature, 
 the body mechanisms for producing loss of heat are stimulated, 
 the bloodvessels dilating, particularly those of the skin. If the 
 centre be cooled, vaso-constriction results. The removal of the 
 cerebral hemispheres and thalamus reduces the animal to a 
 permanently ' cold-blooded ' condition, though life may be main- 
 tained for some time — in the pigeon from one to three months — 
 by keeping the animal in an incubator at 30 C. (86° F.). 
 
 Experimental injury to the corpus striatum, the so-called 
 ' heat puncture,' causes an increased production of heat, which 
 may last for some time without, apparently, causing the animal 
 any inconvenience. It is supposed by some that this experi- 
 ment does not necessarily prove the existence of a heat centre, 
 the effect of the puncture being to produce a more rapid conver- 
 sion of liver and muscle glycogen into glucose, the increased 
 temperature being due to the increased combustion of sugar. 
 Others regard the rise in temperature as being due to stimulation 
 of the vaso-constrictor centre in the medulla. 
 
 Heat Loss. — Unless some arrangements existed in the body 
 for the regulation of the temperature, the heat resulting from 
 metabolic activity would continue to rise steadily until it accom- 
 plished the destruction of the animal. In order to maintain the 
 temperature at a constant point, heat production and heat loss- 
 must balance. This balance may be struck either by heat pro- 
 duction being diminished, or by increased loss of heat. The tem- 
 perature of the body may rise either in consequence of an actual 
 increase in metabolism, or through difficulties in getting rid of 
 
4io A MANUAL OF VETERINARY PHYSIOLOGY 
 
 heat. The process by which, within narrow limits, accurate and 
 prompt adjustment is made is known as heat regulation, or 
 thermotaxis. 
 
 Physical and Chemical Regulation. — The conditions influ- 
 encing the regulation of the body temperature are grouped into 
 physical and chemical. 'By physical regulation are understood the 
 changes occurring in the temperature and humidity of the air, 
 evaporation from the skin and mouth, losses by radiation and 
 conduction, and those occurring as the result of the food and 
 water consumed being raised to the body temperature. The 
 nervous mechanism involved in these processes consists of the 
 vasomotor nerves to the skin, the sweat nerves, and the respira- 
 tory centre. By chemical regulation is understood the mainten- 
 ance of body temperature by metabolism, the nervous mechanism 
 being the motor nerve supply to the muscles, while the fuel is 
 furnished by the food. The thyroid and adrenal glands are 
 believed to participate in the mechanism of chemical regulation, 
 for the thyroid stimulates the secretion of adrenalin, which by 
 increasing the discharge of liver glycogen leads to increased heat 
 production. Adrenalin in sufficient quantity may also diminish 
 heat production by causing constriction of the vessels of the skin. 
 
 These two regulations work in conjunction, and that the utmost 
 nicety exists in their interrelationship is shown by the relatively 
 slight changes in the daily temperature at all hours and at all 
 seasons of the year. We must look at this system of regulation 
 in greater detail. 
 
 If cold water be poured on a hot body, the body is cooled; 
 if the surface of a heated body be wetted and the water allowed 
 to evaporate, the body is cooled. If a cold body be placed in 
 contact with one which is hot, heat is lost. And processes some- 
 what similar to these are occurring in the animal body. 
 
 i. By radiation and conduction heat is lost to surrounding 
 bodies, provided, of course, that they are at a lower temperature 
 than that of the animal; if the surrounding medium is hotter 
 than the animal's body, then heat is gained instead of being 
 lost, so that radiation is of little importance in very hot climates. 
 The natural or artificial covering, be it hair, wool, or clothing, 
 checks the loss by radiation and conduction, as in a dry condition 
 these are bad conductors of heat. When wet, however, they are 
 good conductors, and a considerable amount of heat is lost by 
 sweating or rain. Clothing acts by imprisoning an amount of 
 warm air, the air so confined being a bad conductor of heat on 
 account of its being still. 
 
 2. By evaporation from the skin the sweat is converted into 
 vapour and heat is lost, the rapidity of the process depending 
 on the humidity of the air and its rate of movement. If the 
 
ANIMAL HEAT 
 
 4 n 
 
 air be saturated, no evaporation occurs from the skin, and, 
 consequently, no heat is lost. The value of evaporation as a 
 source of heat-loss in the horse is considerable, probably higher 
 than the figure fixed for man — viz., 14-5 per cent, of the total- 
 but no exact information is available. Evaporation is constantly 
 occurring; the sweat, when the amount is small, is evaporated 
 as fast as it is produced, and this is termed in the chapter on the 
 skin ' insensible perspiration.' The sensible perspiration is that 
 which is not evaporated as rapidly as it is produced, and is the 
 source of the most effective loss of heat. 
 
 3. Evaporation from the mouth ani nostrils, warming of in- 
 spired air, and vapourising of water from the lungs. The former 
 is a very valuable means of heat loss in those animals which do not 
 sweat from the general surface of the skin; the moist nose and 
 open mouth of the dog are good examples of the working of the 
 principle, and in a much smaller degree the bedewed muzzle of the 
 ox. The warmnig of the inspired air and the vapourising of 
 water from the lungs are most important sources of heat loss in 
 those animals which sweat imperfectly. The panting respirations 
 of the dog, and of cattle and sheep in ' show ' condition, are simply 
 a means of cooling the body by warming a larger volume of air, 
 and offering a moist surface for evaporation. All animals in hot 
 countries breathe more rapidly during the heat of the day. 
 
 4. By the urine and faces a loss of heat is incurred through the 
 food and water having been warmed to the temperature of the 
 body, though in both these cases there is no actual loss of heat 
 until they are excreted as urine and faeces. The amount of loss 
 thus brought about must be relatively considerable, especially in 
 winter; the abstraction of heat after drinking may be so great 
 as to cause shivering; observation shows that drinking a pailful 
 of water at 50 F. may cause the body temperature of the horse 
 to fall 0-5° to 0-9° F. A diet of ' roots,' containing 80 per cent, 
 water, is a heavy source of heat loss with cattle in winter. 
 
 The heat lost by conduction, radiation, and evaporation, is 
 greater in small than in large animals, as small animals have a 
 relatively larger surface exposed in proportion to their body 
 weight (see Fig. 114). A dog of 66 pounds weight will lose 
 79-5 per cent, of his body heat by radiation and conduction, 
 and 20-5 per cent, by the evaporation of water; whereas a dog 
 weighing 8 pounds will lose 91 per cent, by radiation, etc., and 
 9 per cent, by water evaporation. 
 
 Bulk of body is a safeguard in large animals against sudden 
 loss of heat, and all warm-blooded animals living in the sea are 
 bulky. It takes considerable heat-loss to lower the body tem- 
 perature of a bulky animal such as the horse; conversely, small 
 animals, in which, as we have seen, the cooling surface is relatively 
 
412 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 greater than in large animals, must be able to increase rapidly 
 their heat production. 
 
 The skin as a source of loss of heat is largely controlled by 
 the nervous system. Through the vasomotor nerves the vessels 
 are constricted or dilated; when the vessels are constricted, the 
 skin becomes pale (though this may not be seen, owing to hair 
 and pigment) and the blood is thrown upon the internal viscera, 
 where it is additionally shielded from loss. In consequence, the 
 skin becomes cold and the loss of heat less, not merely as the result 
 of the lessened radiation, but also and chiefly as the outcome of 
 the diminished evaporation. When the vessels are dilated the skin 
 becomes flushed and hot, the veins stand out with blood, and a 
 large amount of heat is lost. This vasomotor action is an auto- 
 matic reflex act, as also is the nervous control over the sweat 
 
 v 
 
 ^:- 
 
 :! 
 
 V.' 
 
 -M- — 
 
 s 
 
 -S 
 
 s; 
 
 ^=ns 
 
 -n- >- J- - 
 
 I-"- Tv" 
 
 t-' 
 
 
 I 
 NT- 
 
 ±7* 
 
 i 
 
 _vl J _J si 
 
 P 
 
 
 SL 
 
 --& 
 
 -+■ 
 
 
 -i-H- 
 
 Fig. 114. — Diagram to illustrate the Relation between Volume or 
 Weight and Surface (Waller). 
 
 The volumes are 1, 8, 27 cubic centimetres. 
 
 The weights are 1, 8, 27 grammes. 
 
 The surfaces are 6, 24, 54 square centimetres. 
 
 That is to say, their ratio of increase is 1, 4, 9. 
 
 glands, by which more or less water is poured out on the surface 
 of the body and heat lost by its evaporation. This nervous 
 control is normally set in action by changes in the temperature 
 of the surroundings. 
 
 The loss of heat is influenced by the thickness of the natural 
 covering, its colour, etc. That from a rabbit after shaving off 
 the fur is one and a half times greater than before shaving. A 
 sheep before shearing excretes less carbon dioxide and more water 
 than the same sheep after shearing. White rabbits lose 75 per 
 cent, of the heat lost by black or grey, for white not only absorbs 
 less heat during the day, but loses less heat at night. Grey horses 
 are better suited to the tropics than horses of any other colour, 
 and black horses least of all. The black skin of the negro pro- 
 tects the deeper tissues from the sun's rays, from which it might 
 be argued that black horses in theory should stand exposure to 
 
ANIMAL HEAT . 413 
 
 a tropical sun better than grey; but a grey horse has a black skin, 
 and the pigment prevents the rays from penetrating. Loss of 
 heat from the body surface can be experimentally produced by 
 varnishing the skin, and the animal then dies from cold unless 
 wrapped up in cotton- wool (see p. 324). 
 
 Influence of Heat and Cold. — A moderate degree of cold 
 applied to the external surface of the body increases the pro- 
 duction of heat, due to increased oxidations. This results, as 
 we have seen, from reflex impulses discharged through the motor 
 nerves. At the same time, in man, the appetite is increased to 
 meet the extra demand, and foods rich in fat are instinctively 
 sought for. This principle should be observed in the feeding of 
 animals, and an increase allowed in the food to meet the extra 
 oxidations, fat, if possible, forming part of it. The body will 
 stand a considerable degree of cold, but a point may be reached 
 where the rate of heat production is below that of heat loss, 
 and the animal dies from cold. Conversely, the body is adjusted 
 to withstand a moderately high external temperature; the heat 
 of Arabia or India, which renders surrounding objects, such as 
 metal, too hot to hold, and causes the very birds to sit with droop- 
 ing wings and wide-open mouths, is borne with impunity by the 
 acclimatised horse; the heat-regulating mechanisms do not allow 
 the external heat to be stored up. A continuous rise in external 
 temperature cannot be long borne, and a point is reached at which 
 the heat kills; for when the discharge of heat from the body ceases, 
 as it does when the surrounding air goes above a certain point, 
 heat becomes stored up, and heat-stroke follows. A far higher 
 temperature can be borne when the air is dry than when it is 
 moist, as evaporation from the surface practically ceases in a 
 moist atmosphere. Men have been exposed to a temperature of 
 127° C. (260° F.) for a few minutes without ill effects, and with 
 no elevation of the body temperature. When air has its humidity 
 increased by 1 per cent, the loss by radiation and conduction is 
 raised 32 per cent., while an increase of 25 per cent, in the humidity 
 of the air is equal to an increase of 2° C. in the external air. 
 At a temperature of 31° C. (88° F.) in an atmosphere saturated 
 with vapour the regulating mechanism of man is exhausted, and 
 a rise in body temperature occurs. Horses taken from cold to 
 hot latitudes have to learn to compensate, and until they do so a 
 febrile rise in body temperature, with increased respirations and 
 pulse, will occur as the result of standing in a hot sun, even though 
 doing no work. This passes away with acclimatisation, but in- 
 creased respirations on a hot day are always evident even in 
 animals of the country. 
 
 The loss of body heat by animals lying out at night is partly 
 prevented by the fatty covering to the peritoneal cavity, which 
 
414 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 prevents undue conduction of heat. Wet, combined with 
 exposure, causes a more important loss of heat than mere cold. 
 It has been shown from exact observations on man that a limb 
 clothed in wet flannel lost 34-4 per cent, more heat than the same 
 limb in dry flannel. Animals never look so wretchedly miser- 
 able as after a night of cold rain; under the conditions of active 
 service, a cold, wet night is certain to kill off the most debilitated. 
 A physiological resistance to cold can be obtained by training; 
 the body learns to regulate its loss and production of heat, and 
 this brings us to a consideration of the interesting practical point 
 of the necessity of clothing for animals, especially for horses, in 
 a state of domestication. Some animals, such as the horse, ox, 
 and sheep, are born fully developed and clothed ; in a few minutes 
 they pass from a temperature of between 101 to 105 F. within 
 the womb of the parent to perhaps freezing-point on the bare 
 ground. The power of regulating their temperature is fully 
 established, and in a very short time this is assisted by muscular 
 movements of the limbs, which are learnt very quickly; the 
 gambols of young animals serve some other purpose than that 
 of expressing mere lightness of heart. Cold has no effect on 
 these young creatures, if healthy, provided the parent. is able to 
 supply sufficient nourishment. There are other animals, such 
 as pups, kittens, rabbits, and certain birds, such as pigeons, 
 which are born blind, helpless, and more or less naked ; they can- 
 not move, are unable, to regulate their temperature, and if taken 
 from the maternal warmth, their body temperature steadily 
 declines and they die from cold. In these the capacity for regu- 
 lating body temperature does not develop for some little time after 
 birth, and until locomotion becomes possible. 
 . Effect of Low Temperatures. — We have seen, then, that.the 
 horse comes into the world prepared by its heat-regulating 
 mechanism to accommodate its body to the external tempera- 
 ture. In due course this is supplemented by an extra growth 
 of hair for winter use and a lighter covering for the summer. 
 If no interference with the coat be practised, it is undoubted that 
 no artificial covering of any kind is required during the coldest 
 weather, and even where the natural covering is of the lightest, 
 as with the thoroughbred horse, it is sufficient for its purpose. 
 The thoroughbred brood mares of this country, once they go to the 
 stud, live in the open for the remainder of their lives, and never 
 wear a blanket. And practical experience teaches us that this 
 condition may be gradually imposed on all horses with impunity, 
 even on those which have been kept in hot stables. Coughs, 
 colds, and inflammatory chest affections, usually attributed to 
 cold, are practically unknown among horses living in the open, 
 even during the coldest weather, and it is easy to show that these 
 
ANIMAL HEAT 4l5 
 
 diseases are largely the result of the artificial conditions under 
 which working horses have to live. 
 
 Jackson* has proved that the law of adaptability which applies 
 to men living in the Polar regions applies with equal force to 
 horses; for two and a half years he kept horses at 8o° north lati- 
 tude. During this time the thermometer never rose higher than 
 ii° F. above freezing, while 70 to 80° F. below freezing were 
 common temperatures. During the sleighing expeditions these 
 animals lived in the open, and wore one blanket, which, as may 
 be seen in Fig. 115, found its way where most blankets get, under 
 the hind-feet. At the time the photograph was taken the animals 
 
 Fig. 115. — Ponies living in the Open in the Arctic Circle at 45 F. 
 below Zero (Jackson). 
 
 were in a temperature of 45 F. below zero at night — viz., as far 
 below freezing-point as summer heat is above it. It is interest- 
 ing to note they did not suffer, and Major Jackson informs the 
 writer they were never frost-bitten, and never had a cough or cold. 
 Exposure to a low temperature renders the body sensitive to a 
 rise in the thermometer. Jackson tells us that the July tempera- 
 ture of 8° above freezing-point made it feel like midsummer, and 
 far too warm ! Conversely, living in a high temperature renders 
 the body very sensitive to a fall in the thermometer. 
 
 If a part be persistently protected against air-currents, it 
 becomes sensitive to exposure; if it be habitually exposed, a 
 * ' A Thousand Days in the Arctic,' F. G. Jackson. 
 
416 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 considerable degree of cold can be borne with impunity. Our 
 faces are never covered, and the hands by some only. People 
 who wear no gloves do not complain of cold hands, and it takes a 
 winter of Arctic intensity to make the face feel cold. Women 
 commonly wear far less clothing than men, and in consequence, 
 under the necessities of fashion, they exhibit a remarkable degree 
 of tolerance to cold. The bare-footed child is not conscious of 
 cold feet, nor is he, nor the hatless man, marked out as a subject 
 for catarrh. We have the evidence of Darwin* that unclothed 
 man can withstand the most tempestuous climate in the world 
 outside the Arctic Circle. The natives of Tierra del Fuego at the 
 time of Darwin's visit wore nothing; newly-born infants were 
 exposed like their parents. These people did not die from cold, 
 in spite of the rigours of the climate ; their heat-regulating mechan- 
 isms were evidently perfect. 
 
 Clipping. — Now that the clipping of horses can be done ex- 
 peditiously and cheaply, fashion decrees that the horse shall be 
 clipped all over. There is no necessity to remove the whole of 
 the hair of the body; for working horses limited clipping is all 
 that is required, and when this is done they do not feel the effects 
 of being robbed of their natural cover, even though no clothing be 
 supplied. After twenty-four hours they do not feel the cold any 
 more than a man does who has had his hair closely cut ; neverthe- 
 less, they are losing, in consequence, more heat, and therefore 
 require more food. It is economical to clothe the horse which is 
 wholly clipped. Siedamgrotzky observed the effect of clipping 
 on the temperature of horses. He found that the temperature 
 rose after the operation, and fell to normal about the fifth day. 
 Tt was observed that clipped horses had during exercise a higher 
 rectal temperature by i-8° F. than undipped horses, and the 
 return to normal was more steady and regular with them than 
 with undipped. The rise in temperature after clipping may be 
 due to vasomotor action; less blood being in the skin, more will 
 find its way to the viscera — viz., to parts of the body which have 
 a naturally high temperature, the result being that the total mass 
 of blood will have its temperature raised. Another way of ac- 
 counting for the rise in temperature after clipping is by suppos- 
 ing that an actual increase in the production of heat occurs; this 
 may be due to stimulation of the skin influencing the heat pro- 
 duction in the muscles. 
 
 Hibernation. — The effect of a fall in the temperature of the 
 bodies of animals is to produce a depression of metabolism. 
 This is well seen in some mammals — such as the dormouse — which 
 sleep all the winter, during which time they live upon the store 
 of fat laid up in the tissues during the summer. Owing to their 
 * ' Voyages of the Adventurer and Beagle.' 
 
ANIMAL HEAT 417 
 
 depressed metabolism this store is found sufficient to keep them 
 alive, though they wake up at the end of the winter mere skele- 
 tons. On their waking up shivering occurs, and the body tem- 
 perature rises by bounds to the normal, the animal then returning 
 to the condition of an ordinary warm-blooded animal until the 
 recurrence of the next period of hibernation. As to the causes 
 of this remarkable phenomenon we know but little. It is not con- 
 fined to only one class of animals, since it occurs in mammals, 
 amphibians, reptiles, etc. No purely anatomical differences suffice 
 to explain why some animals hibernate and others do not. Ex- 
 ternal cold is usually assumed offhand to be the initiating factor, 
 assisted possibly by the lessened food supply at the approach of 
 winter. But some other more recondite cause than either of 
 these must exist, since marmots may sometimes hibernate in 
 the summer, and dormice will hibernate even if kept warm 
 in the winter; cold will not necessarily cause an animal to 
 hibernate except at the appropriate season, and severe cold 
 may even arouse a hibernating animal from its state of 
 torpor. 
 
 Critical Temperature. — When an animal is fasting, as, for 
 instance, in the experiments made to determine the katabolism 
 of the body (see p. 366), the temperature remains constant, being 
 maintained by the metabolism incidental to the internal work of 
 the body (p. 395), there being a minimum consumption of the fat, 
 glycogen, and protein of the tissues. In addition, there is heat 
 produced from muscle tissues due to the necessary muscular 
 effort connected with standing. As the animal is kept as far 
 as practicable absolutely still, the heat formed in the body during 
 fasting is a measure of the internal work, and experimental in- 
 quiry shows that this quantity of heat is approximately the same 
 from day to day. The amount of heat determined in this way 
 fixes the amount of the maintenance ration (pp. 393, 39 8 )- 
 
 During a fasting experiment there is a favourable air tempera- 
 ture in which the amount of heat produced in the body is exactly 
 counterbalanced by the amount lost. This favourable air tem- 
 perature is known as the critical temperature. Above the critical 
 temperature there is heat to the good, which is got rid of by the 
 ordinary means of physical regulation; on the other hand, below 
 the critical temperature heat is being lost faster than it is being 
 made. It is a mistake to suppose that with every fall m air 
 temperature there is a call for more heat supply. It is not until 
 the critical temperature is reached that the heat production of the 
 animal is at a minimum; below the critical temperature there is a 
 call for more heat, and this is furnished by chemical regulation. 
 Conversely, by raising the external temperature no effect is pro- 
 duced on the quantity of heat produced by the body as the result 
 
418 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 of its expenditure of energy, for the energy required for the in- 
 ternal work is approximately constant. 
 
 The critical temperature has not been determined for all 
 animals: for the dog it is about 20 C. (68° F.); for the guinea- 
 pig, 30 to 35° C. (86° to 95° F.) ; for the pig, 20° to 30° C. (68° to 
 73° F.) ; for cattle, no definite temperature has been fixed (Armsby, 
 from his experiments, regards it as rather low) ; for man, wearing 
 ordinary clothes, it appears to be about 15° C. (59° F.). 
 
 If a fasting animal be placed on a maintenance ration, the air 
 temperature being above the critical point, the effect of the food 
 is to cause an increased production of heat which lasts some hours. 
 The cause of this is the mechanical work involved in mastication, 
 the passage of the material along the digestive tract, the glandular 
 activity excited, the stimulating effect on metabolism produced 
 by the food elements, especially protein, and in the herbivora 
 the fermentations arising mainly from the digestion of cellulose. 
 To these Armsby adds, as the result of his experiments, a variety 
 of undefined substances in the food of the herbivora which stimu- 
 late tissue metabolism. The heat arising in this way has been 
 termed ' the work of digestion.' The whole of the heat formed 
 as above is not the outcome of vital functions, that resulting from 
 the fermentation of cellulose is, if we may so describe it, heat to 
 the good, for though the cellulose is largely, if not entirely; lost 
 to the body as nutriment, the heat resulting from its fermentation 
 remains, and as the critical temperature approaches, instead of 
 being got rid of as an excretion, is used to warm the body. The 
 consequence of this is to lower the critical temperature, and the 
 more liberal the feeding the lower it falls. In the case of the 
 horse 100 grammes of digested crude fibre and other carbo- 
 hydrates produce 697 large calories due to fermentation, and in 
 ruminants 60 calories. All this is available for body heat when 
 the air temperature falls. The advantage of this to the her- 
 bivora is considerable, and explains why, when well fed, they are 
 able to stand cold and exposure with impunity; the heat arising 
 from the fermentation of the cellulose and other carbohydrate 
 material, instead of being got rid of is utilised to warm the body, 
 and by so doing it conserves the chemical regulation and lowers 
 the point at which combustion of the tissues has to make good 
 the loss due to cold. This explains the common mistake of 
 regarding the feelings of herbivora and our feelings to cold as 
 being identical. It cannot be too distinctly stated that if 
 animals are well fed they can withstand practically any degree 
 of cold and exposure without suffering in health. Hot stables, 
 cattle-sheds, and cow-houses are a fruitful source of disease; yet 
 the prejudice against cool surroundings is widely spread; it is 
 largely based on our own subjective feelings to cold, which, as 
 we have shown, are not identical with those of animals. 
 
ANIMAL HEAT 
 
 419 
 
 The Amount of Heat produced by animals depends upon the 
 rate of their metabolism and the surface area of their bodies; 
 the latter factor determines the loss of heat, and hence its produc- 
 tion if the temperature of the body is to be kept constant. A 
 large animal produces actually, but not relatively, more heat than 
 a small one; a small animal, as has been previously stated, has a 
 greater body surface relative to its weight than a large animal, 
 and in this way its loss is greater. As heat production must 
 balance heat loss, the small animal must relatively lose more heat, 
 and therefore produce more heat, than the large animal. 
 
 Thermometer: 
 
 ennometer 
 
 Air inlets 
 
 •') ->Air outlet 
 
 Fig. 116. — Calorimeter. 
 
 The animal is resting in the chamber, which has a double wall containing water, 
 There is an air inlet and outlet to the chamber, and delicate thermometers 
 for ascertaining the temperature of the water, and of the incoming and 
 outgoing air. The whole is placed within another chamber and surrounded 
 by packing of non-conducting material. 
 
 A, Outer insulating chamber; B, middle water-chamber; C, inner chamber. 
 
 The heat produced is measured as heat-units or calories — 
 viz., the amount of heat required to raise 1 gramme of water 
 i° C, known as the small calorie, or the amount required to raise 
 1 kilogramme of water i° C, which is the large calorie, sometimes 
 spoken of as the ' kilo-calorie.' 
 
4 20 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The method by which the heat given off by a body is ascertained 
 is by means of a calorimeter (Fig. lib). This is a chamber with a 
 double wall containing air or water, which absorbs the heat given 
 off, say, from an animal in the chamber. This chamber is contained 
 within another, and the two are separated by non-conducting 
 material in order to prevent loss of heat by radiation. The weight 
 of the water in the calorimeter is known, and the extent to which 
 its temperature is raised during the experiment ascertained; a simple 
 calculation shows the number of calories given off during the observa- 
 tion. The temperature of the animal's body before and after the 
 experiment shows whether the loss of heat has been made good or 
 not, or whether, on the contrary, the body has produced more heat 
 than it got rid of. By the process of calorimetry the heat value of 
 food is also ascertained. In the respiration calorimeter described 
 at p. 360 the heat given off from the body is compared with the heat 
 contained in the food, and the energy expenditure ascertained. 
 
 In the present state of our knowledge it is not possible to state 
 with anything like precision the number of calories required by 
 animals for maintenance or production. The figures as they 
 stand at present have been given in the chapter on Nutrition 
 
 (pp. 398, 399)- 
 
 When the amount of heat produced daily by animals during 
 fasting is compared with the area of the body, a close agreement 
 is found between various animals of different sizes. The explana- 
 tion is that already given at p. 411 ; the large animal produces 
 more heat than the small, owing to its greater katabolism, but 
 the relatively larger surface and more intense katabolism of the 
 small animal eliminate the difference due to size. It has been 
 stated that the body surfaces of animals of geometrically similar 
 figures are proportional to the two-thirds power of their weights, 
 and on this basis calculations of body surface for cattle and a few 
 for horses have been made; but as Armsby properly points out, 
 young and old, and fat and thin animals are not geometrically 
 similar. The following table, however, which gives the number 
 of large calories produced in 24 hours per square metre of 
 surface, shows that the results obtained with some animals 
 closely approximate : 
 
 Number of Large Calories. 
 
 Man . . . . . . . . 1,042 
 
 Dog . . . . . . . . 1,036 
 
 Pig .. .. .. .. 1,045 
 
 Mouse .. .. .. .. 1,188 
 
 The figures for cattle and horses are not reproduced, as they are 
 too doubtful, while those calculated for the sheep by Armsby 
 show that this animal does not obey the law. The metabolism 
 of sheep per unit of surface is little more than 60 per cent, that of 
 cattle. This is not apparent from the table given at p. 398, but 
 is evident when the metabolism of the two animals is compared 
 
ANIMAL HEAT 421 
 
 to the body surface. Armsby suggests that in the sheep the thick 
 coating of wool tends to reduce the amount of heat lost, so that 
 in the process of evolution the intensity of metabolism and the 
 intensity of heat emission in this animal have undergone mutual 
 adjustment. 
 
 Post-mortem Rises of Temperatures are frequently observed, 
 and some of the observed temperatures have been remarkably 
 high. The writer recorded in the horse 109 F. between the liver 
 and diaphragm. The explanation is that metabolism is still 
 occurring in some of the tissues, but since there is no circulation 
 to carry the heat away the temperature of the part rises. 
 
 Pathological 
 
 The actual processes occurring in fever are not known. It cannot 
 be definitely stated whether the increase in temperature is due to 
 an increased production of heat or to a disturbance of the mechanism 
 by which the loss of heat is regulated — probably both causes are 
 in operation. It has been supposed by some that fever may be 
 due to the action of bacterial poisons on the heat-centres, and it 
 has been stated that cultures which produce fever in the intact 
 animal no longer do so on division of the pons, which cuts off the basal 
 ganglia heat-centre from the rest of the body. Others have considered 
 that, though the nervous mechanism is at fault in some cases, in others 
 the heat-forming tissues themselves are at fault, being too active. 
 This view receives some support from the action of agents employed 
 in the treatment of fever; whereas some, like quinine, act on the 
 tissues, others, like antipyrin, appear to produce their effects through 
 the nervous system, particularly by their action on the vessels of 
 the skin. Fever leads to a marked increase in the metabolism of 
 protein. This is shown by an increase in the output of nitrogen; 
 uric acid appears in the herbivora ; ammonium salts of organic acids 
 and creatinine are increased, and creatine, which normally does 
 not appear in the urine, may now be found. The absolute amount 
 of urea is greater than normal, but its proportion to the other 
 urinary nitrogenous substances may be relatively less. Fever is 
 probably a beneficial mechanism in infective processes by assisting 
 in the destruction of bacteria and their products; nevertheless, high 
 temperatures effect great damage to the body-tissues, especially to 
 the heart-muscle, and methods which control temperature appear 
 clinically to give the best results in treatment. 
 
CHAPTER XIII 
 
 THE MUSCULAR SYSTEM 
 
 The muscular system is the largest in the body, the skeletal 
 muscles alone representing 45 per cent, of the body weight. 
 
 The regulation of the blood-supply, the movements of the 
 skeleton, the contraction of the heart, and the transport of the 
 ingesta along the intestinal canal, are all examples of muscular 
 movement, and, further, they are examples of different kinds of 
 movement; the slowly moving muscle of the intestinal canal is 
 very different from the active skeletal muscles, and these, with 
 their long periods of activity and rest, are in great contrast to the 
 rhythmical movements of the heart muscle. 
 
 Structure of Muscle. — There are three varieties of muscle in 
 the body: 
 
 1. Voluntary, skeletal, striped, or red muscle. 
 
 2. Involuntary, pale, smooth, or unstriped muscle. 
 
 3. Heart muscle. 
 
 The voluntary muscles exist generally in large masses known as 
 " flesh,' and their function is to move the skeleton. The muscle 
 mass consists of bundles, the bundles are composed of smaller 
 bundles, the smaller bundles are made up of fibres. The fibre of 
 a muscle does not run the length of the bundle; on the other 
 hand, a primitive fibre is only about 4 or 5 centimetres (ij to 
 2 inches) in length, and of microscopic thickness — viz., -5^ inch 
 on an average. The fibre is as a rule unbranched, its ends are 
 rounded, and it is enclosed within a sheath, or sarcolemma. Each 
 fibre is made up of bundles of minute fibrils or sarcostyles em- 
 bedded in a semi-fluid contractile substance known as sarco- 
 plasm. The fibrils constitute the contractile substance of the 
 fibre; they show markings which produce the transversely 
 striated appearance characteristic of red muscle, and are com- 
 posed of sarcous elements or- sarcomeres, minute bodies of peculiar 
 structure placed end to end in the fibril like beads on a string. 
 
 In some muscles, such as those of the heart and diaphragm, 
 the sarcoplasm exists in relatively large amounts; such muscles 
 
 ♦ 422 
 
THE MUSCULAR SYSTEM 423 
 
 axe deeper in colour than ordinary skeletal muscle; they contract 
 more slowly, and may be said never to tire. Striped muscle may 
 therefore be dark as in the case of the heart, full tinted as in the 
 ordinary skeletal muscles of the larger herbivora, or a pale blood- 
 less-looking structure, such as is found in the rabbit and some 
 fishes. In the rabbit both pale and red fibres may be mixed up 
 in the one muscle. These pale striated muscles exhibit structural 
 differences from ordinary striated muscle. 
 
 The nerve supply to voluntary muscle is both motor and sen- 
 sory: through the sensory nerves the brain is made acquainted 
 with the posture of the body and the condition of muscular 
 tension. This implies the existence of a special muscle sense, 
 which plays such an important part in locomotion. In the 
 muscles this sense is served by special bodies generally found 
 near tendons, called nettro-muscula-r spindles ; these are from 
 I to \ inch in length, and ^5 inch in width ; each spindle is of 
 muscle surrounded by a sheath, and has a sensory nerve entering 
 it at one end. Nervous structures, known as the tendon organs 
 o/Golgi, also exist in the tendons at their junction with the muscle 
 fibres; they consist of spindle-like bodies connected with one or 
 more fine, medullated nerve fibres. These nerves are in communi- 
 cation with the portion of the brain which is devoted to ' muscle 
 sense.' The ordinary degree of sensibility in muscle is not very 
 great unless the part be cramped or inflamed, though pain is 
 caused when they are cut into. By means of the motor nerves 
 the muscle is supplied with impulses which, originating in the 
 brain, passing thence along the spinal cord, and emerging by the 
 ventral roots, finally reach the skeletal muscle concerned, and 
 bring about contraction. Division of the motor nerves, or inter- 
 ference with their function, causes paralysis of the muscle or 
 muscles supplied by them. Each motor nerve enters the primi- 
 tive fibre about its centre, and terminates in a special organ 
 known as an end-plate. By means of curari this end-plate may be 
 paralysed, in which case stimulation of the nerve leads to no 
 muscular response in consequence of the block, though the muscle 
 itself remains irritable, and readily responds to direct stimulation. 
 
 The question of the nerve supply to muscles will receive fuller 
 consideration when the subject of ' muscle sense ' is dealt with 
 in the chapter on the senses (Section IV.). It will also engage 
 attention in the chapter on the Nervous System, when the ex- 
 tremely important subject of reflex inhibition is discussed. It 
 will suffice here to point out that the nerves conveying impulses 
 which pass from muscles, which later on will be known as afferents, 
 are numerous, while the nerves conveying impulses which pass to 
 muscles, known as efferents, are fewer and are limited in function 
 to the transmission of those impulses which stimulate contraction. 
 
424 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 There is no set of efferents to a voluntary red muscle which pre- 
 vents or controls contraction, though obviously this is as im- 
 portant as the initiation of it by excitatory impulses. How 
 this difficulty is overcome is a matter for future consideration. 
 
 Masses of material of the structure outlined above are intended 
 for the transport of the body, for which purpose they are united 
 to the skeleton either by tendons or by the direct insertion of their 
 own fibres. In the muscles of the limbs, and wherever, in fact, 
 the parts are exposed to great strain, the tendon insertion is the 
 most usual. There are certain muscles in the machine on which 
 the strain is so considerable that tendinous material is intimately 
 mixed up with the muscular tissue ; this is well seen in the mas- 
 seters, the muscles of the back, forearm, and thigh. The muscles 
 of the limbs, back, and abdomen are also supported by a ban- 
 dage in the form of fasciee; this is thick and elastic over the abdo- 
 men, fibrous around the limbs, and of great strength. The fascia 
 of the thigh of the horse frequently maintains in position the ends 
 of an undisplaced fracture of the tibia, sometimes for days, and 
 even under the influence of work in draught ! That of the arm 
 is not so powerful. 
 
 During progression the muscular strain is enormous; for ex- 
 ample, in the canter and gallop a weight equivalent to that of the 
 whole body is propelled by one fore-leg (see chapter on locomo- 
 tion) . In the horse provision is made for the muscles of the limbs 
 being rested without necessitating the animal assuming a recum- 
 bent position; this mechanism will be explained when we come 
 to deal with the locomotor system ; by means of it an animal can 
 sleep standing, and may remain standing for some weeks without 
 serious suffering. 
 
 » Involuntary, or Pale Muscle, is not found in masses as is the 
 red, but in thin sheets, which in places, such as the bloodvessels, 
 are only of microscopic thickness. Pale muscle is employed 
 throughout the whole length of the digestive canal from the 
 stomach to the rectum; it is also found in the bladder, uterus, 
 spleen, bloodvessels, and elsewhere. In none of these places is the 
 sharp, short, active contraction of skeletal muscle required; slow, 
 steady, deliberate movements are essential in the digestive canal; 
 slow, steady, expulsive movements are necessary in the bladder 
 and uterus, and even in the bloodvessels, where, as we have seen, 
 the muscular tissue acts like a tap, it is sufficient if this tap is 
 turned on and off slowly and steadily. 
 
 In structure pale muscle consists of nucleated, longitudinally 
 striated, spindle-shaped cells, dovetailed and held together by a 
 cementing substance ; it is through the medium of this substance 
 that the wave of excitation passes from cell to cell, in which 
 respect pale muscle forms a great contrast to voluntary muscle, 
 
THE MUSCULAR SYSTEM 425 
 
 where, as we shall see, the whole contracts, not by the spread of 
 the stimulus from one fibre to another, but as the result of all 
 the fibres being stimulated simultaneously. There are nerves 
 and ganglion-cells in abundance in pale muscle; the nerves, 
 which are chiefly non-medullated, form a fine plexus with the 
 ganglion-cells, which are placed at the junctions of the plexus. 
 It is probably due to these cells that involuntary muscle continues 
 to contract when all connections with the centre are destroyed, 
 though some physiologists see reason for thinking that the con- 
 traction of pale muscle may be carried out just like that of heart 
 muscle^— namely, as a self-acting mechanism, independent of any 
 nervous connections. 
 
 The nerve supply of involuntary muscle is peculiar, and 
 presents a great contrast to that of red muscle; whereas the 
 latter receives only one variety of motor supply, pale muscle 
 receives two, there being one set of fibres which stimulates contrac- 
 tion, and another which inhibits it. Both sources are derived 
 from the sympathetic system. 
 
 Heart Muscle, as we have seen (p. 33), is in structure both red 
 and striated; nevertheless it is involuntary; the fibres are char- 
 acterised by being formed of branched, nucleated, quadrilateral 
 cells, while the sarcolemma is absent. We have also seen that 
 heart muscle possesses automatic action and a double nerve 
 supply which increases and decreases its activity. In these 
 respects it closely resembles pale muscle. In common with all 
 varieties of muscle it possesses tonus (see pp. 407, 434) ; its fibres 
 may be stretched by the dilatation of the cavities of the heart 
 without the stretching producing increased tension on the heart's 
 contents, as is the case with other hollow organs — for example, 
 the bladder. The increased length of fibre thus obtained- gives 
 rise, as in voluntary muscle, to a contraction of greater energy. 
 
 Muscular Contraction. — This apparently simple act is ex- 
 tremely complex, and will require to be dealt with in some little 
 detail. 
 
 Muscles are tissues possessed of irritability and contractility 
 — viz., they possess the power of responding by a movement to 
 the application of a stimulus. The normal stimulus is effected 
 through the motor nerves under the control of the brain or spinal 
 cord, but of the nature of this stimulus we are ignorant. A 
 coarse reproduction of it can be effected by pinching, pricking, 
 chemical, thermal, or electrical stimuli, applied to either the 
 nerve or the muscle itself, and to all these the three varieties of 
 muscle are responsive. When a muscle contracts, the energy 
 set freeis directly proportional to the length of its fibres immedi- 
 ately before they begin to contract. As the result of contraction 
 there is no change in the volume of a muscle, but it becomes 
 
426 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 shorter and thicker; it undergoes changes in its extensibility, 
 elasticity, and temperature; there are also alterations in its 
 electrical condition and chemical composition, and a greatly 
 increased flow of blood through it. 
 
 Most of the information which has been gained regarding mus- 
 cular contraction has been ascertained by employing the muscles 
 of the frog, which retain their irritability for a long time after re- 
 moval from the body. Such a muscle suitably prepared is known 
 as a nerve-muscle preparation, and with certain modifications 
 
 what is found to occur in 
 this as the result of con- 
 traction occurs also in the 
 living mammalian muscle. 
 
 The muscle most usually 
 and conveniently employed 
 for investigating the phe- 
 nomena of muscular con- 
 traction is the gastrocnemius 
 of a frog, dissected out in 
 such a way as to leave 
 its upper tendon connected 
 with a piece of the femur 
 and its lower tendon, the 
 tendo A chillis, intact though 
 free. At the same time care 
 is taken not to sever the 
 connection of the muscle 
 with its motor nerve, the 
 sciatic, which is dissected 
 out for some considerable 
 distance back towards its 
 point of exit from the spinal 
 canal, the central end being, 
 if desired, left connected to 
 a portion of the spinal cord 
 enclosed in a piece of the 
 lower end of the spinal 
 column. The muscle is then suspended by fixing the remains of 
 the femur in a clamp; a small S-hook is attached to the tendo 
 Achillis. This muscle-nerve preparation and its arrangement as 
 above described is shown in Fig. 117.' 
 
 For purposes of experiment the preparation is fixed inside a small 
 chamber with glass sides to prevent the drying of the muscle and 
 nerve; this is effected by placing a few pieces of wet filter-paper in- 
 side the chamber. The sciatic nerve is laid over a pair of electrodes 
 connected by wires to binding-screws outside the chamber; by this 
 means any desired electrical stimulus may be applied to the nerve. 
 A thread attached to the hook in the tendo Achillis passes through 
 a hole in the floor of the moist chamber, and is connected with a hori- 
 zontal lever free to move in a vertical plane, a small weight being 
 hung under the lever to give the muscle the ' load ' necessary for 
 its efficient contraction. The free end of the lever is then brought 
 
 Fig 
 
 117. — A Nerve-Muscle Preparation 
 (Foster). 
 
 The gastrocnemius muscle of a frog; n, 
 the sciatic nerve dissected out back to 
 Sp. c, the lower end of the spinal canal; 
 /, femur; cl, clamp; t a, tendo Achillis with 
 S-hook attached for carrying a weight. 
 
THE MUSCULAR SYSTEM 
 
 427 
 
 to bear against the vertical surface of some recording apparatus — 
 usually a cylindrical drum covered with smoked paper — made to 
 rotate by clockwork about a vertical axis. 
 
 An inspection of Fig. 118 shows at once that if the drum of the 
 
 & .a ja j-. jb 
 § .2 a n o 
 
 a; -5 ■? J= " 
 
 • J3 
 
 5 g.S 
 
 > ; w *• G 
 
 O ^* V yi O 
 
 ^ St 03 2 
 
 ■2 JH a S ^ 
 c 5 8 J M 
 
 3 ~ S .5 % 
 » $. "> m 2 
 
 r/, ^ O 0) 1-1 
 
 -" £ JJ «a ^3 
 
 £ -3 & 9. 
 
 « 
 
 r? H' 
 
 ^ « 
 
 25 tf in 
 
 a> B o !_, _, 
 
 .3 Sd5o 
 
 «fl O O 
 
 ° g 8 1' .2 
 hi 
 
 C 
 
 1> 
 
 3 ?*. cq -3 A" 
 
 5 , w ^ Q ^ 
 «£ • ■- &©_, -^ 
 
 ui °] a ™ <■*-> 
 
 . p ^ -3 o o 
 
 ^ tj o =4 o 
 
 "2 o o ' ±j 
 
 E; 5 co g S g 
 
 § ^ -O S g o 
 
 .- -£> CTJ O c l-> 
 
 ■ -e'S *-« 
 
 ® hi a. 
 
 a = 5 « 's o) 
 
 6 g rt O u *» 
 
 - . >, ■•' to U 
 5-5 ,D J? ,. 
 
 i: is * '5 '3 ^ , 
 
 8 
 
 ■ °, ui « — 
 
 2 .2 <o 6 
 
 ( (D 
 
 ■R 5 
 H 
 
 <D 
 
 [fl 0j 
 
 ££5 
 
 recording instrument rotates, the end ot the lever connected to the 
 muscle must trace out a horizontal line on the smoked surface. 
 If the drum is stationary and the muscle is made to contract, the 
 
428 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 lever will trace a vertical line. If now the muscle is made to con- 
 tract while the drum is rotating, a curve is traced of which the 
 ordinates of the various points and the general shape give us exact 
 information as to the details of the contraction from start to 
 finish. 
 
 The electrical currents employed in stimulating muscles to 
 contraction are of two kinds — the galvanic or constant current, 
 and the induced or interrupted current. 
 
 The galvanic current is conveniently supplied by a Daniell's cell, 
 (E, Fig. 1 1 8), in which zinc is the positive and copper the negative 
 element. These metals are connected with wires to the poles or 
 electrodes. The electrode attached to the zinc element is not 
 however, the positive, but the negative, or kathode, pole, and, 
 similarly, the electrode attached to the copper is the positive, or 
 anode, pole. Within the battery the current flows from zinc to 
 copper outside the battery— viz., between the poles it flows from 
 copper to zinc. The circuit is completed by bringing the poles 
 together, known as ' making ' ; it is broken by separating the poles, 
 known as ' breaking.' When this current is passed through a 
 galvanpmeter — an instrument which detects the existence, direction, 
 and intensity of currents — it flows through this apparatus from the 
 positive to the negative pole. So much confusion results from the 
 use of the terms ' positive ' and ' negative ' — which, as we have 
 seen, have two opposite meanings, depending upon whether the 
 current is passing through the battery or through the poles — that 
 Waller has proposed that, physiologists should call the copper 
 ' galvanometrically positive,' and the zinc ' galvanometrically 
 negative,' in the same way that physicists employ the terms ' electro- 
 positive ' and ' electro-negative ' to indicate the positive and 
 negative elements respectively. 
 
 The induced or interrupted current is produced by passing a gal- 
 vanic current through an induction-coil (D, Fig. 118). This con- 
 sists of a primary coil, pr c, connected with the Daniell's cell, E, 
 and a secondary coil, sc D, capable of a sliding adjustment near to 
 or away from the primary coil. The nearer the coils are together, 
 the stronger the current. To the secondary coil wires are attached 
 conveying the induced current. Between the battery and the 
 primary coil is introduced an appliance for making or breaking the 
 current. This may be done by hand, when single shocks are re- 
 quired, by means of the key F, Fig. 118, or automatically by means 
 of an interrupter, when a rapid succession of make and break 
 shocks are required. This interrupter may be seen in Fig. 118 to 
 the right of the standard supporting the primary coil. 
 
 The most conveniently controllable stimulus for experimental 
 purposes is that of single induction currents or the interrupted 
 current of an induction-coil. These have the advantages of being 
 extremely efficient as stimuli, and of giving rise in the nerve to 
 impulses which may be regarded as the nearest artificial approach 
 to the impulses which in the body are discharged along the nerves 
 by the cells of the central nervous system. 
 
 If by means of a myograph (Fig. 119) a tracing of a muscle 
 contraction is made on a revolving drum, a muscle curve is pro- 
 duced. I A muscle curve consists of three parts: (1) A period 
 
 ■I 
 
THE MUSCULAR SYSTEM 429 
 
 following stimulation during which no contraction occurs; it is 
 known as the latent period, and during this time the muscle is 
 preparing itself for work ; (2) a period of contraction or shorten- 
 ing! (3) a period of relaxation (see Fig. 119). 
 
 The length of time involved in the various phases of a con- 
 traction may be measured by bringing a time-recorder, vibrating 
 fractions of a second, to bear on the smoked surface of the 
 revolving drum; the sinuous line in Fig. 119 below the muscle 
 curve is produced in this way. Though the nerve was stimulated 
 at the point marked 1 on the tracing, it was not until 2 was reached 
 that the muscle responded, the time value for the latent period 
 being T £ T second. As a matter of fact, the latent period is less 
 than this, for a very slight loss in time occurs in the impulse 
 passing along the nerve, and there. are also instrumental defects, 
 such as friction of the lever, to be allowed for. When these 
 allowances are made, 
 the latent period is 
 reduced to about ^ho 
 second. The entire 
 contraction from the 
 instant of stimulation 
 to the end of relaxa- 
 tion occupies about 
 
 •jV Second. p IG , II9 . — The Curve from a Single Con- 
 
 The latent period, traction of the Gastrocnemius Muscle 
 
 contraction and relaxa- ° F THE Frog (Waller). 
 
 tion of a muscle are From 1 to 2 is the latent period; from 2 to 3 the 
 
 affpr+wl Vw rprtain rnn P eriod o£ shortenin g; from 3 to 4 the period of 
 
 attected by certain con- relaxation. The sinuous line below the curve 
 
 ditions. It is found indicates periods of jfa second. 
 
 that with increase in 
 
 the weight a muscle may be called upon to lift, the shorter the 
 contraction and the longer the latent period. If we continue to 
 weight the muscle a point is reached when it is unable to lift the 
 load. With an increase in the strength of the stimulus there is, up 
 to a certain point, increased contraction, and also a shorter latent 
 period; beyond this point, known as a ' maximal stimulus,' an 
 increased stimulus produces no effect. 
 
 If a fresh preparation be kept contracting and recording its 
 movements it will be observed that as the muscle settles to work 
 the first few contractions successively improve, each being 
 stronger than the preceding. This progression is known as the 
 beneficial effects of contraction, and the character of the record is 
 described as 'staircase' (see Fig. 120). The contractions then 
 become less, the latent period, the period of contraction, and 
 especially that of relaxation, become longer as the muscle ap- 
 proaches the condition of fatigue. This may be seen in Fig. 121, 
 
43° 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 where the muscle has been made to contract at the same point 
 at each revolution of the drum. 
 
 If the muscle be now left to itself it will recover, though on 
 restimulation it is more easily fatigued than before 
 
 The effect of heat on a muscle preparation may, in very general 
 terms, be stated as follows: Moderate warmth increases the height 
 of the contraction and diminishes the latent period and length 
 of the curve. Cold decreases the height of the contractions. Very 
 low temperatures cause the muscle to lose its irritability. A 
 high temperature, 42 C. (107° F.), produces heat rigor and kills 
 the muscle by coagulation of its plasma. 
 
 A practical demonstration of the destructive influence of a 
 high temperature on heart muscle is afforded by pneumonia. 
 Horses may be killed by this disease in the course of forty-eight 
 
 hours, at a time when ample 
 area for ventilation remains in 
 the lungs. The sodden parboiled 
 appearance of the heart muscle 
 tells its own story. 
 
 The effect of veratrine poisoning 
 on muscular contraction is very 
 remarkable; apart from the fact 
 that the curve sometimes shows 
 a second contraction, there is 
 a prolonged period of retarded 
 relaxation or maintained con- 
 traction (see Fig. 122). 
 
 Summation. — If instead of a 
 single stimulus being passed into 
 a nerve-muscle preparation two 
 are sent in, so timed that the 
 second follows the first before 
 the muscle has time to relax from the first contraction, the 
 contraction due to the second stimulus is superposed on the 
 first, and the effect obtained is a stronger total contraction. 
 If a third stimulus be sent in before relaxation occurs from the 
 second, the lever of the muscle preparation will describe a curve 
 still higher than the preceding, and so on, until a maximum is 
 reached when it can go no higher. Such a piling up of con- 
 traction on contraction is known as summation of contractions 
 (Fig. 124). 
 
 Tetanus. — If an induction current be applied to a muscle or 
 its nerve, a rapid succession of stimuli is thus introduced, and 
 there is no time for complete relaxation to occur between each 
 successive stimulus. This may be seen in the lowest curve of 
 Fig. 124, in which ten stimuli per second were passed into the 
 
 Fig. 120. — ' Staircase' Contrac 
 tions of Skeletal Mltscle 
 Frog (Stewart). 
 
THE MUSCULAR SYSTEM ^ 
 
 43i 
 
 muscle, and partial relaxation only will be observed to have 
 occurred between each of them. In the middle curve twenty 
 
 Fig. 121. — Fatigue Curve of Skeletal Muscle: Gastrocnemius of 
 Frog (Stewart). 
 
 Time-tracing T J- 5 of a second. The curve is read from right to left. 
 
 wmmwmAMMM^mmwmwnwii:iwyi':y\\:yr::;?^mx!;. , ;<'' '.wftv.VA".' |/av*v/vuvwiai a'.wiwa 
 
 Fig. 122. — Veratrine Curve compared with Normal: Frog's 
 Gastrocnemius (Stewart). 
 
 The tuning-fork marks hundredths of a second. Between 1 and 2 a portion of 
 the time-tracing corresponding to one and a half seconds has been cut out, and 
 between 2 and 3 a portion corresponding to one second. The veratrine curve 
 does not show a peak. At 3 it has not yet fallen to the base-line. 
 
 stimuli per second were used, and the amount of relaxation is 
 represented by a slightly wavy line; in the upmost curve thirty 
 stimuli per second were employed, and the curve shows 
 
432 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 no relaxation; the muscle is in the condition of tetanus. 
 Tetanus, therefore, consists of a series of short contractions with 
 an insufficient interval for intervening relaxation.* The number 
 
 Fig. 123. — Superposition of Contractions (Stewart). 
 
 1 is the curve of contraction due to the first stimulation; 2 is the curve of the 
 second contraction, superadded to r by applying the second stimulus at the 
 moment when the first contraction had nearly reached its maximum. 
 
 of stimuli per second required to provoke tetanus depends on 
 the condition of the muscle, the nature of the muscle, and the 
 class of animal; for example, in the bird, ioo stimuli per second; 
 
 Fig. 124. — Summation Curves of Muscle Contraction (Waller). 
 
 The lowest curve is one obtained by stimulating the muscle ten times every second; 
 the intervals of relaxation are clearly seen, though there is a slight summa- 
 tion shown by the slanting rise of the tracing. In the middle curve the 
 shocks were twenty per second, and the relaxation is only of a very partial 
 kind. The upmost curve is that of tetanus. 
 
 in the tortoise, 3 stimuli per second ; in the rabbit, 20 to 40 stimuli 
 per second for the pale voluntary muscles, 10 to 20 for the red 
 muscles. In insects, whose muscles can contract a million times 
 
 * The tetanus of the physiologist must not be confused with the tetanus 
 of the pathologist; the latter is a bacterial affection producing a poison 
 which causes painful contraction of many of the voluntary muscles of the 
 body, especially of the limbs and jaws. 
 
THE MUSCULAR SYSTEM 433 
 
 an hour, 300 stimuli per second are required to produce tetanus. 
 A fatigued muscle is more readily tetanised than one which is 
 fresh. All the voluntary muscular contractions of the body are 
 regarded as tetanic in nature — namely, as a series resulting from 
 a succession of impulses passed' into the muscle so rapidly that 
 there is no interval for relaxation. 
 
 Phenomena of Contracti'bn: Smooth Muscle. — Though 
 there is a marked difference in appearance of red and smooth 
 muscle, the actual phenomenon- of contraction does not differ 
 excepting in the- matter of rate. The latent period, the contrac- 
 tion and relaxation are-present, but occur slowly and deliberately ; 
 smooth muscle.- may be completely isolated from all nervous 
 connections and still continue to exhibit spontaneous contrac- 
 tions. At p. 424 have been noted the various regions where 
 this type of muscle is found; that of the digestive tract in a 
 recently killed animal is readily available for study, and the eye 
 may easily follow the whole phenomenon of muscular contrac- 
 tion, as the intestines continue to move spontaneously for some 
 little time after death. Mechanical stimulation, such as a tap 
 or pinch, excites a sluggish but marked response of peristalsis, 
 which begins at the precise spot stimulated; even drawing the 
 finger lightly over the stomach wall may produce 'weals ' of 
 contraction, though the stomach is less likely to exhibit spon- 
 taneous contraction than the intestine: 
 
 When stimulated by a continuous or induced' current, the 
 muscle responds in the same way as red muscle, but. from 100 to 
 500 times more slowly; summation also is present, though this 
 is not identical in character with that observed in red muscle, 
 for no contraction -follows the first three or lour stimuli; the 
 stimuli accumulate before producing their effect, and subsequent 
 stimuli produce an increased height of contraction. Tetanus 
 may also be produced in smooth muscle/ 
 
 Muscle Wave. — When the impulse enters a muscle at the 
 middle of each fibre, the part nearest the end-plate contracts 
 first, and the impulse spreads each way to the end of the fibre; 
 this process is so rapid, the fibre being only, as we have seen, 
 about 1 inch in length, that for all practical purposes the whole 
 muscle contracts at one and the Same time. When, however, 
 the nerve-endings in a muscle are paralysed by curari, the part 
 becomes at once practically nerveless, and if under these con- 
 ditions one end of the muscle be stimulated, a wave of contraction 
 passes along it to the other at a rate of about 3 to 4 metres (10 or 
 12 feet) a second in the curarised muscle of the cold-blooded frog. 
 In the muscles of warm-blooded man, where the metabolic 
 processes are more active than in the frog, the rate of progression 
 is greater, and may be taken as 10 to 13 metres (30 to 40 feet) per 
 
434 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 second. The time which the wave takes to pass any one point 
 of the muscle is extremely short, j- 1 ^ second; and if (in frog's 
 muscle) we take its velocity as at least 10 feet per second, jt may 
 be shown, by means of suitable apparatus, that the length of the 
 wave is about i foot (30 centimetres). This is a fact of great 
 interest in connection with the efficiency of muscle as a machine. 
 It insures that each single fibre of which the skeletal muscle is 
 composed, and hence the whole muscle itself, can be placed in a 
 state of complete contraction from end to end at the same moment. 
 Fibrillar Contraction. — The skeletal muscles, when exposed 
 immediately after death, show irregular contractions, bundles of 
 muscle fibre contracting and relaxing irregularly, producing a 
 flickering, quivering appearance. This is known as fibrillar 
 contraction, and, as we have already seen, it becomes a serious 
 pathological condition when it affects the heart muscle. 
 
 Elasticity and Extensibility. — Elasticity is the property a 
 body possesses of returning to its shape after stretching; extensi- 
 bility is the power a material possesses of stretching. A piece of 
 steel is elastic, a piece of putty is extensible. If a steel spring or 
 a piece of elastic be tested by loading them with weights, the 
 stretching is proportional to the weight employed; but if a muscle 
 be gradually weighted, it is found that the greatest degree of 
 extension occurs at the beginning, and as the load is increased 
 the extensibility becomes less. Living muscle is very extensible, 
 and is also elastic. It is therefore stretched by a slight force, and 
 returns to its original length when the extending power is removed. 
 Muscle Antagonism. — Every muscle or group of muscles 
 possesses an antagonist, and though the antagonist may be equal 
 in size, this is not always the case (cf. the mass of the muscles 
 which close the jaws and the trifling size of those which open 
 them). Muscle antagonism insures a smoothness of working 
 not otherwise obtainable, as the muscles, in contracting, are 
 working against an elastic resistance. When the flexor of the 
 metatarsus of the horse is ruptured, the effect of the absence of 
 antagonistic resistance is well demonstrated; the gastrocnemii 
 muscles, owing to the absence of antagonism, jerk the leg behind 
 the body, and in this position the skin covering the cap of the 
 hock is thrown into folds, while the Achilles tendon is kinked and 
 flaccid. 
 
 Tonus. — Muscle tonus has already received a brief notice in the 
 chapter dealing with Animal Heat, its definition being stated 
 as the condition of tension, or stretching, of muscles, even of 
 muscles apparently at rest, which causes them to gape when they 
 are cut across. As a matter of fact, the expression ' tonus ' has 
 not been employed in the same sense by all physiologists. 
 Sherrington says it is the most misused term in physiology. 
 
THE MUSCULAR SYSTEM 435 
 
 Ihis authority places tonus on a new plane.* He reminds us 
 that the function of muscle in producing movement is so striking 
 that we are liable to overlook the equally important fact that it 
 also maintains posture. In maintaining posture the muscles are 
 not producing movement, but they are functioning statically. 
 
 There are numerous postures: standing, sitting, crouching, 
 gazing, and such- like; there are also the postures assumed in graz- 
 ing, in scratching, and even in rearing; the closed sphincters 
 are, in reality, only instances of postural contraction, whether 
 affecting the digestive tube, the bladder, or the pupil of the eye. 
 Nor is tonus (posture) limited to the skeletal muscles; it affects 
 heart and bloodvessels as well as the plain muscle of the viscera. 
 
 The act of standing is due to tonus of the extensor muscles 
 of the fore and hind limbs, combined with tonus of the muscles 
 which bend the spine upwards, and of those which retract the head 
 and neck and lift the tail. The muscles antagonistic to these — • 
 namely, those which do not counteract the effect of gravity — ■ 
 are the flexors of the limbs and the depressors of the spine and 
 tail. These antagonistic muscles are not endowed with tonicity. 
 
 One of the most striking experiments in physiology is that 
 devised by Sherrington, in which an animal, deprived of the whole 
 of its brain excepting the medulla and pons, is found to be able 
 to stand in the upright position, even for several days, and under 
 appropriate stimulation to rest its limbs or employ them as in 
 locomotion. This is evidence of the reflex character of ' posture,' 
 a subject which will engage our attention in the chapter on the 
 nervous system; nevertheless, it is necessary here to make it 
 clear that tonus of the muscles which maintain the body in the 
 upright position exists only so long as the nerves running from 
 the muscles to the nerve centres are intact; if these be divided, 
 tonus, and consequently posture, is lost. 
 
 It is important that we should possess a clear physiological 
 understanding of the method by which, for example, a horse is 
 able to stand for weeks — sometimes, indeed, for years — as a 
 consequence of impulses, not necessarily under the control of the 
 will, passing from the ' standing ' (extensor) muscles to the central 
 nervous system. Such knowledge may help us to appreciate 
 some of the pathological features associated with an inability 
 to maintain the upright position either as the result of disease 
 or injury. The act of standing will be further referred to in the 
 chapter dealing with Locomotion. 
 
 An animal at rest does not maintain the position of ' attention ' 
 indefinitely; for example, a horse standing idle rests his hind- 
 limbs alternately; rarely, unless on apprehension of trouble, does 
 
 * ' Postural Activity of Muscle and Nerve,' Brain, vol. xxxviii., 
 part iii., 1915, from which this section has been compiled. 
 
43& A MANUAL OF VETERINARY PHYSIOLOGY 
 
 he stand with both hind-feet squarely together or both on the 
 ground. This alternate flexion of the hind-limbs indicates the 
 presence of another mechanism in connection with posture; the 
 extensor muscles which have been keeping the limb rigid have 
 yielded to the flexors, but not entirely so, or the limb would double 
 up; the extensors are still acting; they have lost none of their 
 ability for maintaining the body upright, but they have become 
 longer than they were originally; in the words of Sherrington, 
 they have assumed ' a new postural length ' ; a ' lengthening 
 reaction ' has been brought about, and with it, remarkable to 
 say, practically no departure from the muscle's previous state 
 of tonus. Conversely, the extensor muscles may shorten, in 
 which case a ' shortening reaction ' has been produced, though 
 still without any increase in the tension of the muscle. It is 
 by means of the lengthening and shortening reactions of the ex- 
 tensor muscles that the ' latitude of pose ' is permitted in all 
 animals at their convenience, without a forward or backward 
 step being made. As Sherrington says, ' we are so accustomed 
 to regard the muscle fibre as an elastic string that this property 
 of exhibiting different lengths, while exhibiting one and the same 
 degree of tension, appears contrary to our fundamental notions 
 of muscular activity. ' 
 
 The contracted condition of the muscles which produce posture 
 is relatively unf atiguable ; the decerebrated cat may stand for 
 days. It is supposed that the economy with which posture may 
 be maintained is due to some difference between the chemico- 
 physical process of postural, as compared with ordinary, muscular 
 contraction. It has been suggested that there are fibres in 
 muscle specially told off, as it were, for postural work. Accord- 
 ing to another view, the sarcostyles are the contractile elements 
 of the muscle, the sarcoplasm being the postural elements; 
 it is probable that these two elements in muscle are under 
 different sources of innervation, cerebro-spinal nerves supplying 
 the sarcostyles, while the sympathetic supplies the sarcoplasm. 
 The metabolism which occurs in contracting muscles originates 
 in the katabolism of carbohydrate substances ; on the other hand, 
 in muscle tonus it is probable that the nitrogenous substances 
 are broken down. 
 
 It has been shown that the labyrinth of the internal ear and 
 nerve-endings in the tendons and muscles of the neck are con- 
 cerned with the reflex adjustment of the head, eyeballs, jaws, 
 and limbs; if the neck be flexed after destruction of the laby- 
 rinths, there is a decreased contraction of the extensor muscles 
 of the fore-limbs and an increased contraction of those of the hind- 
 limbs; extension of the neck dorsally increases the contraction 
 of the extensor muscles of the fore-legs, and decreases that of the 
 extensors of the hind-limbs. If the head be twisted to one side, 
 
THE MUSCULAR SYSTEM 
 
 437 
 
 the limbs of that side to which it is turned exhibit increased 
 extension contraction; those on the opposite side of the body show 
 decreased extension contraction. 
 
 The lengthening and shortening reactions described above 
 are of peculiar interest in connection with the pale muscle of 
 the hollow viscera. Both the stomach and bladder can alter in 
 capacity without altering the tension of their muscular walls. 
 They are capable of assuming a capacity (posture) suited to the 
 volume of the contents without material increase in tension, 
 from which it is evident that the muscular fibres lengthen or 
 shorten to accommodate the contents, and do not behave as would 
 a rubber bag under similar conditions. ' The living bladder, 
 unlike the dead bladder or rubber bag, enfolds its contents 
 with the same light grip, whether these contents be ample or 
 little.' The length of the abdominal muscles exhibited by a 
 ' tucked up ' horse and a mare in advanced gestation, are other 
 examples of lengthening and shortening reactions. 
 
 Muscle Work. — A muscle is capable of vigorous or slow con- 
 traction, or of any degree of action between these extremes. When 
 it is energetically contracting, all its fibres are employed ; when 
 gently contracting, a few fibres only are at work. The explana- 
 tion is that it is impossible for the nerves to introduce varying 
 strengths of stimulation. There is only one degree of stimulation 
 capable of transmission, so that each fibre has to work at its 
 greatest effort, the all-or-none feature referred to at p. 53 in 
 dealing with the heart muscle. By reducing the number of 
 fibres in action any degree of slowness of contraction of skeletal 
 muscle is obtained. If a muscle preparation be loaded with 
 successively greater weights, it is found that up to a certain 
 maximum the load actually increases the amount of work done 
 by the muscles. This is due to the influence of the tension 
 exercised on the fibres, by. which they are increased in length 
 and thereby rendered more vigorous; they meet the increased 
 resistance by an increased effort, as we have already seen in the 
 case of the heart muscle. By our continuing to increase the 
 weight, a point is reached when the muscle preparation becomes 
 overloaded, and in consequence shortens less. These facts are 
 illustrated in the following table : 
 
 Load in Grammes. 
 
 Lift in Millimetres. 
 
 Work done in Gramme- 
 
 M.llimetres— viz ,■ the Load 
 
 Multiplied by the 
 
 Height. 
 
 5 
 15 
 
 25 
 35 
 
 27-60 
 25-IO 
 
 ii'45 
 630 
 
 138-00 
 376-50 
 286-25 
 220-50 
 
 ■ 
 
438 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The table also shows that when the muscle shortened the least 
 it lifted its greatest weight. It is obvious that if a muscle lifts 
 no weight it does no work, and its energy goes off wholly as heat. 
 Experience shows that there is a load which varies in different 
 animals of the same species, according to the quality* of their 
 muscles, from which the greatest proportion of effective work 
 can be obtained. Finally, there is a load just in excess of the 
 strength of the muscle known as the absolute power of the muscle. 
 This has led to a paradox associated with the name of Weber— 
 namely, that a contracting muscle may have the same length as 
 one uncontracted. Yet we are all familiar with the fact that 
 this is so ; a horse called upon to draw a greater weight than he 
 can move is employing strongly contracting muscles, but they 
 become no shorter; he does no effective work, but his muscles 
 liberate heat, and this is what occurs in the condition known as 
 ' tonus.' In the chapter dealing with Locomotion the important 
 practical questions of the effort a horse can exercise in draught 
 and the weight he can carry are discussed. 
 
 If the weight of the load and the height to which it is lifted be 
 known, the work done by a muscle is readily calculated. Work 
 equals the load lifted multiplied by the height through which it 
 is raised, and may be expressed as pounds or tons lifted i foot, 
 or grammes or kilogrammes lifted i metre, f In the case of animals, 
 the work is expressed as pounds or kilogrammes per foot or per 
 metre of horizontal transport. The amount of work produced by 
 muscles may be calculated in terms of heat. One small calorie 
 =426-5 gramme-metres of work (426-5 grammes raised 1 metre 
 high). Muscular work may also be calculated from the amount 
 of oxygen absorbed: 1 c.c. of oxygen consumed =2-07 kilogram- 
 metres of work. 
 
 * Robertson points out that in the race-horse the difference between a 
 ' stayer ' and a ' non-stayer ' lies in the relative proportion which the pale 
 bear to the dark red fibres in muscle. In the stayer the dark red fibres 
 greatly predominate (' The Principles of Heredity applied to the Race- 
 Horse,' J. B. Robertson, M.R.C.V.S.). 
 
 There is a wide difference in the quality of muscles of individuals ; they 
 differ as greatly as mental characteristics, to which they bear some relation. 
 Slowness of speech and physical slowness are, as we might suppose, fre- 
 quently associated, the laryngeal muscles being as slowly responsive as the 
 skeletal. Precisely the same conditions are met with in animals; there is 
 the keen excitable horse, bursting to ' get away,' restraint merely increasing 
 resistance, in contrast to which there is the ' slug,' on which neither en- 
 couragement nor punishment has anything more than a temporary effect. 
 Even where temperament, ' condition,' and physical development are 
 apparently identical, there are great differences in the muscular effort, as 
 shown by the rapidity of contraction. This is seen in the prize-ring as 
 well as on the race-course, and constitutes what has, for want of a better 
 name, been spoken of above as quality. The quality of a muscle may be 
 due to a structural difference of either muscle or nerve, or both, but it is 
 intimately connected with the effectiveness of the circulatory mechanisms. 
 
 f A 'horse-power,' the unit used in engineering, equals 33,000 foot pounds 
 pf work per minute. 
 
THE MUSCULAR SYSTEM 
 
 439 
 
 The ergograph is an instrument employed for recording voluntary 
 muscular contraction in man; it is shown in Fig. 125. The person 
 experimented upon lifts a known weight to a definite height, which 
 is recorded on a drum. Each contraction is followed by the same 
 interval of rest. Observations with this instrument show that 
 after complete fatigue at least two hours are required for the muscle 
 to recover sufficiently to repeat its first record. If, instead of resting 
 the completely fatigued muscle, further muscular effort is made, 
 the period required by the muscle for recovering is greatly prolonged. 
 This is in accordance with practical experience. The ergograph 
 emphasises the ill-effects of straining the tired machine. The 
 effect of hunger, want of rest, or mental activity, is found to diminish 
 the amount of work produced. When the circulation in the muscle 
 is improved by massage, more work is produced. The effect of 
 improving its nutrition by food, especially by a readily soluble and 
 easily absorbed substance such as sugar, is said to be to increase 
 quickly its power of performing work. Finally, the effect of making a 
 
 Fig. 125. — The Ergograph (Mosso's). 
 
 prolonged call for work upon one group of muscles is to diminish 
 the activity of the others. The results obtained by the ergograph 
 are those which accord with experience, but it is unfortunate that 
 the personal element cannot be excluded from the experiments, 
 especially where the questions of food and work are concerned. 
 
 In connection with the work done by muscle, it is interesting 
 to institute a comparison between the work yielded by the animal 
 body and that by a well-constructed machine. The best triple- 
 expansion engine may yield as work some 10 to 15 per cent, of 
 the available energy in the fuel, the balance being lost as heat. 
 
 A muscle is more nearly allied to an internal combustion engine, 
 and the efficiency of this is greater than that of a steam-engine. 
 A large gas-engine mayhave an indicated efficiency of 34 per cent., 
 which is go per cent, of the maximum possible. The small 
 
44 o 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 engine of a motor-car is relatively better, for under favourable 
 conditions 28 per cent, of the potential energy in the petrol may 
 be given off as work. 
 
 Experiments made on the horse by Zuntz show that 33 per 
 cent, of the body substance katabolised during labour is recovered 
 as effective work. This is described by Armsby as net efficiency, 
 as certain deductions have been made; for instance, it does not 
 include the effort required to move the animal's own body weight. 
 It is obvious, however, that the net efficiency will be affected by 
 
 Fig. 126. — Diagram illustrating the So-called Electric Currents of 
 Rest (Injury Currents) of Muscle and Nerve (Foster). 
 
 Being purely diagrammatic, the figure may serve either for a piece of muscle or 
 nerve, excepting that the currents at the transverse section cannot be shown 
 in a nerve. The arrows show the direction of the current through the galvano- 
 meter. 
 
 a, b, The equator. The strongest currents are those shown by the dark lines, 
 as from a at the equator to x or to y at the cut ends. The current from a to c 
 is weaker than from a to y, though both, as shown by the arrows, take the 
 same direction. A current is shown from e, which is near the equator, to /, 
 which is farther from the equator. The current (in muscle) from a point in 
 the circumference to a point nearer the centre of the transverse section is 
 shown at g, h. From a to b, or from x to y, as indicated by the dotted lines, 
 there is no current. 
 
 the weight of the load drawn or carried, by the pace, also by 
 whether the work is performed on a level or on an incline, and by 
 the degree of gradient. So difficult are these questions to settle 
 by experimental inquiry that 33 per cent, under favourable 
 conditions must be interpreted broadly; with a gradient of 1 in 12 
 the efficiency was reduced to 23 per cent. Nevertheless, we are 
 shown that the work performed by muscles is not inferior to that 
 of the best internal combustion engine. 
 
 Other interesting points were ascertained by Zuntz's work. 
 He found, for instance, that work on a slight ascent was performed 
 
THE MUSCULAR SYSTEM 
 
 441 
 
 more economically than work on a dead level, and this is sup- 
 ported by practical experience. In the days of mail-coaches 
 the horses did not last as long on a level road as on one where 
 there were slight gradients, the explanation being that the slight 
 rise and fall brought different groups of muscles into play, and 
 so afforded a rest to others. Another curious fact brought out 
 by Zuntz was that, though the ox climbed an incline with much 
 the same efficiency as the horse, on a level he expended more in 
 the transport of his own body weight than the horse. These ques- 
 tions will again come under notice in the chapter on Locomotion. 
 
 Muscle Currents. — Great 
 controversy has taken place 
 as to whether currents of 
 electricity exist naturally 
 in uninjured muscle. It is 
 found, for instance, that a 
 piece of muscle isolated from 
 the body, and placed in con- 
 nection with a galvanometer, 
 may be made to demonstrate 
 the presence of electric cur- 
 rents which behave in a 
 perfectly regular manner in 
 passing from one definite 
 point on the muscle to 
 another. These are the so- 
 called natural muscle currents, 
 or currents of rest; they are 
 found to pass in a certain 
 direction — namely, from the 
 longitudinal surface of the 
 muscle to the cut extremity. 
 The natural surface of the 
 muscle, therefore, corre- 
 sponds to the copper of a 
 DanieU's cell, the cut ends 
 to the zinc element; the sur- 
 face of the muscle is in con- 
 sequence galvanometrically 
 
 Fig. 127. — Sartokius Muscle arranged 
 to -demonstrate the diphasic varia- 
 TION of Action" Current in Muscle (or 
 Nerve). 
 
 S, Sartorius; a, stimulating electrodes; b, c, 
 non-polarisable electrodes as ' leads ' to G, 
 the galvanometer, The electrode c is in- 
 tentionally not placed on the injured end 
 of the muscle, as it would be for demon- 
 strating ' negative variation,' since the 
 strong negativity of the injured end would 
 mask the desired phenomenon. A similar 
 arrangement suffices to demonstrate the 
 same phenomenon in a piece of nerve. 
 
 positive, the cut ends galvanometrically negative (see p. 428). The 
 more the ends are injured the more galvanometrically negative they 
 become. It is now known that the currents of rest in muscle are 
 caused by the injury inflicted on the muscle in its course of -prepara- 
 tion for the experiment. They are in consequence frequently 
 described as injury currents. Muscle at rest and absolutely un- 
 injured gives no current whatever. 
 
 If while the galvanometer is registering the direction of an injury 
 current the muscle preparation be stimulated with an interrupted 
 current, a backward swing of the needle of the instrument towards 
 zero indicates that the injury current is diminished; this diminution 
 is termed the negative variation. 
 
 If an uninjured muscle, which is giving no currents, be stimulated 
 by means of an interrupted current into contracting activity, it 
 
442 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 exhibits electrical phenomena, called the current of action. It 
 is the production of this current which accounts for the negative 
 variation of the injury current. The action current is of such a 
 nature as to give rise to what is known as a diphasic variation in 
 the current of a muscle, as shown by the needle of the recording 
 galvanometer swinging first in one and then in the opposite direction. 
 The double variation is due to the fact that the point on the muscle 
 to which the stimulus is applied becomes negative to all points of 
 the muscle at which the wave of contraction resulting from the stimu- 
 lation has not yet arrived. This negativity occurs during the ' latent 
 period ' (p. 429), and passes along the muscles as a wave which pre- 
 cedes, or in some cases accompanies, the wave of contraction. 
 Thus if, as in Fig. 127, a muscle be stimulated at a, while the points 
 b and c are connected through a very sensitive galvanometer, at the 
 moment of stimulation a becomes negative to the rest of the muscle. 
 As this negativity sweeps along the muscle it passes first over the 
 point 6, which* thus becomes negative to c, and the current in 
 
 consequence flows from c to b through 
 the galvanometer. Midway between 
 b and c is a neutral point, as shown 
 by the fact that when the electrical 
 excitation reaches this point, no 
 current is passing through the gal- 
 vanometer. Immediately afterwards 
 the current passes over the point c, 
 and c then becomes negative to b, as 
 shown by the current flowing through 
 the galvanometer in the opposite 
 direction. Every contraction of a 
 muscle gives rise to a diphasic varia- 
 tion, which passes along the muscle 
 slightly in advance of the contraction 
 wave. These phenomena, while of the 
 greatest interest in the case of muscle, 
 become still more important in the 
 case of a nerve, since they provide 
 the only accurate means of following 
 the passage of an impulse along 
 a piece of isolated nerve, which does not, as does a muscle, change its 
 shape or exhibit other obvious changes when stimulated. 
 
 The electrical change in muscle can be observed without the 
 assistance of any apparatus by means of an experiment as old 
 as electricity itself. As employed by Galvani, it was described as 
 electricity without metals. The nerve of a nerve-muscle prepara- 
 tion (M, Fig. 128) is brought into contact with another muscle, 
 M', in such a way as to touch at two points — e.g., the cut end b 
 and the surface of the muscle a. The current of injury in the latter 
 travels along the nerve, and produces a contraction in M. With 
 the same preparation it can also be shown that if M' be stimulated 
 at S and made to contract, the negative variation which accom- 
 panies the contraction of M' will produce a contraction of M. This 
 is known as a ' secondary contraction,' while the above preparation 
 is spoken of as the rheoscopic frog. 
 
 The electrical phenomena in muscle are not isolated examples of 
 electric currents in the body. Closely similar phenomena are de- 
 monstrable in nerves, which we shall shortly study, and electrical 
 
 Fig. 128. — Secondary Con- 
 traction. 
 
THE MUSCULAR SYSTEM 443 
 
 changes accompanying their functional activity occur in secreting 
 glands, in the eye, and to the highest degree in the electrical organs of 
 certain fish. These electrical organs are modified muscle or skin 
 glands. 
 
 Chemical Changes in Active and Resting Muscles.— The 
 physiological changes occurring in muscles are remarkably active. 
 The processes which result in muscular contractions use up at 
 every moment the combustible material contained within the 
 muscle structure, and the products arising from metabolism have 
 to be got rid of at once and repair brought about. Changes are 
 also constantly occurring even during the period of muscle rest. 
 Muscle activity is characterised by muscle waste, muscle rest is 
 characterised by a preponderance of the process of repair; we 
 must therefore learn the nature of the waste and repair occurring 
 in muscles. 
 
 The oxygen carried to resting muscles by the blood is absorbed 
 in considerable quantities, and a volume of carbon dioxide, in 
 slightly less quantity than corresponds to the oxygen absorbed, 
 is returned to the venous blood. Whether a muscle is at rest or 
 active, it is always absorbing and storing up oxygen, and giving 
 off carbon dioxide. From this we might imagine that muscle 
 would readily yield free oxygen, but as a matter of fact none can 
 be obtained from it, and as carbon dioxide cannot be formed 
 without oxygen, it is assumed that the muscle molecules store up 
 oxygen in some form. That such storage occurs appears un- 
 doubted, for a nerve-muscle preparation can continue to contract 
 when placed in a jar of nitrogen, and, what is even more remark- 
 able, continues to produce carbon dioxide. 
 
 The storage of oxygen by resting muscle may, in the case of the 
 cold-blooded frog, enable an excised muscle to yield some hundred 
 contractions when suitably treated. In mammalian muscle, on 
 the other hand, this quiescent store of oxygen does not suffice 
 to maintain irritability for more than the briefest interval after 
 the blood-supply is cut off. This is due to the much greater 
 metabolic activity of mammalian muscle as compared with the 
 muscle of the frog. 
 
 The amount of oxygen absorbed and carbon dioxide given off 
 has already been considered (pp. T43, 145); the amount is 
 greatly increased during work. 
 
 In the active muscle the bloodvessels are more dilated than in 
 the muscle at rest, and this dilatation provides for the increased 
 quantity of oxygen now required by the part. In experimenting 
 on the lip muscle of the horse, Chauveau found that there was a 
 fivefold increase of blood passing through the muscle during 
 contraction as compared with rest. By means of the blood the 
 irritability of the muscle— i.e., its power of contraction— is main- 
 
444 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 tained ; whatever leads to a smaller quantity of blood being de- 
 livered to an active muscle produces partial or complete paraly- 
 sis of the group or groups of muscles affected. This is well seen in 
 a horse suffering from thrombosis of the iliac arteries ; the blood- 
 supply is sufficient during the time the animal is at rest, or even 
 at a walk, but if he is called upon to trot, muscular cramps occur, 
 followed by paralysis. 
 
 The study of metabolism has prepared us for the statement 
 that the chemical changes occurring during contraction do not 
 normally affect the nitrogenous elements of the muscle. The 
 excretion of any increased amount of urea is variable, irregular, 
 may not even take place, and is in no case remotely proportional 
 to the work done. This is true as long as the body is supplied 
 with a sufficiency of carbohydrates and fats. If these are de- 
 ficient, then increased muscular activity does lead to an increased 
 formation of urea, since the muscle now has to metabolise its 
 proteins to provide the energy necessary for the work performed. 
 Carbohydrates and fats are normally the chief sources of muscular 
 contraction. The carbohydrates are represented by glycogen 
 (formed, as we have seen, by the sugar brought to the muscle) 
 and by dextrose. A well-nourished muscle contains 0-5 to 0-9 
 per cent, of its weight as glycogen. This local store of animal 
 starch is, under the influence of an enzyme, converted back to 
 sugar, and in this form is capable of yielding energy under the 
 influence of oxidising enzymes. We have previously seen that 
 the contraction of muscle leads to the disappearance of glycogen. 
 In this condition, though free from glycogen, the muscle still 
 continues to contract, there being sufficient sugar for the purpose 
 existing in the blood, which even under starvation continues to 
 maintain its normal o-i per cent. Under starvation this sugar 
 is furnished by the breaking down of body protein, but under 
 the ordinary condition of nutrition, when work has emptied the 
 muscles of glycogen, there is ample sugar available from the store 
 of carbohydrate taken into the body. Many circumstances 
 seem to point to sugar as being preferable for the work of con- 
 traction. This is probably owing to the ease with which it can 
 be oxidised, but it is quite certain that fat, which is more diffi- 
 cult of oxidation, may also furnish the necessary material used 
 up in muscular work. 
 
 The formation of acid in muscle as the result of contraction 
 has given rise to a good deal of inquiry. Lactic acid is found in 
 muscle; it is not, as we have seen, the ordinary lactic acid found 
 in milk, but a form optically active to which the name ' sarco- 
 lactic acid ' has been given. Resting muscle contains only a 
 trace of lactic acid (0-03 per cent.), while under the influence of 
 contraction a marked amount is present (0-22 per cent.), and rigor, 
 
THE MUSCULAR SYSTEM 445 
 
 whether produced artificially by heating the muscle or naturally 
 after death, leads to the production of as much as 0-3 to 0-5 per 
 cent. The source of the acid is from glucose. 
 
 During muscular activity heat is produced; the blood returning 
 from a muscle has a higher temperature than that going to it. 
 Colin found the temperature of the masseter muscle of the horse 
 to rise 5 F. through feeding. The whole body temperature is 
 raised during work, especially in the horse, and does not fall for 
 some time after. Haldane's observations show that the effect of 
 a rise in body temperature during work is to heighten the excita- 
 bility of the respiratory centre. 
 
 The Cause of a Muscular Contraction. — This is one of the 
 most difficult problems in physiology, and has not been solved. 
 There are many views, the most modern* being that potential 
 energy is stored in the muscle in some unstable chemical form; 
 by its decomposition lactic acid is produced, and the hydrogen- 
 . ions thus set free cause a change in the surface tension where the 
 sarcoplasm and fibrillar of the muscle are in contact; this leads 
 to a condition of tension within the muscle, which consequently 
 shortens and performs work. After contraction has occurred the 
 lactic acid is oxidised, and the energy thus furnished is stored up 
 for the next contraction. The production of lactic acid from 
 glucose is more oxidative in character, but its removal after con- 
 traction has occurred needs an abundant supply of oxygen. 
 During severe muscle exertion the whole of the lactic acid may 
 not be so destroyed, in which case it is eliminated by the 
 kidneys. 
 
 Fatigue. — The influence of fatigue on muscular contraction 
 has previously been referred to. The cause of fatigue is the 
 accumulation in the muscle of the chemical products of contrac- 
 tion. The material in muscles which gives rise to fatigue is 
 sarcolactic acid, and by passing a solution of this acid into 
 muscles the typical phenomena of muscle fatigue may be arti- 
 ficially induced. Similarly, as the blood of a fatigued animal 
 contains fatigue products, transfusion of it into the circulation 
 of a normal animal produces all the symptoms of fatigue. On 
 the contrary, if a fatigued muscle be washed out with normal 
 saline solution and a little weak alkali circulated through its 
 bloodvessels, it becomes restored and regains its power of con- 
 traction. A muscle at work in the body is protected from ready 
 fatigue by the ever-circulating blood, which supplies it with food 
 and carries off the waste products of its activity. 
 
 Attention has been drawn to the fact (p. 423) that muscles 
 
 * A. V. Hill, Fletcher, Hopkins (see ' The Physiology of Muscular 
 Exercise,' by F. A. Bainbridge, M.D., F.R.S., 1919); also Professor Bayliss 
 (see his ' Introduction to General Physiology,' 1919)- 
 
44& A MANUAL OF VETERINARY PHYSIOLOGY 
 
 are connected by elaborated nerve-endings with sensory nerves, 
 to whose existence ' muscular sense ' is due. The sensation of 
 general fatigue which arises from excessive muscular exertion 
 is due to a cerebral appreciation of the changes brought about 
 in the muscles as the result of their contracting activity, and 
 may be looked upon as a protective mechanism against straining 
 the machine. Muscular activity implies the action of central 
 nerve-cells, neurones, in which the impulses which give rise to the 
 contractions of the muscles originate, followed by the passage 
 of these impulses along the motor nerves, and their communica- 
 tion to the contractile fibres by the agency of the end-plates. 
 Hence the phenomena of fatigue may be due to fatigue of the 
 neurones, or of the conducting nerve fibre, or of the end-plate in 
 the muscle, brought about by the injurious action of fatigue pro- 
 ducts. It is known that neurones and nerve fibres are practically 
 impossible of fatigue, and accordingly the end-plate in the muscle 
 itself has been looked upon as the seat of the failure. It is now, 
 however, held that the junction of the motor-nerve with the neu- 
 rone, the synapse, is the seat of fatigue. The energy in the muscle, 
 excepting the organ be absolutely exhausted, is not entirely used 
 up, for a fatigued muscle will contract on direct stimulation, 
 when it is unable to respond to a normal stimulus conveyed 
 through its nerve. 
 
 In the horse (the only animal subjected to fatigue, which is 
 the cause of most forms of lameness from which it suffers) it is 
 not on the muscles that the ill-effects fall, but on their tendinous 
 attachments and the ligaments of the joints. This question is of 
 extraordinary practical importance, and in dealing with locomo- 
 tion the matter will be treated more fully; but the point which 
 it is desired to emphasise here is that when muscles tire the strain 
 of locomotion falls on tendons and ligaments, and these may, in 
 consequence, rupture, as a rule incompletely, but sometimes 
 completely. Rupture of muscle, however, is not unknown; the 
 flexor of the metatarsus is a classical example, but rupture of 
 a muscle is as a rule associated with violent muscular contraction 
 such as a fatigued muscle is incapable of exercising. So power- 
 ful may a muscular contraction be that, should the muscle with- 
 stand the strain, bones directly or indirectly under its influence 
 may be fractured. Most cases of broken back occurring when a 
 horse is cast for operation are due, not to the concussion of the 
 fall, but to the subsequent struggles; under these conditions the 
 neck of the ilium may even be fractured. 
 
 A rupture of the diaphragm (see p. 157) is not due to muscular 
 contraction, but to intra-abdominal pressure, especially tym- 
 panites, and in such cases it is the tendinous and not the muscu- 
 lar structure which gives way. 
 
THE MUSCULAR SYSTEM 447 
 
 ' Condition.' — That remarkable state of the body described as 
 condition,' into which horses can be brought by care in feeding, 
 general management, and carefully regulated work, must be 
 regarded as the state of highest perfection which muscles can 
 attain. In its highest degree it is not a permanent state; no 
 horse can remain in it for any length of time, and many can never 
 
 -be got into condition for severe work. It is easy in the training 
 of horses to overstep the mark and produce ' staleness,' a result 
 which, like the fatigued muscle on the myograph, is usually 
 recovered from by a short judicious rest, to which the system 
 immediately responds. 
 
 During training all superfluous fat and water are removed 
 from the body, the muscle-substance is built up, necessitating an 
 abundant supply of protein, and the respiratory capacity in- 
 creased. It is very necessary to remember that condition, 
 though judged of largely by the state of the muscles, implies the 
 proper working of the respiratory and circulatory systems. 
 In order to sustain severe and prolonged muscular exertion an 
 adequate supply of oxygenated blood must be sent to the muscles; 
 this necessitates a rapid flow of blood and adequate ventilation 
 in the lungs, with strong regular pumping power in the heart; 
 all these factors must work in harmony. As a matter of fact, the 
 ability to endure the strain of violent muscular efforts is far 
 more dependent on the respiratory and cardiac mechanisms 
 having been trained than on the skeletal muscles. Long walking 
 exercise is given as a muscle developer, and judicious gallops 
 to give an animal its ' wind,' yet as a matter of fact the ' wind ' 
 is largely a question of heart. The heart must be large and 
 muscular.* In the horse there are 29 litres (51 -04 pints) of blood 
 flowing through it every minute during rest; during work its 
 
 . output is doubled, or even quadrupled, depending upon the pace. 
 Under stress it fills the pericardial sac. The filling of the heart 
 is a question of venous inflow; if more is being returned than the 
 ventricle can get rid of, it accumulates in the auricle, and when 
 horses are galloped to death it is the auricle which ruptures, 
 generally the right, owing to the resistance offered by the con- 
 gested condition of the lungs. The amount of blood returned to 
 the heart at both auricles must, broadly speaking, be the amount- 
 leaving it by both ventricles. A deficiency in this mechanism 
 leads to ' loss of breath ' ; clogging in the lungs means deficient 
 oxygenation in the tissues, and without an adequate supply of 
 oxygen the muscles are powerless to contract. We are clearly 
 shown, from what may be witnessed in the hunting-field, or 
 
 * For all we know, the difference between a failure and a success on the 
 turf may be a few ounces more or less of heart muscle. A few additional 
 ounces may represent a fortune ! 
 
448 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 wherever horses are exposed to long-continued strain, that the 
 chief value of training lies in the functional improvement of the 
 muscular tissue of the heart and of the circulatory system in the 
 lungs; both of these have to be educated to withstand the extra 
 strain imposed. The only bloodvessels which commonly give 
 way are the pulmonary. A rupture of the aorta is very rare. 
 
 The respiratory movements, as we have learnt, are dependent 
 upon the rhythmic activity of the respiratory centre (p. 137). 
 The powerfully stimulating influence on the respiratory centre 
 of the waste products of metabolism resulting from muscular 
 contraction has suggested to some that ' wind ' may be the result 
 of an increased immunity of the centre to the action of these pro- 
 ducts. 
 
 The voluntary muscles have also to be educated to work in the 
 best and most economical manner; they must be used to. advan- 
 tage, smoothly and in combination; their response must increase 
 in rapidity and power; they must grow and become hard. Sher 
 rington* points out that in properly balanced muscular move- 
 ments there is between each successive contraction a period of 
 rest which is recuperative in nature and opposed to fatigue. 
 One feature in training, which will be more fully appreciated 
 when the study of the nervous system has been made, is the self- 
 education of muscles in the importance of momentary rest follow- 
 ing contraction, while the peculiar innervation of muscles, known 
 as ' reciprocal,' is also improved by education, which insures 
 that antagonistic muscles offer little or no resistance to their 
 opponents. 
 
 Unpractised movements are a serious source of wasted mus- 
 cular effort; by practice the same work can be performed with a 
 greatly reduced expenditure of energy. In man it has been shown 
 that the abnormal use of certain muscles, such as a soldier with 
 an ill-fitting boot would employ when marching, produces ex- 
 travagant combustion and early fatigue. This applies with 
 equal force to the horse; ungreased axles, badly-fitting harness 
 and saddles, sore backs and lameness, all mean excessive wear 
 and tear. Zuntz found that a lame horse at work expended 
 99 per cent, more energy than one which was sound, also that a 
 ' high stepper ' required 33 per cent, more oxygen for doing the 
 same amount of work as a horse without exaggerated action. 
 Condition may further be defined as fitness for the particular 
 class of work required; if horses are used for saddle work without 
 being conditioned to carry weight, they fail; an animal may be 
 in perfect condition for draught work, but quite unfit to carry 
 weight, as the muscles are uneducated. In other words, there is 
 a special education of the muscles for the particular class of 
 
 * ' The Role of Reflex Inhibition,' Science Progress, No. 20, April, 1911. 
 
THE MUSCULAR SYSTEM 
 
 449 
 
 work required, and until horses are ' conditioned ' they cost more 
 and do less work than the trained article. The only way in 
 which this education can be effected is by training. The art of the 
 trainer lies in employing a suitable resistance gradually and skil- 
 fully increased, and it requires long practice and experience. The 
 physiologist can assist, as the pulse, respirations, temperature, 
 and body excretions are an unfailing guide to the progress made. 
 In the following table is shown the mean of forty-five observa- 
 tions on fit and a similar number of unfit horses performing 
 identical work and carrying the same weight : 
 
 Average 
 Pulse 
 Rate 
 be fore 
 Work. 
 
 Average 
 Respira- 
 tion Rate 
 before 
 Work. 
 
 Pulse Rate at End of Work. 
 (Taken for 15 Seconds.) 
 
 Respiration Rate at End of Work. 
 (Taken for 15 Seconds.) 
 
 At 
 Cessation. 
 
 Two 
 Minutes 
 
 after 
 Stopping. 
 
 Five 
 Minutes 
 
 after 
 Stopping. 
 
 At . 
 Cessation. 
 
 Two 
 
 Minutes 
 
 after 
 Stopping. 
 
 Five 
 Minutes 
 
 after 
 Stopping. 
 
 Per 
 Minute. 
 
 Per 
 Minute. 
 
 Per 
 Minute. 
 
 Per 
 Minute. 
 
 Per 
 Minute. 
 
 Per 
 Minute. 
 
 Per 
 
 Minute. 
 
 Per 
 
 Minute. 
 
 
 
 Fit Horses. 
 
 
 40 
 
 IO 
 
 88 55 
 
 46 
 
 41 
 
 21 
 
 16 
 
 
 
 Unfit Horses. 
 
 
 39 
 
 12 
 
 "3 
 
 66 
 
 56 
 
 48 28 
 
 19 
 
 The table shows that the pulse and respirations of fit horses are 
 less disturbed by work, and more rapidly subside to normal 
 after work, than those of unfit. In the above series the heart 
 and respiration rate of the ' fit ' subsided in two minutes to a 
 point only reached by the ' unfit ' in five minutes. 
 
 Chemical Composition of Muscle.— A dead muscle does not 
 possess the same chemical composition as one which is living, and 
 living muscle cannot be analysed without killing it by the methods 
 necessarily employed. Thus any tabular statement of the 
 quantitative composition of muscle gives really the composition 
 of dead muscle. We are, however, assisted to some knowledge 
 of the nature of living muscle-substance by the following facts: 
 
 If contractile, and therefore living, frog's muscle is carefully 
 frozen and then very slowly thawed, it does not lose its irrita- 
 bility : it is still alive. When frozen, it may be minced with a cold 
 knife and ground up in a cold mortar with four times its weight 
 of snow containing i per cent, of sodium chloride. By this 
 process a viscid liquid is obtained which may be filtered, though 
 with difficulty, at o° C. The fluid filtrate is opalescent, neutral, 
 J 29 
 
450 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 or faintly alkaline in reaction, and is known as ' muscle-plasma.' 
 When its temperature is allowed to rise it coagulates in the same 
 Way as does blood-plasma, yielding a clot which, unlike fibrin, is 
 granular and flocculent, and forming a liquid serum. During the 
 clotting the liquid becomes acid, as the result of a formation of 
 sarcolactic acid, and the clot consists of myosin. Assuming, as 
 we may reasonably do, that the muscle-plasma represents more 
 or less closely the muscle-substance in the living fibre, we may 
 take these phenomena of the clotting of the muscle-plasma as 
 indicating the most characteristic chemical differences between 
 living and dead muscle (though there are others), and thus we 
 gain considerable insight into the composition of living muscle 
 as based upon an analysis of the dead tissue. 
 
 With this preliminary caution we may now state the com- 
 position of muscle to be approximately as follows : 
 
 Water 
 Proteins 
 
 75 per cent. 
 
 20 
 
 Fat 
 
 3 
 
 Carbohydrates 
 Nitrogenous waste products 
 Salts - 
 
 o>4 to i per cent. 
 
 0-2 
 
 i to 15 ,, 
 
 The nature of the proteins of muscle is a matter of uncertainty. 
 It must suffice to say that the chief and characteristic protein 
 of dead muscle is the myosin formed in the clotting of muscle- 
 plasma; it belongs typically to that class of proteins known as 
 globulins. Bearing in mind the phenomena of the clotting of 
 muscle-plasma, and using the nomenclature employed for blood- 
 plasma, we may say that living muscle contains myosinogen, 
 which on the death of the muscle is converted into myosin, just 
 as in blood-plasma fibrinogen gives rise to fibrin. It has not as 
 yet been shown that calcium salts play a part in the coagulative 
 formation of myosin, as they necessarily do in the production 
 of fibrin and of the casein-clot in milk. All the proteins of living 
 muscle are not entirely myosinogen, nor are all those of dead 
 muscle myosin. Other members of the globulin class are present 
 in both, as also an ordinary albumin closely resembling serum 
 albumin. 
 
 The carbohydrate material is composed chiefly of glycogen, 
 which diminishes in amount by conversion into sugar on the 
 death or after the contracting activity of muscles; these sub- 
 stances have already been fully dealt with in a previous chapter. 
 The nitrogenous waste products or ' extractives ' are creatine, 
 hypoxanthine (obtainable in the free form in muscle extracts), 
 xanthine, carnine, taurine (in horse-muscle), uric acid in minute 
 traces (more abundant in reptilian muscle), and probably traces 
 of urea, though this is a question not yet decisively settled. 
 
THE MUSCULAR SYSTEM 451 
 
 Of these, creatine is by far the most important ; it is a substance 
 which has already been studied in connection with the production 
 of urea (p. 338). The ash in muscle consists principally of the 
 salts of potassium and magnesium. The gases are carbon dioxide, 
 together with a small amount of nitrogen, but no free oxygen. 
 
 In plain muscle glycogen is found only in traces, if at all; 
 lactic acid and creatine are found in smaller quantities than in 
 striped muscle. Hypoxanthine is conspicuously present, while, 
 in contrast to striped muscle, the chief salts are those of sodium 
 and calcium. 
 
 Rigor Mortis. — After death a muscle passes into the condition 
 of rigor or stiffening, by which it changes both in its physical and 
 chemical aspects. The muscle becomes firm and solid, loses its 
 elasticity, and no longer responds to electrical stimuli; further, 
 it loses its alkaline reaction, and in course of time becomes acid 
 owing to the formation of sarcolactic acid. Through the death 
 of the muscle its proteins pass from a fluid or viscous condition 
 into a solid, and this process is generally believed to be identical 
 with the clotting of muscle-plasma previously described. Rigor 
 mortis and the production of sarcolactic acid are closely con- 
 nected, so that if the formation of the acid be prevented by suit- 
 able means, rigor does not occur. The view now adopted as to 
 the cause of death-stiffening is that it is due to a coagulation of 
 the proteins by the products of metabolism in the muscle, and 
 this explanation accounts for the rapid setting in of rigor in 
 animals hunted to death. Rigor mortis is delayed in a rabbit in 
 which the labyrinth of the internal ear has been destroyed; this 
 is probably due to the fact that the labyrinth is somehow con- 
 nected with the obscure problem of muscle tonus. The muscles 
 in which delayed rigor mortis occurs are those of the same side 
 of the body as the injured labyrinth. During rigor mortis carbon 
 dioxide is produced and heat evolved ; some after-death tempera- 
 tures are remarkably high (p . 42 1 ) . After a certain length of time 
 rigor mortis passes off and decomposition commences. The 
 remarks in Chapter XIX. descriptive of the attitude the body 
 assumes after death should be read. 
 
 There is a condition of muscle known as heat rigor, which is 
 produced by rapidly raising the temperature of a mammalian 
 muscle to 47 C. (117° F.) . It is very closely allied to death rigor, 
 and is due to the same cause — i.e., coagulation of the proteins. 
 It differs from death rigor inasmuch as it does not pass away. 
 
 It is doubtful whether rigor mortis occurs in involuntary 
 muscle; the post-mortem appearance presented in this variety 
 of muscle may be due to cold, for it has been shown that two or 
 three days after death smooth muscle may be warmed up so as 
 to be capable of contraction. 
 
CHAPTER XIV 
 THE NERVOUS SYSTEM* 
 
 Section i. 
 
 Classification of Nerves. — The first step towards a classification 
 of nerves was taken when the discovery was made that some 
 nerves are exclusively sensory and others exclusively motor in 
 function; prior to this it was believed that a nerve could carry 
 either impulse indifferently. It was next found that a nerve in 
 the body could carry impulses only in one direction; for instance, 
 a sensory nerve could only convey impulses from the periphery 
 of the body towards a centre, a motor nerve could only convey 
 impulses from a centre to the periphery. This is the real basis of 
 classification, notwithstanding the fact that the impulses travel- 
 ling to a centre, though spoken of as sensory, and those from a 
 centre to the periphery, though motor, may be widely different 
 in nature. The terms ' sensory ' and ' motor ' prove very 
 misleading in speaking of the function of a nerve, and for long it 
 has been recognised that the two great divisions of nerve fibres 
 are afferent, or centripetal, and efferent, or centrifugal. An 
 afferent nerve is one conveying impulses from the periphery to 
 the centre, no matter what the nature of these impulses may be. 
 An efferent nerve is one conveying impulses from a centre to the 
 periphery, notwithstanding that these impulses may be of widely 
 different characters. The determining factor of whether a nerve 
 is afferent or efferent does not lie in its structure ; the microscope 
 reveals no difference between the two. The character of the 
 nerve is determined by its physiological action and the nature of 
 the nerve-ending in the tissues, and this is proved by the fact that 
 one kind of efferent nerve may be made experimentally to take 
 the place of another. 
 
 Afferent nerves, as we have seen, are those engaged in con- 
 veying impulses towards a centre; the centres are the brain, 
 
 * I am indebted to Professor Sherrington, F.R.S., for kindly supplying 
 that portion of this chapter which deals with the ' scratch,' and ' step' 
 reflex. 
 
 452 
 
THE NERVOUS SYSTEM 453 
 
 spinal cord, and masses of nerve tissue connected therewith, 
 known as ' ganglia.' Whether the afferent nerves be those of 
 special sense — i.e., visual, olfactory, gustatory, and auditory — 
 or those of touch, pressure, heat, cold, and pain, the widely 
 different results obtained on stimulation depend upon the end- 
 organ of the nerve, and not on any difference in the character of 
 the impulses. There are afferent impulses constantly occurring 
 which are not within the knowledge of the animal ; they may be 
 of an excitatory nature, or of a controlling or inhibitory character. 
 We have studied this group characteristically in our consideration 
 of the vascular and respiratory systems, pressor or depressor effects 
 occurring in the circulation in consequence of excitatory or inhibi- 
 tory impulses conveyed by afferent nerves, though not neces- 
 sarily by the same afferent nerve. The same occurs also in the 
 respiratory system. Nor are we limited for illustration to the 
 two systems mentioned; we have met with it in muscle tonus, 
 visceral movements, glandular secretion, and other phenomena. 
 
 Efferent nerves are those in which the impulses are passing from 
 the centre to the periphery, and the largest system of efferent 
 nerves in the body is that passing to the muscles. The term 
 is used in its widest sense, for not only are the skeletal muscles 
 supplied, but also, as we have already seen, the unstriped muscle 
 of the viscera, bloodvessels, and body hairs, and the striped 
 muscle of the heart . Nor are efferent impulses solely motor ; they 
 may be secretory; this we have studied in connection with saliva 
 and gastric juice. Finally, the impulses may be inhibitory; 
 they then control or prevent muscular movement, and control 
 or prevent secretion. 
 
 It must not be supposed that all the tissues of the body are 
 completely endowed with the nerve fibres described. Fibres 
 for the bloodvessels, excitatory and inhibitory, are not found in 
 all bloodvessels, and though excitatory efferent (motor) fibres 
 exist largely in skeletal muscle, there are no inhibitory fibres to 
 this tissue, a remarkable fact, which will be fully considered 
 later, in view of its extraordinary physiological importance. 
 
 From the above it will be seen that a classification of nerves is 
 not a simple matter, and physiologists are not agreed as to the 
 best system to adopt. The afferent scheme given on p. 454 has 
 been suggested by Professor Sherrington. 
 
 Personally we would add to the classification trophic nerves, or 
 those engaged in nutrition. Some physiologists hold that special 
 nerves governing nutrition do not exist, and that the process runs 
 concurrently in the nerves governing wear and repair; the 
 evidence that exists in favour of our belief will be mentioned 
 in due course. 
 
J3 
 
 a 
 
 in 
 
 > 
 
 w 
 
 H 
 Z 
 W 
 
 W 
 
 3-d 
 
 0§ 
 
 m 2 S -^ 
 
 . si fl 
 
 i cu +j 
 '" 3 
 
 ■sua 
 
 ™ Ti Tl CO T^ ' — 1 
 
 3. u 5 *> 
 
 ^•r! en S 
 
 a .. <« cs 
 ■08 "3$ 
 
 3 c 
 
 8 2 w B 
 a; ^ b04_, 
 
 . en m cti en 
 
 !P"'S Co 
 ^~> en br rf to 
 « 13 8 > 3 
 
 
 CO 3 
 
 ,3 hoo 
 
 3 c« o 
 
 fli > en 
 
 ^ d 
 2 3 
 
 3_ 
 o o 
 
 3 
 
 a 
 
 gjsf 
 
 01 o bog 
 
 0) J3 
 
 O h (S 
 •-•^ 1-1 
 
 3*S <3 
 
 3 ri cj ^ dj 
 
 •£ £ °. S S3 
 
 ^•a§ la 
 
 < 
 
 
 XI 
 
 
 c 
 
 
 3 
 
 
 
 
 
 en 
 
 
 , , 
 
 n 
 
 nt 
 
 
 
 
 
 
 3 
 
 3 
 
 n 
 
 m 
 
 .3 
 
 .3 
 
 
 
 
 CO 
 
 Co 
 
 !«( « ■ - - 
 CD T CJktO 
 
 "•■go-o 
 Sjosifll 
 
 n h » >>n g 
 
 U J3 O Ih 3 O 
 o53 0,0 S3 
 
 S ° 11 3,§o 
 
 ■a 
 a 
 
 ce) 
 
 H SS 
 
 v-3 8 
 
 ,3 O HJ 
 
 .SP Ph 3 
 &$ O 
 
 ■sgss 
 
 T3 co O CD 
 
 cs 
 o 
 
 h o 
 
 .51 
 
 CO u 
 
 •a 
 
 a 
 
 C8 
 
 «5 
 0.8 
 
 CO o 
 
 jE^nauiucuiAua \\v gie jinuii^s 
 
 •o^8'pooj' - 5 - a 
 
 -*-> en ce 
 
 CO CO Qj 
 
 ^-* 3 
 
 a 
 
 p. § 
 
 M rk 
 
 .a 
 
 CO CTJ 
 
 3-3 
 
 O-P 
 
 o » 
 
 OK 
 
 o 
 13 
 
 01 
 H 
 
 d 
 
 rt co 
 
 3^ 
 
 3 ft 
 
 3 3 
 
 * — * tn 
 O 
 
 o's a 
 
 a 3*5 
 
 O ™ ™ 
 !25 
 
 >,cu 
 
 5 3 S 
 
 a43 3 
 
 .is o 
 
 <d cd 
 ^ 11 
 
 .Si 
 
 a*] ■+■* 
 
 „ S H 3 rt « 
 
 US 8=3! ■§• 
 
 v$ 3 o a s 
 
 ■ti 3 m « H J8 
 
 «° il 
 
 3*a h co ^ 
 
 1-1 3 u .g 
 
 tiAvo s,Xpoq 3V$ o^ sninrai^s 
 S12 Sui^o'E jps^i itpoq sq^ jo 
 rjjEd gtaos 'sA'Bas.ib uoirjBinnii;s 
 
 1 en 
 'en 3 
 O O 
 ft+3 
 O 
 
 O 
 n O 
 
 rf.5P 
 
 )„ CO 
 
 3 d 
 3^3 
 
 • en en w " >, 
 
 "2 * S3^ 
 
 3 2 >i " o <& in 
 
 g 5 *g .- ?. >-, ^ 
 
 J»5 
 
 en .y M-e 
 3 -P O 
 co 
 I/) 
 
 ■ £ CO co c3 
 en 3 3 o 
 
 ^ 
 
 J 
 
 bo^.'O 3 jj 1 
 
 ^ a S S -3 
 
 c« a .a " 3 
 
 — •- > 
 
 b 1 t ?-. 3 
 
 en h: H 
 
 wsiubSjo jo 3ovf*ns moj^ *j 
 
 £ 
 
 p 3 ^ 
 
 Jh CO o 
 
THE NERVOUS SYSTEM 
 
 455 
 
 .Depressor j 
 
 EFFERENT NERVES. 
 
 fMotor to skeletal muscle. 
 Vasomotor. 
 
 Cardio-motor (not a true motor) . 
 
 Viscero-motor, and includes re- 
 
 , Motor I spiratory(trachea and bronchi) 
 
 and coat of spleen. . 
 
 Pilo-motor and unstriped muscle 
 
 Pressor of skin generally. 
 
 Iris and ciliary muscle. 
 i Salivary. 
 Efferent . v Secretory J Gastric, 
 
 nerves | I Sudorific. 
 
 /Only known for heart, some 
 Inhibito- bloodvessels, some parts of 
 motor digestive canal, iris, and re- 
 
 * tractor penis. 
 
 Conduction in Nerves. — In considering the direction of the 
 impulses conveyed by nerves as a basis of classification, there 
 are two fundamental principles on which something more should 
 be said. It has been shown that the effect produced by a nerve 
 does not depend upon its structure, but upon the nature of its 
 termination in the tissue. Many years ago, before the truth of 
 this doctrine had been conclusively established, the writer had 
 endeavoured to cure a local paralysis by dividing the paralysed 
 nerve and suturing the peripheral end to the central end of a 
 sound motor nerve. He found that when two sound nerves were 
 so dealt with, motor impulses from an entirely new source pro- 
 vided the tissue with motor function, and, moreover, furnished 
 it at the right moment — namely, at the moment the muscle was 
 required to contract, and not during the period when it should 
 relax. Subsequently Langley showed by a similar experiment 
 that the functions of the chorda tympani and sympathetic on the 
 bloodvessels of the submaxillary gland could be reversed. If the 
 peripheral end of the sympathetic were united to the central end 
 of the chorda, the effect was constriction and not dilatation of 
 the vessels. Similarly, if the peripheral end of the chorda were 
 united to the central end of the sympathetic, the vessels dilated 
 on stimulation of the sympathetic instead of contracting. The 
 essential structure was proved to be the nerve termination in the 
 tissue, and not the nerve which carried the impulse. 
 
 It is on these lines that the writer has endeavoured to cure 
 laryngeal paralysis in horses. But though he has succeeded, as 
 stated above, in getting a sound left recurrent sutured to a sound 
 spinal acessory to function properly, he has not succeeded in obtain- 
 ing these results with a paralysed left recurrent, not even when the 
 operation has been performed early, presumably before the end 
 plates had undergone complete degeneration. 
 
456 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The other fundamental principle in nerve conduction relates 
 to the law that the living nerve transmits impulses only in one 
 direction — viz., either to the centre or to the periphery. When 
 a nerve is removed from the body there is no difficulty in trans- 
 mitting an electrical impulse in either direction, but within the 
 
 body the law is as stated. We 
 shall see presently that the dorsal 
 roots of the spinal cord are sen- 
 sory or afferent, but Bayliss has 
 shown that if these roots from 
 the fifth lumbar to the first sacral 
 be divided, and the peripheral 
 end stimulated, vascular dilata- 
 tion of the vessels of the hind- 
 limbs occurs. The assumption 
 is, therefore, that efferent fibres 
 are passing out of the cord 
 through the dorsal roots, and 
 conveying motor impulses to the 
 bloodvessels in a direction oppo- 
 site to that in which the ordinary 
 sensory impulses pass. Bayliss 
 has termed these impulses anti- 
 dromic, as indicating they are 
 occurring in the opposite direc- 
 tion to the natural stream. 
 
 Structure of Nerves. — Nerves 
 were originally grouped under 
 two heads, according to their 
 colour, as white and grey. The 
 microscope revealed the fact that 
 a difference in structure existed 
 between them, the white fibres 
 possessing a thick coating of 
 fatty substance which gave them 
 cleft; R, node of Ranvier. il De- their colour, while the grey fibres 
 generating fibres : mt, drops of were without this covering. The 
 
 myelin; a remains of axis cylinder. white substance is described as 
 III. Further stage of degeneration: , 
 
 mt, drops of myelin; w, proliferating the medullary sheath; nerves SO 
 cells of neurilemma. covered are termed medullated, 
 
 while the ' grey fibres ' — a term 
 now rarely used — are spoken of as the non-medullated. If a 
 medullated nerve be examined microscopically (Fig. 129, I), 
 it is found to consist of a central core, or axis cylinder, con- 
 sisting of fibrils, surrounded by a white substance known as 
 the medullary sheath, and outside this is another sheath, 
 
 Fig. 129. — Normal and 
 ated Nerve Fibres 
 after thoma). 
 
 Degener- 
 (Barker, 
 
 . Normal fibre: S, neurilemma; m, 
 medullary sheath; A, axis cylinder; 
 k, nucleus; L, Lantermann's line or 
 
THE NERVOUS SYSTEM 457 
 
 or neurilemma. The medullary sheath does not extend con- 
 tinuously along the nerve; it is broken at intervals, termed 
 nodes. The portion of nerve included between two nodes has a 
 nucleus somewhere in it lying beneath the neurilemma. A 
 bundle of such fibres enclosed in a sheath — and they may 
 number thousands to the bundle — constitutes a nerve. A non- 
 medullated nerve resembles the above in every respect, ex- 
 cepting in the medullary sheath. Of this it has none. It 
 consists of an axis cylinder covered by a neurilemma. The 
 fibre is freely nucleated, and a bundle of such fibres in a sheath 
 constitutes a non-medullated nerve. This class of nerve belongs 
 solely to one branch of the nervous system — viz., the sympathetic 
 — whereas the medullated fibres are confined to the larger system 
 of cerebro-spinal nerves. 
 
 The essential feature in the nerve is the axis cylinder; it is 
 the true impulse-conducting substance. The function of the 
 medullary sheath is not definitely known; it has been assumed 
 to play the part of a non-conductor and insulate one fibre from 
 another, but evidently this does not exhaust its uses, for, as just 
 mentioned, the fibres of the sympathetic system are without a 
 medullary sheath. Even medullated nerves lose their sheath 
 before terminating in the tissues. Medullated nerves are more 
 sensitive to stimuli than non-medullated, and the longer the 
 nerve the greater the thickness of the sheath, but at present 
 these facts cannot be connected with function. 
 
 The axis cylinder, as previously stated, is the important 
 part of the nerve ; it is the conducting substance, and its nature 
 and origin are of the utmost significance. We shall see presently 
 that the entire nervous system consists of nerve-cells and fibres. 
 The nerve-cell is the essential feature, and no matter whether the 
 brain, spinal cord, or ganglia be examined, nerve-cells character- 
 istic of the tissue are present; the nerve-cells of the cerebellum 
 are absolutely distinct from those of the cerebrum ; the cells in 
 the ganglia are different from either, or from those found in the 
 spinal cord. It does not matter how greatly the nerve-cells 
 differ in type, they all conform to one law, and that is they each 
 furnish from one pole of the cell a process which becomes a nerve- 
 fibre. No cell furnishes more than one such process, and that 
 process constitutes the axis cylinder of the nerve. When the 
 neurone doctrine is dealt with, more will be said on this subject, 
 but one item of information must be anticipated, and that is a 
 nerve-fibre is really an elongated process of a nerve-cell, and may 
 run for a considerable distance without a break; even when it 
 is broken the thread is again taken up, so that the axis cylinder 
 runs from its origin to its termination. There is no union of 
 nerve-fibres; of the thousands in a large bundle each and every 
 
458 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 one is complete in itself, and it is this which enables the same 
 nerve-trunk to convey impulses of many and opposite kinds. Of 
 such a nerve-trunk no better example could be given than that of 
 the vagus, with its fibres to the larynx, lungs, heart, stomach, and 
 intestines, each functioning in its own way and independently 
 of its neighbours, with which it may have no more in common than 
 if they did not exist. 
 
 Nerves are remarkable for their want of elasticity, but they 
 are capable of very considerable stretching without breaking. 
 In man the nerves of the limbs require a weight of from 18-2 kilo- 
 grammes (40 pounds) to 54-6 kilogrammes (120 pounds) to break 
 them. Nerves are also very indifferently supplied with blood- 
 vessels. 
 
 Nerve Terminations. — There are some structures, such as 
 glands, where the nature of the nerve termination is not satis- 
 factorily made out; there are other places, such as muscle, 
 where definite and distinct motor nerve-endings have been 
 found; and on many sensory and sympathetic nerves special 
 terminations, known as ' Pacinian corpuscles ' and ' Krause's 
 end-bulbs,' exist. Nerve terminations are found in the muzzle 
 of animals, in tendons, in muscles, in the generative organs, 
 conjunctiva, mouth, tongue, epiglottis, etc.; some are known as 
 ' Krause's end-bulbs,' those in tendon are described as the ' organ 
 of Golgi,' in muscle they are known as ' end-plates,' whilst in the 
 skin of the muzzle the nerves terminate in small swellings or 
 enlargements known as ' tactile cells,' which are placed between 
 the epithelial cells of the epidermis; cells of this kind also exist 
 in the foot of the horse. The nerves of special sense have each a 
 distinct termination peculiar to themselves, such as the hair-cells 
 of the internal ear, the rods and cones of the retina, taste-bulbs 
 of the tongue, etc. 
 
 Chemistry of Nerve-Fibres. — The chemistry of these tissues is 
 very imperfectly known. Advances in physiological chemistry 
 have shown that substances like protagon, which at one time was 
 believed to represent the essential composition of the medullary 
 substance or myelin, are really a mixture of substances. The 
 myelin furnishes three substances which possess definite chemical 
 characteristics — viz., cholesterin, lecithin, and cerebrosides. 
 Cholesterin is a substance containing neither nitrogen nor phos- 
 phorus, and in chemical nature is allied to a group of bodies 
 found in plants known as terpenes. Its silvery crystalline for- 
 mation is characteristically shown in the tumours on the choroid 
 plexus of the horse. In the body it occurs with lecithin, though 
 the nature of the physiological connection, if any, is unknown. 
 Lecithin is a phosphorus-containing nitrogenous fat, especially 
 characteristic of the nervous system, but found elsewhere in the 
 
THE NERVOUS SYSTEM 459 
 
 body. When lecithin is decomposed it yields, among other 
 products, a fatty acid ; it is this which is blackened in osmic acid 
 staining of normal and degenerated fibres. Cholin, another 
 product of the decomposition of lecithin, is very poisonous. 
 Cerebrosides are a group of glucoside bodies containing nitrogen, 
 of which very little is known, excepting that they yield galactose 
 on decomposition. 
 
 Irritability and Conductivity. 
 
 Two features of nerve-fibres are irritability and conductivity. 
 By irritability is understood the reaction to a stimulus, by con- 
 ductivity the propagation of an excitation. The nature of the 
 change occurring in a centre or receptive surface which induces 
 an impulse is as yet unknown, but whatever the change may 
 be it is experimentally possible to imitate it artificially by 
 means of chemical, mechanical, or electrical stimuli, and when 
 nerves are so stimulated they function as if the normal stimulus 
 had been applied. Electrical stimuli are most commonly em- 
 ployed, and if a motor nerve be so stimulated the muscle con- 
 tracts ; if a sensory nerve, pain or a sensory impression is observed. 
 If a secretory nerve be stimulated, secretion results. At one time 
 the normal stimulus to a nerve-cell was believed to be electrical in 
 nature; it is now known that this is not so, and that though 
 electrical stimuli are capable of causing a nerve to function, this is 
 a coarse method compared with the natural stimulus. A sudden 
 change in temperature acts as a stimulus to a sensory, but not 
 to a motor nerve. A motor nerve is quite unaffected by sudden 
 cooling or heating, whereas a sensory nerve so treated produces 
 pain, and it has been suggested that this difference in reaction 
 indicates some marked difference in structure. When nerves are 
 ligatured or divided impulses are no longer transmitted. Ex- 
 perimental inquiry shows that even after long-continued excita- 
 tion nerves are still irritable, which has given rise to the belief 
 that they never suffer from fatigue (see p. 446). 
 
 The stimulation of a nerve-muscle preparation by means of 
 a weak constant current causes a ' twitch ' of the muscle at 
 ' make,' and another at ' break '; during the period the current 
 is passing through the nerve, though the muscle gives no evi- 
 dence of this, yet important changes are occurring. These 
 changes are concerned with the irritability and conductivity of 
 the nerve, and must here be examined. 
 
 Electric Phenomena in Nerves. — When studying a muscular con- 
 traction, we saw that a perfectly uninjured muscle was iso-electric 
 — viz., it gave no evidence to the galvanometer of the existence of 
 a current. If, however, the muscle was injured, the seat of injury 
 
460 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 became electrically negative to the uninjured part, and the current 
 was called the current of injury. The figure there employed to 
 explain injury currents in muscle is again used here to explain the 
 same currents in nerves. If the nerve be injured, the current of 
 injury flows from the equator to the cut end outside the nerve, 
 and from the cut end to the equator inside the nerve. If a nerve, 
 while exhibiting the presence of a current of injury, be stimulated, 
 the uninjured portion becomes electrically negative to the injured 
 part. There are now two currents in the nerve — viz., the current 
 of injury and the new one just created; each is flowing in the 
 opposite direction, the result being that the current of injury 
 becomes diminished. This diminution or reduction in the current 
 of injury is termed negative variation. 
 
 In the two observations described the nerve is assumed to be in 
 an injured condition. If now a constant current be passed through 
 
 Fig. 130.— Diagram illustrating the Electric Currents of Injury in 
 Nerve and Muscle (Foster). 
 
 The diagram serves either for nerve or muscle, excepting that the current gh 
 is not present in nerve. The strong currents are shown by the dark lines; 
 the arrows show the direction through the galvanometer, ab, The equator; 
 ax, current from equator to cut end; xa, current within the nerve, cut 
 end to equator. 
 
 an uninjured nerve, a wave of electrical negativity travels along it, 
 and this current is termed the current of action. It will be obvious 
 that it was the presence of the current of action which diminished the 
 current of injury in the previous experiment. Let a nerve be taken 
 and arranged as shown in Fig. 131, A — viz., a constant (polarising) 
 current passed through its middle piece, while a galvanometer is 
 connected with both regions of the nerve outside the anode and 
 kathode poles of the battery. Under these circumstances, a polar- 
 ising current passes through the nerve in the direction of its circuit — 
 viz., from anode to kathode and back to the battery; but, in addi- 
 tion to this, there is a current in the extra-polar regions, as indicated 
 by the attached galvanometers. These extra-polar currents are 
 termed electrotonic. The piece of nerve beyond the kathode is 
 increased in excitability and conductivity, and this is termed kath- 
 electrotonus . The portion of nerve beyond the anode is decreased 
 
THE NERVOUS SYSTEM 
 
 461 
 
 Bnltenj. 
 
 AxitiMrulVuit current J 
 
 Area oj decreased excitability . 
 
 KaleleetroWic current 
 ». ]|| Area of increased excitability 
 
 JV£M£ 
 
 A/sxrs 
 
 */£/??£ 
 
 c 
 
 ConTraction occurs ah 'make 1 
 
 only , uij (it kathode 
 
 Very strong current 
 descending 
 
 Contraction occurs al" 'make' 
 only. 
 
 D. 
 
 Very strong current 
 ascending 
 
 , A/SWS 
 
 Contraction, occurs at break' only 
 w^ at anode. 
 
 Fig. 131. — Diagram of Electrotonus. 
 
462 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 in excitability and conductivity, and is in a condition known as 
 anelectrotonus. It will be observed that, though the battery current 
 is sent only through the middle piece of the nerve, it gives rise to 
 a current which passes through the whole length of the nerve in 
 the same direction as the exciting current, as shown by the galvano- 
 meters. Turning again to Fig. 131, A, it is evident that between the 
 region of increased and that of reduced irritability there must be a 
 region of neither increased nor diminished irritability. This is known 
 as the neutral point. When the polarising current is weak, this point 
 is nearer the anode ; when it is strong, it is nearer the kathode, 
 so that with very strong currents the whole intrapolar region may 
 be in a condition of anelectrotonus. A clear understanding of this 
 fact will render the subsequent observations intelligible. It will 
 be remembered that the muscle of a nerve-muscle preparation gave, 
 on stimulation, a twitch at ' make,' and another at ' break,' the 
 make contraction always starting from the kathode, and the break 
 contraction from the anode. These contractions are due to the 
 development of electrotonic currents, and, provided the stimulating 
 current always remains of the same strength, the twitch at ' make ' 
 and another at ' break ' will always be obtained. But it is known 
 that an increase or decrease in the strength of the polarising current 
 gives other results in the muscular response; for instance, if the 
 current be weak (Fig. 131, B), contraction of the muscle occurs only 
 at ' make,' the kathode end of the nerve being the region of increased 
 excitability and conductivity, the anode end being in both these 
 respects decreased, and with a weak current lowered below the point 
 of possible electrical response. If the current be made stronger, 
 contraction occurs both at ' make ' and ' break.' If a very strong 
 descending current be passed through the nerve, contraction occurs 
 at ' make ' only, owing to the area of increased excitability lying 
 next the muscle (see Fig. 131, C). If the muscle be attached to the 
 opposite end of the nerve — viz., nearest the anode pole — and a very 
 strong current sent through, the battery current will be passing up 
 the nerve, as it is termed — i.e., from the muscle to the spinal cord 
 (Fig. 131, D); under these conditions there is no difficulty in a 
 ' break ' contraction occurring, as the area of increased excitability 
 is again in the myopolar region; but the reduced excitability and con- 
 ductivity of the nerve prevents the current from reaching the kathode, 
 so that no contraction occurs at ' make.' Pfluger, who investigated 
 these phenomena, formulated a Law of Contraction, which, after the 
 above explanation, may now be stated : 
 
 Strength of 
 Current. 
 
 Ascending Current. 
 
 Descending Current. 
 
 Making. 
 
 Breaking. 
 
 Making. 
 
 Breaking. 
 
 Weak 
 
 Moderate 
 Strong - 
 
 Contraction 
 
 Contraction 
 No contrac- 
 tion 
 
 No contrac- 
 tion 
 Contraction 
 Contraction 
 
 Contraction 
 
 Contraction 
 Contraction 
 
 No contrac- 
 tion 
 Contraction 
 No contrac- 
 tion 
 
 When a particularly irritable nerve-muscle preparation is stimu- 
 lated with a constant current, it may pass into tetanus at either 
 
THE NERVOUS SYSTEM 463 
 
 ' make ' or ' break.' This is known as Ritter's tetanus. The nature 
 of electrotonic currents is not fully agreed upon ; they are markedly 
 shown in fresh medullated nerves, but not in those which are dead ; 
 they are only feebly shown in non-medullated nerves. Though 
 physical in origin, they can be obtained only in nerves which still 
 possess the characteristics of living. They may be demonstrated 
 in an artificial nerve in which the axis cylinder is represented by 
 platinum wires, and the medulla by a glass tube containing a solution 
 of zinc sulphate. It is believed that electrotonic currents are due 
 to polarisation occurring between the medullary sheath and axis 
 cylinder, but nothing is yet settled. 
 
 The facts which have been ascertained experimentally as to effects 
 which follow the application of constant and induced currents have 
 been employed in the diagnosis of nerve degeneration. A healthy 
 motor or sensory nerve, or healthy muscle, reacts to both stimuli. 
 A degenerated nerve conveys no sensory impulses, nor is stimulation 
 of a motor nerve so affected followed by muscular contraction. 
 Muscles in the horse, if we except the effect of azoturia, do not 
 appear to suffer from atrophy, unless as the result of injury or 
 degeneration of the nerves. A muscle will grow smaller if thrown 
 out of use, but the result is very different from the atrophy occurring 
 in consequence of nerve degeneration, which is remarkable for its 
 completeness and the relative rapidity with which it occurs. Muscle 
 fibres will respond to direct stimulation of a constant current, even 
 when the nerve is degenerated ; but the contraction is slower, and is 
 brought about, not through the nerves, but through the muscle 
 fibres. A healthy muscle responds as readily to a constant as to 
 an induced current. With complete degeneration of a motor nerve, 
 no response on its stimulation is evoked from the muscle ; nor can the 
 latter be stimulated by induction shocks, but it may be stimulated by 
 the constant current. This is employed in human practice as a test 
 of degeneration. A normal contraction, we have seen, occurs at 
 the negative pole on closing. To obtain a contraction at the positive 
 pole on closing, a stronger current must be employed"; but in par- 
 alysis, which is not of central origin, the muscles do not respond to 
 an induced current, but react to a continuous current, and the same 
 strength of current, both of opening and closing, will readily produce 
 a response. This is the Reaction of Degeneration. 
 
 Negative Variation. — -We have seen (p. 460) that when a nerve is 
 indicating through the galvanometer the existence of a current of 
 injury, its stimulation leads to a current in the opposite direction, 
 or negative variation. In living nerves negative variation has been 
 observed in consequence of stimulation of the brain, and the associa- 
 tion of negative variation with normal nerve impulses is accepted. 
 Negative variation marks in a nerve what a contraction represents 
 in a muscle — viz., the passage of an impulse. The capillary electro- 
 nometer shows that the onward rush of negativity travels at the 
 same rate as a nerve impulse. By means of the string galvanometer 
 of Einthoven (an instrument far surpassing the capillary electrono- 
 meter in delicacy), a negative variation may be seen to occur in 
 the vagus at each inspiration. Movements due to an electrical change 
 can be detected passing both up and down the nerve in consequence 
 of nervous impulses. 
 
 Diphasic Variation. — When a muscle contracts, a wave runs along 
 it, and an electrical change occurs. The contracting part of the 
 muscle becomes negative, and then, as the muscle returns to the 
 state of rest, this negativity disappears. This two-phase electrical 
 
464 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 condition is termed diphasic variation ; its presence in muscle is 
 concurrent with a contraction, while in nerve it is identical in point 
 of time and rate with the transmission of a nervous impulse. We 
 have seen that the current of injury may be detected by the rheo- 
 scopic frog; the current of action may be detected in the same way. 
 If the nerve of a nerve-muscle preparation be laid on the heart of 
 a frog which is still active, the current of action liberated from the 
 heart by its contraction causes the muscle of the limb to contract 
 at each heart-beat. 
 
 The string galvanometer shows that when a muscle contracts two 
 electrical waves pass over it, due to changes in potential. Electro- 
 meter records of the contraction of the heart muscle may be ob- 
 tained by placing leads either on the exposed heart, or leading off 
 
 through the limbs. Waller 
 was the first to show how an 
 electrocardiogram could be 
 obtained in an intact animal 
 at each beat of the heart. If 
 a dog be placed with one fore- 
 paw in a basin containing a 
 solution of salt, and a diagonal 
 hind-paw in another, and the 
 two basins connected with the 
 galvanometer, at every beat of 
 the heart the electrical changes 
 resulting from its contraction 
 are conducted through the 
 body to the electrometer, and 
 the minute movements of the 
 mercury rendered visible by 
 a microscope. Contraction 
 begins at the base of the 
 heart; the base is therefore 
 negative to the apex. When 
 the ventricles contract, the 
 cardiogram shows that the 
 apex is then negative to the 
 base; the interpretation of 
 the curve is not, however, so 
 simple as might appear. 
 
 The electrocardiogram rep- 
 resents the algebraic sum of the 
 electrical changes taking place at the two electrodes (base and apex). 
 As is evident from the figure, the electrical changes slightly precede 
 the form or pressure changes. 
 
 Nature of Nerve Impulse. — The velocity of a motor impulse in 
 man has been ascertained to be about 100 metres (333 feet) 
 per second; the velocity in sensory fibres is unknown. In the 
 frog the velocity of the action-current in motor nerves agrees 
 exactly with the rate at which a motor impulse travels along the 
 nerve, and in discussing the nature of nerve-impulses it is 
 natural that the action current (negative variation) should 
 be considered to be intimately connected with the transmission 
 of ordinary normal impulses. It is necessary, however, to be 
 
 Fig. 132. 
 
 Simultaneous records of A , intraventricular 
 pressure; B, electrocardiogram of cat's 
 heart; P, corresponds to the auricular 
 systole; R and T, to the ventricular 
 systole (modified from Piper). 
 (From Bayliss' ' Principles of Physiology.') 
 
THE NERVOUS SYSTEM 465 
 
 careful and avoid considering this change and a nerve impulse 
 as identical. 
 
 The most valuable additions to our knowledge of the subject 
 of nervous conduction we owe to the work of Keith Lucas* and 
 his school, which deals with the phenomena exhibited by con- 
 duction along peripheral nerve-fibres. The problems which 
 have received most careful study may be briefly referred to. 
 In the first place, the intensity of the motor nervous impulse 
 cannot be gauged by the magnitude of the resultant muscular 
 contraction, for the latter may be determined by the number 
 of nerve-fibres stimulated rather than by the variability in the 
 intensity of the unit fibre excitation. Lucas, Adrian, Verworn, 
 and others have shown that the intensity of the impulse is not 
 influenced by the strength of the stimulus. In fact, nerves 
 under stimulation behave as muscles do, and exhibit only one 
 degree of activity; it is ' all or nothing.' The strength of the 
 normal stimulus is adjusted by varying the number of fibres in 
 action. 
 
 At certain parts in the neuronic or interneuronic tracts there 
 is evidence of a decrement in conduction — for example, at the 
 synapse in the central nervous system, and also in the junctional 
 tissue between nerve and muscle. The decrement in conduction 
 in the nerve-fibre itself has been studied by observing the effects 
 produced by the localised action of narcotics on the nerve-trunk. 
 Among the most interesting phenomena exhibited by nerve 
 in this condition is the rise to the normal intensity of the nervous 
 impulse after its passage through a narcotised area, which is too 
 short to give rise to complete disappearance of the impulse. The 
 progression of the nervous impulse evidently depends not solely 
 upon the energy supplied at its place of origin, but also upon 
 the energy set free locally by the disturbance itself, as in the 
 firing of a train of gunpowder. The chemical changes, however, 
 accompanying the disturbance are so slight as to be difficult 
 of detection. There is evidently a consumption of oxygen and 
 evolution of carbonic acid. The passage of the nervous impulse 
 leaves behind it a condition in which the nerve is temporarily 
 refractory (fatigue) to a second stimulus, followed by a period 
 of partial recovery and subsequently by one of increased ex- 
 citability. The phenomena of summation and inhibition are 
 exhibited by peripheral as well as by central conduction. 
 
 The Neurone Doctrine. — It is only within the last thirty years 
 that it has been possible to understand the elements concerned in 
 the working of the nervous system, not only in respect of their 
 structure, but also their function. That the nervous system 
 
 * "The Conduction of the Nervous Impulse,' by Keith Lucas, 1917. 
 ' Monographs on Physiology.' 
 
 30 
 
466 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 consisted of fibres and nerve-cells had long been known, that the 
 cells were capable of nourishing the fibre was shown years ago by 
 the experiments of Waller on the dorsal roots of the cord, but 
 there was still something lacking. The origin of the nerve-fibres 
 was unknown, while the distribution of the cells in the grey 
 substance did not appear to be made on any system with a 
 physiological basis. The doctrine of the neurone supplies what 
 
 was wanting. A neurone is 
 a nerve-cell with its various 
 processes and nerve -fibres. 
 The cell-body,* the fibre, and 
 the processes are a physio- 
 logical unit, a nervous system 
 in miniature; myriads of such 
 microscopic systems consti- 
 tute the nervous system. 
 
 A nerve-cell consists of a 
 mass of protoplasm intersected 
 by fine fibrils known 'as neuro- 
 fibrils. Towards the centre of 
 the cell is a refractile nucleus, 
 and lying between the neuro- 
 fibrils are certain peculiar 
 bodies, which, though granular 
 in nature, stain with methy- 
 lene blue — an exception to the 
 general staining reactions of 
 cell granules (Fig. 133). These 
 bodies are known as Nissl's 
 granules, or chromophile sub- 
 stance ; they exist in the form 
 of angular-shaped masses or 
 rods, and extend into all the 
 processes of the cell, excepting 
 that from which the nerve- 
 fibre arises. These granules 
 
 Fig. 133. — Structure of Multipolar 
 Cell (Barker). 
 
 <rt,vAxon t hillock (the portion of the cell 
 from which the axon comes off), con- 
 taining no Nissl bodies, and showing 
 neuro-fibrils ; ax, axis cylinder or axon; 
 m, medullary sheath, outside of which 
 is the neurilemma; c, cell-substance 
 (cytoplasm), showing Nissl bodies in a 
 lighter ground substance; d, proto- 
 plasmic processes or dendrites con- 
 taining Nissl bodies ; », nerve-cell body 
 or perikaryon ; »', nucleolus; nR., node 
 of Ranvier; s.f., collateral fibril. 
 
 are intimately concerned in 
 the nutrition of the cell and its fibre, but whether as fluid or 
 granules is unknown. In a working cell the granules diminish, 
 and under great muscular exertion they may be reduced to fine 
 particles, resembling dust in appearance. This is due to the 
 chromophile substance breaking up; the chromatolysis so pro- 
 
 * The ' nerve-cell body ' and the ' nerve-cell ' are not the same ; the latter 
 includes the dendrites and axons — it is, in fact, the neurone. The former 
 is the cell which gives birth to the neurone, and it is best distinguished as 
 the perikaryon. 
 
THE NERVOUS SYSTEM 
 
 467 
 
 duced also occurs when the cell is separated from its fibre. It is 
 evident from this that nerve-cells, unlike nerve-fibres, are capable 
 of fatigue. Nerve-cells are not all of the same shape, size, or 
 
 Fig. 134. — Cell from the Nuclei of the Oculo-Motor Nerve of the 
 Cat (Barker, after Flatau). 
 
 The nucleus, nucleolus, and Nissl bodies are shown on a larger scale 
 than in Fig. 133. 
 
 general arrangement ; some have only two processes growing from 
 the cell, and hence are called ' bipolar cells ' ; other cells have 
 numerous processes, and are called ' multipolar.' All the pro- 
 
 Fig. 135. — Multipolar Nerve-Cell (Barker, after Kolliker). 
 m, Axon; c c, collaterals; d, dendrites. 
 
 cesses of a nerve are not nerve-fibres ; of all the processes in the 
 multipolar cell shown in Fig. 135, only one is the beginning of a 
 nerve-fibre. It is called the axon ; it is the process of the cell 
 which transmits the impulse. The other processes are called 
 
468 A MAN.UAL OF VETERINARY PHYSIOLOGY 
 
 dendrites ; they are the receiving portion of the cell. The actual 
 conducting materials in the cell are the neuro-fibrils previously- 
 spoken of. These pass from the dendrites of the cell into the 
 cell-body, and from the cell-body they pass into the axon. The 
 axon is therefore merely a bundle of neuro-fibrils, a nerve is the 
 immensely elongated process of a nerve-cell. Cells with these 
 processes vary in type and arrangement, but may be classified 
 into the two groups previously mentioned of bipolar and multi- 
 polar. Bipolar cells are found typically in the ganglia on the 
 roots of the spinal and cranial nerves. These cells are peculiar, 
 inasmuch as they possess no dendrite process. The axon issues 
 as a single process from the cell, and then divides T-shaped into 
 two fibres (see Fig. 136); such cells would have been called 
 one-poled, but for the fact that they are known by embryological 
 studies to be bipolar. This type of cell is especially associated 
 with the sensory nerves, and the axon is accordingly long, and 
 
 Fig. 136. — Cells from the Gasserian Ganglion of a developing Guinea-Pig. 
 
 The originally bipolar cells are seen changing into cells apparently unipolar. 
 The same process occurs in the cells of the spinal ganglia (Van Gehuchten). 
 
 may extend, say, from the foot to the spinal cord. The multi- 
 polar cells (Plate I., 3, and Fig. 135) are more widely distributed, 
 and are found in the grey matter of the brain and spinal cord. 
 They are furnished with processes and the usual axon, but the 
 axon may be long or short, and this peculiarity enables multipolar 
 cells to be divided into two groups, known as Golgi cells of the 
 first and second type. Golgi cells of the first type may have an 
 actual or relatively long axis cylinder. It may be a cylinder 
 reaching from the cerebrum to the medulla, which would be re- 
 latively long, or from a segment of the spinal cord to the foot, 
 which would be actually long. This type of cell is associated 
 with motor nerves. Golgi cells of the second type possess a 
 short, widely-branched axon, which never leaves the grey matter; 
 this type of cell may be seen in the cerebrum and cerebellum. 
 Their short, widely-branching axons lead to the belief that such 
 cells are distributive in function. 
 
THE NERVOUS SYSTEM 469 
 
 The processes leading to a cell are called dendrites (Fig. 135). 
 Some cells, like the bipolar, have no such processes; the multi- 
 polar, on the other hand, may be richly endowed. The neuro- 
 fibrils found in the cell extend into it from the dendrites. The 
 function of the dendrites is to collect impulses for transmission 
 to the cell. The remaining process belonging to a cell is termed 
 the axon (Figs. 133 and 135); it is the process which gives origin 
 to the nerve-fibre, and into its substance the neuro-fibrils from 
 the cell-body pass. Before the axon of a nerve becomes a nerve- 
 fibre it gives off slender branches known as collaterals (Fig. 135); 
 they may be few in number or numerous, as in the case of Golgi 
 cells of the second type. A nerve-fibre terminates by ending 
 in a fine tuft of branches in the neighbourhood of another cell ; 
 the tuft is termed arborisation, and the junction thus formed 
 with the neighbouring cell is termed a synapse (Fig. 138). It is 
 believed that in the neighbourhood of synapses there exists a 
 receptive substance, possessed of certain physiological properties, 
 which favours the transmission of impulses to the neighbouring 
 cell, with which, it will be observed, it does not come into actual 
 contact. 
 
 A nerve-cell, its dendrites, axon, collaterals and synapse, con- 
 stitute a neurone. A neurone is a nervous system in miniature; 
 millions of such placed end to end, like the links in a chain, and 
 side by side, like a series of chains, enable the nervous system 
 to be visualised. Some of the links are short, others, we have 
 seen, are long, but no link — -i.e., no single neurone — runs from the 
 brain to the periphery of the body. The majority of the neurones 
 originating in the brain extend no farther than the nearest cell 
 station, such as the basal ganglia or medulla; as a rule, either in 
 one or the other, or in both of these places, the neurone ends by 
 arborising around a cell in a ganglion, and a fresh neurone is 
 formed. Even when it gets into the cord it has not necessarily 
 a long run; it may run the length, but the majority of the cord 
 neurones do not ; they dip into the grey matter at intervals, and 
 arborise around cells from which fresh fibres arise. This may 
 or may not be the last cell station; if it is the last, the axon 
 passes out into a peripheral nerve, but even when outside the 
 spinal cord a fresh break may occur — as we shall see in dealing 
 with the sympathetic system — a further arborising around cells 
 in the sympathetic ganglia, and finally new fibres. This is what 
 is meant in speaking of the neurones being arranged end to end 
 like the links of a chain. In the simplest conceivable form of 
 nervous mechanism the smallest number of neurones is two; 
 it is seldom that any such simple number exists in the body 
 mechanisms; as a rule, there are several breaks between the 
 periphery and the brain, or the brain and the periphery. A 
 
470 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 single motor cell in the cerebrum, with its axon, is not connected 
 with a single spinal motor neurone ; the latter may be connected 
 with many such cerebral neurones. The sum of all the iibres in 
 the white substance of the brain is, therefore, obviously larger 
 than the sum of those in the cord. 
 
 The conducting path is always from dendrites to cell, and 
 from cell to axon. In a neurone a stimulus can be transmitted 
 only in one direction — i.e., from dendrite to axon, so that the 
 dendrite is the receiving portion of the cell, the axon the issuing 
 or distributing portion. No matter how many neurones the 
 impulse has to pass through, nothing is lost excepting time. The 
 impulse is delayed, but it is carried from the termination of the 
 axon across unoccupied fields to the nearest dendrites, and so 
 transmitted to the next cell. There is no contact of the cellular 
 elements of one axon with those of another. This is an essential 
 part of the neurone doctrine. There is everywhere contiguity, 
 but no structural continuity. 
 
 Such is the principle of structure existing in the nervous 
 system, brain or spinal cord, afferent or efferent nerves, cerebro- 
 spinal or sympathetic. There may be variations, and here and 
 there the general rules of construction may be slightly departed 
 from, but the principle is maintained — a principle which explains 
 not only the anatomical, but the physiological side of the system, 
 and enables the mind to grasp the unity of construction of the 
 entire nervous system. 
 
 The cell is the life of the fibre. If the fibre be cut off from the 
 cell, it degenerates. Nor is the nutritional change confined to 
 the fibre. The cell also suffers in consequence of loss of function, 
 there being no impulses transmitted to it by its dendrites. The 
 neurone concept consists in regarding the nerve-cell, its den- 
 drites, its collaterals, axon, and terminal arborisation as a 
 physiological unit. Innumerable such units, never touching, 
 but yet brought closely together, constitute the nervous system. 
 
 Degeneration and Regeneration of Nerves. — The cell 
 nourishes the axon; if, therefore, the axon be cut off from its 
 cell, degeneration occurs, and this law applies to all cells cut 
 off from their nucleus. We shall see presently that the great 
 inflow and outflow of nerves from the spinal cord is divided into 
 groups — afferent and efferent — broadly speaking, sensory and 
 motor. Every axis cylinder is a nerve, and has a cell-station. 
 In the case of the sensory nerves the cell-station is the ganglion 
 on its root just outside the spinal canal; in the case of the motor 
 fibres the cell-station is in the ventral course of the grey matter 
 of the cord. If a spinal sensory nerve be divided below the gang- 
 lion, the whole length of nerve below the ganglion, being cut off 
 from its cell-station, degenerates. If the nerve be divided above 
 
THE NERVOUS SYSTEM 471 
 
 the ganglion, those fibres which enter the cord from the T division 
 of the two-poled cell alone degenerate, for it is only these which 
 are affected by division above the ganglion (Fig. 137, 1). If the 
 efferent or motor roots be divided, the cell-station being then 
 above the division— viz., in the grey matter of the cord— the 
 nerve degenerates downwards (Fig. 137, 4). If both roots of the 
 spinal nerves be divided below the level of the spinal ganglion, 
 both nerves degenerate in a downward direction (Fig. 137, 3)— 
 These profoundly interesting facts, discovered many years ago 
 by the late Dr. Waller, are known after him as Wallerian 
 degeneration. By this method of observation it became possible 
 to trace the afferent paths in the spinal cord in consequence of 
 the degeneration produced when these were divided above the 
 ganglion. 
 
 The nerve-fibre, as has been stated above, is but a branch 01 
 a nerve-cell. If a portion of a cell be separated from the part 
 containing its nucleus, it soon dies. Thus, when a large amoeba, 
 or a Radiolarian, is torn up into several pieces, the portions 
 containing no nucleus degenerate and die; but that portion 
 containing the nucleus repairs itself and re-forms a perfect cell. 
 The nerve-fibre dies down after being cut, just in so far as it is 
 a piece of cell cut off from its nucleus. The sensory nerve 
 divided in neurectomy, as practised on the horse, degenerates 
 towards the foot, and not up the limb, for it is the part below 
 the wound which is cut off from its nutrient centre, and not the 
 portion above. Had this been a motor nerve, the degeneration 
 would still have taken place below the wound, and for the same 
 reason. All spinal nerves have their seat of nutrition either in 
 the spinal cord or in the ganglia just outside it. The nearer 
 to the spinal cord the point at which the section is made, 
 the greater the length of nerve which degenerates ; the farther 
 away from the cord the point at which section is practised, the 
 shorter the length which degenerates. When the nerve-degener- 
 ates, the fatty medullary sheath breaks up, forming globules 
 around the axis cylinder. The latter also degenerates and ulti- 
 mately breaks up. The remarkable fact about these changes is 
 the rapidity with which they occur, especially in the dog. Four 
 days is sufficient to show their commencement. Small nerve- 
 fibres degenerate more quickly than large. 
 
 The microscopical changes occurring in degeneration are very 
 characteristic. In medullated nerves the axis cylinder swells 
 and breaks up, and the medullary sheath disintegrates into 
 droplets of myelin. Each detached piece of these undergoes a 
 similar change, until the nerve is represented by a bundle of 
 connective tissue without axis cylinder or medullary sheath 
 (Fig. 129, II, III). Along the course of the degenerating 
 
472 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 fibres proliferation of the neurolemma nuclei takes place to form 
 a scaffolding for the regeneration process. In non-medullated 
 nerve, degeneration is characterised by the disappearance of the 
 axis cylinder. The nerve above the seat of division, though still 
 in communication with the cell, undergoes a limited degeneration 
 as the result of injury, and even the nerve-cells which are intact 
 undergo temporary atrophic changes in consequence of their 
 axons having been cut (Nissl degeneration). 
 
 No. i. — Degeneration of afferent 
 fibres caused by a section of superior 
 root above the ganglion. 
 
 No. 2. — Degeneration of afferent 
 fibres following a section of superior 
 root below the ganglion. 
 
 No. 3. — Degeneration of efferent and 
 afferent fibres following a section of 
 the entire nerve. 
 
 No. 4. — Degeneration of efferent 
 fibres following a section of inferior 
 root. 
 
 Fig. 137. — Diagrams to illustrate Wallerian Degeneration of Nerve 
 
 Roots (Waller). 
 
 The black portions indicate degeneration. 
 
 By the suturing of divided nerves union is established, and 
 though the act of division causes degeneration, yet, when union 
 has taken place, regeneration of fibres occurs. A fresh axis 
 cylinder grows through the length of the degenerated nerve, and 
 after some months motion or sensation is perfectly or imperfectly 
 restored. Sensation is always much later in appearing than 
 motion. After union of severed cutaneous nerves vasomotor 
 tone is recovered sooner than the sensory functions, or the tone 
 of the other smooth muscle innervated tissues — sweat glands 
 
THE NERVOUS SYSTEM 473 
 
 and hair follicles. Even suture of divided nerves is not always 
 necessary for union. It is known clinically that the plantar 
 nerves of the horse will often unite in a few months, in spite of 
 a piece having been excised, the portion of nerve above sending 
 down an axis cylinder, which soon finds out the divided portion 
 below. If the gap between the divided ends of a nerve is con- 
 siderable, a new axis cylinder cannot find its way across. 
 
 Trophic Nerves. — -Not only is the nutrition of the nerve itself 
 affected by nerve division, but also the nutrition of those parts 
 supplied by it. Ulceration more or less severe has been known 
 to follow injury of certain nerves. Sloughing of the cornea 
 occurs in animals when the ophthalmic division of the fifth is 
 divided, but this may be due to other causes than loss of trophic 
 influence; and many practitioners are acquainted with the 
 sloughing of the entire foot which sometimes, though fortunately 
 rarely, follows the operation of neurectomy. It thus appears 
 that nerves influence the nutrition of a part. It is also well de- 
 monstrated in cases of intense muscular atrophy due to nerve 
 injury, and in the dry papillated condition of the nose of the dog 
 after division of the cervical sympathetic. The existence of 
 special trophic nerves has been denied — i.e., of nerves exclusively 
 devoted to maintaining the nutrition of the part. But sloughing 
 of the hoof occurring a few days after neurectomy is evidence of 
 the existence of some special nutrition having been cut off through 
 division of the afferent fibres. The rapid degeneration of 
 muscle after nerve injury is also suggestive of special trophic 
 nerves; the muscles of the scapula or of the hind-quarter of the 
 horse appear to ' melt ' away, so rapid in the atrophy. 
 
Section 2. 
 Reflex Action. 
 
 Nerve-fibres do not under ordinary circumstances generate 
 impulses; they transmit them, but without modifying them. 
 Modification can occur only in nerve centres, such as the brain 
 and spinal cord, and these centres always consist largely of 
 nerve-cells, of which, as we have seen, the nerve-fibres leaving 
 or entering the centre are simply processes or branches. The 
 spinal cord may be described, not as one long centre, but as a 
 series of centres lying end to end, each capable to a greater or less 
 extent ot acting independently of its neighbour, and each centre 
 possessing its afferent and efferent roots. 
 
 In these segments of spinal cord complex acts can be initiated 
 by the arrival of simple afferent impulses. Such acts may be 
 carried out without any assistance from the brain, for they can 
 readily be demonstrated in an animal in which the brain has been 
 destroyed. These acts are known by the name of 'reflex,' from 
 which it must not be inferred that an afferent impulse is simply 
 reflected into an efferent channel, but rather that an afferent 
 impulse reaches the cord, and, passing into the grey matter, 
 stimulates the ganglionic cells which generate the efferent im- 
 pulse. The structures necessary for a simple reflex act are— 
 
 (1) an afferent nerve to convey the impression to a nerve-centre; 
 
 (2) a nerve-centre in which the outgoing impulses are generated; 
 
 (3) an Afferent channel for their transmission (Fig. 138). More 
 complex acts may need a greater number of afferent nerves, 
 of excitable centres, and of efferent fibres. Such a nervous chain 
 is known as a Reflex Arc, and can never consist of less than two 
 neurones. 
 
 A classical example of a reflex act is exhibited when a frog 
 from which the brain has been entirely removed draws up its 
 leg when the foot is pinched. According to the degree of 
 pressure applied to the foot, it draws up either one leg or both — 
 i.e., the reflex movements are unilateral or symmetrical, accord- 
 ing to the number of ganglionic centres in the cord which have 
 been stimulated. Still greater violence applied to the foot of 
 this brainless frog will affect a larger number of centres farther 
 forward in the cord, so that the fore-limbs may share in the 
 reflex. The brainless frog reacts more regularly to this experi- 
 ment than one possessing a brain, which is evidence that the 
 brain is capable of exercising a controlling influence or inhibitory 
 
 474 
 
THE NERVOUS SYSTEM 
 
 475 
 
 effect upon reflex actions. One very prominent feature of a 
 reflex act is its apparently purposeful character. An oppor- 
 tunity for studying this in detail will be given presently when 
 the ' reflex frog ' is considered. 
 
 In the dog very characteristic reflex actions occur after division 
 of the cord, such as those of walking, running, scratching, mic- 
 turition, and defalcation; and some of these will shortly be con- 
 
 The parts within the dotted 
 line lie within the grey 
 matter of the nerve centre. 
 
 Re, Re, Receptive surface, 
 in this case the skin, im- 
 pressions from which are 
 conducted by afferent 
 fibres, dd, ax, to a nerve 
 centre, pk', where out- 
 going impulses are gener- 
 ated; these are conducted 
 by an efferent channel, 
 ax', to a discharging body, 
 Ef, in this case muscle. 
 
 The figure is also employed 
 to illustrate conduction 
 and transmission of im- 
 pulses along a nerve arc. 
 The dotted line encircles 
 a nerve centre (in this case 
 the spinal cord) ; dd then 
 become dendrites on then- 
 way to the perikaryon (see 
 note, p. 466), pk (in this 
 case the ganglion on the 
 dorsal root of the spinal 
 nerve), or if the peri- 
 karyon be short-circuited, 
 the impulse passes direct 
 to the axon, ax ; this con- 
 stitutes the neuronic path, 
 sy, sy are synapses, by 
 which the impulse is con- 
 ducted to the next neu- 
 rone, through the medium 
 of dendrites, dd', dd', and 
 thence topk', the cell-body 
 or perikaryon ; this consti- 
 tutes the synaptic path. 
 
 sidered in detail, owing to their deep physiological importance. 
 The higher the animal scale is ascended, the less easy is it to 
 obtain evidence of free spinal reflexes— viz., reflexes which take 
 place without any guidance from the brain. This may perhaps 
 be due to a more constant influence exercised over them by the 
 brain. Nevertheless, animals as highly developed as the horse 
 and ox give most undoubted evidence of reflex spinal acts. 
 If the spinal cord of either be severed behind the medulla, a cut 
 
 Fig. 138. — Diagram of a Reflex Arc 
 (Sherrington). 
 
476 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 on the skin of the abdomen will evoke a kick. The excitability 
 of the cord lasts for a few minutes after death ; in the ox it lasts 
 longer, but no comparative observations have as yet been made. 
 
 Locomotion is often essentially a reflex act. The exact group- 
 ing of muscles, and the regulation of the degree and rapidity of 
 their contraction, would appear at first sight to need the super- 
 vision of the highest centres in the brain; but this is not the 
 case. A pigeon will fly after decapitation; a brainless dog can 
 walk, and a headless cat, under appropriate stimulation, flexes 
 and extends its legs alternately. If a horse thought of every 
 step he had to take, he would soon be worn out and blunder. 
 That the higher centres also come into play is shown by the 
 judgment which the animal exercises when jumping — viz., he 
 estimates the proper distance at which to take off, the amount of 
 muscular contraction required to lift the body, and the height to 
 which it should be raised. Locomotion, however, is not purely 
 a reflex act, as it is carried out with the knowledge and consent 
 of the animal, but it functions as such. A true reflex act is 
 involuntary, and carried out without the knowledge of the 
 animal. 
 
 By a Co-ordinate Movement is meant one in which the con- 
 traction of various related groups of muscles is so adjusted that 
 the degree of their contraction and everything necessary for a 
 perfect movement is present and faithfully carried out. Co- 
 ordination of movement may occur without the assistance of 
 the brain. In the spinal cord, therefore, not only reflex but 
 co-ordinate movements are generated. The crossed or diagonal 
 movements of locomotion in quadrupeds are of this nature, and 
 are carried out by the spinal cord. Movements which are 
 irregular and purposeless, or in any way fail to co-ordinate, are 
 termed inco-ordinate. We shall see later that the co-ordination 
 of muscles is a complex reflex mechanism. All reflex actions are 
 co-ordinate. 
 
 The Reflex Frog. — It is usual to illustrate reflex action by 
 reference to the decerebrated frog. In this animal the spinal 
 cord is capable of carrying out the most complex reflex acts, far 
 higher in character than are exhibited by animals with a greater 
 nervous development. If a decerebrated frog be placed in water, 
 it swims; if it be stroked, it croaks; if stimulated, it springs; if 
 placed on its back, it recovers its normal position; if acid be 
 applied to the right thigh, the left foot will be employed to wipe 
 it off, or if this be held, the right leg is flexed. Still more re- 
 markable is the fact that if a brainless frog be placed on a 
 board which is gradually brought from the horizontal to the 
 vertical position, the animal gradually crawls up and, when the 
 board is vertical, sits at the top. If the board be lowered to the 
 
THE NERVOUS SYSTEM 477 
 
 horizontal, the creature descends. It is beyond the belief of 
 a layman that such acts are not of a purposive character. 
 As a matter of fact, they fall within the definition of a true 
 reflex act. 
 
 If a headless frog be lightly stimulated on one hind-leg, a 
 reflex contraction of the limb occurs ; if the stimulus is made 
 stronger, both limbs react. By increasing the strength of the 
 stimulus the fore-limbs may be involved, so that there is a pro- 
 portion between the magnitude of the reflex acts produced and 
 the strength of the stimulus. There is a latent period in a reflex 
 act (see p. 493), and with each successive increase in the strength 
 of the stimulus the latent period becomes longer in consequence 
 of a greater length of cord being involved. If the frog be injected 
 with strychnine and stimulated, convulsive movements occur, 
 and sensitiveness to touch is greatly increased, so that even a 
 current of air will cause muscular contraction. In this case it 
 is supposed that there is an overflow, as it were, into all motor 
 paths, the strychnine breaking down the barriers which usually 
 direct the impulse. The strychnine does not open new paths, but 
 it converts inhibitory effects into excitatory, so that contractions 
 appear in muscles at the moment they should under normal 
 conditions relax. Incidentally, the sensitiveness shown by the 
 strychnine preparation demonstrates that there are sensory paths 
 leading to all the motor neurones of the body. 
 
 The inhibition of reflexes is an extremely important question, 
 which will receive separate consideration. 
 
 The reflex acts exhibited by the higher animals are more 
 complex than those of the lower. The normal actions of the 
 latter are almost entirely reflex, whereas those of the former 
 are normally reflexes controlled, modified, or set free, as the case 
 may be, by cerebral centres. If in the dog the cord be divided in 
 the anterior thoracic region, the animal becomes paralysed, 
 there is loss of sensation as well as of motion; yet in process 
 of time recovery occurs, and the isolated portion of cord is 
 capable of carrying on the reflex acts of micturition, defaecation, 
 impregnation, parturition, scratching, and stepping, without the 
 knowledge of the animal. Such a dog may even be able in 
 course of time to walk or run. In the first instance it learns to 
 stand, then takes a few steps before subsiding, and gradually the 
 reflex paths are educated to the new condition which has ariserb 
 Locomotion, we have stated, is not normally a true reflex act, 
 yet under purely reflex conditions it may be carried out for hours. 
 The horse has not to think of the order in which he uses his four 
 legs, and how each step is to be taken; the fact is that this is 
 carried out by the spinal cord without the knowledge of the 
 animal; in other words, the work is done for him. It is only 
 
47» A MANUAL OF VETERINARY PHYSIOLOGY 
 
 when judgment must be exercised or instructions issued that the 
 higher centres take any share in locomotion. 
 
 There are movements which may be excited in the limbs after 
 death in both horse and ox, but particularly in the latter, which 
 remind one very strongly of the reflex frog. If immediately 
 after a horse is destroyed an attempt be made to open the 
 abdomen the animal kicks. Some minutes must be allowed to 
 elapse before the irritability of the cord disappears. In the ox 
 the period is longer, and even after decapitation the apparently 
 purposeful movements are very remarkable. For these reasons 
 we believe the cord plays in these animals a more independent 
 part in locomotion than is generally considered. How largely 
 locomotion is reflex may be indicated by the walking of the chick 
 out of the egg. A volitional act requires some experience and 
 training; a reflex act is innate, and may be complete at birth. 
 
 Impulse Paths. — When an impulse enters the cord — and it can 
 gain entrance only by the dorsal spinal roots — it may be dealt 
 with locally by a single spinal segment; it may be distributed 
 by several local segments ; or it may pass the entire length of the 
 cord and be dealt with by the cerebellum or cerebrum (Fig. 145, 
 A and B). The strength of the entering stimulus may deter- 
 mine whether one or more segments of the cord are involved, 
 as we have seen in' the strychnine experiments on the reflex 
 frog. Such a spreading of impulses is termed irradiation, and 
 at one time it was believed that this could occur only in a 
 forward direction. Sherrington showed that it could also 
 occur down the cord, though it remains limited to certain 
 lines. We may here trace the path of an impulse, selecting 
 any spinal reflex act. What is true for this is true for all im- 
 pulses passing to the cord by afferent nerves, and the principle 
 is equally true for those, like sight, hearing, etc., which do not 
 communicate with the cord, but pass direct to the brain. We 
 have seen that a reflex arc (Fig. 138) consists of a chain of 
 nerve-cells, each complete link being called a ' neurone,' the 
 neurones following each other end to end like the links of a chain;' 
 further, that a complete neurone is made up of a nerve-cell body 
 (perikaryon*) with its processes, some of these, the dendrites, 
 being the receiving and another, the axon, being the transmitting 
 process. We also know that the axon terminates by arborising 
 around the dendrites of the next link. The passage of an impulse 
 in a nerve-arc must lie during part of its course within the neurone 
 (in all cases travelling from dendrite to axon, though not neces- 
 sarily traversing the perikaryon, which may be short-circuited). 
 Having arrived at the end of one neurone, the impulse has then 
 to cross the space existing between it and its neighbouring 
 
 * See footnote, p. 466. 
 
THE NERVOUS SYSTEM 479 
 
 neurone; the region in which this occurs is the synapase. Con- 
 duction is therefore in part within the neurone or intraneuronic, 
 shortly neuronic, and in part between the neurones, interneuronic 
 or preferably synaptic, as it occurs in the area of a synapase 
 (Fig. 138). The whole of the conduction and transmission of 
 impulses in the nervous system then become describable as 
 (1) neuronic and (2) synaptic, and this distinction is physio- 
 logically fundamental, because the nature of the conduction 
 cannot be the same in the two cases. In all nerve-centres synaptic 
 conduction is added to neuronic conduction ; synaptic conduction 
 is irreversible in direction, neuronic conduction is reversible. 
 
 The Receptor System has been revealed by the work of Sher- 
 rington; it is a system engaged in the transmission of impulses 
 from the periphery to the centre. It forms the basis of the 
 classification of afferent nerves on p. 454. The impulses are 
 received in what he has termed ' fields of distribution,' and of 
 these there are two main divisions — surface and deep. A surface 
 field may be external, or exteroceptive — for example, the skin — or 
 internal {interoceptive), such as the mucous membrane of the 
 nostrils. The deep field, or proprioceptive system, lies in the 
 muscles, joints, tendons, viscera, etc. Whereas the surface field 
 is brought into operation by its surroundings, such as touch, 
 pressure, heat, cold, sight, hearing, smell, the deep field is activated 
 by something derived from itself — for example, mass, weight, 
 pressure or alteration of pressure, such as occurs in a contracting 
 or relaxing muscle. The first step, then, in the conveyance of an 
 impulse — say, from the skin to the cord — is that the nerve-path 
 shall originate in a receptive field. An area of skin consists of 
 points forming a receptive surface, from which the nerve-path 
 starts. The receptive neurone extends from the receptive surface 
 to the central nervous organ, and it is the sole avenue which 
 impulses generated at its receptive point may use, no matter 
 whither they may be proceeding or how distant their destination. 
 
 A single receptive point may play reflexly upon a number of 
 different effector organs — i.e., organs connected with the efferent 
 system, muscles, glands, and suchlike — yet all its reflex arcs 
 spring from the one single shank — viz., from one afferent neurone, 
 which conducts from the receptive point at the periphery into 
 the central nervous organ. This neurone dips at its deep end 
 into the spinal cord or brain, and in this network forms manifold 
 connections. So numerous are its potential connections that, as 
 shown by the general convulsions induced under strychnine- 
 poisoning, its impulses can discharge every muscle and effector 
 organ in the body. Yet under normal circumstances the im- 
 pulses conducted to the central network do not irradiate in all 
 directions. Their spread, as judged by the effects produced, 
 
480 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 increases with increase of stimulation of the radiant path, but 
 the irradiation remains limited to certain lines. Under weak 
 stimulation these lines are few. The conducting network offers, 
 therefore, to any given path entering it, some communications 
 that are easier than others. This is sometimes expressed by 
 saying, borrowing from electrical terminology, that the conductive 
 network from any given point offers less resistance along certain 
 circuits than along others. This recognises the fact that the 
 conducting paths in the great central organ are arranged in a 
 particular pattern. This pattern reveals something of the inte- 
 grative function of the nervous system. It shows us what organs 
 work together in true relationship. The impulses are led to this 
 and that effector organ, gland or muscle, in accordance with 
 the pattern. 
 
 The receptive neurone iorms, as we have seen, the sole avenue 
 by which impulses generated at its receptive point can be con- 
 veyed to their destination. It is a path exclusive to the impulses 
 generated at its own receptive points, and receptive points other 
 than its own cannot use it. The receptive neurone forms a 
 private path exclusively devoted to impulses from a single recep- 
 tive point. Our study of the skin has shown that its entire 
 surface is a collection of receptive points. On reaching the cord 
 the impulses pass along certain association tracts or internuncial 
 paths — i.e., paths which connect the various segments of the 
 cord. These are paths common to groups of private paths, and 
 at their terminations the impulses pass from the synapses across 
 1 the space which separates these from the first link or neurone 
 in the chain of the efferent path. The efferent path passes out, 
 in the case of the cord, by the ventral spinal nerves to the gland 
 or muscle concerned, where it terminates in a final neurone. 
 The motor or efferent path differs in one important respect from 
 the sensory or afferent, inasmuch as it is not exclusively devoted 
 to the transmission of impulses generated at a single receptive 
 source, but receives impulses from many receptive sources situ- 
 ated in various regions of the body. It is, however, the sole path 
 by which all impulses, no matter whence they come, must travel 
 if they would reach the muscle, gland, etc., concerned. It is a 
 public path common to impulses of all kinds, such as tissues are 
 constantly receiving. Reflex arcs arising, therefore, in different 
 sense-organs can pour their influences into one and the same 
 muscle. A limb-muscle is the terminus ad quem of nervous arcs 
 arising not only in the right eye, but in the left ; not only in the 
 eyes, but in the organs of smell and hearing ; not only in these, 
 but in the otic labyrinth, in the skin, and in the muscles and joints 
 of the limb itself, and of all the other limbs as well. Its motor 
 path is a nerve common to all these arcs. It is the final common 
 
THE NERVOUS SYSTEM 4 i5i 
 
 path, and a motor nerve to a muscle is a collection of such final 
 common paths. 
 
 The afferent neurones comprising the private paths are several 
 times more numerous than the common paths; in other words, 
 the outlet is much smaller than the inlet, and in consequence 
 there is competition for the right of way. In this»matter there 
 is no compromise in a conflict, say, between two opposite 
 reflexes. One or the other must pass, both cannot. We shall 
 see this well illustrated in the ' scratch ' and ' foot reflex ' of 
 the dog. The victory in this struggle lies, as usual, on the side 
 of big battalions. The stronger the stimulus, the more likely is 
 it to occupy the road, especially if it be of a painful nature. The 
 stronger inhibits the weaker. The weaker is not necessarily 
 destroyed; it may simply be held back, and permitted to follow 
 when the stronger rival has left the road open. Inhibition does 
 not take place in the motor nerve itself. The field of competi- 
 tion between the rival arcs seems to lie in the grey matter, 
 where the converging neurones come together at the commence- 
 ment of the common path, and here it is that some arcs force 
 the final path into one kind of action; others force it into a 
 different kind of action; and others, again, preclude it from 
 being employed by the rest . In studying the reflex act of stepping, 
 we will see this feature well illustrated in the reciprocal innerva- 
 tion of antagonistic muscles. We have previously learnt (p. 436) 
 that while the flexor muscles of a limb are contracting the tone 
 of the extensor muscles is inhibited ; this inhibition arises ;n the 
 nerve-centre, the stream of motor impulses along the motor 
 neurone being for the time cut off in the struggle for the common 
 path. This fact is of the utmost importance to a clear under- 
 standing of the question of muscular co-ordination, and further 
 consideration of the question will be given later. 
 
 On the receptive surface the various impressions, such as light, 
 touch, heat, cold, taste, smell, and so forth, are selected by the 
 special afferent nerves devoted to their transmission. The spinal 
 connections of different nerve-endings in the same area of skin 
 are assumed to be different, since stimuli suitable for one set of 
 movements are unsuitable for another. The scratch reflex needs 
 fielding or stimulation of a hair for its production. The reflex 
 of the ' extensor thrust,' which has yet to be spoken of, can only 
 be excited by pressing between the plantar cushion and the toe- 
 pads of the dog, and no other form of stimulation can invoke 
 it. A consideration of the special spinal reflexes now to be 
 described will prove object-lessons in the principles of the common 
 path, and the remarkable mechanisms it is capable of effecting. 
 
 The Stepping Reflex. — In the dog, when the spinal cord has 
 been severed in the hinder part of the cervical region, and the 
 
 31 
 
482 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 ' shock ' from the transection has passed off, reflex walking is 
 observable. The walking movement includes alternate flexing 
 and straightening of the limb. The forward movement of the 
 hind-leg in taking a step is produced by flexion at the hip, and 
 to prevent the foot brushing against the ground as the leg swings 
 forward flexion occurs at the stifle and hock, so as somewhat to 
 raise the foot. The limb is then straightened again, so that the 
 foot may reach the ground and bear the weight of the body. 
 In order to prevent the limb doubling up under this burden, the 
 extensor muscles which support the patella joint and hock from 
 bending have to contract with sufficient power. Stiffened by 
 the contraction of these muscles, the limb serves as a prop to 
 support the body. While the foot rests on the ground the body 
 moves forward, so that in due course the hip advances in front 
 of a vertical drawn upward from the foot. The extended hind- 
 limb at this time is sloped somewhat backward as well as down- 
 ward. When this posture is reached, the extensor muscles are 
 thrown into further action, and give the limb a push off from 
 the ground, propelling the body forward. The hind-limb thus 
 makes its contribution to the forward progression of the body; 
 in galloping in the normal dog this extensor thrust is very 
 marked. 
 
 In this reflex spinal stepping we may study first the flexion 
 of the limb which occurs in the forward movement of the step. 
 Flexion similar but more pronounced can be easily excited in 
 the spinal dog* by exciting the skin of the foot electrically. 
 Though the flexion occurs at hip, patella joint, and hock together, 
 it will be simpler to confine our examination to the flexion at 
 one of these joints only, for what occurs in the muscles of each 
 of the three joints is, so far as concerns us now, the same. The 
 chief muscles which flex the stifle are the semitendinosus and 
 biceps at the back of the thigh. The electric stimulation of the 
 skin of the foot is found to throw these muscles into contraction, 
 and, with them, also the psoas muscles (flexors of the hip) and 
 the tibialis anticus, etc. (flexors of the hock). But this is only 
 part of what happens. At the same time as the flexors of the 
 stifle contract, the extensoi muscles of the stifle, the vasti and 
 crureus, are relaxed. The same stimulus which excites the motor 
 neurones of the flexors to discharge motor impulses into those 
 muscles causes the motor neurones of the antagonistic muscles, 
 the extensors of the stifle, to cease discharging impulses, and 
 keeps them prevented from discharging impulses. The stimulus 
 sets up an intraspinal excitation of the motor neurones of the 
 
 * ' Spinal dog ' is the term used to indicate that the reflexes are entirely 
 spinal, owing to the brain having been destroyed, or the cord having been 
 cut off from the brain. 
 
THE NERVOUS SYSTEM 
 
 483 
 
 flexor muscles and an intraspinal inhibition of the motor neurones 
 innervating the extensor muscles. 
 
 When the flexion phase of the act of stepping has been passed 
 through, the leg extends again by return of the activity which 
 had been inhibited in the motor neurones of the extensor muscles. 
 In due course the foot reaches the ground. When it does so 
 the weight of the body gradually comes upon it, and presses 
 the sole of the foot hard against the ground. A stimulus is thus 
 given to nerve-endings in the sole. This stimulus can be 
 imitated— for instance, by pressing against the toe-pads with a 
 finger. This, in the spinal dog, even when the animal is lying 
 on its side, excites a strong reflex extension of the limb, the 
 ' extensor thrust.' Just such an extension occurs when the 
 
 Fig. 139. — The Scratch Reflex (Sherrington). 
 
 The ' receptive field,' as revealed after low cervical transection, a saddle-shaped 
 area of dorsal skin, where the scratch reflex of the left hind-limb can be 
 evoked. Ir marks the position of the last rib. 
 
 foot is pressed against the ground by the weight of the body 
 in the act of stepping. This extensor thrust gives the propulsive 
 forward movement of the body, which is the contribution made 
 by the limb in its reflex step toward the progression of the animal. 
 The extensor thrust is particularly marked in the gallop. 
 
 The Scratch Reflex. — Good opportunity for study of this cor- 
 relation between reflexes is given in the ' scratch reflex.' This 
 reflex can be easily elicited in many normal dogs; when the 
 spinal cord has been transected in the neck, it becomes ab- 
 normally responsive. Stimuli applied within a large saddle- 
 shaped field of skin (Fig. 139) excite a scratching movement 
 of the leg. The movement is rhythmic flexion at hip, stifle, and 
 hock. It has a frequency of about four per second. The stimuli 
 
484 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 provocative of it are mechanical, such as rubbing the skin or 
 pulling lightly on a hair. The nerve-endings which generate the 
 reflex lie in the surface layer of the skin, about the roots of the 
 hairs. A convenient way of exciting these is by feeble faradisa- 
 tion. 
 
 Prominent among the muscles active in this reflex are the 
 flexors of the hip. If we record their rhythmic contraction we 
 obtain tracings, as in Fig. 141. A series of brief contractions 
 succeed one another at a certain rate, whose frequency is inde- 
 pendent of that of the stimulation. The contractions are pre- 
 sumably brief tetani. The stimulus to the hair-bulbs of the 
 shoulder throws into action a lumbar spinal centre, innervating 
 the hip-flexor, in much the same manner as the bulbar respiratory 
 
 Hli 
 
 Fig. 140. — Spinal Arcs involved in Scratch and Foot Reflexes 
 (Sherrington). 
 
 Diagram of the spinal arcs involved in Fig. 139. L, Receptive or afferent nerve 
 path from the left foot; R, receptive nerve-path from the opposite foot; 
 Sa, S/3, receptive nerve-paths from hairs in the dorsal skin of the left side; 
 Pa, P/3, neurones originating within the cord ; FC, the final common path, in 
 this case the motor neurone (nerve-path) to a flexor muscle of the hip. 
 
 centre drives the spinal phrenicus centre. In the case of the 
 respiratory muscle the frequency of the rhythm is, however, 
 much less. 
 
 The reflex is unilateral : stimulation of the left side of the back 
 evokes scratching by the left leg, not by the right. In the 
 lateral column of the spinal cord fibres exist directly con- 
 necting the spinal segments of the shoulder with the spinal 
 segments containing the motor neurones for the flexor muscles 
 of the hip, and stifle, and hock. We thus arrive at the follow- 
 ing reflex chain for the scratch reflex: (1) The receptive neurone 
 (Fig. 140, Sa), from the skin to the spinal grey matter of the 
 corresponding spinal segment in the shoulder. This is the 
 exclusive or private path of the arc. (2) The long descending 
 neurone within the cord (Fig. 140, Pa), from the shoulder segment 
 
THE NERVOUS SYSTEM 485 
 
 to the grey matter of leg segments. (3) The motor neurone 
 (Fig. 140, FC), from the spinal segment of the leg to the flexor 
 muscles. This last is the final common path. The chain thus 
 consists of three neurones. It enters the grey matter twice — 
 that is, it has two neuronic junctions, two synapses. It is a 
 di synaptic arc. 
 
 Now if, while stimulation of the skin of the shoulder is 
 evoking the scratch reflex, the skin of the hind-foot is stimulated, 
 the scratching is arrested. Stimulation of the skin of the hind- 
 foot causes the leg to be flexed, drawing the foot up. This is the 
 foot reflex. The drawing up of the foot is effected by strong 
 tonic contraction of the flexors of hock, stifle, and hip. In this 
 reaction the reflex arc is : (1) The receptive neurone or nerve-path 
 (Fig. 140, L), from the foot to the spinal segment; (2) perhaps a 
 short intraspinal neurone; and (3) the motor neurone or nerve- 
 path (Fig. 140, FC) to the flexor muscle — e.g., of hip. Here, 
 therefore, we have an arc which embouches into the same final 
 common path as So,.. The motor neurone FC is a nerve-path 
 common to it and to the scratch reflex arcs; both arcs employ 
 the same effector organ, a hip-flexor. 
 
 The channels for both reflexes finally embouch upon the same 
 common path. The flexor effects specific to each differ strikingly 
 in these two cases. In the scratch reflex the flexor effect is an 
 intermittent contraction of the muscle; in the foot reflex it is 
 steady and maintained. The accompanying tracing (Fig. 141) 
 shows the result of conflict between the two reflexes. One 
 reflex displaces the other from the common path. There is no 
 compromise. The scratch reflex is pushed aside by the 
 reflex arc provoked from the foot. The stimulation which pre- 
 viously sufficed to evoke the scratch reflex is no longer effective, 
 though it is continued all the time. But when the stimulation 
 of the foot is discontinued the scratch reflex returns. 
 
 This remarkable result indicates that there is interference 
 between the two reflexes, and that one has inhibited the other. 
 Though there is no cessation -of activity in the motor neurone, 
 one form of activity that was being impressed upon it is cut out 
 and another has taken its place. 
 
 Suppose, again, during the scratch reflex, stimuli are applied 
 to the foot, not of the scratching, but of the opposite side 
 (Fig. 140, R). This stimulation of the foot causes flexion of its 
 own leg and extension of the opposite. If, therefore, when the left 
 leg is executing the scratch reflex, the right foot is stimulated, 
 the scratching is cut short, though it is the left leg's flexors which 
 are involved. This inhibition of the flexor scratching movement 
 occurs sometimes when the contraction of the extensors is, 
 minimal or hardly perceptible. 
 
486 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 o SfS-5 • 
 
 ■2 3 5 ti 5 j5 
 -2 o d <o -*■ 5 
 <a u » ■>,*" 
 
 aS.S.Sflo 
 
THE NERVOUS SYSTEM 487 
 
 It is obvious from this that the final common path, FC, to 
 the flexor muscle can be controlled by,, in addition to the before- 
 mentioned arcs, others that actuate the extensor muscles, for 
 it can be thrown out of action by them. The final path, FC, is 
 therefore common to the reflex arcs, not only from the same 
 side foot (Fig. 140, L) and shoulder skin (Sa, S/3), but also 
 to arcs from the opposite foot (R),.in the sense that it is in the 
 grasp of all of them. In this last case we have a conflict for 
 the mastery of a common path, not, as in the previous instance, 
 between two arcs, both of which use the path in a pressor manner, 
 although differently, but between two arcs that, though both of 
 them control the path, control it differently* one in a pressor 
 manner heightening its activity, the other in a depressor manner 
 lowering or suppressing its activity. 
 
 We said that the scratch reflex is unilateral. If the right 
 shoulder be stimulated, the right hind-leg scratches; if the left 
 shoulder be stimulated, the left hind-leg scratches. If both 
 shoulders be stimulated at the same time, one or the other 
 leg scratches, but not the two together. The one reflex that 
 takes place prevents the occurrence of the other. The reason is 
 that, although the scratch reflex appears unilateral, it is not 
 strictly so. Suppose the left shoulder stimulated. The left leg 
 then scratches. If the right leg is then examined, it is found 
 to present steady extension, with some abduction. This exten- 
 sion of the leg which accompanies the scratching movement 
 of the opposite leg contributes to support the animal on three 
 legs, while it scratches with the fourth. 
 
 Suppose now we stimulate the left shoulder, evoking the 
 scratching movement of the left leg, and that the right shoulder 
 is then appropriately and strongly stimulated. This latter 
 stimulus often inhibits the scratching movement in the opposite 
 leg, and starts it in its own. In other words, the stimulus at the 
 right shoulder not only sets the flexor muscles of the leg of its own 
 side into scratching action, but it inhibits the flexor muscles of the 
 opposite leg. It throws into contraction the extensor muscles 
 of that leg. In the previous example there was a similar co- 
 ordination. The motor nerve to the flexor muscle is therefore 
 under the control not only of the arcs of the scratch reflex from 
 the shoulder on the same side, but of those from the opposite 
 shoulder as well. But in regard to their influence upon this 
 final common path, the arcs from the homonymous shoulder 
 and the opposite shoulder are opposed. 
 
 The scratch reflex occurs in many animals besides the dog — 
 e.g., the cat, guinea-pig, sheep, rabbit, rat, parrot. In small 
 animals its rate of rhythm is greater than in large; the scratch 
 is quicker in small dogs than in large. 
 
488 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The Falling Reflex of cats is a well-known reaction. A fully 
 inverted cat, if allowed to fall, alights squarely on all four feet. 
 The reflex is not abolished by the destruction of the motor 
 areas in both hemispheres of the cerebrum, but ablation of 
 both hemispheres prevents it from occurring. The phenomenon 
 is dependent on influences from the semicircular canals of the 
 internal ear and eyes, controlled by the cerebral cortex, for 
 though destruction of the labyrinths does not prevent the reflex, 
 this follows when the eyes are occluded. 
 
 The Pinna Reflex. — By gently twisting the tip of the pinna 
 of the ear of the cat a reflex is elicited consisting of a quick 
 retraction and folding back of the pinna. The motor nerve 
 is the seventh cranial, while the second cervical nerve con- 
 tains the afferent fibres. It is also observed in the decerebrate 
 preparation. 
 
 The Head-Shake Reflex. — By blowing into the ear of the cat 
 a rotatory shake of the head is caused; it can also be observed 
 in the decerebrate preparation, the occipital stump rotating. 
 A few drops of water introduced into the concha produces the 
 same effect. The reflex is protective to the auditory meatus, 
 the left ear producing a shake in the direction of the hands of 
 the clock, the right ear reflex being anti-clockwise. 
 
 Tendon Reflexes. — One of the best known of the tendon 
 reflexes in man is the knee-jerk, a jerking forward of the leg 
 when the straight ligament of the patella is struck. This is 
 caused by a momentary single spasm of the extensor muscles 
 of the knee, due to a sudden pull on the muscle when the tendon 
 is tapped; this excites the afferent nerves within the muscle, 
 and a reflex response through the motor nerves follows. It is 
 well seen in the dog, cat, rabbit, etc. It is dependent on the 
 reflex tonus that is maintained in the muscles by the spinal arcs 
 connected with them; if that tonus be much lowered, as by 
 severance of the nervous reflex arc, the jerk can no longer be 
 elicited. Sherrington describes the knee-jerk as the prototype 
 of the proprioceptive class of reflexes. Another brisk ' jerk ' 
 in the dog is the ischial, obtained from the hamstring muscles 
 by tapping the tuberosity of the ischium. 
 
 Tendon reflexes have not, so far as we are aware, been studied 
 in the horse ; nor is it known whether the existence of any limb 
 reflexes has been demonstrated, if, perhaps, we except the im- 
 mediate lifting up of the foot, which generally follows pinching 
 the so-called ' chestnut ' found on the inside of the fore-arm of 
 the horse. 
 
 Reflex Inhibition. — In the reflex movements of ' stepping ' and 
 " scratching ' in the spinal dog, attention has been drawn to 
 acts of inhibition; for instance, it was shown that the stimulus 
 
THE NERVOUS SYSTEM 489 
 
 which excites the flexors to contract causes the extensors to 
 relax. The stimulus to produce contraction in the extensors is 
 still present, but it is' inoperative, owing to inhibition. It was 
 also shown that the scratch reflex, having been started on one 
 side of the body, could be inhibited by starting it on the opposite 
 side. In this case the stimulus starts one set of flexor muscles 
 going, and inhibits the corresponding set of flexors in the opposite 
 limb. Similarly, in the conflict bet-ween the two reflexes of 
 scratching and stepping, all the conditions needful to maintain 
 the scratch reflex in operation are present, but they are inhibited 
 so long as the pad of the foot is pressed upon; when that 
 stimulation is withdrawn, the scratch reflex returns. We have 
 therefore to consider the question of reflex inhibition. 
 
 The visceral muscles receive, as we have already seen, a double 
 efferent nerve-supply. The heart muscle and that of the in- 
 testines and bloodvessels can be stimulated or inhibited; this is 
 effected by one efferent conveying impulses of an excitatory 
 character, while the other efferent conveys inhibitory impulses. 
 These work reflexly under the control of an afferent service, 
 and the nature of the impulses in the afferent determines 
 which of the efferent impulses are to be placed in operation. 
 Muscles, as we have seen, are provided with an effective 
 afferent system of nerves, more than half the fibres belonging to a 
 muscle being of this nature. Sherrington has shown that where 
 the muscle passes into the tendon, a ' nest of afferents ' exists, 
 and he has also proved that the Golgi organ found in tendon is 
 afferent in nature. In spite of this liberal afferent system, there 
 is only a limited efferent outflow; muscles are well supplied with 
 nerves conveying excitatory (motor) impulses, but there are no 
 nerves conveying inhibitory impulses. Notwithstanding, in- 
 hibitory functions in skeletal muscle are of a most important 
 and widespread character, and they are effected in a purely 
 reflex manner by inhibiting the neurone which conveys the 
 excitatory impulses. 
 
 Under the title of reciprocal innervation Sherrington de- 
 scribes what occurs in all normal movement — i.e., a relaxation 
 of antagonistic muscles. For instance, while the flexors of a 
 limb are contracting, their antagonists, the extensors, are re- 
 laxing. This effect is brought about by the tonus (p. 436) of 
 the antagonistic muscles being reduced, owing to an inhibition 
 of those motor cells in the cord which supply the extensor muscles. 
 Reciprocal innervation thus saves a waste of nervous energy in 
 overcoming the contraction of antagonists, and it insures 
 muscular co-ordination. In certain muscular movements antago- • 
 nistic muscles may contract concurrently. This is evident in 
 those cases where a muscle is connected with two or more joints, 
 
490 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 being a flexor of one and an extensor of another. In this case 
 antagonistic muscles are capable by their contraction of inhibiting 
 the extensor movement of one joint in order to permit the muscle 
 to act as flexor of another. The innervation of the antagonistic 
 muscles so employed is no longer reciprocal, but identical, as they 
 must contract and relax at the same time, and not alternately. 
 
 The purposes served by reflex inhibition in skeletal muscular 
 actions are classified by Sherrington as follows :* 
 
 i. Reflex inhibition cuts short the contraction of one set of 
 muscles when another set is called into play; it also guards and 
 maintains a reflex already in operation by preventing its being 
 interrupted by other stimuli 
 
 2. It grades the degree of intensity of the discharge from nerve 
 centres by diminishing it to any required extent, so that the needful 
 discharge from a motor centre can be adjusted with the greatest 
 precision. 
 
 3. Reflex inhibition exhibits itself in respiration, stepping, eyeball 
 movements, and the reciprocal innervation of antagonistic muscles, 
 the underlying principle being that by means of reflex inhibition 
 the contraction of a muscle causes relaxation of its antagonist. 
 
 4. Inhibition is the main means by which rhythmic reflexes are 
 produced. For example, the rhythmic reflex in stepping being 
 excited, its execution produces a stimulus which brings into activity 
 a reflex inhibition; this cuts the rhythmic reflex short. The muscle 
 having ceased to contract, the inhibition is removed, and the original 
 stimulus is once more in undisputed possession of the field, and so 
 rhythmicity is maintained. 
 
 5. On the withdrawal of inhibition, there is a great augmentation 
 of activity in the inhibited centre. This Sherrington describes as 
 post-inhibitory rebound ; this rebound favours the change from 
 flexion to extension, and vice versa. He regards it as responsible for 
 one of the two phases of such diphasic muscular reflexes as stepping, 
 mastication, and respiration. It is evident that when one or more 
 pairs of legs are concerned in locomotion, some of the limbs are in a 
 condition of flexion, the others are in extension; and the change from 
 one phase to the other is brought about by post-inhibitory rebound. 
 
 6. Strychnine converts reflex inhibition into reflex excitation, and 
 the toxins of tetanus and rabies have the same effect. These agents, 
 as pointed out by Sherrington, ' work havoc ' with the elemental 
 co-ordination of the skeletal muscles, changing reciprocal innervation 
 into identical innervation. Hence, in tetanus patients, the greater 
 the effort to open the jaw, the more tightly is it closed; and in the 
 hydrophobia patient, the greater the attempt made by the sufferer 
 to inhibit his inspiration in order to swallow, the deeper and more 
 prolonged the inspiration which ensues, inhibition ' being converted 
 by the virus into excitation.' 
 
 7. Chloroform and ' fatigue ' strengthen inhibition. An inhibitory 
 reflex stimulus, which is ineffective on a normal centre when that 
 centre is fresh, becomes effective when the centre is under chloroform, 
 or when the active driving of the centre has been pushed for a 
 little time. 
 
 * 'The Role of Reflex Inhibition,' British Medical Journal, March 25, 
 1911, and Science Progress, No. 20, April, 1911. 
 
THE NERVOUS SYSTEM 4 or 
 
 In complex reflexes many stimuli are at work together, and 
 co-operate harmoniously for a co-ordinate result. In standing, 
 walking, running, etc., very important sources of the reflex lie 
 in the muscles and joints of proximal parts of the limb — namely, 
 in the joints of the hip and stifle and the great muscles acting 
 on them. These joints and muscles are liberally supplied with 
 afferent nerves. The importance of these as sources of the 
 reflex of stepping is indicated by several facts. In the first 
 place, a dog or cat is found still to walk well when the foot 
 reflex is cut off in all four feet by division of the nerves, both 
 superficial and deep. In the second place, when the spinal dog 
 is lifted so that its limbs do not touch any solid support what- 
 ever, reflex walking and galloping are performed, although the 
 limbs are stepping wholly in the air. But to excite this reflex of 
 walking in the air, it is necessary that the limbs hang down. 
 The reflex ceases if the dog be inverted, when gravity is no 
 longer acting on the joints and muscles as it does in the position 
 usually accompanying acts of stepping. It is evident that 
 impulses which stimulate contraction are passing from muscles 
 and joints to spinal centres. The spinal centres which execute 
 reflex walking, running, etc., receive much help and direction 
 from afferent arcs which arise in the labyrinth ot the ear. The 
 stimuli which are the source of reflex walking, etc., arise, there- 
 fore, in many receptive organs. 
 
 It is by means of the deep stimuli that the proprioceptors of 
 the limbs maintain the extensor muscles in a state of tonic 
 activity, and so enable the upright position of the body to be 
 maintained without effort and without the knowledge of the 
 individual. When, therefore, the spinal dog is held in the air 
 legs downwards, gravity acting on the joints and muscles — 
 namely, weight and pressure — is the cause of impulses passing 
 from the deep fields to the spinal cord, and so maintaining the act 
 of stepping. When the animal is inverted, the deep fields no 
 longer receive their natural stimulus, and the movement ceases. 
 We shall see shortly that the chief ganglion controlling the pro- 
 prioceptive system is the cerebellum. 
 
 The Standing Reflex. — When the brain of a dog or cat is re- 
 moved between the anterior and posterior colliculi * the extensor 
 muscles of the limbs, the extensor muscles of the neck, back, and 
 tail, and those which close the jaw, are in a condition of mild tonic 
 contraction. Their antagonists, the flexors, exhibit relaxation. 
 The phenomenon is known as Decerebrate Rigidity. The 
 rigidity of the extensor muscles is due to impulses passing out 
 
 * The term ' colliculi ' is now used to denote the corpora quadrigemina . 
 Its shortness is convenient, and it avoids confusion in the case of reptiles, 
 birds, fish, etc., where these structures are bigeminate, 
 
492 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 from the cord as a reflex effect, for if the afferent nerves of the 
 muscles be divided the rigidity ceases. The reflex effect originates 
 in the muscle, for if the nerves of the skin be divided the rigidity 
 continues. The decerebrate preparation, if. stood on its legs, 
 remains there, the rigidity of the extensor muscles of the above- 
 named parts sufficing to maintain the erect attitude, even for 
 days at a time. When, however, the brain stem is divided behind 
 the pons the preparation can no longer remain erect, nor can a 
 decapitated dog or cat stand. 
 
 An animal, when standing, is frequently altering its pose (see 
 P- 435); we have learned that this is effected by the lengthen- 
 ing and shortening contraction of the extensor muscles, by which 
 means the muscle, though counteracting the same load — namely, 
 the weight on the limb — can adjust itself at different lengths. 
 There are also modifications of posture effected through the 
 influence of the otic labyrinth and deep afferents of the neck. 
 If the head of the decerebrate preparation be flexed, the fore- 
 quarters sink owing to inhibition of the extensor muscles of the 
 fore-legs, while there is increased tension of the extensors of the 
 hind-quarters ; the posture assumed by the body resembles that 
 seen in the horse when ' stretching.' If the head be tilted 
 upwards and backwards, the fore-quarters are raised owing to 
 contraction of the extensor muscles, while the hind-quarters sink, 
 the final attitude resembling that assumed by a horse when in the 
 act of rising from the ground, or a cat looking up at an object. 
 When the head is turned to one side, as in the familiar posture 
 adopted by a horse when looking round in his stall, the extensor 
 muscles of the fore and hind limbs on the same side of the body 
 exhibit increased extensor action, while on the opposite side 
 of the body there is a decrease in the extensor effect. The in- 
 fluence of the labyrinth in relation to the position of the body 
 in space will be referred to again in the chapter on the senses. 
 
 Other Reflex Acts. — We have seen that reflex acts are not con- 
 fined to skeletal muscles; the act may be secretory or nutritive, 
 or one involving the contraction or relaxation of pale muscle; 
 for example, the contraction and dilatation of the bloodvessels 
 under the influence of the vasomotor, system, the peristaltic 
 movements of the intestines, the contractions of the bladder 
 and uterus, the movements of the pupils, and the secretions from 
 various glands, are all examples of reflex acts. 
 
 Reversal of Reflex Effect. — The influence of the nervous system in 
 regulating as a reflex effect the calibre of the bloodvessels has been 
 dealt with elsewhere (p. 86). It is not proposed to refer again to 
 this, excepting to illustrate a phenomenon known as reversal of 
 reflex effect. When an ordinary afferent nerve is stimulated in the 
 dog, the effect produced on the bloodvessels is that of constriction, 
 
THE NERVOUS SYSTEM 493 
 
 and the blood-pressure is reflexly raised. Bayliss has shown that a 
 rise in pressure can be converted into a fall by a sufficient dose of 
 chloroform. In observations by Sherrington and Sowton on 
 skeletal muscle, it was found that the excitatory reflexes brought 
 about by strychnine could be undone by chloroform or ether, so that 
 the reflex response could be changed back from contraction to 
 inhibitory relaxation, and on permitting the chloroform narcosis 
 partially to pass off, the strychnine influence reappears. In the de- 
 cerebrate preparation the above observers showed that reflex con- 
 traction of skeletal muscle or reflex inhibition could be produced at 
 will through the same afferent nerve by varying the strength and 
 form of the electrical stimulation, and in other ways. Instances 
 begin to be numerous in which the reflex obtainable from one and 
 the same afferent nerve may be diametrically reversed on alteration 
 of some one definite factor in the conditions of the reaction. 
 
 Peripheral Reflex Centres. — Centres for reflex action out- 
 side the brain and spinal cord have long been carefully 
 looked for, especially in connection with the sympathetic 
 system. So far as is known, there are no sympathetic acts purely 
 reflex; nor are all the necessary structures present in the 
 sympathetic system for the formation of a reflex arc. The 
 ganglia on the dorsal roots of the spinal nerves are similarly 
 structured, and they are not seats of reflex action. We have 
 seen that a reflex arc requires an afferent nerve, a centre, and 
 an efferent nerve. All three of these are not present in connection 
 with a sympathetic or spinal ganglion. 
 
 The Time occupied by a Reflex Act varies dependently upon 
 the strength of the stimulus and the nature of the reflex; the 
 sharper the stimulus, the more immediate the reflex; the more 
 active the centre, the more rapid the response. Impulses which 
 have to cross the cord take longer to produce their effects than 
 those which enter and return from the same side. It is mainly 
 during this appreciable delay, or latent period, as measured by 
 delicate apparatus, that the changes which lead to an efferent 
 response are occurring in the grey substance. In the dog the 
 time occupied by a reflex on the same side is estimated at from 
 0-022 up to 2-3 seconds, according to circumstances. 
 
Section 3. 
 
 Spinal Cord. 
 
 The spinal cord extends from the atlas to about the second,or 
 third_sacxaL_ vertebra, and is completely enclosed in a dense 
 membrane, the dura jnater. The canal in which it is lodged 
 is very much larger than the cord, es pecially at those parts 
 where the greatest ajnojint_ fi_f mccffiinexjJu3ccurs, as in the neck . 
 The cordTisngt the same, shape, nor the same size .throughout,.; 
 oval in the cervica l region, it becomes circular in the dorsal , and 
 again ovajjn the Jnrn har portion. It is largest w here any con- 
 siderabTeJjulk of nerves is .bfiing_given_oi£, and thus there is an 
 enlargement cofresponding to the fore, and another to th ejiind 
 limbs (Fig. 142). On e xposing the s pinal ca nal, a largejiurnber 
 of nerves are found to be gassing through the durajnater- either 
 
 A B C 
 
 Fig. 142. — Transverse Section of Spinal Cord of Horse. 
 
 A, At level of first dorsal vertebra; B, at level of first lumbar vertebra; 
 C, at level of first sacral vertebra. 
 
 outwards or inwards, and these gain an exit. from or entrance to 
 the spinal canal by means of the f oramen f orrnecL at the_j-unctk«i 
 of the vertebras. 
 
 Spinal Nerves. — These are gjyerj_pfT frnm or enter the _£0jd 
 from the fi rst bon e of the neck to the fourth or fifth- sacral 
 vertebra. A_bunch of nerves passes through every intervertebral 
 "foramen _on_eaeh— side, and these_ represent the spijaaL_nexyes. 
 When the dura mater of the cord is opened, it is observed that 
 the nerves within this cover are arranged in two groups: 
 one derived from the dorsal, the other from the ventral, aspect of 
 the cord. These are the superior and inferior roots of the 
 spinal nerves, or, to maintain the preferable nomenclature, the 
 dorsal and ventral roots. In the horse the dorsal and ventral 
 roots leave the cord, not as a single bundle, but as several. 
 On the dorsal root a nodule is found where the nerve passes 
 through the intervertebral foramen. This is the spinal ganglion, 
 and as the roots are in bundles, so there are bundles of ganglia 
 at each outflow between the vertebrae (see Fig. 145). There are 
 
 494 
 
THE NERVOUS SYSTEM 495 
 
 no ganglia on the ventral roots. Dorsal and ventral roots unite 
 outside the intervertebral foramen to form the ordinary mixed 
 spinal nerve. The functions of these two roots are quite opposite. 
 In the dorsal roots the impulses are passing from the periphery 
 to the centre ; 'they are afferent fibres, and from the fact that 
 they convey impulses which give rise to sensations of various 
 kinds, they are also known as ' sensory.' In the ventral roots 
 the impulses are passing from the centre to the periphery; they 
 are efferent nerves, and, from the fact that the majority of the 
 efferent impulses result in movement, they are also known as 
 ' motor ' nerves. A portion of cord embracing a pair of spinal 
 roots is spoken of as a segment, and the spinal cord consists of 
 a series of such segments united end to end. 
 
 Passing away from the spinal cord in company with the 
 ventral roots is a branch known as the white ramus communicans. 
 So soon as it gets outside the vertebras it leaves the ventral root 
 and passes to a portion of the nervous system known as the 
 ' sympathetic' The sympathetic system is frequently regarded 
 as a something quite distinct from the ordinary cerebro-spinal 
 system, and in function this is very largely true; but the white 
 ramus communicans serves to remind us that the two systems 
 are very closely linked, and are under one central authority. 
 It is unnecessary here to follow the white ramus farther; it will 
 be fully considered with the sympathetic system. 
 
 Function of the Spinal Nerves.- — If the dorsal spinal roots 
 be divided, all parts supplied by them below the division lose 
 sensation; if the portion of nerve in connection with the spinal 
 cord be irritated, pain is produced. The spinal sensory fibres 
 endow the whole body with sensation, with the exception of 
 certain parts of the face. If the ventral roots be divided, all 
 parts supplied by the nerves below the seat of division suffer 
 motor paralysis; if the cut end of the nerve still in connection 
 with the tissues be irritated, the muscles contract vigorously; 
 while if the piece of nerve in connection with the cord be irritated, 
 nothing happens. In this way it is demonstrated that the 
 sensory impulses pass into, whilst the motor impulses pass out 
 of, the cord. Sometimes pain is felt when the motor roots are 
 divided, due to one or two branches of the sensory nerves finding 
 their way back into the cord by this channel. The phenomenon 
 is known as re^MZSMLs&tisiMliiy. 
 
 Columns of the Cord. — If a cord be suitably prepared, a 
 transverse section (Fig. 143) shows it to consist 01 two similar 
 halves, united by a comparatively small central mass of tissue, 
 through the centre of which a minute longitudinal canal runs. 
 The halves are separated by fissures on the dorsal and ventral 
 surfaces of the cord. The ventral median fissure is wide, and 
 
496 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 does not reach down to the centre, while the dorsal median fissure 
 is narrow and deep. Each half of the cord is seen to consist of 
 dorsal, lateral, and ventral columns, separated from each other by 
 a shallow longitudinal groove. A section of the cord shows it 
 to be made up of both white and grey matter, the latter, internally 
 placed, forming the medulla. This is arranged something like 
 the letter h, or like two inverted commas placed back to back. 
 The shape of the grey matter on transverse section depends 
 entirely on the region of the cord examined. This may be seen 
 
 Fig. 143. — Transverse Section of the Spinal Cord in the Cervical Region 
 x 8d. The Lines in the Lateral and Superior Columns running from 
 the Outer Margin are Laminae of the Pia Mater (M'Kendrick). 
 
 a, Processus reticularis; 6, dorsal horn; c, grey commissure; d, dorsal septum; 
 e, Goll's column; /, superior column; g, point of entry of dorsal root; h, sub- 
 stantia gelatinosa; i, lateral column; j, large multipolar nerve-cells; k, ventral 
 horn; I, white commissure; m, inferior longitudinal fissure; n, inferior 
 column; 0, central canal; p, point of exit of ventral roots. 
 
 in Fig. 142, which shows three sections of the cord. The dorsal 
 and ventral ends of the comma, or crescent-shaped halves of the 
 grey matter, are spoken of as the cornua. From the dorsal 
 cornu the afferent or sensory fibres run; from the ventral run the 
 efferent or motor roots. The white substance of the cord also 
 varies in thickness (see Fig. 142). Stated generally, the cord 
 increases in white matter from the tail to the head. The grey 
 matter is largest in the cervical and lumbo-sacral enlargements, 
 and this increase and decrease in size corresponds with the 
 
THE NERVOUS SYSTEM 497 
 
 increase and decrease in the number of nerves entering and 
 leaving the cord in these regions. 
 
 The Grey Matter of the Cord may be regarded as a pair of 
 long columns extending throughout its length. These columns, 
 when examined microscopically, are found to consist mainly of 
 nerve-cells, some of which are connected with the dorsal, others 
 with the ventral roots of the spinal nerves. A collection of 
 nerve-cells possessing afferent and efferent nerves is essentially 
 a ganglion, and, regarded in this light, the grey matter of the 
 spinal cord may be considered to be built up of a series of gan- 
 glia placed end to end and communicating. In the ventral 
 cornu of the grey matter the cells are largest and arranged in 
 groups. The cells, as we have already seen (p. 46S), are multi- 
 polar; their axons are the oiigin of the fibres of the ventral or 
 motor spinal root. Every motor nerve obtains its nerve-cell 
 origin in the inferior cornu, and wherever the outflow of nerves is 
 greatest, as in those parts of the cord opposite to the limbs, 
 there these cells are largest and most definitely grouped. There 
 is "another group of cells in the ventral cornu which is not 
 connected with the building up of motor nerves; their axons pass 
 from the grey into the white matter, and travel up, down, and 
 across the cord, where they constitute short tracts of white 
 matter, which knit together the various segments. These are 
 known as ' association fibres.' Between the ventral and dorsal 
 cornua is a portion of grey matter known as the ' intermedio- 
 lateral column.' In this part are the cells connected with the 
 nerves which link up with the sympathetic system, and supply 
 the viscera by means of the white rami communicantes. The cells 
 in the dorsal horn of the cord are smaller, and the cell-groups 
 not so well marked. There is, however, a group of large cells 
 extending throughout the dorsal grey matter in the thoracic cord, 
 known as ' Clarke's column.' It furnishes fibres which pass into 
 the white substance of the cord, and form there what is known 
 as the direct cerebellar tract. ^-~"' 
 
 The White Matter of the Cord consists of medullated nerve- 
 fibres, generally running longitudinally; structurally, these 
 fibres differ from those found in the body-nerves by the fact 
 that they possess no neurilemma, the medullary sheath being 
 contained within a supporting material known as neuroglia. 
 The white matter of the cord is divided into certain columns — • 
 dorsal, lateral, and ventral. These anatomical columns give no 
 notion of the physiological paths which exist between the brain, 
 cord, and body. These paths in the white matter have been 
 mainly worked out by the degeneration method — viz., by study- 
 ing the degeneration which follows division of the spinal nerves. 
 If, for example, cutting the dorsal roots above the spinal gan- 
 
 33 
 
498 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 glion leads to the degeneration of traGts in front of the injury, it 
 is known this must be a sensory or ingoing path ; if the degenera- 
 tion occurs behind the injury, it is evident that the affected 
 fibres have their cell-station farther forward, and that the path 
 is motor or outgoing. Another method of tracing the tracts is 
 the developmental. In the early embryo the fibres have no 
 myelin sheath; when, later on, this appears, it is observed that 
 all the fibres belonging to the same group are simultaneously 
 invested with myelin. In this way it is possible to determine the 
 fibres possessing a common course. There are other methods 
 of inquiry which need not here be referred to; sufficient has 
 been said to indicate the nature and difficulties of this class of 
 investigation. 
 
 Paths in the Cord. — The white matter forms paths in the cord, 
 which are~spoken of as ascending and descending tracts.* 
 
 The ascending and descending tracts in the spinal cord 
 of man have been mapped out only after years of laborious 
 research, in which pathological as well as physiological results 
 have been utilised. In man the tracts are numerous and com- 
 plex; as the animal scale is descended simplification occurs. The 
 monkey is less complex than man, but more complex than the 
 dog, and so on. Tracts present in man are absent in the lower 
 animals; for instance, a tract known as the direct pyramidal, 
 which connects some motor centres in the brain with the limbs 
 and muscles concerned, has no representative outside of man 
 and the higher apes. Another, known as the crossed pyramidal 
 tract, and well represented in man and the monkey, is but 
 insignificant in the dog (see Fig. 144). The columns of white 
 matter in the spinal cord of the domesticated animals have 
 not been clearly made out, with an exception to be mentioned 
 presently, and we shall, in consequence, be compelled to refer 
 to the columns in man in order to illustrate the principle on 
 which the work is carried out. The white tracts do not run 
 unbroken throughout the length of the cord; some are long 
 and others short; some disappear for good; others change 
 their relative positions at different levels. Speaking generally, 
 the descending tracts diminish in size from the head towards the 
 tail; the ascending tracts increase from the tail to the head. 
 The shorter tracts are probably the older ones development ally, 
 for long tracts are more conspicuous in highly developed animals 
 in which the independent activity of the cord is imperfectly 
 retained, while short tracts are associated with more indepen- 
 dent function of the spinal cord and less development of the 
 higher centres. 
 
 * These terms have been retained for quadrupeds in preference to the 
 expressions ' headwards ' and ' tailwards,' or ' forwards ' and ' backwards.' 
 
THE NERVOUS SYSTEM 
 
 499 
 
 Descending Tracts in the Cord. — These run from the brain to 
 the spinal cord in the ventro-lateral columns of the white matter, 
 and give off aborisations to the motor neurones of the ventral or 
 motor roots of the spinal nerves (Fig. 145). Most probably there 
 are other descending paths not yet discovered, but those which are 
 clearly known are as follows : 
 
 Direct Pyramidal Tract. 
 Lateral Pyramidal Tracts — 
 
 [a) Crossed or Hetero-Lateral. 
 
 (6) Homo-Lateral. 
 Rubrospinal Tract. 
 Proprio-Spinal Tracts. 
 Vestibulospinal Tract. 
 
 Direct Pyramidal Tract. — This is found only in man and the 
 anthropoid apes. It is associated, and the same holds good for the 
 Lateral Pyramidal Tract, with complex, skilled, and delicate muscular 
 movements, such as occur in man ; it is through these channels that 
 
 Py.d 
 
 MAN MONKEY 00C 
 
 Fig. 144. — Diagram to illustrate the Relative Size of the Crossed Pyra- 
 midal Tract (Py) in the Dog, Monkey, Man (Foster, after Sherrington). 
 
 In B, Py' is an outlying portion of the pyramidal tract separated from the rest 
 by the cerebellar tract. Py.d in A is the direct pyramidal tract. 
 
 these movements are produced and directed. The more highly 
 developed the brain, the larger the direct pyramidal tract. In 
 Fig. 144 the position and relative size of this tract in man is shown. 
 Both pyramidal tracts are spoken of as corticospinal , to indicate 
 that they connect the cortex of the cerebrum with the spinal cord. 
 Crossed or Retero-Pyr amidol Tract. — This is the main descending 
 motor path in animals (Figs. 144, C; 145, A). Nevertheless, it is 
 relatively smaller than in man, for the size of this tract bears some 
 relation to the size of the motor areas in the brain. Where these are 
 small, as in the horse, ox, and sheep, the tract is small. Small 
 motor areas suffice for animals such as the horse, where the limb 
 movements are of a simple pendulum type. The combined areas 
 of the two pyramidal tracts in man constitute nearly 12 per cent, 
 of the total cross area of the cord. The following shows the propor- 
 tion the crossed pyramidal system bears to the cord in other animals : 
 Cat - 7-76 per cent. 
 
 Rabbit 5"3° 
 
 Guinea-pig - 3 -00 
 
 Mouse i-oo' 
 
 p ro g - - Absent, 
 
500 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The crossed pyramidal tract is so named on account of its fibres 
 crossing in the medulla on their way to the cord. In the direct 
 pyramidal tract there is no crossing. The effect of the fibres 
 crossing is that the right cerebrum controls the muscles on the left 
 side of the body, and vice versa. If an animal be shot in the right 
 brain it falls on the left side, as these muscles are the first to be 
 paralysed. 
 
 In the dog, and perhaps in other of the lower animals, there 
 are other paths than the pyramidal conveying impulses which lead 
 to motion, for if all the pyramidal fibres be divided, complete 
 paralysis does not follow, while stimulation of the motor areas 
 of the cerebrum continues to produce muscular contraction. It is 
 believed that the supplemental path is the rubrospinal tract, which 
 takes its origin in the red nucleus of the mid-brain, and is con- 
 sidered by Dexler* to replace in animals the direct pyramidal 
 tract. 
 
 J. L. King has investigated the crossed pyramidal tract of the 
 sheep. t He finds it to be imperfectly developed, and that it does 
 not extend beyond the first cervical segment. In the medulla 
 some of its fibres decussate, others are uncrossed. This short tract 
 represents all the motor fibres having origin in the cortex of the 
 cerebrum ; from this we should expect that not only are the cortical 
 motor areas small, but that motor fibres must be derived from some 
 other portion of the brain. King has found this to be the case. In 
 the ventro-lateral columns of the cord of the sheep are two well- 
 developed descending paths running its entire length. One of these 
 he identifies as the rubrospinal tract. These two tracts represent 
 in the sheep the chief primary motor paths in the cord. Centrally 
 they are connected with the mid-brain, pons, and medulla. The fibres 
 are consequently subcortical in origin. The two motor paths are 
 reinforced by proprio-spinal fibres, which bring them into intimate 
 relation with near and distant segments of the cord. King concludes 
 that the chief motor paths in the spinal cord of lower mammals do 
 not, as in man, originate in the central cortex, but from a point 
 lower in the brain. 
 
 Homo-Lateral Pyramidal Tract. — Some uncrossed fibres lie in the 
 same position as the crossed pyramidal tract. 
 
 Rubrospinal Tract. — Known also as the Pre-pyramidal or Mona- 
 kow's Tract, lies just below the crossed pyramidal tract(Fig. 145, A) . It 
 arises in the mid-brain from the red nucleus. King considers that 
 one of the two well-marked motor paths in the sheep represents 
 the rubro-spinal tract found in some other animals. This tract in 
 the dog is referred to above. 
 
 The Proprio-Spinal Descending Tract, or tracts, consists of fibres 
 which connect the more anterior spinal segments with the more 
 posterior. The fibres of these in the dog are much more numerous 
 than the pyramidal tract fibres, and their importance lies in 
 the fact that they connect the four limbs together in locomo- 
 tion. 
 
 Vestibulospinal or Ventro-Lateral Descending Tract. — This tract 
 has its origin in one of the vestibular nuclei (Deiters' nucleus) . In 
 the cord it lies in the ventro-lateral column near the surface of the 
 cord. 
 
 * ' Veterinary Anatomy,' S. Sisson. 
 
 f The Quarterly Journal of Experimental Physiology, vol. iv., No. 2, 
 June, 1 91 1. 
 
THE NERVOUS SYSTEM 50I 
 
 Ascending Tracts. — These are as follows : 
 
 The Direct or Dorsal Spino-Cerebellar Tract (tract of Flechsig). 
 The Median Superior Tract (column of Goll). 
 The Lateral Superior Tract (column of Burdach). 
 The Ventro-Lateral Ascending or Ventral Spino-Cerebellar Tract 
 (tract of Gowers) . 
 
 The Direct or Dorsal Spino-Cerebellar Tract (Fig. 145, A) arises in 
 the peculiar group of cells in the grey matter on the inner side of the 
 dorsal horn. The axons from these cells run to the cerebellum _ 
 without crossing. This tract forms a long afferent path, and it has 
 been made out that the fibres from the lumbar portion of the cord 
 lie outermost; the dorsal fibres lie within these; and, finally, the 
 fibres from the anterior dorsal and lowest cervical nerves are inner- 
 most. All the fibres comprising this tract do not reach the brain; 
 many terminate in the cord. The tract terminates in the cerebellum, 
 which it enters by the posterior peduncle. 
 
 The Median Superior Tract lies on either side of the dorsal median 
 fissure. The fibres composing it are small, and the path terminates 
 at the bulb. This tract is made up of long fibres of the dorsal roots 
 of the tail, pelvis, and hind-limbs, and runs up and ends mainly in- 
 the bulbar nucleus of Goll, often called the leg nucleus. Degenera- 
 tion of the tract occurs when the dorsal roots of the cord are divided. 
 In Fig. 145, A, this tract corresponds to the sensory path marked 
 ' hind-leg ' and ' tail.' 
 
 The Lateral Superior Tract is derived from long fibres of the 
 dorsal roots of the thoracic region, fore-limbs, and neck. It runs 
 up and ends mainly in the bulbar nucleus of Burdach (nucleus 
 cuneatus) commonly spoken of in man and the monkey as the arm 
 nucleus. In Fig. 145, A, this sensory path is indicated by the 
 portion marked ' fore-leg,' ' neck,' and ' trunk.' 
 
 The Ventro-Lateral Ascending or Ventral Spino-Cerebellar Tract, — 
 In the case of this tract the precise seat of origin is unknown. 
 Division of the dorsal roots of the suinal nerves produces no effect 
 upon it. Its origin must therefore be within the cord. The tract 
 terminates in the cerebellum, part entering by the anterior peduncle. 
 The fibres terminate by arborising around cells in the middle lobe 
 of the organ. 
 
 Association Fibres. — Certain short fibres, previously mentioned, 
 arise in the cord, and form tracts uniting the various segments. 
 They originate from ' tract cells ' in the grey matter of the cornua, 
 and from here run .into the white substance, but after a short course 
 they run back and re-enter the grey matter. These tracts may be 
 ascending or descending; their function is to knit together the 
 various spinal segments. 
 
 The above ascending and descending tracts do not account for the 
 total amount of white matter; there are still unmapped regions in 
 the cord. 
 
 When the various ascending tracts reach the medulla, they undergo 
 change in form, position, and distribution. Some, as we have seen, 
 terminate in cells in the bulb ; in fact, there are only two upward 
 paths which pass through unbroken — viz., the two cerebellar. All 
 the descending tracts which are passing from the cerebrum to the 
 cord, pass through unbroken. 
 
 Functions of the Ascending Tracts.— In the median superior 
 and lateral superior tracts and in the dorsal columns of the cord 
 
m « 
 
 c g 
 
 •V £ 
 U O 
 
 ID ".£ 
 J3 O 
 
 
 
 ? 
 
 
 ^•5 
 
 J3 
 
 C 
 
 o 
 
 •a 
 
 
 o 
 
^ 
 
 CD 
 'J, 
 
 
 3 
 
 S - i/) 
 
 Cm 
 
 
 
 ^ — i t! 
 
 fj 
 
 i— CJ 
 
 .5 
 
 ado 
 
 U H ™ O 
 
50 4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 impulses are conveyed which lead to a sensation of pressure 
 (touch), but more especially impressions from skeletal muscles, 
 tendons, and joints, known as muscle-sense, to which is due the 
 co-ordination of muscles in locomotion. It will be remembered 
 that the lateral superior tract is almost wholly composed of 
 sensory fibres from the muscles. These impulses are carried up 
 the side of the cord into which they entered. In the bulb the 
 paths end, and fresh fibres are formed, which cross to the opposite 
 side to terminate in the cerebrum, in the area known as that of the 
 body-senses. Injury to the fibres of body-sense produces an 
 awkwardness in gait; the animals show a want of skill in using 
 the hind-limbs. The muscles are there, but, owing to the loss of 
 muscle-sense, they are not conscious of them, and consequently 
 are unable to direct their movements. 
 
 The dorsal cerebellar tract and the ventral cerebellar tract carry 
 impulses connected with deep sensibility arising from muscles, 
 tendons, and joints. Each of these tracts terminates in the cere- 
 bellum; it is believed they are also associated with muscular 
 co-ordination, which, as we shall see later, is the special function 
 of the cerebellum. 
 
 The nerve-fibres which give rise to sensations of pain and 
 temperature after entering the cord end in the grey matter; fresh 
 axons are then formed, which run up both sides of the cord. 
 In the medulla the fibres terminate, and fresh axons arise from 
 its cells,-which pass to the optic thalamus of the opposite side. 
 This is the chief station for all varieties of afferent stimuli. In 
 both the ascending and descending paths there are fibres terminat- 
 ing in the cord — i.e., not running the full length of the tract. 
 This is especially the case with the afferent fibres, the larger number 
 of which enter the grey matter, where many terminate by 
 arborising around tract cells. Many tract cells furnish further 
 fibres, which continue the path. 
 
 All sensory impulses pass to the brain on the side opposite to 
 their origin, and all motor impulses leave the brain on the 
 opposite side to that to which they are distributed, so that injury 
 to a left motor area leads to a right body paralysis (see Fig. 147). 
 
 The termination of the sensory fibres in the cord has been in 
 part referred to. Having passed through their cell-stations in 
 the ganglia on the dorsal spinal nerves, they enter the cord, but 
 do not directly pass along it as an afferent tract, but penetrate 
 the grey matter, where many terminate (Fig. 145, A); others are 
 provided with fresh axons, and then continue the headwards 
 passage as a spinal tract. As a matter of fact, the entry of the 
 fibres into the cord is by no means so simple as the above would 
 suggest, and it is further complicated by the fact that the fibres 
 on entering the cord divide into a Y or T, one branch passing 
 backwards for a short distance, the other forwards, both, perhaps, 
 
THE NERVOUS SYSTEM 
 
 505 
 
 giving off collaterals (see Fig. 146). Some of these collaterals 
 establish cell communication with the grey matter, and so with 
 the motor system, a reflex arc being thus formed (Fig. 145, A and 
 B). The reflex arc may be complicated by the introduction 
 of an intermediate set of neurones, connecting the cell termination 
 of the sensory nerve with the cell origin of the motor. 
 
 Special Centres in the Spinal Cord. — In the cord certain 
 centres exist, which, though ordinarily under the control of a 
 chief centre in the bulb, yet are 
 capable, as we have seen, of carry- 
 ing on peculiar reflex actions even 
 after the cord has been divided. 
 
 The cilio-spinal centre lies between 
 the cervical and dorsal portions of 
 the cord ; in it fibres originate which 
 through the cervical sympathetic 
 supply the dilator muscle of the 
 iris. Destruction of the region in 
 question causes a contraction of the 
 pupil, whilst irritation of it causes 
 the pupil to dilate. The ano-spinal 
 centre, found in the lumbar portion 
 of the cord, controls the act of 
 defalcation; it would appear to 
 be highly developed in herbivora, 
 which possess the power of bringing 
 it into play not only when the body is 
 at rest, but during movement. The 
 
 Fig. 146. — Branching of Dorsal 
 Root Fibres in Cord (Donald- 
 son, after Ramon y Cajal). 
 functions of the ano-spinal centre DR, Dorsal^root fibresjintering the 
 
 appear rather complex, inasmuch as 
 
 cord, and dividing Y- or T-wise 
 into fibres running forwards and 
 
 backwards, which give off colla- 
 terals, Col. Cells, CC, can be 
 seen in the grey matter of the 
 cord with which the collaterals 
 establish communication. 
 
 it has not only to maintain the tone 
 of the sphincter, but also to relax 
 it during defalcation, and under 
 the latter condition simultaneously 
 to contract the abdominal muscles 
 
 ' and diaphragm. These contractions, however, are brought about 
 quite simply through the reciprocal innervation of antagonistic 
 muscles (see p. 489). The vesicospinal centre also exists in the 
 lumbar portion of the cord, and governs micturition; its action 
 is similar to that of the ano-spinal centre. In the lumbar 
 portion of the cord other centres are found— for example, the 
 erection centre, the genito-spinal centre, which contains the 
 nervous apparatus employed in the emission of semen, and the 
 parturition centre. Vasomotor centres are found throughout the 
 cord; they are principally under the control of similar centres 
 in the bulb, but may act independently. All these centres in 
 the cord are activated through the sympathetic system. 
 
Fig. 147. — Diagram of the Afferent and Efferent Paths passing to and 
 from the Brain by the Cord (Sherrington). 
 
 L, Left; R, right; CBM, cerebrum; cbm, cerebellum; mo, medulla oblongata, containing the 
 decussation of p, the pyramidal tract, and of /, the fillet; the decussation of / should really be a 
 little higher instead of a little lower than that of p ; «G, nucleus gracilis (Goll's) ; oT, optic thalamus ; 
 pvc, the superior vesicular column, or column of Clarke; sp g, spinal ganglion; cG, median superior 
 column (Goll's); dc, direct cerebellar tract. 
 
 " ,The arrows show the direction of the impulses. An afferent impulse, r say from the skin, passes 
 along the sensory nerve, through the spinal ganglion, and enters the dorsal columns of the cord; it 
 may pass to the cerebrum direct via the medulla by cG, the median superior column, which crosses 
 in the bulb, and so gains the opposite^side of Jthe brain ; or J the impulse may pass by dc, the cerebellar 
 tract, to the cerebellum, entering it on the^ame side, and from here crossing over to the opposite 
 cerebral hemisphere. An efferent impulse originates in the cerebral cortex, gains the pyramidal tract, 
 passes through the bulb to the opposite side of the cord, enters the cells in the inferior cornu of the grey 
 matter, and passes out in the inferior spinal nerves. The arrows denote the direction of the impulses. 
 
Section 4. 
 
 Bulb (Medulla Oblongata), Pons, and Mid-Brain. 
 
 The change undergone by the spinal cord in merging itself into 
 the bulb may be studied by successive sections of the latter. 
 Briefly it may be stated that the central canal of the, cord widens 
 in order to become subsequently the fourth ventricle; the sym- 
 metrical and regular arrangement of the grey and white matter 
 of the cord becomes broken up, not only in order that it may find 
 its way to its destination in the higher centres, but that t hose , 
 fibres w hich are passing-autHEnay be c ollected from the various 
 ouEIyTng__cerj±xes, and brought togetherln an orderly manner i n 
 the smaller and more CQ.ro pa.cf stra rjjjrg, the medulla. But the 
 medulla is not only a highway for nerve-fibres passing in two 
 opposite directions — it is also a cell centre ; and besides restarting 
 all the tracts which terminate in the bulb o n their way up or down, 
 it also gives origin_to six of the most, important cramal^ nerves. 
 The origin of these m~fne medulla, in the various grey nuclei, 
 is an additional complication in the rapidly changing appearance 
 presented by the bulb in successive sections from rear to front. 
 Apart from the complexity of the subject we are now entering on, 
 which increases as we approach the cerebrum, there is also the 
 important fact that in the large herbivora, but with few exceptions, 
 the course of the fibres and the collocation of cells, not only in the 
 bulb, but also in the other centres, are not known with any degree 
 of accuracy. 
 
 There is now good reason to hope that the experiments of King* 
 and other observers on the larger herbivora will be the means of 
 laying the foundation of exact physiological knowledge with regard 
 to their nervous system. 
 
 The bulb in the horse is wider and more flattened than the 
 cord; it is about 2 inches in length, and is wider anteriorly than 
 posteriorly; it is considered to begin at the firjt_i^rvicjTnerve 
 and end at the pons . In this short length ofnervous material, 
 functions of vi tal importance^ are carried out, through the 
 medium of those cranial nerves wh ich take their origin from 
 this p art. On the'ventral surface of the medulla_twg well-marked 
 structures exist, known as the pyramids. There is no surface 
 enlargement indicating the presence of the olivary bodies, so 
 prominent a feature in man, but in the substance of the medulla 
 
 * Op. cit. 
 5°7 
 
508 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 well-markftd olivary nuclei exist. The pyramids are formed by 
 the" collection of motor fi bres descendirigfrom" the Tjnun/lmd 
 brought •tp-ggthCTln twob undles in the 5uT DT~Tn the pyramids yi^ 
 they decussate, so that" fibres from thex igh-t-b r ain- paso-down the 
 left side' of the cord, and vice versa. It is more convenient to 
 build up the bulb from below, so that, though the pyramidal 
 fibres are passing out of the brain, they are, from a constructional 
 point of view, spoken of as if they were passing the other way. 
 This being so, it is usual to describe the pyramidal fibres in the 
 cord as crossing in the medulla, and, by so doing, cutting through 
 the ventraLhorns of the__grsy_raafcter, and event ually causin g , 
 
 these to disappear, as may be seen from sections taken at a ^ v 
 higher level. In this way the p}TamidS- J areJormed,-.and little. is f*,!^ 
 left of the original grey matter. On the dor sal surface of th e 
 bulb the two ' horns open ni^ t L : the tip of each swells, and forms 
 the substantia gelgHnosa, while the columns of Goll and Burdach, 
 which, iTwill be remembered, have their fibres passing into the 
 brain, end in nuclei known as the nudejiS-SM^ ,s an( i n ucleu s 
 cuneatus respectively. These are often called the ' leg^jiuEESs. 
 and ' arm ' nucleus. The fibres from the above columns ter- 
 minate by arborising around the cells in their nuclei; t hose /i 
 from the anteri or half of the body t erminate in the nucleus 'w 
 
 cuneatus, while'those from thepostenor half terminate in_the / 
 k nucleus gracilis. From the cells in these riucleifresh axons axe/ 
 tormed, which pass forward into tJieJaigh<ss-<»5rrrTfts.' and scTsfill ,, 
 further increase the com plex arr ang-eme-nt- of the Jjulb. Inci-jf^ 
 dentally attention may be draw n to the fact that in thesjyiiicleiih£\^ 
 firs t sensoryjjpinlE7pTn^on_ e. whose seat of origin is in the dorsal 
 root ganglia/has its ending. From these nuclei the second 
 sensory neurone fibres arise, which, passing forwards TxftTie 
 higher centres mentioned above, form the internal arcuate Jibres. 
 In their journey forwards these decussate above the pyramids, 
 continue their course through the bulb and pons under the 
 name of the_/£ZZfi2,-.-and so reach the optic thal^n-ms- here the 
 secoj^j]£uroneffltids and the third ariges^jvhich passes forwards 
 to end in the posJ^ce-atraL gyrus. The fil let, or lemniscu s, is 
 consequently the sensory path connecting the"between-brain and 
 the bpdy™ 
 "The changes made in the arrangement of the medulla having 
 brought the central canal of the cord close to the dorsal surface, 
 it soon opens out into the fourth ventricle; and now a further 
 complexity in the arrangement of the bulb is evident from the 
 fact that the nuclei-giving -origin, to the cranial nerves have 
 here their position, those of the te nth an d twelf th" pairs bei ng 
 seen forming the,grey matter of the floor QFjJfiKJSSllil. ventricle. 
 The internal arcuate fibres cross to the opposite side, enter the 
 
 \J 
 
THE NERVOUS SYSTEM 509 
 
 restiform^jM&y*-wh.ich forms the post erior peduncle pi the- cerjg- 
 beHum, and so gain access to the cereBellum. Some of these 
 arcuate fibres enter the restiform body on its own side. There are 
 also fibres which pass from the olivary body through the restiform 
 body of the opposite side to the cerebellum. The restiform body 
 is formed in part from the direct cerebellar tract of the cord. 
 The fibres of this tract terminate in the medianJaemisphere, 
 or verrmsj'&t trTe""cerebe*lrOTH"l3y arborising around the Purkinje 
 cells of that body. 
 
 Such, briefly, is the arrangement of the bulb. It is better for 
 the purpose of description to continue carrying the sections 
 forward both through the pons and mid- brain. 
 
 The Pons lies between the bulb and the cerebral peduncles. If 
 a section be made through it, the appearance presented does not 
 differ materially from that furnished by a section of the upper 
 extremity of the medulla, with the exception that bundles of 
 fibres taking a transverse course are seen passing to the middle 
 peduncles of the cerebellum. Between these fibres is the grey 
 matter, or nuclei pont is. From the dorsal surface of the pons 
 are formed the anterior cerebellar peduncles. FiJ^es^course 
 throu gh the p ons frgm_qng side of the cerebellum ifi thftftther, ' 
 arid 'from the r-PT-p^iim +r> tl3P p nr is The pyramidal fibres 
 rea ch into the po ns, and are spoken of as the cmiico-ponHne. 
 Though the fibres are described as reaching into the pons from 
 the pyramid, it will be remembered the flow is in the opposite 
 direction, and that these fibres are passing from the cerebral 
 cortex backwards into the cord. In this section can be seen the 
 nucleus of Deiters , a collection of large multipolar- cells— ia-the 
 floorof__the^ourth ventricle... which is intimately connected with 
 afferent and efferent mechanisms, and is one of the cell-ter- 
 minations of the vestibular branch of the eighth nerve. From 
 this nucleus arises t he ve stibulo-spinal tra ct (see p. 500). From 
 the pons arises the important fif th pair o f cranial nerves . 
 
 A section through the Mid-Brain shows that the^reticular 
 structure of the pons is continued forward; it isJ ealarged by 
 fibres derived from the oppjjsite-eeffibellar cortex, and fibres 
 from the nuclei of the fifth and eighth cranial nerves. The 
 nervous masTTormed by these structures bifurcates into the 
 crura cerebri, or cerebral peduncles, and these constitute^the^ ventral 
 po rtion of the mirl-fr r j iirr ' "Facn peduncle is 3Ivid"ed by a groove 
 into two portions— a dorsal, known as the tegmentum, and a 
 ventral, or crusta. Between the two is a collection of grey 
 matter known as the substantia nigra. In the crusta the motor, 
 and in the tegmentum the sensory fibres are contained; th e mid-_ 
 brain therefore constitutes an im portant highway between th e 
 cerebrum, and the body. 
 
5io A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Func±icuis-oflhe_Bulb.- — We have seen that this is the path by (]) 
 which the br ain com municates with the periphery and the p eri- 
 phery with t he bra irH It gives origin t o, or is connected with, Q 
 all the cranial nerves but those of ^mell, vision, and of the 
 muscle's~"t5f~th'e eyeball. ) It is the su preme rpfW rpntrp for all (|) 
 ,the mechanisms of the body most important to life, such as 
 respiration (p. 137), circulation (p. 87), the action of the heart 
 (p"p.58-bo), and of the dfgesjEwajjpjratus from the pharynxio the 
 large intestine (pp. 165, 197, 217, 234). It is astonishing that 
 these varied functions can be discharged by a few inches of 
 nervous tissue. If the medulla be cut off from the brain in front 
 of its various centres, the animal continues to live, respirations 
 are regulated, the blood-pressure is maintained, and the heart 
 continues under control. If the section be made behind the 
 centres, all is at once changed, because the respirations cease 
 instantaneously; the blood-pressure sinks dangerously, but then 
 partially recovers, rising in the cat and dog to a height of about 
 90 mm. (instead of the normal 130). The heart beats slowly but 
 well, and the animal dies from asphyxia (see p. 138). If artificial 
 respiration be established, the beat of the heart may be main- 
 tained for some days. The animal is conscious, feels pain if the 
 lip be pinched, blinks when the finger approaches the eye, feels 
 hunger, and will even seize food within its reach, masticate and 
 swallow it (Chauveau). 
 
 The centres lo cated i n the bulb are those for mastication , 
 swallowing, Clicking, vomiting, respiration, phonati©n, coughing/V y 
 the regulation, of the hejrt-beat and arterial calibre, movements i 
 of the Ms, the secteti on. oi saliva, the "glycosuria centre, and a 
 centre for the sweat g lands. The bulb and pons contain centres 
 which are intimately associated, through centres in the spinal 
 cord, with the reflex act of standing. 
 
 The Mid-Brain is composed of the c orpora quadrieemina and 
 c erebr cdjj&kmcles ; these form the basal ganglia. 
 
 I HeCorpora Quadrigemina. — The white matter of the anterior 
 of these bodies is derivedfrom the optic n erve, the fibres of 
 which arborise around ItsceUs. fibres from "the corpora quadri- 
 gemina also run backwards to the cord, and in man gain a tract 
 known as the infero-lateral descending. The anterior pair of 
 bodies is concerned in vision. It fofms ajreflex arc connected Q) 
 with the movements of the pupil, while, through itTconnectlon 
 with the optic nerve, it works upon_ the mu scles, .of .the eyebal l. 
 The posterior corpora quadrigemina are important in the higher 
 group of vertebrates. — viz., birds and mammals which have a 
 cochlea — that is, a ' hearing ' ear, besides an ' equilibratin g ' ear . (J) 
 They receive fibres jrom the coc hlear nerve, and have reflex centres 
 concerned with the lower auditory functiona^u^withwcalisatioTn 
 
THE NERVOUS SYSTEM 511 
 
 The Optic Thalamus. — As we have previously seen (p. 504), fa 
 this is the j unction for all afferent jmpr essions. prjpr to Hktrihn- ** 
 ti on to their destination. It* is" especially the main sensory 
 station, the seat not only of painful, bu t also of pjeasura ble 
 impressions. Its activities, especially in the higher animal, are 
 c ontroll ed by the_cerebral.,cortex, and failure to exercise the 
 necessary control was the explanation of the neurotic condition 
 produced in many during the Great War. It may be argued that 
 among the lower animals cerebral control over the emotional 
 thalamus is difficult to prove, while it is easy to show how 
 frequently it is never exercised. Most animals respond to painful 
 impressions and endeavour to escape from them; the painful 
 impression may be either physical or .moral. The horse of all 
 others is susceptible of panic, and in these cases there is an 
 inability on the part„ aLthe-GQPtex- to cont rol the thalamus . 
 On the other hand, a dog while fighting wilfsubmitto severely 
 painful impressions without relaxing its hold, and a ' jibber ' 
 may be utterly indifferent to whip or spur. In such cases the 
 thalamus is controlled. 
 
 Head,* however, points out that the normal control of the cortex 
 is an automatic and not a voluntary a,ct, and that the moment 
 it becomes voluntary a conflict ensues which leads to a break-down. 
 It is difficult to determine the extent to which this holds good in 
 animals. Horses when first taken into military training meet with 
 sights which to them are terrifying, but confidence is soon estab- 
 lished. We never remember an animal having to be rejected from 
 the service owing to a failure to overcome its natural nervousness 
 during training, though the flutter of a lance flag or wave of a sword 
 frequently create more apprehension than the discharge of a rifle. 
 The artillery horse who does not blink an eyelid at the noise made by 
 his own guns dislikes the crackle of rifle-fire when he first hears it, 
 and the cavalry horse who is indifferent to rifle-fire may be felt 
 suddenly to ' open out ' and contract under the rider when he first 
 meets with artillery fire. In all cases the control exercised by the 
 cortex must at first be voluntary; later, it is of course automatic. 
 On the other hand, the elephant, in spite of his intelligence, can 
 never be trusted under fire. 
 
 The structure of the thalamus suggests that it is also connected 
 with motor functions, for destruction of portions of the cortical 
 area leads to degeneration 6¥" their corresponding nuclei in the 
 optic thalamus. Connected with the thalamus is the red nucleus, fa 
 which gives origin to an important spinal motor path in the lower 
 animals (see p. 500). Some observers have considered that 
 secretory, vasomoto r and other fibres connected with the sym- 
 pathetic system, are represented in the thalamus. Cauterisation 
 of the thalamus in pigeons has recently been stated to render 
 
 * ' Elements of the Psycho-Neuroses,' British Medical Journal, March 20, 
 1920. 
 
512 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the animal permanently cold-blooded (F. T. Rogers), so that 
 h£) it would appear in some way to be associated with the regulation 
 of bo dy temperature . J 
 
 The Corpus Striatum. — Lesions of this body are said to be 
 fh associated with_ajiae_in temp erature ; hence it has been looked 
 upon as governing heat production. Little is known of the 
 subject, but it is supposed that impulses, pass to the skeletal 
 muscles, causing an increased production of heat. Develop- 
 mentally the corpus striatum is a part of the cerebral cortex, 
 and not a portion of the old mid-brain. The corpus striatum 
 is interesting clinically on account of the comparative frequency 
 with which it is diseased in the horse. There are no symptoms 
 pointing to cerebral trouble until a day or two before death. 
 It is remarkable to what size a growth in this body may attain 
 before any symptoms of pressure are shown. These cases 
 should enable the question to be settled of the connection, 
 if any, between the corpus striatum and body temperature. 
 
 Experimental injury of the corpus striatum of the horse was 
 carried out by Colin, who found that a simple puncture caused 
 no inconvenience or interference with locomotion. Three fresh 
 punctures made in the same corpus produced immediate paralysis 
 of the hind-legs, and the animal could only be kept standing by 
 a support under the belly. When walked thus supported, the 
 fore-limbs were quite unaffected, the hind-limbs dragged. If the 
 animal fell, power appeared to be regained in the hind-legs, for 
 he was able to recover the standing position. The paralysis 
 gradually passed away, and the horse was able to stand without 
 assistance. 
 
Section 5. 
 The Cerebellum. 
 
 The cerebellum is broadly divided into a central body, known 
 as the vermis, or worm, and two hemispheres which lie on either 
 side of the central body. It is situated above and in front of 
 the medulla, and is remarkable for the curious foldings of its 
 grey and white matter, which give it a characteristic appearance 
 on section. The grey matter is placed externally, a reversal 
 of the order found in the cord. The 
 grey matter of the cerebellum is found 
 to consist microscopically of two layers, 
 an outer and inner. In the outer or 
 molecular layer two types of cells are 
 found: one is known as a ' basket cell,' 
 from the peculiar manner in which its 
 axon dips down and encloses the large 
 characteristic cells of Purkinje, which 
 are placed between the outer and 
 inner layeis of the cortex (Fig. 148). 
 In the second type of cell found in 
 the outer layer of the cortex the 
 axons run longitudinally to the surface 
 of the cerebellum, giving off collaterals 
 which dip down and arborise around 
 the dendrites of the cells of Purkinje. 
 In the inner or granular layer of the 
 cortex the cells are small, their axons 
 travel towards the surface of the 
 cerebellum, where a T-piece is formed, 
 which, running parallel to the surface, 
 makes connections with the dendrites 
 of the Purkinje and other cells, such 
 as those of Golgi's second type, which 
 are also present. Purkinje's cells, as above mentioned, are 
 placed between the two layers of the cortex. They are distin- 
 guished by their considerable size and by possessing a single 
 dendrite, which breaks up into a remarkable branching tuft and 
 runs towards the surface of the organ, while the axon passes 
 into the white matter, giving off collaterals (Figs. 149 and 150). 
 
 The white substance of the cerebellum consists of both. afferent 
 and efferent fibres; the afferent fibres— viz., fibres afferent for 
 
 513 33 
 
 Fig. 148. — Pericellular 
 Baskets (Schafer, after 
 Ramon y Cajal). 
 
 Two cells of Purkinje from the 
 cerebellum are seen sur- 
 rounded by end ramifica- 
 tions forming a basket-work : 
 b, derived "from the branch- 
 ing of axons; a, of small 
 nerve-cells in the molecular 
 layer. 
 
514 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the cerebellum — are found microscopically to consist of two 
 varieties, which, from peculiarities in their arrangement and 
 distribution, are known as ' moss ' and ' tendril ' fibres. The 
 ' moss ' fibres terminate by branching in the nuclear layer of the 
 grey matter, while the ' tendril ' fibres pass into the molecular 
 layer, and arborise by climbing around the dendrites of the 
 cells of Purkinje. The efferent fibres are the axons of the cells 
 
 Fig. 149. — Cerebellar Cortex: Section 
 in Direction of Lamina (Cajal). 
 
 A, Outer or molecular layer; B, inner, 
 nuclear, or granular layer ; C, white matter. 
 
 a, Purkinje's cell; b, granule cells of inner 
 layer; c, dendrite of a granule cell; d, axon 
 of a granule ceU passing into the molecular 
 layer, where it bifurcates into two fine 
 longitudinal branches (Golgi's method). 
 
 Fig. 150. — Cerebellar Cor- 
 tex: Section across a 
 Lamina (Cajal). 
 
 a, Purkinje's cell; the numerous 
 dots in the molecular layer 
 represent cross-sections of the 
 bifurcated axons of the granule 
 cells (Golgi's method). 
 
 of Purkinje, which are essentially efferent in function. The 
 above outline affords some notion of the complexity of "the struc- 
 ture of the cerebellum; it is anticipating matters to say that this 
 body is the centre of an important reflex arc, but it is evident 
 from its structure how well it is arranged for reflex processes. 
 
 The cerebellum, by means of its three peduncles, is brought 
 into connection directly or indirectly with all the other parts of 
 the brain. The incoming fibres sweep through the peduncles 
 
THE NERVOUS SYSTEM 515 
 
 and terminate in the grey matter in the manner already de- 
 scribed. The efferent fibres originating in the cells of Purkinje 
 do not pass out through the peduncles directly as fibres derived 
 from the cortex; these cortical fibres first terminate in nuclei, 
 and from the cells of the nuclei fresh fibres are formed. 
 
 Through the anterior peduncle fibres pass from the cells of 
 Purkinje to the mid-brain. This is the only purely efferent tract 
 assigned to the cerebellum, and the distribution of the fibres is 
 peculiar and complex. The peduncles converge and meet in the 
 mid-brain; by their so doing their fibres decussate, so that 
 impulses from the cerebellum pass to the opposite side of the 
 brain, where they are conveyed to the red nucleus. This is the 
 second cell station for the efferent fibres. In the red nucleus, 
 some of the fibres are sent upwards to the cerebral cortex by 
 means 01 fibres known as thalamo-cortical, and others pass back- 
 wards by a special tract known as the bundle of Monakow, or 
 rubrospinal tract, which in its passage to the medulla crosses the 
 central line, so that impulses pass down the lateral columns of 
 the cord on the same side as they issue from the cerebellum. 
 In this way the cerebellum is brought into communication with 
 its own side of the cord, an arrangement which is in contrast 
 to the connection existing between the cerebrum and spinal cord, 
 which is crossed. By means of the anterior peduncle the fibres 
 of the ventro-lateral ascending tract of the cord (tract of Gowers) 
 also gain entrance to the cerebellum, being distributed to the 
 lower part of the vermis. 
 
 By means of the middle peduncle the cerebellum is brought 
 into- communication with the pons, fibres passing from nuclei in 
 the pons to the cerebellum. This is the second indirect path 
 between the cerebrum and cerebellum, but in this case it is 
 afferent — viz., the communication is from cerebrum to cerebellum. 
 Through the posterior peduncle, which is formed by the resti- 
 form body of the medulla, runs another important afferent path 
 to the cerebellum, but in this case from the spinal cord. The 
 direct cerebellar tract passes unbroken through the bulb, through 
 the posterior peduncle, its fibres being mainly distributed to 
 -the vermis on the same side. Other very important fibres con- 
 stituting a connection between the cerebellum and the labyrinth 
 are conveyed by this peduncle from the eighth pair of cranial 
 nerves. The eighth pair of nerves arises from two roots; one 
 from the vestibule of the ear is in no way connected with the 
 sense of hearing, but is entirely devoted to the question of body 
 equilibrium and allied matters. This branch gains access to 
 the cerebellum through the posterior peduncle. It will be ob- 
 served that the posterior peduncle is a second afferent path 
 to the cerebellum. 
 
516 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The cerebellum is connected, as we have now seen, with 
 the mid-brain, cerebrum, medulla, and spinal cord. It is 
 doubtful whether its two halves are connected. Afferent 
 impulses may pass to the cerebrum via the cerebellum,, and 
 efferent impulses from the cerebrum to the muscles may reach 
 them through the cerebellum. Afferent impulses may pass to the 
 cerebellum and efferent impulses issue from it without the inter- 
 vention of the cerebrum. 
 
 Functions of the Cerebellum. — Our knowledge respecting the 
 uses of this organ is still very incomplete. Flour ens regarded 
 it as being connected with locomotion and the problem of body- 
 balance. The study of comparative anatomy lent support to 
 this theory; it was observed how highly the cerebellum was 
 developed in birds and in swift-swimming fishes with great 
 locomotive power, such as the shark. In birds the vermis of 
 the cerebellum is large and deeply folded, though the hemispheres 
 are almost wholly absent, excepting in those birds which remain 
 some time in the air and possess wings and feet of considerable 
 strength. The experiment of Flourens on the cerebellum con- 
 sisted in its removal in the pigeon; as the result of the operation 
 the bird was unable to fly, stand, or feed itself. When it 
 attempted to walk, spasm of the extensor muscles of the legs 
 occurred, and it fell, struggling wildly, in a state of evident 
 panic. There was no muscular paralysis, but the pose of the 
 body was awkward, the head being drawn back and neck 
 twisted. During the muscular spasms the contractions led to the 
 animal turning somersaults. In course- of time the violent 
 symptoms disappeared, and the bird was able to fly, but for a 
 long time was unable to perch, through spasms of the extensor 
 muscles of the limbs. When the cerebellum is sliced away, and 
 not entirely removed, the gait is rendered uncertain and stag- 
 gering, and there is inability to maintain equilibrium. 
 
 In the dog injury to one hemisphere of the cerebellum leads 
 to the animal moving in a circle or rolling towards the injured 
 side, and to a disturbance of co-ordination. The entire cere- 
 bellum, however, has been removed in the dog, and the animal 
 remained alive for many months. In these cases there are 
 muscular spasms of the head, neck, and fore-legs, weakness of 
 the hind-limbs, and when the eyes are closed standing is im- 
 possible. When the acute symptoms pass away the animal 
 is left with a deficiency of muscular tone. It is interesting to 
 note that this lesion does not prevent the dog from swimming. 
 
 Colin exposed the cerebellum in a draught horse, and punc- 
 tured the middle lobe, or vermis, with a scalpel. The animal at 
 once shook his head, but there were no convulsions. After a 
 second and third puncture the gait became staggering and the 
 
THE NERVOUS SYSTEM 517 
 
 limbs splayed to preserve the equilibrium. When the animal 
 walked, the body was balanced alternately to the right and left, 
 as if at each step he expected to fall. In the walk he showed 
 a marked tendency to lean forward in the attitude of draught, 
 as if determined to negotiate a stiff hill ; it was only with difficulty 
 that seven or eight assistants held him back. 
 
 The cerebellum has been regarded as the centre of muscle 
 sense — i.e., as the centre by which the position of the boctyand its 
 movements are recognised in the absence of visual and tactual 
 impressions. The necessary impulses are brought to it from 
 the foot-pads in soft-footed animals, also from the depths of the 
 body — i.e., the muscles, tendons, ligaments, joints (pp. 491, 504) 
 —and, as we shall presently see, from the internal ear. Loss 
 of sensation in the soles of the feet in man leads to disturbances 
 of equilibrium; in animals this does not appear to be so. A cat 
 may have its feet desensitised by division of all sensory nerves 
 with no evident interference with locomotion, and in the solid- 
 footed horse robbing the part of sensation gives similar results. 
 
 The importance of maintaining the body equilibrium, a power 
 markedly lost as the result of injury or disease of the cerebellum 
 (cerebellar ataxia), is explained by the connection which exists 
 between the cerebellum and the internal ear. This connection 
 is the most important path by which afferent impressions 
 associated with the mechanism of equilibration are conveyed. 
 The- nerve from the semicircular canals enters the pons and 
 connects with the nucleus of Deiters, which in turn connects 
 with the cerebellum. Through the nucleus of Deiters the 
 vestibular nerve is brought into indirect communication with a 
 nucleus which gives rise to the nerves supplying the muscles of 
 the eyeball, and by means of the vestibulo-spinal tract (see 
 p. 500) it establishes connection with the motor cells in the spinal 
 cord. The influence of the semicircular canals on equilibration 
 will receive consideration in treating of the organs of special sense; 
 briefly it is due to afferent impressions received by the cerebellum 
 from the semicircular canals that the cerebellum is enabled to 
 judge of the position of the body in space. The receptors of the 
 labyrinth and the deep receptors of joints and muscles are 
 stimulated by the animal itself, such as by movements of the 
 joints, while the surface receptors (extero-ceptors) are stimulated 
 from without. The deep receptors (proprio-ceptors) of a limb 
 can, however, influence only the tonicity of the muscles of that 
 limb, while those of the labyrinth influence not only the limbs 
 and the trunk, but the head and muscles of the eyeball. In 
 consequence the limb influences are mainly concerned with the 
 relation of the limbs to other portions of the trunk, while the 
 influences from the labyrinth regulate the position of the body 
 
5i8 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 generally towards the external world and the anti-gravitational 
 reflex of standing (Sherrington). 
 
 The maintenance of equilibrium and the sense of direction are 
 associated with movements of the eye muscles, the labyrinth 
 furnishing the necessary information as to the relationship of 
 the body to surrounding objects. One of the earliest indica- 
 tions of disease of the spinal cord may be an inability to walk 
 in the dark, and we have seen that a dog without a cerebellum 
 cannot move with the eyes covered. The cerebellum is rurnished 
 with the needful information through its indirect connection 
 with the third and fourth pairs of cranial nerves, which govern 
 the movements of the eyeball. It would appear that the biped 
 derives more information from perceiving the position of the 
 limbs relatively to the body than is afforded the quadruped, 
 whose eyes are in advance of the limbs. The writer has never 
 known a horse refuse to advance, even on the darkest night, 
 provided the ground be good. If it is broken, he travels with 
 care; if boggy, he may refuse to advance. Without seeing his 
 limbs or surrounding objects, he moves over unbroken ground 
 as safely in the dark as in the light, while his sense of direction 
 over ground he has previously travelled is so excellent that the 
 man who has lost his way may safely leave the solution of the 
 problem to his horse. 
 
 Each half of the cerebellum controls the muscular system on 
 its own side of the body, and each half receives impulses from 
 the opposite cerebrum. It appears likely that definite regions 
 of the cerebellum govern definite body areas. It can be shown 
 experimentally that a particular region of the cerebellar cortex 
 controls movements of the fore-limb, another governs a hind- 
 limb ; others cause rolling or bending movements of the body, or 
 conjugate movements of the eyes. The posterior portion of the 
 vermis is concerned mainly in muscular co-ordination, and in 
 some of the lower animals is the only portion of the cerebellum 
 represented. 
 
Section 6. 
 The Cerebrum. 
 
 The cerebrum is composed of grey and white matter, the grey 
 being externaUy placed like a mantle and thrown into convolu- 
 tions. The use of the convolutions is to increase the area of 
 the grey matter, and their depth has been supposed to bear 
 some relationship to the intelligence of the animal. There are 
 many animals whose cerebra are quite smooth ; there are others, 
 like the bear, seal, and whale, whose cerebra are much more 
 convoluted than in man. 
 
 Weight of brain in relation to body weight has been suggested 
 as a measure of intelligence, and when the comparison is made 
 among animals of the same group it may be possible that the 
 heavier brains are the more intelligent. Weight of brain may, 
 however, depend upon such factors as fluid, white matter, 
 and grey substance. Colin compiled an elaborate series ot 
 tables of weight of the cerebral nervous system in animals, and 
 showed that the proportion of brain to body weight was as 
 follows : 
 
 Man 
 
 Cat 
 
 Dog - 
 
 Rabbit 
 
 Sheep 
 
 Ass - 
 
 Pig 
 
 Horse 
 
 Ox 
 
 to 52 
 99 
 235 
 295 
 317 
 332 
 369 
 593 
 682 
 
 For every kilogramme of body weight — 
 
 Man requires 19-00 grammes of brain. 
 
 Cat requires 
 Dog requires 
 Rabbit requires 
 Sheep requires 
 Ass requires 
 Pig requires 
 Horse requires 
 Ox requires 
 
 n-37 
 4-80 
 
 3-3 1 
 3-00 
 2-46 
 1-90 
 1-68 
 1-47 
 
 In proportion to its size, a mouse has more brain than a man, 
 thirteen times more than a horse, and eleven times more than an 
 elephant; a cat has much more than a dog; a rabbit nearly 
 approaches a dog. It is evident that outside the animal group the 
 
 519 
 
520 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 relative weight of brain to body is of limited value as a standard 
 of intelligence. The modem method of determining the mind of 
 an animal from the structure of the cortex of the cerebrum will 
 be dealt with later on. 
 
 Structure of the Cortex. — The cerebral cortex is made up 
 of several layers of cells, the number of which is not agreed 
 upon by histologists, but it may be regarded as consisting of 
 four principal layers. These layers vary in different animals, 
 and we are compelled, as in other portions of the physiology of 
 the nervous system, to fall back upon what is known of the 
 subject in the dog, pig, and man, in the absence of direct infor- 
 mation regarding the horse. The layer of cells immediately 
 beneath the pia mater is spoken of as the molecular ; it consists 
 of some very small nerve-cells with their dendrites and axons, 
 also of dendrites projecting from cells deeper seated in other 
 layers, and the terminal processes of axons belonging to fibres 
 coming from other regions. Perhaps the distinct feature of the 
 layer is the structure first spoken of, the small cells with their 
 dendrites and axons. These begin and end in this layer, and 
 appear to be of a linking-up, or, as it will be described later, 
 associative nature. In the second layer are small pyramidal 
 cells, with the apex towards the surface and the axon passing 
 inwards to the white substance. The depth of this layer in- 
 creases the higher the animal scale is ascended. Next comes 
 a layer of large pyramidal cells, sometimes called ' giant pyramids 
 of Betz,' found mainly in that portion of the brain anterior 
 to the fissure of Rolando,* and in particular in the region of that 
 fissure. These cells are of great physiological importance, as 
 from them the fibres constituting the great motor tracts are 
 derived. The last layer is that of polymorphous cells, which are, 
 generally speaking, small, and many of them fusiform in shape. 
 With these are cells of Golgi of the second type; but whereas 
 the axons of the former pass inwards to form white fibres in 
 the medullary portion, the axons of the Golgi cells pass outwards 
 and end by arborising in the molecular layer. The functions of 
 the cerebral cortex have been examined, not only by the method 
 of direct experimental excitation, but also from the histological 
 side ; this has enabled the cortex to be mapped out into different 
 regions, and in this connection it is desirable to make clear 
 that the histological method does not wholly consist in com- 
 paring the structure of one fully-developed area with another, 
 but also in studying the periods at which the fibres leading to the 
 
 * It is convenient for the present to retain the term ' fissure of Rolando ' ; 
 it will be pointed out later that there is no fissure of Rolando in the dog, 
 nor is the crucial fissure of this animal, in the vicinity of which the motor 
 area is located, the equivalent of the fissure of Rolando. 
 
THE NERVOUS SYSTEM 
 
 521 
 
 area assumed their sheath of myelin, both in the embryo and 
 young animal. This line of research is especially associated with 
 the name of Flechsig. The study of the structural features of the 
 brains of imbeciles and of other pathological conditions has also 
 largely contributed to a more exact knowledge of the uses of the 
 various parts of the cortex. 
 
 The structure of the cortex points to its being a collection of 
 reflex arcs, the complexity of which differs according to the 
 animal scale. The cortices of man and of the frog possess the 
 
 Fig. 151. — Pyramidal Cells of Cerebral Cortex in Different Animals 
 (Donaldson, after Ramon y Cajal). 
 
 A — D shows the several degrees of complexity in the fully developed pyramidal 
 gv. cells in different vertebrates: A, frog; B, lizard; C, rat; D, man. a — e shows 
 
 , the development of the pyramidal nerve-cells of the cerebral cortex in a 
 typical mammal: u, neuroblast with commencing axon; b, ^ dendrites 
 
 ! appearing; d, commencing collaterals. 
 
 same anatomical elements, but the complexity of the neurones 
 of the former are in marked contrast to the simplicity which 
 distinguishes those of the frog (see Fig. 151). A rich system of 
 dendrites means more numerous and complex connections, 
 and evidence has recently been brought forward to show that 
 there are structural peculiarities accompanying specialisation 
 in function, to which attention will be drawn later. 
 
 In the white substance of the cerebrum are nerve-fibres con- 
 veying impulses to the cortex, either from different parts of 
 itself or from the basal ganglia, bulb, and cord. There are also 
 
522 A MANUAL OF VETEBINARY PHYSIOLOGY 
 
 present efferent fibres which have their origin in the cells of the 
 cortex, and which pass to different parts of the brain, or out 
 of the brain via the bulb and cord. Fibres which pass from 
 one part of the cortical convolutions to another belong to the 
 linking or association system (Fig. 152). Fibres which pass to 
 the opposite side of the brain via the corpus callosum belong to 
 the commissural system, while the masses sweeping out from the 
 cortex and connecting it with the mid-brain, pons, bulb, and 
 spinal cord, belong to the projection system. In Fig. 153 the 
 projection system in the human brain is shown. It is not wholly 
 efferent— that is to say, it does not consist entirely of motor 
 fibres passing out to the pons, bulb, and spinal cord. The motor 
 
 Fig. 152. — Association Fibres (after Starr). 
 
 Human cerebral hemisphere seen from the side. A, A, Association fibres between 
 adjacent convolutions; B, between frontal and occipital lobes; C, cingulum 
 connecting frontal and temporo-sphenoidal lobes; D, uncinate fasciculus be- 
 tween frontal and temporal regions ; E inferior longitudinal bundle between 
 occipital and temporo-sphenoidal lobes; O.T., optic thalamus ; C.N., caudate 
 nucleus. 
 
 tracts form only a part of the projection system, which in 
 addition contains sensory, visual, and auditory tracts, and 
 fibres travelling from the frontal lobes to the cerebellum. 
 
 The Great Efferent or Motor Path in man is formed, as we 
 have just seen, from the large pyramidal cells in the cortical 
 layer of that portion of the cerebrum anterior to and bordering 
 on the fissure of Rolando, precentral gyrus, and travels back- 
 wards and downwards through the corona radiata, internal 
 capsule, and peduncles of the cerebrum to the pons (Fig. 153), B. 
 From this large tract are detached the motor fibres connected 
 with the cranial nerves, and these cross to the opposite side of 
 the brain. The larger mass sweeps onwards through the bulb, 
 
THE NERVOUS SYSTEM 5 2 3 
 
 and there in the pyramids the system decussates, the fibres 
 crossing to the opposite side of the cord. In the cord, cell connec- 
 tions are formed all the way along, either by means of collaterals 
 or by the fibres actually terminating; in either case synapses are 
 formed around the cells in the ventral horns of grey matter. 
 From these axons arise which eventually form the motor roots 
 of the spinal nerves. From the motor roots the fibres pass to the 
 muscles, where they terminate in end-plates. It will be observed 
 
 Fig. 153. — Paths from Cortex in Corona Radiata, Human (Starr). 
 
 A, Tract from frontal convolutions to nuclei of pons and so to cerebellum; 
 B, motor pyramidal tract; C, afferent tract for tactile sensations (represented 
 in the diagram as separated from B by an interval for the sake of clear- 
 ness); D, visual tract; E, auditory tract; F, G, H, superior, middle, and 
 inferior cerebellar peduncles; J, fibres from the auditory nucleus to the 
 posterior corpus quadrigeminum; K, decussation of the pyramids in the 
 bulb; FV, fourth ventricle. The Roman numerals indicate the cranial nerves. 
 
 that there are two neurone systems in this path, one between 
 the cortex and the cord (pons for the facial muscles), the other 
 between the cord and the muscle. From the moment these fibres 
 start on their journey from the cortex connections are constantly 
 being formed with neighbouring structures by means of synapses, 
 not only in the cortex, but in the mid-brain, pons, bulb, and, 
 as we have seen above, in the cord. It has even been considered 
 that in their passage along the cord the two tracts exchange 
 
524 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 fibres. In this way is formed the tract for voluntary body 
 movement which we have studied at p. 499 under the name of 
 the ' crossed pyramidal tract.' It is the path by which move- 
 ments of which the animal is distinctly conscious are effected. 
 The brain starts or instructs the movement, the mechanisms 
 in the cord carry it out. The apparently purposeful muscular 
 movements initiated by the cord of the animal immediately after 
 death warrants the supposition that quadrupeds are in much the 
 same position as the decerebrated frog. The limbs are repre- 
 sented in the cortical area of the fissure of Rolando, and they are 
 also repiesented in the grey matter of the spinal cord, the great 
 difference between the two centres being that one is exercised 
 voluntarily, while the other is involuntary, until the moment 
 arrives when the pulling of the needful switch brings it under the 
 control of the higher centres. 
 
 We shall see presently that the entire cerebral cortex may be 
 removed from the frog, pigeon, and dog, and the motor area 
 from the sheep, without producing in these mammals more than 
 a marked loss of power, which is regained in time. In such cases 
 the centres in the cerebrum controlling the muscles are entirely 
 cut off from them, and other motor tiacts have to be found. In 
 the dog it is suggested that the rubro-spinal tract (p. 500) fulfils 
 this function. In the sheep we have seen (p. 500) that motor 
 fibres are only in part derived from the cerebral cortex, and that 
 a subcortical supply is furnished by the mid-brain, pons, and 
 medulla. There can be little doubt that a similar arrangement 
 exists in the other herbivora. 
 
 The neurone relay between cortex and end-plate is the ex- 
 planation of a peculiarity observed in muscular paralysis in 
 man. When the neurone between the cord and the muscle is 
 affected, the paralysis is complete; but when the pyramidal 
 neurones — i.e., those between the cortex and the grey matter of 
 the cord — are alone affected, there is a path left open for reflex 
 stimulation, and those impulses normally passing to muscles 
 from the cerebellum, which have been spoken of as ' tonic,' 
 throw the paralysed limbs into a condition of continuous con- 
 traction. This is spoken of as spastic paralysis. 
 
 The Great Afferent or Sensory Path conveys impulses evoking 
 sensations — -heat, cold, touch, and pain — -also such senses as are 
 not normally recognised — viz., muscle sense, joint, tendon, and 
 viscera sense, and the senses of vision, hearing, smell, and taste: 
 This path is broken by at least three sets of synapses: (1) in the 
 cord; (2) in optic thalamus; and (3) in cortex cerebri. The optic 
 thalamus is practically the meeting-place or junction of the 
 whole afferent system on its way to the cerebral cortex (Fig. 147). 
 
 Impulses conveyed by sensory nerves pass through the spinal 
 
THE NERVOUS SYSTEM 525 
 
 ganglia, enter the spinal cord, and reach the medulla by the 
 dorsal columns of the cord. In the dorsal columns of the cord 
 they travel on the side on which they enter, and finally reach the 
 medulla in the gracilis and cuneate nuclei, where they arborise. 
 Through the fresh fibres arising from the nerve-cells of these 
 nuclei the impulses cross to the other side of the medulla, and 
 by means of the fillet reach the mid-brain and optic thalamus. 
 From the thalamus the fibres of the third sensory neurone are 
 distributed by means of the internal capsule to that portion of 
 the cerebral cortex lying behind the fissure of Rolando — viz., 
 the post-central gyrus. Impulses which, though afferent, are 
 probably not sensory, but for reflex work, gain the medulla via 
 Clarke's column. The spinal roots on entering the cord arborise 
 around the cells in this column, and by means of the fibres form- 
 ing the dorsal and ventral cerebellar tract they reach the brain 
 as follows: The dorsal (direct) cerebellar tract passes by means 
 of the restiform body into the superior part of the vermis, while 
 the ventral cerebellar tract runs along the lateral columns 
 to the pons, passes beneath the roots of the fifth nerve, and then 
 bends back to end in the superior vermis of the cerebellum. 
 It is by means of the lateral columns that pain-provoking im- 
 pressions are conveyed from both halves of the body. These 
 impulses having reached the pons, pass to the optic thalamus, 
 which is the sensory relay station to the cortex and the seat of 
 pain. 
 
 The Association System of fibres is employed in bringing the 
 various parts of the brain into connection — convolution with 
 convolution, lobe with lobe, cerebrum with mid-brain, cere- 
 bellum, and pons. Some fibres, the commissural, connect 
 the right side with the left in all its anatomical parts, the 
 corpus callosum being the largest member of this system. The 
 system of association fibres links up the various parts of the 
 brain and affords routes innumerable for the passage of impulses 
 to and fro . It has been previously mentioned that the complexity 
 of the dendrites of the cells bears some relation to the scale of 
 brain development in the animal kingdom ; it may now be noted 
 that the more profusely branched the dendrites the larger the 
 number of paths there are in the association system. 
 
 Functions of the Cerebrum. 
 
 The cerebrum is without sensation ; it can be handled, cut, or 
 otherwise injured without any sign of pain being elicited. For 
 many years it was considered, in consequence of the experiments 
 of Flourens, that the cerebrum was a homogeneous organ, 
 and all of its parts functionally of the same character. It was 
 
526 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 known to be connected with the. higher faculty of intelligence, 
 but that it was an organ containing within itself other organs 
 functionally distinct, yet intimately connected, was never 
 anticipated. 
 
 We have seen that the decerebrated frog is a reflex machine, 
 but from animals so low in the scale it is difficult to obtain 
 any real knowledge of the function of the organ. 
 
 Removal of the Cerebrum. — The pigeon with the cerebrum 
 removed becomes converted into a drowsy, lethargic animal, 
 unable to feed itself, though it may try in an aimless manner 
 to do so. It can fly and perch, may be awakened by a loud noise, 
 shows no fear, and is not possessed of maternal feeling. It may 
 awake sufficiently to preen itself and gape, and then once more 
 returns to its condition of somnolence. __ So far as can be 
 judged, the chief loss this animal has experienced is that of 
 memory. 
 
 Colin removed the cerebrum in horses. He notes that the 
 animal fell almost before the superficial layer had been taken 
 away. A horse so dealt with was unaffected by stimulation, 
 gave no indication of feeling pain, noises were not heard, light 
 had no effect upon the pupil, and there was no eyelid reflex. 
 Ammonia applied to the nostrils caused no irritation, and taste 
 and all special senses were lost. In the case of the decerebrated 
 heifer the animal was able to stand, but could not see; hay was 
 held in the mouth and not masticated,* and no notice was taken 
 of the blowing of a horn. 
 
 If partial destruction of one hemisphere in the horse be prac- 
 tised, the animal may be kept on its legs for a short time. The 
 superficial layer of the right cerebral lobe was removed in the ass, 
 and the animal remained standing for nearly an hour. The 
 limbs on that side were slightly bent under the influence of the 
 body weight, and the animal moved with difficulty, and, if left 
 to himself, remained immobile. When made to walk he walked 
 very quickly, now and then in a circle in the direction opposite 
 to the lesion. If he walked into anything he fell, but could be 
 raised. In another ass the left hemisphere was incised in the 
 direction of its length, and immediately hemiplegia occurred 
 on the right side; the animal fell on that side, and could not be 
 got up again. In many horses a puncture through the entire 
 thickness of the cerebral lobes sometimes sufficed to cause the 
 animal to fall and to be unable to rise. Bovines withstood far 
 better than equines mutilation of the cerebral lobes. A heifer 
 with one cerebral lobe removed remained standing for more than 
 half an hour, retained its vivacity, and walked with such ease 
 that it was extremely difficult to observe the muscular deficiency 
 
 * A common symptom in the horse suffering from brain trouble. 
 
THE NERVOUS SYSTEM 527 
 
 on the side opposite to the lesion. When the remaining lobe was 
 removed, she could still stand up, but could move only forwards, 
 backwards, and turn with great difficulty. 
 
 Injury to one cerebral peduncle in the rabbit was found by Longet 
 to lead to menage movements towards the opposite side ; but if the 
 peduncle were cut through, the movements did not occur. Colin 
 found that menage movements were performed by the horse when 
 a cerebral peduncle was injured. He pricked the left peduncle, and 
 observed that the head was at once carried to the right, and the 
 neck and body bent, so that the muzzle became applied to the flank, 
 and sometimes to the thigh, as if the animal were bent in halves; 
 the limbs were gathered in a bunch under the body, and the animal 
 rotated on a small pivot. When turning, it frequently fell, but the 
 body still remained curved; when raised, the circular movements 
 were repeated. The horse could go backwards, but could neither 
 advance nor turn to the left. 
 
 Colin destroyed the corpus callosum, and observed that he pro- 
 duced neither pain nor convulsions. Longet cut into it throughout 
 its length in young horses and young rabbits, and found there was 
 no sign of pain. The animals remained standing, but could walk 
 or run about if made to. Colin repeated the above experiment on 
 two horses. Both remained standing for a short time; one then fell 
 backwards after some movement of the muscles of the eyeball; the 
 other showed great muscular weakness, and fell on his side. There 
 were no convulsions in either case. 
 
 All these observations by Colin were made before anything 
 was known of the function of the cerebrum in connection with 
 skeletal muscles. By his experiments the crossed nature of the 
 pyramidal tract in these animals was clearly proved, and the great 
 independence of the spinal cord in bovines demonstrated. 
 
 Goltz removed both cerebral hemispheres by successive stages 
 in a dog, and succeeded in keeping the animal alive for eighteen 
 months. It became a most interesting psychical study. The 
 animal was a mere reflex machine. It could see, but not compre- 
 hend; it would show signs of hunger, eat when the nose was 
 brought in contact with the food, but could not recognise food 
 placed near it. After the paralysis succeeding the operation had 
 passed away, the dog could walk slowly and stupidly with its 
 head down. It would growl or bark, turn its head to any spot 
 stimulated, but did not bite. The face was expressionless, and 
 the tail was never wagged. It rejected food of a disagreeable 
 nature, such as meat soaked in a solution of quinine. No matter 
 how hungry it was, this was refused, though the gustatory centres 
 had been removed. The animal slept, but did not dream. . She 
 exhibited no sexual excitement or oestrum. Memory, affections, 
 and the capacity to learn were absent; anger was a prominent 
 feature. Goltz's dog exhibited it every day for eighteen months 
 each time she was fed. 
 
 One observation in the history of this dog is of extraordinary 
 
528 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 veterinary interest. The animal in its roaming on one occasion 
 wounded its hind-foot ; it was then observed to walk lame, holding 
 the injured foot off the ground. It is possible to urge that pain 
 was felt, as some portions ot the brain-stem had been left intact; 
 but Sherrington's remarkable experiment on the decapitated cat 
 disposes of this view.* A cat so treated cannot stand, but it 
 can perform stepping movements with its limbs. ' If, as the 
 preparation lies on its side, one hind-foot be forcibly pinched, 
 this limb is flexed . . . and the other limbs at once begin rapid, 
 co-ordinated stepping movements. The injured foot is held 
 up out of harm's way, and the other legs run away.'f 
 
 Removal of an anterior lobe of the cerebrum in the dog leads 
 to unilateral motor and sensory paralysis; the motor paralysis is 
 recovered from, but the loss of muscle sense remains. Removal 
 of the posterior lobes of the cerebrum leads to blindness; there 
 is no paralysis, sensory or motor; the dog remains obedient, but 
 sluggish. Sherrington cut the brain off from the heart and viscera 
 in the dog by division of the cord in the lower cervical region. 
 The animal showed joy, anger, fear, and sorrow. Nothing would 
 induce it to eat dog's flesh, but in this case the olfactory and 
 gustatory paths were still open; in Goltz's dog they had been 
 removed. That * dog will not eat dog ' is a very old maxim, but 
 travellers in the Arctic have recorded the fact that they will do 
 so under stress. Nevertheless, it is remarkable that both the 
 idiot created by Goltz and the dog deprived by Sherrington 
 of all knowledge of its stomach should refuse to eat their own 
 kind. In this matter they exhibit a far nicer discrimination 
 than man. 
 
 Observations begun on man in the Franco-German War and 
 continued on animals have demonstrated that the cortex, which it 
 must now be made clear is the elaborating part of the cerebrum — 
 as distinct from the white or connecting matter — contains an area 
 which controls the voluntary muscles of the body, and is known 
 as the motor area. Another area connected with body sensi- 
 bility, such as muscle sense, touch, pressure, and temperature, 
 is known as the sensory area, and comprises not only the body 
 senses above mentioned, but the special senses of sight, hearing,- 
 taste, and smell. The position of these areas is known in the 
 sheep, dog, monkey, and man with considerable accuracy, so that 
 
 * Brain, part cxxix., vol. xxxiii., 1910. 
 
 f These astonishing results show that lameness may be purely reflex in 
 origin. The question of the fitness or unfitness of chronically lame cases 
 to perform work will in time have to be considered afresh. The above 
 observations show with what justice the veterinary profession, as the 
 result of careful observation, has urged in a court of law, though generally 
 without success, that an animal suffering from lameness is not necessarily 
 suffering pain. 
 
THE NERVOUS SYSTEM 
 
 529 
 
 maps of the hemispheres have been drawn up indicating the 
 
 function of the various localities; the subject is generally spoken 
 
 of as the localisation of cerebral function. (.. j 
 
 The Motor Area. — A region of the brain of the monkey or 
 
 man, known as the fissure of Rolando, is intimately connected 
 
 with the cells which give rise to the motor nerves passing to 
 
 muscles. In the dog there is no fissure of Rolando, but there is 
 
 a fissure, known as the frontal or crucial sulcus (Fig. 154, c.s.), 
 
 in which the motor cells for the muscles are localised as in the 
 
 fissure of Rolando, and 
 
 n, 
 
 it 
 
 lirf "M 
 
 similar localities must 
 exist in other animals, 
 although little has been 
 done to identify them. 
 
 If the brain in the 
 region of the crucial 
 sulcus of the dog be 
 exposed, and the part 
 stimulated electrically, 
 that muscle, or group of 
 muscles, contracts which 
 is connected with those 
 cells of the cortex im- 
 mediately beneath the 
 electrode. It is remark- 
 able that a great Euro- 
 pean war was necessary 
 to demonstrate this fact, 
 
 hnt nhvsinlnp-ists for n ' Neck; f X > fore - limb ; *■'■■ hind-limb; t, tail; 
 DU pnysiOlOglStS 10, f facg (the position here assigned to the 
 
 Fig. 154. — Motor Areas of Dog's Brain 
 (Stewart). 
 
 years remained in this 
 matter under the influ- 
 
 f acial muscles is unusual ; it is generally in- 
 dicated more laterally and farther forward); 
 ■f +v, + V.' c r,f c - s -> crucial sulcus; e.m., eye movements; 
 ence 01 tne teacmngs 01 ^ dilatation of the pupil in both eyes, but 
 Flourens and Majendie, especially in the opposite eye. 
 who stated that the The areas are marked only on the left side of 
 rprphral rnrtPY was in the figure except the eye areas, whose position, 
 
 cereDrat cortex was in- t0 avoid con f us i on> i s indicated on the right 
 
 excitable. Gradually hemisphere, 
 knowledge derived from 
 
 stimulation of the cortex has accumulated, until a very re- 
 markable picture of the cortical functions can be obtained. 
 Fig. 155 shows the motor area of the chimpanzee; that for 
 man is still more complex. The size of a motor area bears 
 a relation to the degree of complexity of limb movements. 
 The limb movements in quadrupeds are simple; for instance, 
 in the horse they are mainly pendular; in the dog they are 
 more complicated, and still more so in the cat, in which 
 the delicate movements of face- washing are necessarily of a 
 
 34 
 
53° 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Knee 
 
 Anus fy va,6ifi&. 
 
 Fingers 
 t, thumbs _. 
 
 ^'t/cidiure 
 
 of jaw. 
 
 Opening 
 ofjaw. 
 
 SuZcila centralis. 
 
 Voca.L \ 
 
 cords. Mastication 
 
 Fig. 155. — Motor Area of Cortex of Chimpanzee (Grunbaum and 
 Sherrington). 
 
 Lateral aspect of the hemisphere. 
 
 Fig. 156.- 
 
 -Dog's Brain with Lesion 
 (Stewart). 
 
 A portion of the cortex indi- 
 cated by the shaded area was 
 destroyed by cauterisation. 
 The symptoms were complete 
 blindness of the opposite eye 
 (in this case the right); weak- 
 ness of the muscles of the 
 limbs and of the neck on the 
 right side; slight weakness of 
 the limbs on the left side. 
 When the animal walked, 
 there was a tendency to turn 
 to the left in a circle. • In 
 eating or drinking, the head 
 was turned to the left, so that 
 the mouth was oblique, and 
 the right angle of the mouth 
 was lower than the left. The 
 tail movements were normal, 
 and there was no deviation of 
 the tail to one side. 
 
 complex type. In those areas which have been mapped out 
 in the dog, the representation of the muscles of the neck is 
 
THE NERVOUS SYSTEM 53I 
 
 at n, Fig. 154; the fore-limb at f.l. ; the hind-limb at h.l. ; 
 face at / (see note on Fig. 154); tail at t; eye movement at 
 e.m. ; dilatation of the pupil at p. The movements of flexion, 
 extension, and rotation of the various limbs, -head, neck, and 
 trunk, and the more complex movements in the higher animals, 
 have each their separate station in the cortex. The more complex 
 and delicate the movements, the larger the cortical representa- 
 tion, so that the head and arm in the ape and man are more 
 largely represented than the trunk and lower limbs, where the 
 movements are of a simpler character. The thumb area is 
 relatively larger than the shoulder or hip area. In Fig. 156 a 
 lesion is shown of the left hemisphere of the dog, involving 
 most of these centres. The symptoms shown are detailed' in the 
 description of the figure. In destructive brain lesions it is 
 observed that muscles, like those of inspiration, which act bilater- 
 ally, are not affected by a unilateral lesion. We have previously 
 drawn attention to the fact (p. 154) that the muscles of the larynx 
 have a double representation in the cortex — viz., that both sides 
 are represented in each hemisphere, and the same holds good 
 for the respiratory muscles of the chest and abdomen.* 
 
 The motor area of the cerebral cortex of one of the larger 
 herbivora has been investigated by Simpson and King.f 
 
 These observers have shown that the motor area in the sheep is 
 situated in the superior frontal convolution, and is very limited. 
 This convolution, with the fissures surrounding it, is shown in 
 Fig. 157, while the positions of the motor areas are seen in Fig. 159. 
 The motor area comprises from front to rear centres for (a) face, 
 mouth, and tongue; (b) head and eyes; (c) fore-limb; (d) hind-limb. 
 Of these, the fore-limb area is the most, and the face area the least, 
 excitable. The hind and fore limb centres are separated by the 
 splenial sulcus ; stimulation above the sulcus produces flexion and 
 extension of the opposite hind-limb and spreading of the digits. 
 Flexion is always more evident than extension. Stimulation below 
 the sulcus produces movements of the opposite fore-limb, flexion in 
 this case also being more frequent than extension. Where the fore 
 and hind limb areas meet, and extending from the splenial to the 
 coronal sulcus (Fig. ^57), is an area common to fore and hind limbs, 
 stimulation of which produces movements of the fore and hind 
 limbs of the opposite side, and sometimes of the hind-limb of the 
 same side, but never of the fore. Stimulation of the head and eyes 
 area leads to turning of the head to the opposite side and conjugate 
 deviation of the eyes . The head movement is deliberately performed 
 as though the animal were turning to look at something behind it. 
 
 * The two-sided representation in each hemisphere is limited to respira- 
 tion. In other cases each hemisphere causes contraction of chest and 
 abdominal muscles strictly limited to its crossed side, and this is true 
 even of the perineal muscles and sphincter ani (Sherrington). 
 
 t ' Localisation of the Motor Areas in the Sheep,' Professors Sutherland 
 • Simpson and J. L. King, Quarterly Journal of Experimental Physiology, 
 vol. iv., No. 1, 1911. 
 
532 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Great longitudinal fissure 
 
 Superior frontal, 
 convolution 
 
 Middle frontal 
 convolution 
 
 Sylvian 01 arcuate, 
 convolution 
 
 Fourth parieto- 
 occipital convolution 
 
 Third parieto- 
 occipital convolution 
 
 Second parieto- 
 occipital convolution 
 
 First parieto- 
 occipital convolution 
 
 Cerebellum 
 
 Coronal sulcus 
 
 Splenial sulcus 
 Cruciate sulcus 
 
 Ascending limb of 
 Sylvian fissure 
 
 Supra-Sylviau fissurt 
 
 'Lateral fissure 
 
 Intermediate sulcus 
 Medial sulcus 
 
 Fig. 157, — Brain of Sheep, Dorsal Aspect (Simpson and King). 
 
 8 7 
 
 Fig. 158. — Cerebrum of Horse, showing Motor Areas (Arloing). 
 
 Movements of limbs; _:, closing movements of jaws; 3, movements of nostrils 
 and npper lip; 4, retraction of the tongue; 5, depression of jaws and flexion 
 of the neck to the opposite side; 6 and 7, blinking and closing of the eyelids; 
 8, elevation of the upper lid and adduction of the ear. 
 
THE NERVOUS SYSTEM 
 
 533 
 
 Below this area is found that for the face, tongue, and lips, which is 
 relatively the least excitable of all, and of which the localisation is 
 the most uncertain. Stimulation produces lip movements, pro- 
 trusion and retraction of the tongue, but attempts at mastication 
 are rarely made. In lambs this centre is connected with that of 
 sucking, even to the peculiar tail-movements which accompany 
 the act in this animal. No tail-movements can be evoked from 
 this area unaccompanied by sucking. 
 
 The exposed cerebrum was found, as in other animals, to be 
 deficient in sensation, painful or otherwise; and extirpation of the 
 motor area led to no limb paralysis. The animal walked as well 
 without the superior frontal and middle frontal convolutions as 
 with them. Further, it was quite unaffected as the result of the 
 experimental procedure, and, when liberated, at once began to eat. 
 
 Face, mouth, and 
 tongue 
 
 Head and eyes 
 
 Fore-limb 
 
 Hind-limb 
 
 Fig. 159.— Diagram showing Position of Motor Areas on Dorsal Aspect, 
 Brain of Sheep (Simpson and King). 
 
 The motor areas in the horse as determined by Arloing are 
 shown in Fig. 158. As with the sheep, a remarkable feature is 
 their comparative unimportance, which has long been anticipated 
 clinically (see below). We have already seen that the chief 
 outflow of motor fibres from the brain of the sheep, and pre- 
 sumably of other herbivora, has a subcortical origin from 
 the mid-brain, pons, and medulla (p. 500). 
 
 The effect of removing the motor areas varies according to 
 the species of animal. In the monkey it results in permanent 
 motor paralysis of hand or foot, but not of parts with less skilled 
 movements— e.g., shoulder or knee. In the sheep, as we haye 
 
534 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 just seen, there is no paralysis. In the dog paralysis is not 
 necessarily produced, and it has been supposed that the basal 
 ganglia are capable in this animal of taking up the duties of the 
 cortex. The destruction which has been observed at times in 
 the cortex of the horse is commonly unaccompanied by any 
 symptoms until shortly before death. Colin draws attention 
 to the difficulty of producing paralysis experimentally in the 
 horse from lesions of the hemispheres. Neither the artificial 
 production of a clot in the falciform sinus nor the introduction 
 of pieces of lead the size of a pea into the convolutions gave 
 rise to hemiplegia. This quite bears out a frequently observed 
 clinical fact, that it is possible for horses to have tumours the 
 size of an egg in their lateral ventricles without exhibiting any 
 disturbance. The writer has seen several such cases, the tumours 
 being of variable size, and the clinical history has never given 
 more than a few days' illness, though the growths must have 
 been forming for a considerable period. 
 
 Strong stimulation of the motor areas produces epilepsy. By 
 observing the groups of muscles first affected and knowing the 
 region of the cortex to which they are related, it is possible 
 in man to localise with considerable exactitude the seat of this 
 trouble. 
 
 The Sensory Areas. — It will be remembered that these serve 
 the higher senses of vision, smell, taste, and hearing, as well as the 
 senses of the lower order, touch, temperature, and muscle sense. 
 The term ' sensory ' is employed in its physiological sense. 
 The lower order of afferent impulses are those of which we are 
 generally unconscious. Of muscle sense we are, perhaps, under 
 normal conditions, wholly unconscious; towards the perception of 
 heat and cold the consciousness can be awakened, but these are 
 ordinarily unrecognised unless the stimulation is severe. The 
 areas connected with temperature and muscle senses are situated 
 above the crucial sulcus. The Centre for Vision is situated in the 
 occipital lobe, and destruction of the centre, say on the right 
 side, is followed by blindness in the two right halves of the 
 retinae in those animals where decussation of the optic nerves 
 is incomplete. In those in which it is complete a right-brain 
 lesion leads to a left-eye blindness. We shall deal again with 
 this centre in considering the subject of vision. The Auditory 
 Centre has been located in the temporal lobes, and the' facts con- 
 nected with it will be considered in the chapter on hearing. 
 The auditory and visual centres are capable of eliciting a response 
 in the motor areas, for the ears are pricked, and the head and 
 eyes turned towards an object or a sound, and therefore con- 
 necting paths between the special sense centres and the regions 
 in the motor area, connected with the muscles in question, 
 
THE NERVOUS SYSTEM 535 
 
 are believed to exist. The Olfactory Centre : In animals with 
 the sense of smell acutely developed the olfactory bulb and 
 tract are large. The dog, rabbit, horse— in fact, most, if not 
 all, of the domestic animals — have the sense of smell highly de- 
 veloped. It is not necessary here to consider the reason in detail, 
 but it is obvious that in the natural state the whole question 
 of their lives hinges largely on the question of smell; food, the 
 presence of an enemy, sexual instinct, are capable of exciting 
 in the cortex psychical reactions, which, in the higher animals, 
 are brought about through other channels. In the primitive 
 brain, swellings first develop in connection with smell, and in 
 the process of evolution the cortical centre for smell was one of 
 the first to be established. In the brain with no cerebrum — ■ 
 as, for instance, in the brain of the shark- — the olfactory centre, 
 contrary to the general rule in fishes, is immensely developed. 
 Fishes have excellent sight, but generally no sense of smell; 
 but the shark lives by the sense of smell, and in the old type of 
 brain with which it is furnished the olfactory nerves spread out 
 into an area of considerable size. In the new type of brain — 
 viz., one with cerebral hemispheres — the olfactory lobes form 
 the principal part of the earlier structures, and it is noteworthy 
 that their fibres pass direct to the cortex; all other paths con- 
 nected with special sense organs have first to pass through the 
 corpus striatum. In man, who has a poor sense of smell, they 
 have atrophied, and in the whale they have practically disap- 
 peared. The cortical centre for smell lies in the region of the 
 hippocampus and temporal lobe, but the central connections of 
 the olfactory apparatus are not fully known. The Centre for 
 Taste is ultimately mixed up with that of smell, but its precise 
 locality is unknown. With the Centre for Speech we have nothing 
 to do. It is interesting to know that stimulation of the corre- 
 sponding area in the ape does not lead to the production of voice.' 
 
 When the sense and motor areas are removed from the cortex, 
 there is a considerable amount of substance left not associated 
 with either of these functions. These have been termed by 
 Flechsig association areas ; the term silent areas, employed by 
 some physiologists in speaking of them, appears especially 
 suitable to veterinary physiology. These association or silent 
 areas are the region of the higher intellectual faculties, the organs 
 of thought, the region in which impressions conveyed to the 
 sense centres are interpreted, for it is through the sense centres 
 that intelligence is developed. An association area in the 
 anterior part of the frontal region of man is an important intel- 
 lectual centre, though of secondary importance to one situated 
 in the parietal region posterior to the fissure of Rolando. 
 
 It will be observed that up to this point the functions of the 
 
536 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 various parts of the cerebrum have been largely determined by 
 direct experiment or pathological observation. These, however, 
 as we indicated previously, are not the only methods of inquiry 
 open. Flechsig's embryological method has furnished important 
 results. It will be remembered that it is based on a knowledge 
 of the periods at which the fibres in the various tracts acquire 
 their medullary sheath. In the human embryo the nerve-fibres 
 are sheathed three or four months after the axis cylinder is formed, 
 and the order in which this occurs appears invariable. The 
 afferent fibres are first myelinated, then the efferent, and lastly 
 the association fibres; this holds good for the nerves of the 
 whole body. At birth the human infant has the afferent 
 system myelinated, but not all the efferent. This, of course, does 
 not apply to those animals which are born with the power of 
 locomotion, but the matter is deeply interesting; also the fact 
 that the functional activity of nerve tracts is largely dependent 
 on the amount of myelin present. In man the myelination of 
 the fibres of the brain goes on for years after birth, but ceases at 
 forty, and in old age diminishes. We have indicated but briefly 
 the field which awaits the veterinary histologist of the future; 
 in the absence of direct experimental inquiry- — which nothing 
 can supplant — important information may be gained by an 
 application of the methods of Flechsig. 
 
 Function revealed by Structure. — The development of nervous 
 tissue from its earliest laying down to its maturity has, as we have 
 seen, been turned to account in the interpretation of its function. 
 The next step, on somewhat similar lines, was the interpretation 
 of function by a comparison of the structure of the various parts 
 of the cortex in normal and insane persons. Of the structure of the 
 cortex an outline has already been given; the four, or, as some prefer, 
 six, layers of cells and fibres into which this has been divided have 
 been given another classification by Bolton, * which has been generally 
 adopted. It is spoken of as the five-layered type of cortex, and the 
 following are described in man : 
 
 i. Outer fibre lamina ; Two-thirds developed at birth. 
 
 2. Outer cell lamina : Half developed at birth. 
 
 3. Middle cell lamina: Three-fourths developed at birth. 
 
 4. Inner fibre lamina ; Two-thirds developed at birth. 
 
 5. Inner cell lamina • Fully developed at birth. 
 
 Watsonf has studied the development of these layers in many 
 orders of mammals. He shows that layers 4 and 5 are the earliest 
 to appear, and whether in the highest or lowest mammals there is 
 very little difference in their thickness (Fig. 160). Layer 3 is the 
 
 * ' Further Advances in Physiology,' edited by L. Hill, M.B., F.R.S. 
 Goulstonian Lectures, J. S. Bolton, M.D.: Brain, part cxxix., vol. xxxiii., 
 1910. 
 
 f ' The Mammalian Cerebral Cortex, with Special Reference to its 
 Comparative Histology,' G .A. Watson, M.B., C.M., Archives of Neurology, 
 vol, iii., 1907. 
 
THE NERVOUS SYSTEM 
 
 537 
 
 next in order of development. Layers i and 2 are the last to appear, 
 and the slowest in development. In the lowest mammals layer 2 is 
 generally poorly developed, and even at best obtains but a slight 
 absolute depth as compared with the human brain. 
 
 The great difference between the human brain and that of the 
 insectivora lies chiefly in the degree of development of layer 2, 
 whereas, notwithstanding the difference between the brain of man 
 and that of the mole — one of the smallest in the mammalian, series-^ 
 layers 3, 4, and 5 are of much the same thickness (depth) in each 
 
 Molecular 
 
 Supra-graDulcir 
 (pyramidal) 
 
 Granular 
 
 Infra- grcmuhr 
 
 NORMAL 
 HUMAN ADULT 
 
 I 
 
 n 
 
 277 
 
 •356 
 
 1077 
 
 THE MOLE 
 
 538 
 
 Tola! deplh 1-892. 
 
 V "344- 
 
 f 34-1 
 
 838 
 
 Fig. i6o.- — The Approximate Relative Depths ofTthe^Cerebral Cortical 
 Layers in Man and the Mole (Watson.) 
 
 animal. Fig. 1 60 illustrates approximately the relative depths of the 
 cerebral cortical layers in the normal human adult and the mole (after 
 Watson-). From the point of view of intelligence, layer 2 is of the 
 utmost importance; it is under-developed to different degrees in 
 idiots and imbeciles, and in chronic and recurrent lunatics the same 
 fact is observed, though in a lesser degree. The examination of the 
 various layers of the cortex in cases of congenital blindness has 
 shown that certain portions of layer 3 are atrophied; this has 
 enabled the cortex to be mapped out into a projection centre for 
 visual impressions, known as the visuo-sensory area. At the peri- 
 
55« A MANUAL OF VETERINARY PHYSIOLOGY 
 
 phery of this area is another, which can be distinguished by an 
 abrupt modification in the arrangement of the layers. It is known 
 as the visuo-psychic ; it undergoes no change in congenital blindness, 
 and therefore has no visuo-sensory function. The visuo-psychic 
 layers are not affected in dementia. A third region of the cortex is 
 the prefrontal ; it is the last to reach maturity, and is the region 
 specially affected in mental disease. Its cell layer, No. 2, has been 
 previously referred to as showing marked retrogressive changes in 
 mental affections. Layer No. 2 is the most important of the human 
 brain, and it is the only one which varies in depth in normal in- 
 dividuals. 
 
 The functions of the cell laminas of the cortex, according to the 
 observers previously mentioned, and generally accepted by neuro- 
 logists, are as follows : 
 
 Layers 1 and 2 carry out the psychic, or associational, functions of 
 the cerebrum; they represent educability and general intelligence. 
 Hence they are rudimentary in the insectivora, better developed in 
 rodents, again better developed in ungulates and carnivores than in 
 rodents, and markedly more developed in the primates than in the 
 carnivores. They have to do ' with all those activities which it is 
 obvious the animal has acquired (or perfected) by individual experi- 
 ence, and with all the possible modifications of behaviour which 
 may arise in relation to some novel situation; hence, with what is 
 usually described as indicating intelligent as apart from instinctive 
 acts, the former being not merely accompanied, but controlled, by 
 consciousness ' (Watson). The region of the brain where these pro- 
 cesses are carried on is mainly in the prefrontal, or region of higher 
 associations. Of less importance in this respect is the visuo-psychic, 
 or region of lower associations ; and of least iiirportance is the visuo- 
 sensory or projection-sphere region. Layer 3 deals with the recep- 
 tion or transformation of afferent impressions, whether from within 
 or without the brain. Layers 4 and 5 govern the lower voluntary 
 and instinctive activities of the body, which require neither experience 
 nor education. They deal with the preservation and perpetuation 
 of the species, with such things as food, shelter, and sexual consort, 
 accompanied, though not necessarily controlled, by consciousness. 
 Preservation is deeply, firmly, and early implanted in the brain of 
 the lowest mammals, and in them is as fully, developed as in' man. 
 It is the common platform on which they meet man. In Fig. 161 
 is shown the microscopical structure of the Betz-cell area in man 
 and the dog, and the motor area of the pig, the depth of the laminse 
 in each case being indicated.* 
 
 Coverings of the Brain. — The dura mater is a dense fibrous 
 membrane, which acts the part of a protective covering for the 
 brain ; between it and the arachnoid there exists a lymphatic space 
 known as the subdural. The arachnoid contains but few vessels 
 and no nerves, and covers the extremely vascular pia mater; 
 between the arachnoid and the pia mater is formed the subarach- 
 noid space, which contains the subarachnoid or cerebral fluid. 
 
 Cerebral Fluid. — The cerebral fluid is secreted by the pia 
 mater and choroid plexus; in horses it is normally small in 
 
 * For these I am indebted to Dr. G. A. Watson. The structure of the 
 cortex of the brain of the pig had not previously been investigated. 
 
IwPI'''. f ' ' 
 
 
 LI 
 
 -Hi 
 
 •>■:■;' it i\ l * ".] 
 
 »'...vj - , * • V »'. j 
 
 LI 
 
 LI1 
 
 ...-.F! 
 
 *!-■ <-■«&- 
 
 L 1 
 
 H&Xi 
 
 > L.II 
 
 
 L.III 
 
 LIViV 
 
 i>'*';" 
 
 
 
 3i 
 
 
 
 • ; v ■ 
 
 V- ' 
 
 - 4 * 
 
 
 ViVl 
 
 >« 
 
 - / . Si 
 
 
 •* : v ; ; 
 
 . * ! 
 
 t • * 
 
 -• 
 
 ■-. 
 
 >";. 
 
 " -, . ■ ," ■ 
 
 ■ '". T 
 
 - _ -> 
 
 ■;• ;: i 
 
 VL.IVtV 
 
 (1) Man. (2) Dog. (3) Pig. 
 
 X 74 diameters. X 89 diameters. X 109 diameters. 
 
 Fig. 161. — Cortex of the Betz'Cei.l> Area of Man and of the Corresponding 
 Areas in the Dog and Pig (Watson). 
 
 The magnification of (1 ) is less than that of (2) and (3). The depth of Ire 
 different layers of the cortex is approximately indicated by the trainings to 
 the right of the figure.,. 
 
 In man, L.II. — the pyramidal lamina — is much trie best developed layer of the 
 cortex, being considerably deeper than the conjoined layers III. to V. It is 
 the lamina of 'psychic, associative, and educative significance.' In the dog 
 andTpig, L.II. is comparatively poorly developed, having a depth of only one- 
 third to one-half of the conjoined layers III. to V. The conjoined layers 
 IV. and V., which are of ' organic and instinctive significance,' are of more 
 nearly equal depth in all three specimens. In this region L.III. — the granule 
 or receptive lamina — is reduced to a minimal depth and distinctness in all 
 the cortices. L.I. — the outer fibre lamina — is relatively deep in the pig, 
 possibly owing to a failure of development in the upward direction of the 
 pyramidal layer. 
 
 The larger cells of L.IV. in the ' motor ' area of the pig have not the characteristic 
 shape and appearance of the Betz cells of the human motor cortex; the cor- 
 responding cells in the cortex of the dog more nearly approximate to the 
 human type. 
 
 In the cortex of the pig the individual nerve-cells, though fewer in number than 
 in the dog, have a less embryonic appearance. 
 
54^ A MANUAL OF VETERINARY PHYSIOLOGY 
 
 quantity, amounting to about 5 or 6 grammes (8o or 90 grains). 
 The use of this fluid is to equalise the pressure on the brain, and 
 to afford protection to it ; by the manner in which this organ is 
 suspended inside the skull by the dura mater it is saved from 
 jar and concussion. Both cerebrum and cerebellum half float 
 on water-cushions. Withdrawal of the cerebral fluid leads to 
 convulsions, while an increase in the amount may cause coma, 
 owing to the pressure exercised. When the arteries in the brain 
 dilate, the skull being unyielding, some room is made for the 
 extra blood by displacement of the cerebro-spinal fluid. 
 
 Halliburton's researches have shown that the cerebro-spinal 
 fluid is constantly being secreted, and as constantly got rid of. 
 Its channel of exit is not by the lymphatic system, but by 
 diffusion into the blood-stream. This fluid takes the place of 
 lymph, though it is a true secretion, and not in part an exudation 
 as lymph is. There is an abundant exchange between the 
 cerebro-spinal fluid and blood, but very little, if any, between the 
 blood and cerebro-spinal fluid. Halliburton regards this fluid 
 as an ideal physiological saline, in which the sensitive neurones 
 are bathed in order to obtain their osmotic equilibrium. 
 
 Movements of the Brain are dealt with at p. 99. 
 
 Sleep. 
 
 The actual cause of sleep is not known. The tissues require 
 rest and repair, and these are undoubtedly effected during sleep; 
 but no explanation which has been generally accepted has 
 accounted for the loss of consciousness which occurs during the 
 process of anabolism. The amount of sleep required by animals 
 appears to be greater in the carnivora than the herbivora. 
 The dog and cat spend a considerable time in this condition of 
 unconsciousness, but the herbivora sleep much less, and for 
 only short periods at a time. Neither class sleep with the same 
 depth nor intensity as man; it is not conceivable that a dog or 
 cat could remain in a condition of protound slumber while con- 
 siderable noise was occurring. In the case of the horse he is such 
 a light sleeper that the faintest footfall suffices to wake him up. 
 He sleeps with his eyes open or semi-open and on his side, fully 
 extended. The ruminant sleeps resting on the chin, with the 
 head extended, or with the nose turned in to the side. The 
 horse sleeps for short periods together; his immense weight 
 does not admit of his lying for any length of time on one side, 
 for during this period the lower lung does no work and the lower 
 muscles get cramped. He rises and lies on the opposite side, 
 or rolls over to that side ; or may rise, eat for a short time, and 
 again lie down. He requires little sleep, but it should be of good 
 
THE NERVOUS SYSTEM 54I 
 
 quality. Hard-worked horses cannot do without it. The horse 
 has the power of sleeping while standing, for which the limbs 
 are provided with certain necessary mechanisms in connection 
 with the muscles, which will be examined in the chapter on loco- 
 motion. The horse that sleeps while standing drops the head 
 below the level of the withers, the eyelids partly fall over the eye- 
 ball, and the limbs are brought rather more under the body than 
 usual. The extensor muscles of the limb cannot be relaxed, or 
 the body would fall (p. 435); they must accordingly continue 
 to have impulses poured into them in order to maintain their 
 contracted condition. Nor is this confined to the limb muscles; 
 those which sling the body between the fore-legs must also have 
 tone imparted to them if the erect attitude is to be maintained. 
 The impulses dealing with body equilibrium must also continue 
 in operation. None of the mechanisms of the limbs which aid 
 the muscles and tendons are in any way adequate to explain 
 these facts. The extensor muscles, on which everything depends, 
 receive assistance from the fascia of the arm and thigh. The 
 muscles themselves are extremely powerful, and possess in their 
 substance a large amount of tendinous material, such as is 
 evident in all muscles constantly in action — for example, those of 
 the back, masseter muscles, and others. During sleep the horse 
 is unconscious, and the muscles are under no kind of cerebral 
 control, yet they remain strong and in operation without the 
 knowledge of the animal. This is due to the reflex actions of the 
 spinal cord, which, as has previously been urged, are so evident 
 in the herbivora (p. 478); likewise the reflex control involving 
 the cerebellum is still in full operation in maintaining posture, 
 so that, asleep or awake, through these agencies, the tonus 
 of the muscles is maintained, not only of the limbs, but of the 
 back and chest wall, which are equally essential to the maintenance 
 of the erect attitude. The attitude of the sleeping bird, on one 
 leg, is a similar instance of the maintenance of reflex balance 
 when consciousness is in abeyance. 
 
 The rest obtained by sleeping in the erect attitude is insuf- 
 ficient for hard- worked horses. They need the complete re- 
 laxation of their muscles, and this can only be furnished in the 
 recumbent position. When from anchylosis of the vertebra; or 
 other causes the horse does not lie down, he must be placed in 
 slings at night in order to obtain the needful rest. If not hard 
 worked, they may stand for years without this aid; on board a 
 ship, or for surgical reasons, they may be kept in a confined 
 position for a considerable period without lying down, especiaUy 
 if the muscles be exercised for a few minutes each day. Horses 
 are not free rrom falling when asleep; they may even actually 
 come to the ground, but this is rare; what generally happens is 
 
542 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 that relaxation of the extensors of the fore-limbs occurs, and the 
 animal knuckles over on to the fetlocks and at once recovers 
 itself,but not without inflicting an injuryto the skin over the joint. 
 The fall always occurs in front, and not behind; probably the 
 extra weight on the fore-legs may have something to do with this. 
 
 Observations made on man and those animals which lend 
 themselves to inquiry show that during sleep the respirations 
 become slower and deeper, and this can readily be observed in 
 the horse. The secretion of urine becomes lessened, the pulse- 
 rate and blood-pressure fall, and the brain shrinks, so that 
 the volume of blood in the other parts of the body is slightly 
 increased, as might be expected from the general inaction of the 
 muscles. The production of carbon dioxide is lessened, while 
 the loss of nervous control over the production of heat causes a 
 fall in temperature. The essential physiological factor in sleep 
 appears to be the anaemia of the brain. 
 
 ■ The Theories of Sleep. — The theories formed to account for 
 sleep may shortly be stated. One is based on the accumulation 
 of acid waste products in the system. The source of these lies 
 mainly in the muscles. These tissues, as we have already seen 
 (p. 444), are capable of producing lactic acid. Lactic acid injected 
 into the circulation is said to produce fatigue, and it certainly 
 produces unconsciousness. Sleep after severe muscular work 
 is a necessity, but the acid theory is obviously incomplete, or 
 the idler would need but little sleep. The doctrine of the neurone 
 has been evoked to explain sleep, the suggestion being that the 
 synapses in the cells of the cerebral cortex shrink, and in conse 
 quence fail to make connection with the incoming fibres from the 
 outside world. The view has been put forward by Salmon* 
 that sleep is presided over by an organ of internal secretion. 
 The secretion, he considers, is furnished by the cells of the 
 cerebral cortex. 
 
 Hibernal Sleep. — The long period of sleep enjoyed by hiber- 
 nating animals appears to be protective in character, and repre- 
 sents the tiding over of a period when food-supply is defective. 
 A study of the processes which attend this remarkable pheno- 
 menon might be expected to throw some light on the cause 
 of ordinary sleep. It can hardly be urged that in this ease 
 the cerebral cells are secreting a substance, which keeps the 
 animal in a state of such profound sleep that evidences of life have 
 frequently to be carefuUy looked for. The metabolism occurring 
 during hibernation has been referred to at p. 416. Salmon 
 draws attention to the remarkable activity occurring in the 
 hibernal gland. This gland lies beside the thymus, and when 
 stored may reach the whole length of the body. It is highly 
 
 * British Medical Journal, July 8, 1911. 
 
THE NERVOUS SYSTEM 543 
 
 vascular and charged with a fatty substance, rich in lecithin, 
 which accumulates during the summer and is consumed during 
 the period of sleep, so that at the end of the winter the gland, 
 like the animal, may be a mere shred. This gland contains a 
 colloid substance, and it is significant that a similar material is 
 found in the blood of hibernating animals and in no others. 
 No definite statement can, however, at present be made as to 
 whether the gland furnishes the sleep-inducing material as well 
 as the food-supply. 
 
 Psychical Function. — In attempting to define to what extent 
 the faculty of reasoning exists in animals we are treading on 
 distinctly controversial ground. Probably this question can 
 be positively answered in the affirmative for two animals only— 
 namely, the elephant and the dog. In the horse the moral sense"* 
 is very small; we do not think he knows he is doing anything 
 wrong when he periodically kicks his stable into matchwood, or 
 when he 'runs away,' but he understands that he should not 
 refuse a jump, and a horse careless in his walk or trot knows 
 exactly what every stumble" will be followed by, and anticipates 
 matters accordingly. Strength of will most animals lose as the 
 result of domestication. They become mere reflex machines 
 or automata, but there are notable exceptions — for instance, 
 the ass, mule, and occasionally the horse. The so-called stupidity 
 of the ass and provoking obstinacy of the mule are not indica- 
 tions of want of intelligence; on the other hand, these animals 
 show a determination of purpose and strength of will, which, it 
 more deeply marked and combined with aggressiveness, would 
 keep them as free from civilisation as the zebra. The majority of 
 horses, on the other hand, have no great strength of will; they 
 ca"n be rendered docile and tractable, they will gallop until they 
 drop, work at high pressure when low would suffice, can never 
 apparently learn the obvious lesson that it is the ' willing horse ' 
 which suffers, and that the harder they work the more they get 
 to do. All this is due to defective intelligence and a want of the 
 higher faculties; they cannot reasonf like the dog or elephant, 
 
 * The use of this term is open to objection in the case of animals, but it 
 appears to the writer that something equivalent to the moral sense does 
 exist in them. The expression on a dog's face when he has done something 
 he knows to be wrong, or, at any rate, which he knows is against the rules 
 laid down for his life, conveys a conception of the existence of moral sense. 
 
 f Exception may be taken to the employment of the term ' reason.' 
 To reason is impossible without general concepts, and in the absence of 
 speech general concepts are difficult to suppose.. Nevertheless, the term 
 is convenient, especially as the writer considers that the more intelligent 
 of the lower animals possess what in them amounts to reasoning power. 
 
 The absence of speech suggests that animals have no power of inter- 
 communication. The question cannot bs discussed here, but those con- 
 cerned in closely watching the habits of animals must frequently feel that 
 communication between them is not impossible. 
 
544 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 and are more flexible than the ass or mule. Some horses show 
 signs of reasoning and are capable of grasping a position. A 
 load so heavy as to be beyond the limit of his power, or some 
 other cause, has taught him to refuse to work; to use the familiar 
 expression, he ' jibs,' he has learned to disobey, he has learned 
 his own strength, and the comparative powerlessness of his 
 master, and this through an exercise of reason. In other words, 
 the horse which refuses to wear himself out in the service of 
 man is one possessing too much intelligence and strength of will 
 for a slave; a ' jibber ' is an intelligent and not a stupid horse. 
 As a rule, the intelligence and affection of the horse exist only 
 in books and in the imagination of those who have least to do 
 with him; in whatever region of the brain affection is located, 
 it does not occupy much space in the equine. Taking the 
 dog as the standard to judge by, it may he said with the greatest 
 truth that the large majority of horses have no affection what- 
 ever, either for their own kind (excluding maternal affection) or 
 for human beings. There are exceptions; a kind groom is 
 appreciated, and a pair of horses may become greatly attached, or 
 a horse may become very fond of a cat . The liking of mules for 
 the grey mare which leads the troop is well known. Neverthe- 
 less, two strange horses cannot as a rule be put together without 
 disagreeing, and no one ever heard of a horse pining away 
 through the prolonged absence of his master. The often- 
 quoted example of a horse jumping over a man on the ground 
 rather than treading on him is an act misunderstood; it is 
 true the horse jumps over the man, but he does so because 
 he is taught to jump over every obstacle, and the man on the 
 ground might, for all he knows, be a bush. In other words, it 
 becomes largely a reflex, and only to a very limited extent a 
 volitional act. If the leading members of a flock of sheep are 
 made to jump over an obstacle, and the obstacle then be removed, 
 all the succeeding sheep will continue to jump on arriving at the 
 same spot. 
 
 If the horse possesses but little affection, this defect is com- 
 pensated for by his cherishing no ill will ; to all his hard life and the 
 abominable cruelties of domestication he shows no sign of resent- 
 ment ; water and feed him, and give him a place to lie in, and he 
 forgets the past in his anxiety for the present. He is a peculiar 
 mixture of courage and cowardice; physical suffering he can 
 endure, no animal bears pain better; when his blood is up 
 nothing is too big or too wide for him in the hunting-field, and 
 he has a keen enjoyment of both chase and race in spite of the 
 punishment they may entail. But the same horse is frightened 
 out of his life by a piece of paper blowing across the road, or by 
 his own shadow, and an unusual sight, or a heap of stones on the 
 
THE NERVOUS SYSTEM 545 
 
 side of the road has cost many a man his life. No animal is 
 more readily seized with panic, and this spreads amongst a body 
 of horses like an electric shock. Yet panic must not be held to 
 indicate an absence of reason, though the rapidity of its spread 
 in the case of a stampede may suggest it. There are other 
 animals than the horse affected by panic. The dog, with all 
 his intelligence, is acutely affected by a pot tied to his tail, but 
 it does not cause all the dogs he meets to stampede. Panic is 
 not unknown in the highest animal, and reason does not prevent it . 
 Reasoning power in the majority of horses is very small; an 
 animal runs away because he is seized with panic, or because his 
 spirits are bubbling over. Yet he has sufficient reasoning powers 
 to learn that for the time being he was master of the situation, 
 and if intelligent above the average he becomes a confirmed 
 ' bolter.' Distinct acts of reasoning are rare; of the lack of these 
 we daily see examples in our hospitals — namely, horses injured 
 in the most severe manner through their own struggles when 
 placed in a little difficulty, such as a head rope around the leg, 
 or an inability to rise when down, owing to being too close to the 
 wall, or some trifling circumstance of this kind. In these diffi- 
 culties, if he employed any reasoning powers he would remain 
 quiet until released, instead of which he behaves like a lunatic, 
 inflicting on himself in a short time injuries which may lay him 
 up for months. Or take the case of a horse which gets his tail 
 over the reins when being driven; instead of lifting the tail in 
 response to the exertions of the driver, he draws it closer down 
 to his quarters, gripping the reins as in a vice, and is so astonished 
 and frightened at the new state of things that he becomes un- 
 controllable. We can hardly point. to a single act in the horse 
 in which the powers of reasoning are clearly brought into play, 
 unless it be that he knows punishment follows refusal to obey, 
 and that he often learns to ' jib.' Further, he recognises in a 
 moment whether the man on his back intends to be master, or is 
 somebody with whom he can trifle. Every horse knows a truss 
 of hay or straw by sight. The point need not be laboured, 
 yet no horse will pass a truss of either lying in the road. He 
 appears unable to reason that what he is familiar with in the 
 stable may be no more dangerous when met with in an unusual 
 situation. 
 
 The horse is very conservative; he does not like anything new 
 or any departure from his ordinary mode of life; he will starve 
 himself for days rather than take a new feeding grain. He will 
 not at first drink out of a trough if he has been used to a bucket 
 or stream, and he dislikes a change of stable or a new place. 
 His gregarious instincts are proverbial ; he frets at the absence of 
 his companions, and if used to work amongst a body of horses, as 
 
546 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 in- cavalry, he will take any degree of punishment rather than 
 leave them for a few minutes. During the absence of his 
 companions he neighs, sweats, paws with the fore-legs, and 
 almost screams with delight on rejoining them, not because he 
 loves them, but because he dislikes being alone. 
 
 The horse has an excellent memory for locality, probably 
 nearly equal to that of the dog or cat; he never forgets a road, 
 and, automaton-like, if he has once stopped at any place on it, 
 he wants to stop at the same place next time, no matter how 
 long the interval may be between the visits. Finally, his pre- 
 dominant feature, and the feature of all animals below adult 
 man, is the childishness present throughout life; probably the 
 absence of care, worry, and anxiety may account for this. The 
 horse will play all day with a piece of rope, or nibble his neigh- 
 bour persistently; even the oldest horses, when ' fresh,' will 
 perform the antics of a foal, and imitation amongst them is so 
 universal that, if one of a string of horses being led along happens 
 to kick out, this repeats itself all along the line as if by pre- 
 conceived arrangement. 
 
 Sydney Smith defined the difference between reason and 
 instinct as follows : ' If, in order to do a certain thing, certain 
 means are adopted to effect it, with a clear and precise notion 
 that these means are subservient to that end, the act is one of 
 reason; if, on the other hand, means are adopted subservient 
 to an end, without there being the least degree of consciousness 
 that these mean's are subservient to the end, then the act is one 
 of instinct.' L. Morgan* believes that between instinct on the 
 one hand, and reason on the other, we may insert as a middle term 
 'intelligence,' while Romanes f and others use the word 'in- 
 telligence ' as synonymous with ' reason.' Morgan defines 
 instinct as a motor response to a certain stimulus- — i.e., a reflex 
 act, but one accompanied by consciousness. Animals come into 
 the world endowed with this innate capacity for motor response; 
 but these instincts are not quite perfect ; they need training and 
 experience, and their instructor is ' intelligence.' Intelligence, 
 according to this authority, does not imply a conscious knowledge 
 of the relation between the means employed and the end attained; 
 such a conscious knowledge would be reason. In other words, 
 we are to regard animals as simply reflex machines, their brain 
 being very little higher in function than their spinal cord, and 
 in respect of some, but certainly not of all, such a position is 
 tenable. Morgan's definition of instinct and intelligence offers 
 no reasonable explanation of why a dog buries a bone, or why 
 they fight or worry cats; nor does it explain why a horse So 
 
 * Fortnightly Review, August, 1893 • 'Habit and Instinct,'. 1896. 
 f ' Animal Intelligence,' 1883 ; ' Mental Evolution in Animals,' 1883. 
 
THE NERVOUS SYSTEM 547 
 
 inclined will turn his quarters towards another as he passes, 
 and rapidly let both hind-legs fly in the direction of his objective ; 
 nor why he uses his fore-legs to strike when he knows his hind- 
 legs cannot reach the object of his irritation. It fails completely 
 to explain why an elephant employed piling timber refuses to 
 work one minute beyond the allotted hours. Nor can we believe 
 that the extraordinary sense shown by a sheep-dog is not directed 
 by reason. It is absolutely impossible to believe that such acts as 
 we refer to imply no conscious knowledge of the relation between 
 the means employed and the end attained. 
 
 The higher animals are capable of a limited amount of reason- 
 ing; in some the faculty is even relatively well developed, with 
 others it is extremely imperfect. The elephant and dog are 
 here at the top of the scale, the ox and sheep at the bottom, 
 the horse comes midway. We do not see how to separate reason 
 from intelligence, but there is no difficulty in separating these 
 from instinct. 
 
 Lord Avebury, in one of his lectures on ' The Senses and Intel- 
 ligence of Animals,' drew attention to the fact that in spite of 
 the wisdom and cleverness of the dog, there was much doubt as 
 to whether he could realise that two and two made four, though 
 he could be taught to call attention to his daily needs by picking 
 up cards bearing certain words as 'food,' 'water,' 'door'; 
 the same animal could not, however, be taught to bring the 
 duplicates of cards having distinctive colours or marks. 
 
 Avebury remarked that he had always felt a great longing to 
 know how the world appeared to animals. It seems impossible 
 to believe that their minds are a blank. A dog in search of 
 his master has his mind occupied, and as during sleep he dreams, 
 it is evident that the thoughts which have passed through it 
 have left an impression. Even with the horse it is not possible 
 to believe that daylight and darkness, food and water, work 
 and rest, form the only subjects of thought as they present them- 
 selves. The whole question bristles with difficulties, and, as 
 Avebury says, we have tried to obtain information of the senses 
 and intelligence of animals by teaching them our ideas, rather 
 than devising a language or code of signals by means of which 
 they might communicate theirs to us. 
 
 Some animals are born with such complicated reflex acts as 
 walking, galloping, jumping, etc., so highly developed that they 
 are employed at once. No member of the human family has 
 been seen to walk and run about a few hours after leaving the 
 womb, for both brain and spinal cord are incompletely developed, 
 and the acts have to be learned. This is not so with animals 
 (excepting the dog and cat): the chick walks out of its shell, 
 foals, calves, lambs, goats, etc., are born prepared to feel their 
 
548 A MANUAL OF VETERINARY PHYSIOLOGY * 
 
 feet at once ; they require no teaching and exercise no imitation, 
 their senses are perfect, they can recognise their mother or a 
 stranger, can see, smell, hear; in fact, they have nothing to learn, 
 for they are born with as much intelligence as their parents, 
 and differ from them in one respect only, and that is, they are 
 born wild, and so have to learn confidence. Domestication and 
 obedience are not properties transmitted from parent to offspring. 
 
 ' Thinking Horses.' — ' Some years ago extraordinary interest was 
 aroused by the report that some horses in Germany had been thsight 
 to think. A mathematical master, Von Osten by name, discovered 
 that the horse he drove responded intelligently to verbal instructions, 
 such as ' Turn to the left — or right,' and he succeeded in a short time 
 in driving this animal by spoken directions only through the crowded 
 thoroughfares of Berlin. This horse died, and was replaced by one 
 which was put through a regular course of instruction in arithmetic 
 and spelling. Von Osten now came in contact with one Krall of 
 Elberfeld, who knew nothing whatever of horses, but was nevertheless 
 convinced that the animal belonging to Von Osten was genuinely 
 intelligent . He accordingly purchased two Arab stallions , which were 
 given a course of elementary school instruction, and he obtained 
 results much higher than those of Von Osten. They were taught the 
 first four rules of arithmetic, and then more complex problems, such 
 as the extraction of the square root. Spelling was also taught. The 
 horse then began to spell out spontaneous statements, and in these a 
 phonetic system was employed — for instance, ' essen,' to eat, was 
 spelt ' sn,' and ' gehen,' to go, ' gn.' 
 
 The Society for Psychical Research in this country attempted to 
 discover the explanation of these extraordinary results, but their 
 investigation led to nothing. The report was published in The 
 Times of 4th July, 1913, and on this we have drawn. Several Com- 
 missions had sat in Germany with equally negative results. 
 
 The general impression was that the animals worked by signals 
 given consciously or unconsciously by the trainer, or someone 
 present, but from a letter to The Times of 19th June, 1914, it 
 appeared that a blind horse had been educated, and was thoroughly 
 reliable in his answers. The problem accordingly has never been 
 solved. 
 
 It must be evident from the views expressed by the writer in his 
 account of the intelligence of the horse that he regards the perform- 
 ances of the ' thinking horses ' of Germany as dependent on a skilful 
 deception. 
 
Section 7. 
 
 The Cranial Nerves. 
 
 There are twelve pairs of cranial nerves; of these two only 
 are required to take part in functions other than those con- 
 nected with the head. This leaves ten pairs, the function of 
 which is wholly connected with the head, either as nerves of 
 special sense, or in a motor or sensory capacity. 
 
 Modern Classification of Cranial Nerves. 
 
 The modern theory of the architecture of the cranial nerves 
 conceives them as built on the same lines as spinal nerves. In 
 this scheme the first and second pairs are not included, the olfactory- 
 bulb and retina being regarded as outgrowths from the brain. From 
 the third pair to the twelfth the cranial nerves originate from the 
 brain stem — i.e., the bulb, pons, and mid-brain — which is only an 
 extension forward of the spinal cord. Scattered irregularly in this 
 position, but mainly in the region of the fourth ventricle, are groups 
 of nerve-cells, or nuclei, some of which are sensory, others motor. 
 The motor nuclei are arranged in two longitudinal rows on either side 
 of the primitive neural axis. From the median row arise the third, 
 fourth, sixth, and twelfth nerves; from the lateral row the motor 
 branches of the fifth, seventh, ninth, tenth, and eleventh. 
 
 Nuclei which have been termed sensory are found in the same 
 region connected with the fifth, eighth, ninth, and tenth nerves. 
 These nuclei are the terminations of the sensory cranial nerves, 
 which are arranged in the same way as the spinal nerves, and have 
 ganglia on their sensory roots. The cranial nerves with their ganglia 
 are outside the cranial cavity. The ganglia consist of cells, each, 
 like a spinal nerve ganglion, possessing a T-shaped process, one end 
 of which grows to the centre, and one to the periphery. It is the 
 central end of the T-piece which terminates in the sensory nucleus of 
 the brain stem by arborising around its cells. It will be observed 
 that the sensory fibres do not arise in the nuclei, but terminate 
 there. The resemblance between the cranial and spinal nerves is 
 completed by the existence of sympathetic fibres in the cranial 
 system. 
 
 In this scheme the sensory portion of the fifth pair is represented 
 by the dorsal roots of the following motor (ventral) cranial nerves — 
 i.e., seventh and twelfth. The tenth pair becomes the dorsal root of 
 the eleventh. The ninth becomes a dorsal, with no ventral root. 
 The eighth nerve has its dorsal root from the cochlea, and its ventral 
 from the vestibule and semicircular canals . 
 
 The special senses for which provision is made are smell, sight, 
 hearing, and taste. Only two of these senses — viz., smeU and 
 sight — have nerves exclusively devoted to their functions; 
 
 549 
 
55o A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the senses both of hearing and taste are furnished by nerves 
 which perform additional functions: 
 The afferent cranial nerves are : 
 
 First, or olfactory. 
 Second, or optic. 
 Eighth, or auditory. 
 
 The efferent nerves are: 
 
 Seventh pair: Motor to the muscles of the face. 
 Eleventh pair : Motor to the muscles of the neck and 
 
 shoulders. 
 Twelfth pair: Motor to the muscles of the tongue. 
 
 Mixed nerves : 
 
 l ira pair » motor and sensory to the muscles of 
 
 liThpaTf «* eyeball. 
 
 Fifth pair ) Resemble spinal nerves in consisting of 
 Ninth pair \ a motor and a sensory root, with a 
 Tenth pair ) ganglion on the latter. 
 
 Afferent Group. 
 
 The nerves of special sense will be dealt with in the chapter 
 devoted to the senses. 
 
 Efferent Group. 
 
 Seventh Pair, or Facial. — This nerve has been grouped as 
 efferent, but in its course it receives sensory fibres, though at 
 its origin it is exclusively motor. It arises from the medulla 
 behind the pons, at the lateral part of the corpus trapezoides. 
 The deep-seated origin is from the facial nucleus in the medulla. 
 In company with the eighth or auditory pair, it passes through 
 the internal auditory meatus, but escaping from the internal ear 
 by the aqueduct of Fallopius, it leaves the eighth nerve, and pass- 
 ing beneath the parotid gland, finds its way to the face, about 
 1 1 inches below the articulation, over the external masseter 
 muscle. During this short course the nerve has supplied the 
 small superficial petrosal nerve, a nerve to the stapedius muscle 
 of the middle ear, and the chorda tympani. The latter takes a 
 remarkable course through the petrous temporal bone ; emerging, 
 it supplies secretory fibres to the submaxillary and sublingual 
 glands, and eventually joins the lingual nerve of the fifth pair, 
 and is distributed to the anterior two-thirds of the tongue. 
 Besides the above branches, the main trunk of the facial, on its 
 way to the face, has supplied motor fibres to the muscles of the 
 external ear, and sensory fibres to the skin of that part; motor 
 fibres to the digastric muscle, to the orbicularis of the eye, and 
 
THE NERVOUS SYSTEM 
 
 55i 
 
 ^ 
 
 the corrugator of the brow. The main trunk on the surface of 
 
 the masseter muscle divides into two branches, which between 
 
 them supply all the muscles of the face, lips, cheek, and nostril 
 
 with motor power. The seventh nerve is the nerve of expression. 
 
 In this connection it supplies the external ear and all the muscles 
 
 of the face and lips. Paralysis of this nerve produces a typical 
 
 facial expression (Fig. 162). The ear droops, the eye remains 
 
 open, the upper lip is drawn towards the sound side, is elongated 
 
 and flabby; the lower lip droops, and saliva runs from the mouth; 
 
 the nostrils are elongated, and the face possesses a stupid, vacant 
 
 look. Nutritive change may occur to the eye in consequence of 
 
 exposure, and deafness is present 
 
 owing to paralysis of the muscle of 
 
 the middle ear. Bernard pointed 
 
 out that if both facial nerves be 
 
 divided in the horse, and the 
 
 animal galloped, suffocation results 
 
 in consequence of the nostrils being 
 
 unable to dilate, and the horse 
 
 unable to breathe through the 
 
 mouth. Though the facial is 
 
 intimately connected with the 
 
 masseter muscle, as may be seen 
 
 in the face of a well-bred horse, 
 
 no motor fibres are supplied to 
 
 this, nor, in fact, to any of the 
 
 principal muscles of mastication; 
 
 it does, however, supply fibres to 
 
 the stylo-maxillaris, in part to 
 
 the digastricus, and to two small 
 
 muscles connected with the hyoid 
 
 bone. 
 
 The seventh pair of nerves in 
 all animals control the muscles of 
 expression, of emotion, pain, and 
 
 pleasure. In the horse they are in addition respiratory nerves 
 of the first importance, as they control every movement of the 
 nostrils. 
 
 Eleventh Pair, or Spinal Accessory.— This is a purely motor 
 nerve, with two roots of origin— viz., the bulb and the spinal 
 cord as low down as the fifth cervical vertebra. This latter 
 branch passes up the cord between the motor and sensory roots, 
 receiving filaments on the way, and entering the foramen 
 magnum it joins the root arising from the bulb. It leaves the 
 skull in company with the vagus; then, separating from this 
 nerve, it divides into two branches, which supply the sterno- 
 
 Fig. 162. — Characteristic Facial 
 Expression of the Horse with 
 Paralysis of the Seventh Nerve. 
 
552 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 maxillaris, levator humeri, and trapezius muscles with motor 
 power. 
 
 This nerve furnishes motor fibres to the vagus, and is con- 
 nected with the superior cervical ganglion of the sympathetic 
 (see p. 559). Its course is very remarkable; having run all the 
 way up the neck within the spinal canal, its dorsal branch passes 
 externally down the neck as far as the upper part of the scapula, 
 and its object in doing so is to innervate the trapezius. It 
 passes other large muscles of the neck, such as the splenius, 
 without supplying them with any fibres, while all the muscles 
 in the neighbourhood of the trapezius are supplied with motor 
 power direct from the spinal cord. 
 
 Owing to its connection with the vagus nerve, division of the 
 accessory causes loss of voice. Bernard showed that crowing 
 in birds was no longer possible after, dividing the root within the 
 spinal canal. 
 
 The Twelfth Nerve, or Hypoglossal, is the motor nerve 
 of the tongue. It is the last cranial nerve to arise from the 
 medulla, which it does by means of several filaments. The deep- 
 seated origin is the hypoglossal nucleus, which is situated in 
 the floor of the fourth ventricle, near the mid-line. The nuclei 
 of both sides are connected by fibres. On its passage to the 
 tongue this nerve establishes connection with the first cervical 
 and the superior cervical ganglia of the sympathetic, and 
 subsequently supplies' all the muscles of the tongue, with fibres 
 to those muscles which depress the larynx. 
 
 Mixed Group. 
 
 This group, it will be remembered, resembles spinal nerves in 
 consisting of a sensory and a motor portion, the fifth, ninth, and 
 tenth pairs possessing a ganglion on the sensory root. 
 
 Third Pair, or Motor Oculi. — This nerve has a deep-seated 
 origin in the anterior corpora quadrigemina and peduncles of 
 the cerebrum. It supplies with motor power all the muscles of 
 the eyeball, including the retractor and muscle of the upper lid, 
 but does not supply the external rectus nor the superior oblique. 
 Through its connection with the ciliary ganglion it supplies 
 fibres to the sphincter of the iris and to the ciliary muscle (see 
 The Eye) . At its origin this nerve is connected with the two other 
 motor nerves of the eyeball — -viz., the fourth and sixth pairs. 
 Division of the third pair causes the eye to turn downwards 
 and outwards, owing to the unbalanced action of the superior 
 oblique and external rectus muscles. There is also depression 
 of the upper lid, immobility of the eyeball, and dilatation of the 
 pupil. The sense of direction and the reflex tonus of the eyeball 
 
THE NERVOUS SYSTEM 553 
 
 muscles are also conveyed by these nerves . The movement of the 
 eyeball muscles will be considered in detail later. 
 
 Fourth Pair, or Pathetic— This is the smallest cranial nerve. 
 It arises immediately behind the corpora quadrigemina in the 
 anterior cerebellar peduncle. The deep-seated origin is from a 
 nucleus in the floor of the cerebral aqueduct. Sisson* points out 
 that the fibres of this nerve decussate totally in the anterior 
 medullary velum, so that the fibres for the left eye are derived 
 from the right brain. The fourth cranial nerve supplies motor 
 power to only one muscle — viz., the superior oblique of the 
 eyeball. 
 
 Sixth Pair, or Abducens. — This nerve arises from the bulb 
 behind the pons, and external to the pyramid. It supplies the 
 external rectus or abductor muscle of the eyeball, a portion of 
 the retractor, and the deep recti with motor power. 
 
 A large number of fibres in the third, fourth, and sixth pairs 
 of nerves are afferent, and supply receptive nerve-endings to the 
 various muscles and their tendons. These branches are con- 
 cerned in the postural action (tonus) of the eyeball muscles 
 (Sherrington). 
 
 Fifth Pair, or Pars Trigemini. — The large sensory root of this 
 
 nerve arises from the pons, close to the middle peduncle of the 
 
 cerebellum, and at the foramen lacerum has upon it a large 
 
 ganglion known as the Gasserian. The deep-seated origin of 
 
 the sensory root is from the trigeminal nucleus, which, according 
 
 to Dexler,-)- extends from the pons to the sixth cervical segment 
 
 of the cord, and is known as the spinal trad of the trigeminus. 
 
 The connections of the trigeminal sensory root in the brain are 
 
 with the thalamus and cerebral cortex of the opposite side. In 
 
 ungulates it is also, according to Wallenberg,! connected with the 
 
 thalamus' of its own side. Further, it is connected with the 
 
 motor nuclei of the fifth, seventh, ninth, tenth, and twelfth 
 
 crania] nerves. The Gasserian ganglion behaves like a spinal 
 
 ganglion. From its axons the sensory fibres arise which pass 
 
 outwards to the structures of the face and head, and confer on 
 
 them ordinary, and, in the case of the lips, acute tactile sense. 
 
 Fibres passing from the Gasserian ganglion to the brain are linked 
 
 up, as shown above, with the optic thalamus, the central seat of 
 
 sensory impressions. The motor root arises from the pons on the 
 
 inside of the sensory root. From these two roots three branches 
 
 of nerve are formed — the ophthalmic, superior maxillary, and 
 
 inferior maxillary. The Ophthalmic branch is the smallest, and 
 
 is purely sensory. It supplies the lachrymal gland, upper eyelid, 
 
 membrana nictitans, temporal region, mucous membrane of 
 
 * ' Veterinary Anatomy.' t Sisson, op. cit. 
 
 \ Sisson, op. cit. 
 
554 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 septum nasi, and superior turbinated bone, and provides the 
 sensory root of the ciliary ganglion. The Superior Maxillary 
 Branch is of great size, and purely sensory in function. It 
 supplies the septum of the nostril, hard palate, teeth, gums, soft 
 palate, upper lip, guttural pouch, parotid gland, external ear, 
 skin, and other structures of the face. The immense bundle 
 which issues from the infra-orbital canal of the horse is out of 
 all proportion to the extent of tissue to be supplied either with 
 ordinary sensation or with tactile sensibility. The Inferior 
 Maxillary Branch is a mixed nerve, its sensory fibres being 
 derived from the Gasserian ganglion, its motor fibres from the 
 motor root of the main trunk. It is through the motor fibres 
 of this branch that mastication is carried on — in fact, it has 
 been termed the masticatory nerve. It supplies all the muscles 
 of mastication excepting the digastricus. Its sensory fibres 
 are distributed to the lining membrane of the lips and mouth, 
 to the molar and incisor teeth in the lower jaw, and the 
 structures connected therewith, also to the anterior two-thirds 
 of the tongue, to the parotid gland and guttural pouch, to 
 the integument covering the lower half of the head, and to 
 the muscles of mastication. In addition, the lingual fibres not 
 only supply sensation to the tongue, but are distributed to the 
 fungiform papilla, and so assist in the sense of taste. 
 
 If the fifth pair of nerves be divided, there is complete loss 
 of sensation to one half of the face, part of the ear, cornea, 
 conjunctiva, nasal mucous membrane, and anterior two-thirds 
 of the tongue. There is paralysis of the muscles of mastication, 
 the mouth and tongue become injured by the teeth, in consequence 
 of the loss of sensation; the food collects on the paralysed side 
 of the mouth ; the cornea ulcerates, either in consequence of the 
 loss of trophic influence, or, as most physiologists think, from 
 irritation caused by foreign bodies, of which the animal has no 
 knowledge, in consequence of sensation being lost. Division of 
 the superior maxillary branch in the horse — known as Bell's 
 experiment — prevents the animal from feeding, owing to in- 
 ability to grasp the food with its lips. Now, as this is a sensory 
 and not a motor nerve, the question arises, Why is the animal 
 prevented from grasping its food ? It is the duty of the sensory 
 branches of the fifth to keep the muscles informed of the position 
 of objects. When sensation is cut off, owing to the entire loss 
 of sensibility in the lips, the animal cannot know how to take hold 
 of the food. It can see the material in the manger, but, in conse- 
 quence of loss of sensation, does not know how to employ the 
 muscles of the lips to collect it. 
 
 There are certain reflex acts in which the sensory branches 
 of the fifth are intimately concerned. When sensation is cut 
 
THE NERVOUS SYSTEM 555 
 
 off, the ' feelers ' growing from the orbit are unable to excite 
 the reflex act of closing the eye. Irritation of the Schneiderian 
 membrane produces no sneezing, and irritation of the conjunctiva 
 or of the cornea does not produce tears, while the loss of sensation 
 in the tongue means failure in the stimulus for the secretion of 
 saliva. 
 
 Ninth Pair, or Glosso-pharyngeal.— This nerve, consisting of 
 a motor and a sensory portion, arises by several roots from the 
 anterior and lateral part of the bulb. As it leaves the cranium, 
 a ganglion is found on it- — the ganglion petrosum. This ganglion 
 is connected with the superior cervical ganglion of the sympathetic 
 and the jugular ganglion of the vagus. There are three branches 
 of nerves distributed from the petrous ganglion: one passes to 
 the cavity of the tympanum, and supplies sensory branches to the 
 mucous membrane of the tympanum ; a second is a motor nerve, 
 and supplies the muscles of the pharynx ; while the third branch 
 is sensory, and supplies the posterior third of the mucous membrane 
 of the tongue, part of the pharynx, and anterior face of the 
 epiglottis. In this branch are taste fibres, which end in the 
 ' taste bulbs ' of the circumvallate papillae. 
 
 Tenth Pair, Pneumogastric or Vagus. — This is a mixed 
 nerve, containing motor, sensory, and secretory fibres. There is 
 no other nerve possessing such a wide distribution, for its fibres 
 extend from the bulb to the anterior mesenteric ganglion in the 
 abdominal cavity. 
 
 The nerve arises from the floor of the fourth ventricle and the 
 nucleus of the solitary tract; its motor fibres are derived from 
 the spinal accessory. Leaving the bulb, it passes through the 
 foramen lacerum, and here the sensory root has a ganglion on it, 
 the jugular ganglion. In conjunction with the spinal accessory, 
 the vagus courses its way on to the guttural pouch; here the 
 two nerves separate. The vagus now joins with the cervical 
 sympathetic, from which results, in the horse and most other 
 animals, a single cord, which passes down the neck in company 
 with the carotid artery. As it enters the chest, the vagus 
 separates from the sympathetic, and the right and left trunks 
 proceed on their way. They both give off important branches, 
 known as the ' recurrent laryngeals,' fibres to the heart, trachea, 
 bronchi, and cesophagus, and then each divides into two branches, 
 dorsal and ventral, which come together from opposite sides 
 and form the dorsal and ventral oesophageal branches. These 
 penetrate the diaphragm, after running above and below the 
 cesophagus. The dorsal branch, composed mainly of fibres 
 from the right vagus, passes to the cardia of the stomach, gives 
 off many fibres to that organ, and, continuing its course back- 
 wards, joins the anterior mesenteric ganglion (solar plexus) of 
 
5jb A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the sympathetic (Fig. 81 ) . The ventral branch of the oesophageal 
 nerve proceeds to the lesser curvature of the stomach, while fibres 
 pass on to the duodenum and liver. In this way the vagus 
 supplies fibres to the larynx, trachea, bronchi, lungs, heart, 
 oesophagus, stomach, duodenum, liver, and through the sympa- 
 thetic ganglion fibres to the small and to a portion of the large 
 intestines. In the chapters dealing with the heart, circulation, 
 respiration, and digestion, the special functions of this nerve 
 have received full consideration. Nevertheless, there are some 
 features of the vagus which may be conveniently dealt with here. 
 Through the jugular ganglion the vagus is brought into relation 
 with the facial, glosso-pharyngeal, and spinal accessory nerves, 
 and it is intimately connected in the neck, thorax, and abdo- 
 minal cavity, with the sympathetic system (see p. 234). In the 
 neck it gives off a pharyngeal branch, which forms a plexus with the 
 pharyngeal branch of the ninth pair; and from this plexus 
 motor fibres proceed to the middle and posterior constrictor 
 muscles of the pharynx, and finally distribute themselves on the 
 cervical portion of the oesophagus, to which they furnish motor 
 power. The vagus next gives off the superior laryngeal, a mixed 
 nerve, which supplies acute sensation to the mucous membrane 
 of the epiglottis and larynx, and inhibitory fibres to the respiratory 
 centre. It is this nerve which is refiexly excited in the act of 
 coughing, and is capable of arresting inspiration, a very necessary 
 provision at the moment of swallowing. The motor fibres in 
 this nerve are the external laryngeal, given off before the main 
 trunk enters the larynx, and supplying motor power to the 
 crico-pharyngeus, crico-thyroid muscles, and in part to the 
 oesophagus. Anatomists are not agreed as to the innervation 
 of the crico-thyroid muscle in the horse. Moeller regards its 
 motor supply as being derived from the first cervical pair; 
 Chauveau and others, and recently Sisson, described it as being 
 innervated by the external laryngeal nerve. The function of 
 the crico-thyroid muscle being to render the vocal cords tense, 
 division of the nerve produces a hoarse voice. Section of the 
 superior laryngeal causes loss of sensibility in the larynx, and 
 allows food to enter it. The inferior or recurrent laryngeal nerves 
 are given off from the vagus within the chest. The right comes 
 first, being given off opposite the second rib, and winding around 
 the dorso-cervical artery from without inwards, while the left 
 is given off above the base of the heart, the nerve winding from 
 without inwards around the posterior aorta. Both nerves leave 
 the thoracic cavity and return up the neck. This complicated 
 arrangement resembles very closely the ' out-of-the-way ' course 
 taken by the spinal accessory in order to reach the trapezius. 
 From the points where each recurrent is given off within the chest 
 
THE NERVOUS SYSTEM 557 
 
 their courses are not identical; the left, for instance, having a 
 greater distance to travel, is somewhat mixed up with the struc- 
 tures around the base of the heart, while in its passage up the neck 
 it is more superficially placed than the right recurrent. Both 
 nerves supply motor power to all the muscles of the larynx, 
 with the exception of the crico-thyroid. All the fibres in the 
 recurrents are not motor; in their passage up the neck sensory 
 branches are given off to the trachea and oesophagus. At p. 152 
 sufficient consideration has been devoted to the subject of 
 paralysis of the laryngeal muscles and its influence on respiration. 
 The question is of intense practical interest, owing to the frequency 
 with which the innervation of these muscles is destroyed on the 
 left side. At present there is no satisfactory explanation of this 
 unilateral paralysis. After division of both recurrent nerves, 
 both sides of the larynx are paralysed, and in horses asphyxia 
 is gradually produced. The writer has, however, seen bilateral 
 paralysis not seriously interfering with slow work. In such cases 
 it is believed that the age of the horse is the saving factor ; the 
 cartilages, becoming rigid with age, prevent the arytenoids from 
 completely collapsing over the opening of the glottis. If the 
 recurrent nerve on either side of the neck be divided and the horse 
 galloped after the operation, he is found to be a ' roarer.' Not 
 only does paralysis of these muscles interfere with respiration, 
 but it also affects the voice. The altered character of the voice 
 of the horse may be observed either during neighing or coughing. 
 So distinctive, indeed, is the cough of a ' roarer,' that such cases 
 may be at once recognised. Longet found that dividing the 
 recurrent led to loss of voice in old animals, while in the young 
 the voice was rendered unnatural and shrill. This shrill con- 
 dition was entirely lost by cutting the external laryngeal, which, 
 by paralysing the crico-thyroid, prevented the vocal cord from 
 being stretched, and so rendered the animal mute. If the 
 peripheral end of a divided recurrent be stimulated, spasm 
 of the larynx is produced. There are certain vegetable poisons, 
 such as those contained in Lathyrus sativus, and other plants of 
 the Leguminosse, which appear especially to single out this nerve. 
 Horses fed on grain containing Lathyrus are soon rendered 
 incapable of work, as asphyxia occurs through spasm of the 
 larynx. 
 
 Chauveau, in his experimental inquiry on the vagus of the 
 horse, found that if both vagi be divided in the neck and the 
 animal fed, the stomach and whole length of the oesophagus 
 frequently became greatly distended with food, in consequence 
 of loss of motor power. It seems certain that the sensation of 
 repletion in these cases is lost; this conjecture would account for 
 the animal continuing to eat. 
 
Section 8. 
 The Sympathetic or Autonomic System. 
 
 The function of the system of nerves derived from the spinal 
 cord, with which we have hitherto dealt, is connected with the 
 movements of the skeletal muscles and the question of sensation ; 
 but there are other highly important matters which need nervous 
 control and yet are not within the knowledge of the animal. To 
 the old physiologists these were known as the ' vegetative ' 
 functions, presided over by a system of nerves described as the 
 sympathetic, which was believed to be independent of the cerebro- 
 spinal. These vegetative functions comprised the movements 
 of the heart, bloodvessels, and abdominal viscera. The work of 
 Gaskell and Langley has altered the entire conception of the 
 sympathetic system, and even changed its name; it is now 
 known as the autonomic. The nerve-centres in the brain and 
 spinal cord are enclosed within a bony framework; contrarily, the 
 nerve-centres in the sympathetic system are lodged in various 
 tissues and organs of the body, outside and at varying distances 
 from the cranium and spinal canal. The old conception of the 
 sympathetic system was that of a cord of nervous matter which 
 extended from the base of the skull to about the first bone of the 
 tail, and on which were situated certain nodules of nervous 
 matter known as- ganglia; from these ganglia plexuses of nerves 
 arose to supply the heart and bloodvessels; connected with this 
 long cord were other ganglia situated in the abdominal cavity, 
 the plexuses of nerves from which supplied the abdominal and 
 pelvic viscera. There is nothing in the modern conception 
 which conflicts with this account of the anatomical arrangement, 
 but there is a great deal which amplifies and extends the former 
 description. It is now known that it is not a system indepen- 
 dent of the brain and spinal cord, but that it is intimately con- 
 nected with both, and obtains from each the nerve-fibres with 
 which it works. Through the activity of the various ganglia it 
 possesses it elaborates these fibres into a system peculiar to its 
 own requirements, and then proceeds on the lines of self-govern- 
 ment to carry out its functions without any further reference to 
 the brain or spinal cord. It thus possesses an abundant supply 
 of efferent nerves derived from the brain, but especially the spinal 
 cord; in return, however, it gives little, for afferent nerves are 
 few and far between, and, moreover, their sensibility is blunted. 
 
 The structure of the fibres in the sympathetic system is much 
 
 558 
 
THE NERVOUS SYSTEM 559 
 
 the same as in the cerebro- spinal; the fibres are white or grey, 
 but principally grey, and in point of size they are much finer. 
 Similarly, the ganglia do not differ in structure from the nerve- 
 centres found in the cord; they contain multipolar cells. It was 
 the presence of these multipolar cells which at one time led to 
 the belief that the ganglia of the sympathetic system were centres 
 of reflex action, but there are no definite reflex actions in this 
 system of nerves, although it works as a reflex effect— that is to 
 say, not under the control of the will. Nevertheless, there are 
 some well-known facts which suggest that sympathetic ganglia 
 may initiate reflex action; the most obvious example of this is 
 the peristaltic movements of the intestines when removed from 
 the body, and after aU nervous connections with the spinal cord 
 have been severed. 
 
 Construction of the Autonomic System. — The way in which 
 the old sympathetic system of the anatomists is linked up with 
 the cerebro- spinal system to form the autonomic system of nerves 
 must now be explained. The linkage with the brain is furnished 
 by the third, seventh, ninth, tenth, and eleventh pair of cranial 
 nerves. On the sympathetic trunk in the neck are two, some- 
 times three, ganglia — namely, the superior and inferior cervical, 
 and in the horse occasionally a middle cervical. The superior 
 cervical ganglion is connected with the last four cranial nerves 
 and with the first cervical from the spinal cord; the inferior 
 cervical ganglion is connected with the sixth and seventh cervical 
 nerves and through a band of nerves accompanying the vertebral 
 artery it is also linked up with the spinal cord from the second 
 to the sixth cervical nerves. The arrangement in the remaining 
 portion of the spinal cord is different: the sympathetic trunk 
 leaves the inferior cervical ganglion and gains the ribs close to the 
 heads of these; here it may be distinctly seen beneath the pleura, 
 and at each intercostal space it possesses a nodule or ganglion. 
 Throughout the length of the dorsal, lumbar, and sacral verte- 
 brae this cord runs, and throughout its length it is brought into 
 close linkage with the spinal cord in the following way : 
 
 With the ventral roots of the spinal nerves there pass out of 
 the canal white medullated fibres, known as the white rami, 
 which leave the ventral root and run to the ganglia on the 
 sympathetic cord; these ganglia are known as the vertebral. 
 Many of the white rami connect with the vertebral ganglia and 
 issue from them generally as grey fibres, known as grey rami, 
 which then rejoin the ventral roots of the spinal nerves and 
 proceed to their destination in the mixed nerves (see Fig. 163). 
 When the fibre enters the ganglion as a white ramus, or pre- 
 ganglionic fibre, its axon breaks up and arborises around the den- 
 drites of a sympathetic cell; it then'reissues, as we have stated, as 
 
560 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 a grey ramus, or post-ganglionic fibre. Not all the white rami ter- 
 minate in the vertebral ganglia; some having reached this point 
 pass through, and proceed to other and more distant ganglia of 
 the sympathetic — for example, the mesenteries and the semi- 
 lunar, which form the system designated the collateral or pre- 
 vertebral ganglia. The nerves intended for these enter as white 
 rami, arborise around the cells, and emerge as grey or post- 
 ganglionic fibres, and proceed to their destination; those in- 
 tended for still more distant ganglia, such as those of the intes- 
 
 DORSAL ROOT 
 
 SPINAL GANGLION 
 
 MIXED NERVE 
 
 P0ST-6ANGLIONIC FIBRE 
 
 PREGANGLIONICS 
 FIBRE 
 
 Ft/ 
 SYMPATHETIC 
 
 GANGLION 
 
 Fig. 163. — Diagram showing the Arrangement of the Dorsal and Ventral 
 Roots of the Cord and the Connection of the White and Grey Rami 
 with the Ventral Root. 
 
 The sensory fibres are shown passing through the spinal ganglion and entering the 
 cord through the dorsal root. The motor fibres leave the cord by the ventral 
 root. Shortly after leaving the white ramus is given off, shown in the 
 diagram as the preganglionic fibre. This passes to a lateral (vertebral) 
 ganglion of the sympathetic system, makes a cell connection as shown in the 
 diagram, and issues as a grey ramus, or post-ganglionic fibre, which rejoins the 
 ventral root. 
 
 tines, do not arborise in, but pass through the collateral ganglia 
 still as white rami or preganglionic fibres, and so reach their 
 destination, the terminal ganglia, there finding their cell-stations 
 (see Figs. 163, 164). 
 
 The cell-stations, then, for a preganglionic fibre must be a 
 vertebral (or lateral) ganglion, a collateral (or prevertebral) 
 ganglion, or a terminal ganglion. An immense amount of work 
 has been done by Langley and his fellow-workers in determining 
 the cell-stations of the whole system of spinal linkage with the 
 
THE NERVOUS SYSTEM 
 
 561 
 
 sympathetic system. Langley found that nicotine applied to the 
 ganglion had the effect of paralysing the nerve-cells, but not the 
 nerve-fibres. In order to ascertain whether a nerve-fibre had 
 its cell-station in any given ganglion, the ganglion was painted 
 with nicotine, and if the nerve on stimulation continued to func- 
 tion it was evident that it made no cell connection. 
 
 Every preganglionic fibre must have its origin in the mid- 
 brain, bulb, or cord, so that every autonomic fibre consists of two 
 neurones, the first lying between the cell of origin and the sym- 
 pathetic ganglion in which it terminates, the second neurone being 
 placed between the ganglion in which it terminated and the tissue 
 
 SPINAL CORD 
 
 PREVERTEBRAL 
 GANGLIA 
 
 POST-GANGLIO 
 FIBRE 
 
 Fig. 164. — Diagram of the Autonomic Ganglia. 
 
 The preganglionic fibres issue from the spinal cord, and in the upper figure are 
 shown as passing through, but forming no cell connection with, the lateral 
 (vertebral) ganglion. As a preganglionic fibre it continues, and makes a 
 cell connection in the next, or prevertebral ganglion, from which the post- 
 ganglionic fibres issue. 
 
 In the lower figure some of the preganglionic fibres have formed a cell connection 
 with the lateral ganglion, and issue as post-ganglionic fibres. Others pass 
 through, still as preganglionic, and some of these form cell connections in the 
 prevertebral ganglion; others do not, and, continuing as preganglionic 
 fibres, reach the terminal ganglion, where they arborise; from this post- 
 ganglionic fibres issue. 
 
 supplied. In other words, the preganglionic and post-ganglionic 
 fibres represent the two neurones in the autonomic chain; to this 
 there is no exception; it is a fundamental principle in the con- 
 struction of this peculiar system. It is not necessary that a pre- 
 ganglionic fibre on its way to join a vertebral ganglion should 
 pass to that which is nearest to it; it may pass forwards or back- 
 wards among the vertebral ganglia until it finds the one in which 
 its cell-station exists. In consequence, a post-ganglionic fibre 
 derived from a vertebral ganglion does not always rejoin the 
 spinal nerve with which it left the cord, while the post-ganglionic 
 fibres derived from the prevertebral or terminal ganglia never 
 
 return to the spinal nerve. 
 
 36 
 
562 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Post-ganglionic fibres are more numerous than preganglionic; 
 this is effected by the nerve arborising around several cells in a 
 ganglion, from each of which a fresh neurone issues; by this means 
 considerable economy of space is effected in the spinal canal. 
 
 Functions of the Autonomic System. — The function of this 
 curious system of nerves is to govern the movements of the heart, 
 viscera, bloodvessels — in fact, of all involuntary muscles- — and 
 to bring about the various secretions; it regulates those functions 
 over which no voluntary control can be exercised. 
 
 Very little reflection will show that the autonomic system 
 cannot be confined solely to nerves derived from the spinal cord. 
 In the head are bloodvessels and involuntary muscles, such as 
 the iris, which require ' innervation, ' and glands which are called 
 upon to secrete. These tissues are supplied by cranial and not 
 by spinal nerves. Evidently, then, some cranial nerves function 
 as autonomic, and no better example of this can be given than 
 the pneumogastric. The autonomic system consists, therefore, 
 of a cranial and a spinal system. 
 
 The Cranial Autonomic System. — -The ciliary ganglion on 
 the third pair of nerves furnishes post-ganglionic fibres to the 
 involuntary muscle of the iris and ciliary muscle. The seventh 
 pair furnishes the chorda tympani so intimately concerned in 
 the secretion of saliva, while fibres from the seventh and ninth 
 pair supply dilator fibres for the bloodvessels of the mouth, nasal 
 chambers, and salivary glands, through various ganglia, such as 
 the otic, submaxillary, and sublingual. The tenth and eleventh 
 pair have abundant autonomic fibres which furnish viscero- 
 motor fibres to the oesophagus, stomach, small, and part of the 
 large, intestines (see p. 235), motor fibres to the involuntary 
 muscle of the bronchi, inhibitory fibres to the heart, secretory 
 fibres to the gastric glands and pancreas, and nerves to the liver, 
 spleen, and kidneys. The cell-stations of these are not known 
 with precision; most of them are probably located in the tissues 
 supplied. 
 
 The Spinal Autonomic System is far better known, and con- 
 sists of vasomotor nerves, dilator and constrictor, secretory 
 nerves for the skin and viscera, erector fibres for the hair, and 
 viscero-motor nerves, both excitatory and inhibitory. Through- 
 out the dorsal, and part of the lumbar, area vasomotor (chiefly con- 
 strictor), sweat, and pilo-motor fibres are abundant— especially 
 the vasomotor. Those having their cell-stations in the vertebral 
 ganglia are intended to supply the skin of the trunk and limbs; 
 the fibres for the skin of the head and neck do not terminate in 
 the vertebral ganglia, but join the cervical sympathetic, and 
 issue as post-ganglionic fibres from the superior cervical ganglion. 
 The vasomotor, especially constrictor fibres for the abdominal 
 
THE NERVOUS SYSTEM 5 6 3 
 
 viscera, do not return to the spinal mixed nerves for distribution, 
 but proceed, mainly in the splanchnic nerves, to the solar and 
 mesenteric ganglia, and issue from these as post-ganglionic 
 fibres. The splanchnic nerves are the great vaso-constrictor 
 nerves of the abdominal cavity— in fact, the largest constrictor 
 nerves in the body, also the viscero-motor, both excitatory and 
 inhibitory, of the intestinal wall. 
 
 From the sacral cord the autonomic fibres in the ventral roots 
 pass out without making connection with the vertebral ganglia, 
 and proceed as the nervus erigens to the pelvic plexus. They 
 supply the external organs of generation with vaso-dilator fibres, 
 the single colon and rectum with viscero-motor. The vaso- 
 constrictor nerves for these parts are derived from the dorsal 
 and lumbar cord, reaching their destination via the sympathetic 
 by means of the hypogastric nerve or the pudic nerve. The 
 bladder receives two sources of viscero-motor supply, which has 
 been described (p. 355). 
 
 It will be seen that through the autonomic system of nerves 
 impulses of an opposite nature pass to the various tissues — for 
 instance, to the bloodvessels, mainly constrictor, but also dilator 
 effects, to the visceral muscle contractile or inhibitory impulses. 
 The same branch of nerve, though not the same fibre, may be 
 conveying vasomotor and viscero-motor impulses of an opposite 
 type, so that, as in the case of the heart, there are nerves which 
 are bringing about increased activity, katabolic, and inhibitory 
 nerves effecting repair, anabolic. While the cell-stations for the 
 katabolic nerves are generally in the vertebral or else the pre- 
 vertebral ganglia, those of the anabolic nerves are generally in 
 the prevertebral or else the terminal ganglia. 
 
 Activation of the Sympathetic System. — The sympathetic 
 muscular system is activated by the ordinary nervous impulse 
 and by the secretion of the adrenal bodies; one acts by direct 
 contact with the plain muscle, the other operates from a distance . 
 At p. 304 it was stated that the medulla of these endocrine 
 glands was a portion of the sympathetic system, and that they 
 poured their secretion, adrenaline, into the blood-stream. Adrena- 
 line cells and sympathetic ganglion cells are brother and sister 
 (T. R. Elliot). Constriction of the bloodvessels and secretory 
 activity are maintained by adrenaline, while the rate and rhythm 
 of the heart's action are stimulated by its influence on the 
 sympathetic nerve-endings in that organ. The effect of adrena- 
 line on the sympathetic system is obliterated by the action of a 
 substance obtained from ergot, known as ergotoxine or acetyl- 
 choline. 
 
 Sensory Phenomena in the Autonomic System.— It has 
 already been stated that the majority of fibres in the sympathetic 
 
564 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 system are efferent ; afferent fibres are known to exist, but there 
 is very little information concerning them. As a rule, the parts 
 supplied by the sympathetic system are devoid of ordinary 
 sensibility; the existence, therefore, of afferent fibres must be 
 very limited. The intestines can be handled, burned, cut, or 
 sutured without the animal evincing pain; the heart can be injured 
 in many ways without giving any sign of sensibility; the liver, 
 spleen, and kidneys are equally devoid of touch sensibility, and 
 are ordinarily free from painful impressions. 
 
 It is no wonder that to the majority of people the interior of 
 the body is non-existent as a sensitive area. Notwithstanding 
 the normal insensibility of the part, the most acute pain experi- 
 enced by the horse is referred to the abdominal cavity. There is 
 no pain to equal that caused by a twisted condition of the intes- 
 tine, and an attack of acute colic comes next in order of intensity 
 The explanation lies in the fact that the parietal peritoneum is 
 extremely sensitive, and that the mesentery with its attachments 
 when dragged upon produces acute pain. Nor is tension as a 
 cause of pain confined to the mesentery; great distension of the 
 • hollow viscera causes acute suffering, as also may any irritation 
 within the hollow viscera; the passage of a gall-stone in man is 
 attended with intense pain, due to the stretching of the duct. 
 This class of pain is not referred to the viscera, but to the ab- 
 dominal wall, and the explanation which has been offered is that 
 the sensory cutaneous nerves belong to the segment of cord in 
 which the afferent nerves from the viscera end, and the pain is 
 thus referred to the skin; it is known as referred pain. 
 
 Nevertheless, Flourens observed that the sympathetic ganglia 
 were sensory in different degrees, but always less so than the 
 nerves of the cerebro-spinal system. He frequently noticed 
 that stimulation of the semilunar ganglion of the rabbit caused 
 acute pain, while stimulation of the cervical ganglion was un- 
 noticed. Many observers following Flourens record the same 
 fact regarding the sensory character of the semilunar ganglion 
 when stimulated. Colin, in his observation on the degree of 
 sensibility of the sympathetic ganglia in horses, oxen, dogs, and 
 rabbits, agreed with Flourens that these tissues were endowed 
 with different degrees of sensation, and that the semilunar and 
 thoracic were more sensitive than the upper cervical. He found 
 that the rapidity with which a painful reaction showed itself 
 depended on the strength of the stimulus, weak stimuli taking 
 several seconds before any evidence of pain was given. He also 
 found that if the gastric, splenic, hepatic, and intestinal arteries 
 were pinched with forceps, stretched, or damaged, great pain was 
 shown. These facts, perhaps, enable the pain of colic, and 
 especially of intestinal twists, to be explained, though why horses 
 
THE NERVOUS SYSTEM 565 
 
 can carry in their intestines stones, sand, gravel, and calculi, 
 week after week and month after month, without pain being 
 produced is still unknown. 
 
 Pathological. 
 
 The absence of alcoholism and syphilis in the lower animals 
 reduces the incidence of nervous affections. It cannot truly be said 
 that worry or anxiety are absent among them, and that hence 
 nervous affections are not so frequent. Nervousness is charac- 
 teristic of all horses, while to realise that anxiety cannot be denied 
 to the lower creation, one has only to witness what occurs when 
 their young are taken from them. 
 
 We have seen that the brain is relatively lowly organised in the 
 inferior animals, while the cord is relatively highly organised. Brain 
 trouble is consequently not so frequent as disorders of the cord. 
 
 In the horse the old class of disorders known as megrims and 
 staggers, with its varieties of sleepy and mad, are now rarely de- 
 scribed, and probably as rarely seen. They appear in the main to 
 have been due to two causes — dietetic errors and the obliteration, 
 more or less complete, of the jugular veins as the result of ' bleeding.' 
 We still meet with ship staggers in bad weather at sea, the result, 
 apparently, of the horse's inability to vomit. The condition would 
 appear to be produced by some reflex effect through the vagus. 
 Another reflex nervous affection is hysteria. Abscess of the brain 
 may result from septic infection, especially that of strangles. Tumours 
 of the choroid plexus are not uncommon in the horse ; they are com- 
 posed of cholesterine. Hydatids in the substance of the brain of 
 sheep and cattle are well known. True inflammation of the brain 
 and its coverings is probably unknown in the herbivora, excepting 
 as the result of surgical conditions or of the virus of rabies. 
 
 Paralysis of the hind-quarters, partial or complete, is common, 
 and a variety of disorders are probably concerned, quite apart from 
 those due to injury. Paralysis in the horse is common in India, and 
 some cases may be associated with parasitic invasion ; in temperate 
 climates, noxious substances in the food are recognised as a cause. 
 In azoturia, venereal disease of equines, and embolism of the iliac 
 arteries it occurs as a symptom. Enzootic paralysis is frequently 
 recorded in connection with feeding on certain kinds of grass and 
 hay, while other nervous affections exist as epizootics of sheep; 
 louping ill has been traced to infection by a specific microbe . Of 
 the local paralysis known as roaring sufficient has already been said 
 at pp. 152, 156. Local paralysis in the fore-leg may also occur as 
 a result of a fracture of the first rib. The fact, however, remains 
 that we know little or nothing of the nervous diseases of animals ; 
 take, for example, shivering and springhalt,* spasmodic affections, 
 the latter characterised by more, or less violent flexion of a hind- 
 limb, due to the unbalanced action of the flexor muscles. The cause 
 of one spasmodic disease, tetanus, is clearly known, but the others 
 are still wrapped in obscurity. 
 
 * Vulgarly known as stringhalt, a. meaningless expression, a degeneration 
 of the word springhalt accurately employed by Shakespeare (' Henry VIII,,' 
 I.iii. 11-13). 
 
CHAPTER XV 
 THE SENSES 
 
 Section i. 
 
 Sight.* 
 
 Light. — The energy which produces light is transmitted from 
 the sun to the earth by means of wave motions in the ether; the 
 wave motion is produced by oscillations of the ether in all directions 
 at right angles to the line of propagation of the energy. The velocity 
 of the ether waves is 186,000 miles a second, but their rate of 
 oscillation varies. Some are relatively slow, and are not perceived 
 as light, but as heat; still slower oscillations (from 3,000,000 to 
 15,000 per second) are characteristic of waves used in wireless 
 telegraphy. On the other hand, there are oscillations of incredible 
 velocity, some of which produce X rays, others ultra-violet rays, 
 and others, of slower movement, visible violet rays; the last are 
 produced by oscillations in the ether of 1,200 billions per second; 
 red light has only half this number of oscillations. The rate of 
 oscillation determines the length of the wave; the more rapid 
 the oscillation, the shorter the wave length; a wave length is the 
 distance from crest to crest. 
 
 Ordinary light consists of a mixture of red, orange, yellow, green, 
 blue, and violet lights. When brought together, these colours form 
 white light, and when white light is passed through a prism it is 
 decomposed into its primary colours (see Fig. 165). A beam of sun- 
 light entering a prism becomes bent or refracted ; the various rays 
 are unequally refracted, the red being the least, the violet the most 
 bent. There are rays beyond the red and the violet which are 
 invisible to our eyes; they are known as the ultra-red (heat rays) 
 and ultra-violet (chemical rays) . Every lense disperses light in the 
 same way as a prism, and both in the eye and in every optical 
 instrument dispersion is corrected. 
 
 The Eyeball, surrounded by a protective laye r ofkt, is placed 
 in a socket partly of bone and partly f ibrous which, together with 
 the muscle s, affords the bulb and optic nerve complete grpiection. 
 A very severe blow is required to break the bony rim, but the 
 
 * In the revision of this section many points have arisen necessitating 
 observations on the living animal for which the writer has no facilities. 
 He is accordingly greatly indebted to Captains Minett and Oxspring, of 
 the Army Veterinary School, Aldershot; Professors Linton and Mitchell, 
 of the Royal (Dick) Veterinary College; and Messrs. H. Gray and J. B. 
 Robertson, M.R.C.V.S., for several observations. 
 
 566 
 
THE SENSES 
 
 5°7 
 
 transparent, refractive part of the eye is necessarily exposed and 
 is readily in] ured. The size of the orbital rim is such that 
 
 Fig. 165. — Refraction and Dispersion by a Prism (Stewart). 
 
 The beam of light is bent towards the normal N as it passes across B, A, and away 
 from the normal N' as it passes across A, C. At both surfaces it is bent 
 towards the base of the prism A, C. At the same time the light suffers dis- 
 persion — that is, the rays of shorter wave-length are more refracted than 
 those of greater wave-length. This splits up light into its constituent colours. 
 
 infinitely more harm may be inflicted on the eye by small than 
 by large bodies. The external tissues of the eye are dense and 
 
 Fig. 166. — Section of the Eyeball of the Horse. 
 
 1, The cornea; 2, sclerotic; 3, choroid; 5, the lens (the dotted line around the 
 equator indicates the position of the suspensory ligament of the lens) ; 6, ciliary- 
 processes; 7, the iris; 8, corpus nigrum; 9, corneo-scleral junction; 10, ciliary 
 body (where the dotted line terminates is the position of the narrow ciliary 
 muscle); n, the optic nerve with its ' kink ' penetrating the sclerotic very low 
 down; 12, the optic disc. 
 
 resisting, but the internal structures are delicate and readily 
 displaced from their position. 
 
568 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The eye as an optical instrument is not perfect. It has been 
 stated above that there are certain rays beyond the red and 
 violet which it cannot see, and, furthermore, there are 
 frequently defects in its refraction, more or less considerable, 
 which prevent the formation of a clear picture. The principle 
 of its construction is that the "rays of light enter through its 
 transp arent anterior coat , then pass through a fluid medium 
 of greater refractive index than the air7 tUen through a lens, 
 next through a fluid, glass-like substance , and" finally reach 
 a _nervous ~e xpansibn at the back of the eye , where the picture 
 is formed. There are also present mechanisms for limiting the 
 amount of light entering the eye, and for bringing the rays of 
 light to a correct focus. The familiar instrument known as the 
 camera is a very close reproduction of the eye. It possesses its 
 lense for refracting the rays, its diaphragm for limiting the number 
 admitted, its sensitive plate for receiving them, and its focussing 
 arrangement for producing a sharp image. The various structures 
 of the eye and its accessories will now be looked at for their 
 physiological importance, the eye of the horse being taken as 
 the standard.* 
 
 The Eyelids are remarkable for the fineness of the skin entering 
 into their structure, for the absence of fat, and the mobility of 
 the upper as compared with the lower lid. The surface next 
 the eyeball is lined by the conjunctiva. The slit formed by 
 the union of the two lids is longer than'the cornea, so that a portion 
 of the sclerotic may be seen; ordinarily, since the conjunctival 
 membrane covering it is pigmented, the white sclerotic is not 
 obvious, excepting the palpebral fissure be widely dilated, or 
 the part be depigmented. The size of the fissure during repose 
 is sufficient to expose the greater part of the cornea, but during 
 excitement or anger it is much deeper, and the whole cornea 
 is completely exposed. The eyelids lie in close approximation to 
 the eyeball, especially the lower. This approximation of the 
 lower lid is essential for the drainage of the conjunctival sac; 
 otherwise the tears would run over the face. The lids a re given 
 firmness by, the presence in them of a tarsal cartilage ; along 
 the edge is situated a rim of glands which secrete a somewhat 
 unctuous material which assists to prevent the tears from 
 running over the face. The eyelids are closed by the orbicularis 
 muscle, which mainly operates on the upper lid; its antagonist 
 arises far back within the orbit and runs to the upper lid; the 
 shape of the eyeball alters the direction of the pull of the levator. 
 When the eyeball is removed the upper lid can no longer be raised. 
 
 * For a detailed account of the comparative anatomy of the eye, see 
 ' Veterinary and Comparative Ophthalmology,' by Dr. E. Nicolas, edited 
 and enlarged by H. Gray, M.R.C.V.S., 1914. 
 
THE SENSES 5 6g 
 
 Excepting by a distinct effort, the palpebral fissure is never 
 entirely closed in the horse, even during sleep; the lid droops 
 over the cornea, so that it may be said the animal sleeps with 
 his eyes open. If he is lying down, the lowermost eye is closed. 
 The dog and cat sleep with both eye s closed ; the sheep and ox 
 behave much as does the horse. 
 
 The Eyelashes on the lower lid are few and very fine, while 
 on the mid-region of the upper lid they are abundant, long, and 
 strong, and exist as a double row, the rows crossing each other 
 trellis-wise, but without interlacing (Fig. 167). A few long pro- 
 tective hairs grow from the brow and below the lower eyelid; 
 tfiey may be as much as 4 or 5 inches in length in some horses. 
 
 I 
 
 
 Fig. 167. — The Eye of the Horse. 
 
 They are connected with special nerve terminations, and the 
 delicacy of their sense of touch is remarkable. This function is 
 protective; they give the eyes warning of danger. 
 
 The Membrana Nictitans, or third eyelid, is functionally active 
 in all animals below man and the monkey, though not to the 
 same degree in each. Least active in the cat, owing to the use 
 made of the fore-limbs, it is seen in its highest development in 
 the horse, boldly and rapidly sweeping across the globe of the 
 eye when required, and when out of action lying alkttfet un- 
 observed away from the line of sight on the inner aspect of the 
 eyeball. Its basis is a curved piece of cartilage, thin on that 
 edge which can be seen lying next to the globe of the eye, but 
 
57 o A MANUAL OF VETERINARY PHYSIOLOGY 
 
 thick and embedded in fat at its deep part within the orbit. 
 The movement of the membrana is brought about indirectly; 
 the retractor musclejjulls the globe of the eye well within the 
 socket, which causes a forward displacement of the fatty cushion 
 connected with the cartilage. The cartilage is naturally curved, 
 but b ecome s flattened and straightened out by the pressure 
 caused by the retractor, and in consequence is s hot forward ; 
 when the retractor "relaxes the eyeball comes forward, the pad of 
 fat recedes, and the cartilage retires, taking up its curvature 
 through its own elasticity. The earliest symptom of tetanus 
 in the horse is a protrusion of the membrana nictitans, the result 
 of spasm of the retractor muscle. I n connection with the 
 membrana is found a glandj re sembfing the lachrymal ; it isTcnown 
 as the gland of Harder , and is developed to a remarkable degree 
 in th e^ ox. 
 
 ^The Lachrymal Apparatus.— The lachrymal gland l ies under 
 the o rbital process: in the ox.i t is very larg e. Its duct s, of 
 which there are several, open into the conjunctival sac high 
 up unde r the upper lid . Tne~lachrymal fluid tor the lower hd 
 is obtained by grav ity. The fluid is ctiHveye d from the eye 
 by means of a duc twhich has* twT> openings, one, on the uppe r 
 and another oh tn"e tower eye lid, the lower being the large r. 
 The fluid is (Erected into the sfct-like openings of the duct by 
 means of the lachry mal caruncle . The ducts enter the lachrymal 
 sac, which com municates with a canal that "finally opens into 
 fne* nasal chamber by a passage which is sometimes duplicated 
 in the horse, and is relatively large. 
 
 The fyjiciiQft- of the l achrymal secretion is to preserve the 
 b rilliancy of the surface of t he_c orjiea, and to flush the l achrym al 
 sac when this is o ccupieji bv an irritan t. Man is said to be the 
 only animal capable of weeping, but H. Gray has seen it in the 
 cat, and it is recorded as occurring in the elephant and seal. A 
 few tears may run down the cheek of the horse when the cord 
 is divided behind the medulla, but tears are never seen during life. 
 
 The Orbit. — The entire content s of the orbit — viz., eyeball, 
 muscles, gland, nerves, vessels, ~ and fa~t— are e nclose d in a 
 fibrous, funnel-shaped membrane which posteriorly obtains 
 attachmen t where the optic nerve issues from the skull, and in 
 front is attached to the orbital rim. The sheath is held in 
 position by~"several bands. "The 1 structures are thus not only 
 protected by, but confined within, a membranous case; the 
 interva l b etwee n the sheath and the e ye-socke t is filled with fai, 
 which acts as a cujhjanT" There is fat within this~sheath also, 
 the largesTmass rjemgon the inner side of the eyeball. The use 
 of this pad of fat, in pushing forward the membrana nictitans 
 when the retractor muscle contracts, has already been mentioned. 
 
THE SENSES 
 
 571 
 
 The Muscles of the Eyeball are seven , in number — four 
 recti, t wo obliq ue, and o ne retractor . The use of the rectfl s 
 to rotate the "eyeball upward s, d ownwards , i nwards, and out- 
 wards. All^ the recti muscles are attached squarely to "the 
 g-'erbtic at slightTy~Varying chstances*"~rrom the corned; the 
 nearest to the cornea is the external rectus; judging by its 
 weight, it is a stronger muscle than the internal rectus. These 
 four muscles would be capable of doing all that was required 
 in the matter of movement were it not that the visual axis 
 of the eye must always be kept parallel to the horizon, and this the 
 recti muscles cannot accomplish for the reason that the position 
 of the visual field'of the eye has to be adjusted to the movements 
 of the animal's head. No matter what position this occupies, 
 whether it be depressed, as in grazing, or elevated by the muzzle 
 
 Fig. 168. 
 
 -Various Positions of the Horse's Head relative to the 
 Direction of the Pupil. 
 
 The pupils cannot be seen, but the alterations in the position of the orbits are 
 well shown as the result of depressing or elevating the head. 
 
 being pressed upwards, the pupil remains practically horizontal. 
 This peculiarity can be especially well studied in animals with a 
 horizontal pupil, which acts like a spirit level, and demonstrates 
 unmistakably the wheel-like rotation of the eye (Fig. 168). When 
 the head is in the normal position the pupil is horizontal; if there 
 was no torsion of the eyeball the nasal side of the pupil during 
 grazing would be placed obliquely downwards, and in the elevated 
 head obliquely upwards — almost vertical, indeed, resembling 
 that of the cat. The oblique muscles cause the globe of the 
 eye to be twisted obliquely either upwards or downwards 
 around its antero-posterior axis, in order to insure that the visual 
 axis of the eye is always so presented to the outer world that 
 the picture on the retina will be parallel to the horizon. When 
 the superior oblique contracts it pulls the temporal side of 
 
572 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the eyeball upwards, and if it were not antagonised by the 
 inferior oblique, it would render the pupil almost vertical; 
 the inferio r oblique pulls the tempo ral side of the eyeba ll down- 
 ward's! This wheel-lTEe movement ls~regulated from the semi- 
 circular canals of the ear. 
 
 If the head be greatly elevated, the eyeball is depressed in the 
 socket, the cornea partly disappears beneath the lower lid, 
 and the sclerotic shows considerably above (see Fig. 168) . This 
 is part of the mechanism for keeping the visual axis of the eye 
 parallel to the horizon, and is due to a conjugate action of the 
 inferior rectus with the superior oblique. 
 
 It has been remarked that animals largely supplement eye 
 movements by head movements, and this is true. The chief 
 movements of the eyeballs are backwards and forwards, corre- 
 sponding to outwards and inwards in man, and for this action 
 the internal and external recti are responsible, but neither 
 of these can maintain a horizontal pupil; they have accordingly 
 to work in conjunction with the oblique muscles. In grazing, 
 the superior rectus is of especial use by pulling the eyeball up- 
 wards, and so keeping the visual axis of the eye exposed to the 
 horizon, while the oblique muscles keep it level. Animals do 
 not require to look to the ground in grazing; though the face 
 narrows below the eyes, it is impossible for them to see what they 
 are eating; they are guided entirely by the sense of smell, their 
 eyes being ' on guard.' 
 
 The anatomical arrangement of the oblique muscles enables 
 them to carry out their function of maintaining a horizontal 
 field of vision. The loop through which the superior passes before 
 joining the eyeball alters the direction of the pull, while the 
 inferior oblique has its origin remote from the recti and superior 
 oblique, and is so placed as to act at right angles to the globe. 
 Both muscles are worked from the nasal side of the orbit, but 
 inserted into the temporal side of the globe, which arrangement 
 affords a large range of movement. 
 
 The re tractor muscle envelops the optic nerve and encircles the 
 eyeball ; it 1^ inserted into the surface of the sclerotic behind the 
 recti and oblique muscles. Its chief function is to withdraw the 
 eyeball within the socket, and so work the membrana nictitans. 
 By some it has been supposed to prevent the eyes projecting 
 during grazing, but the recti could effect this; in fact, the recti 
 acting simultaneously could retract the eyeball. It is not 
 improbable that the retractor assists the recti, especially the 
 inferior rectus. It is a voluminous muscle, and its lowest 
 bundles run a long way forward beneath the eyeball. The 
 antagonistic resistance to the backward pull of the recti and 
 retractor muscles is said to be furnished by smooth muscle fibres 
 
THE SENSES 573 
 
 in the palpebral portion of the capsule of Tenon; this capsule is 
 inserted into the margin of the orbit as well as the upper lid. 
 
 Nerve Supply of Ocular Muscles. — -No fewer than three pairs 
 of cranial nerves are connected with the motor innervation of the 
 seven muscles (p. 552). The third fair supplies all the muscles, 
 with the exception of the external rectus and superior oblique, 
 as well as the raiser of the upper lid. The sixth -pair supplies the 
 external rectus and part of the retractor. The superior oblique 
 is innervated by the fourth fair, so that this one muscle has an 
 exclusive cranial nerve-supply. The retractor muscle is 
 supplied partly by the third and partly by the sixth pair. 
 It is evident from what has been said that the muscles of the 
 eyeball exhibit co-ordinate action; for example, if the sixth pair 
 is acting on the external rectus, the fourth pair must also be 
 in operation on the superior oblique, or else the pupil would not 
 be horizontal. Similarly, if the internal rectus is contracting 
 and a portion of the third pair thus functioning, the oblique 
 muscles must also be in action to maintain the pupil horizontal. 
 There is no movement of any of the recti muscles which is not 
 associated with movement of the oblique muscles. When the 
 movements of one eye are considered in relation to those of its 
 fellow, the matter becomes more complex. Where the eyes are 
 placed to the front it is evident that during the to-and-fro 
 movements different muscles are being employed in either eye. 
 If, for example, the eyes rotate to the right, the external rectus 
 of the right eye is co-operating with the internal rectus of the 
 left; such movements are called .conjugate. Where the eyes are 
 placed wide apart and situated towards the lateral sides of the 
 face, as in the horse, the movements are not the same as in 
 man; there is still, as with him in binocular vision, the outward 
 and inward movements, better described as backward and 
 'forward, in which the muscles of the right and left eyes are 
 conjugate, but there is, in addition, binocular vision, in which 
 the movements of the eyes are in offosite directions, the right, 
 for example, turned in to the left, and the left turned in to the 
 right. This produces, so to speak, a double internal squint, 
 and is the only position of the eyes in which the horse can see 
 the same object with both of them at one and the same time. 
 In this movement it is evident that the muscles of the eyeball 
 are no longer conjugate, but offosite ; for the internal recti and 
 the oblique muscles of both eyes contract simultaneously. 
 
 An interesting question here arises: When a horse is looking 
 back at an object with one eye, does the opposite eye, which 
 cannot see it, also look back ? The answer is in the negative. 
 The movements of the two eyes are conjugate— i.e., while the 
 right is looking back, the left is looking forward. There are 
 
574 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 animals, however, capable of looking behind with both eyes, in 
 which case the movement is opposite and not conjugate; this 
 occurs in the hare and in birds.* The horse is also capable of 
 seeing an object behind him with both eyes. The vision is very 
 limited, for the animal invariably turns the head, if ever so 
 slightly, in order to view the object with one eye and so obtain 
 a larger visual field. 
 
 We can now turn to the globe of the eye, and consider its con- 
 struction in special relation to its function. 
 
 The Cornea in most animals is circular, but in the horse and 
 other domestic herbivora it is oval. If it be divided into two 
 portions by a vertical line through the centre of the eye, it 
 will be found larger on its nasal than on its temporal side. 
 As a consequence of this, the curvatures of the cornea in the 
 two meridians of the eye are not the same; the meridian in the 
 horse, which is nearly always the flatter, is the horizontal. This 
 excess of curvature of one meridian of the cornea over that 
 of the meridian at right angles to it produces a defect in vision 
 known as astigmatism. The cornea is a remarkably tough 
 membrane, non- vascular, and richly supplied by nerves. It is 
 nourished by lymph, which circulates freely through it. Glisten- 
 ing and transparent, it possesses in the horse -a very narrow, 
 opaque rim confined to the nasal and temporal portions of its 
 circumference, where it is fitted into the sclerotic. The fitting 
 is similar to that of a watch-glass into its case, for it thins out 
 and runs under the sclerotic, which is here also thinned out. 
 The conjunctiva of the eyeball ends at the rim of the cornea, 
 and is here deeply pigmented. There are several layers to the 
 cornea, one of them, the membrane of Descemet, being extra- 
 ordinarily tough and quite vitreous in specimens hardened in 
 formaline. The epithelium which lines the inner concave face is 
 for the purpose of protecting the cornea against the watery fluid* 
 with which it is in contact — i.e., the aqueous humour. 
 
 The cornea is one of the refracting surfaces of the eye; in 
 man it is regarded as the principal refracting surface, but in the 
 horse and dog the lens refracts more strongly than the cornea 
 (Matthiessen). By reason of its prominence and of the great 
 length of its horizontal meridian in herbivora, the cornea 
 extends the field of vision, and so allows very oblique rays to 
 be refracted. Johnsdhf points out that the cornea is large in 
 all animals, having a divergence of more than 50 degrees to 
 60 degrees, and that in addition such animals have nearly 
 spherical lenses, the spherical lense affording a wide angle of view. 
 
 * W. Harris, M.D., ' Binocular and Stereoscopic Vision,' Brain, part cv., 
 1904. 
 
 t ' Comparative Anatomy of the Mammalian Eye,' G. L. Johnson, M.D., 
 Phil. Trans., vol. cxciv., B. 194, 1901. 
 
THE SENSES 575 
 
 The Sclerotic makes up the greater part of the surface of the 
 eyeball. It is a dense, bluish- white, fibrous tunic; thickest 
 behind, it thins at the equator, but is thinnest where its edge 
 overlaps the cornea; it is also thicker on the temporal than on 
 the nasal side, where it is protected from injury. It has a 
 limited blood-supply, as might be supposed from its fibrous 
 nature. It surrounds the globe of the eye to the extent of 
 about four-fifths, and gives shape to the part. The eyeball is 
 not a sphere in the herbivora; it is distinctly flattened posteriorly 
 (Fig. 166), whereas in the carnivora it is more spherical. The 
 cornea, which is let into its anterior face, is placed, not in the 
 centre, but below the centre of the globe. This is especially marked 
 in the horse (see Fig. 166). The length of the globe, measured 
 from the cornea to the retina, averages in the horse 43-68 mm. 
 (Nicolas and Gray); in the ox it is much shorter, 35-34 mm. being 
 a mean, and the eyeball of this animal generally is much smaller 
 than that of the horse. As a matter of fact, the antero-posterior 
 diameter of the eye is liable to variation, which affects the re- 
 fraction, as will be seen later. We have found the extreme 
 measurements in the horse to be from 39-00 mm. to 42-75 mm. 
 (if inches to 1^ inches). 
 
 The optic nerve, running in the substance of the retractor 
 muscle, pierces the posterior wall of the sclerotic low down 
 on its temporal side, after making a peculiar curve or dip 
 (Fig. 166). This curve is necessitated by the horizontal move- 
 ments of the eyeball; when the eye looks back the curve is 
 increased, when it looks fogvard the ' slack ' of the nerve is taken 
 up and the curve entireflpaisappears. 
 
 The Choroid lies within the sclerotic. It may be regarded as 
 the tunic which principally attends to the vascular supply; 
 it furnishes an internal reflector to the retina by means of the 
 tapetum, while its extension into the anterior region of the eye 
 provides the iris, ciliary body, and ciliary processes. 
 
 The greater part of the choroid lies between the retina and 
 sclerotic, enveloping and lining the whole of the vitreous or dark 
 chamber as far forward as the ciliary processes. It consists of 
 several layers : one, the vascular, for large vessels ; a second, the 
 capillary; and a third, the tapetum, which is the interesting 
 physiological coat. 
 
 The Tapetum Lucidum is a peculiar, brilliant area of the 
 posterior wall of the eye found in all domesticated animals 
 but the pig. Its brilliant and beautiful iridescent colouring 
 of yellow and bluish-green gives the appearance of pigment having 
 been removed from the choroid.* The area of the tapetum is 
 
 * The pigmentary layer of the retina is always adherent post-mortem to 
 the choroid, and for a long time was believed to be a choroidal tunic. 
 
576 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 limited in size, being in the horse about 40 mm. in width and 
 25 mm. in depth, but there is a good deal of the tapetum 
 still possessing its peculiar colouring, which is covered by dense 
 retinal pigment, and therefore not exposed. The shape of the 
 tapetum is roughly triangular, the base being just above the 
 optic papilla. The tapetum is, therefore, in the optical axis of 
 the eye. The colour in herbivora and carnivora, though not 
 due to identical structural arrangement, is in both cases brought 
 about by the decomposition of light, such as is diagrammatically 
 shown in Fig. 165. In herbivora the decomposition is effected 
 by the presence of a laminated fibrous structure; in carnivora 
 by iridescent cells (Nicolas and Gray). 
 
 In whatever way the colour is caused, it produces a most 
 beautiful object as viewed through the ophthalmoscope (Plate I., 
 Fig. 4). Metallic and bluish-greens and yellow predominate in 
 herbivora, yellow and orange in carnivora. There are no vessels 
 in the tapetum; the surface in the horse is delicately stippled 
 with brownish-green spots. The tapetum ends abruptly, there 
 being almost a sharp line between the brilliant reflecting area 
 and the dull brown retinal pigment of the surrounding choroid, 
 a part which is sometimes spoken of as the tapetum nigrum. 
 In albino and piebald horses the appearance of the tapetum 
 is still more beautiful ; there being no pigment, the whole of the 
 back of the eye appears as a brilliant rainbow of colours in a 
 pink setting. The appearance of the tapetum varies in different 
 animals; that of the ox resembles the tapetum of the horse, 
 but without the stippling; in the sheep the colour may be 
 green or sky blue; in the dog it is usually golden-yellow in the 
 middle, brilliant green at the periphery (Nicolas and Gray). 
 Gray, however, points out that the tapetum of the dog varies 
 greatly in colour owing, as he says, to inbreeding. 
 
 The function of the tapetum is to enable animals to see better 
 in a dim light; it cannot, of course, enable them to see in the 
 dark. It acts as a concave reflector to the sensitive layer of the 
 retina, so that the rays, having passed through the retina, are 
 reflected back to it by the tapetum. In this way the retina is 
 twice impressed. This is the view expressed by Johnson, who, 
 however, thinks that in animals with a tapetum the usual 
 image is formed, not on the retina, but on the tapetum, and from 
 there reflected forward on to the innermost layer of the retina. 
 
 The Ciliary Body (Fig. 166) is a large, deeply pigmented, ring- 
 like structure with stellate processes, placed at the anterior part 
 of the globe; the processes indicate the termination of the choroid 
 as a tunic. The ciliary body is attached at its circumference 
 to the sclerotic; it is pierced centrally by a circular opening 
 into which is fitted the lens of the eye. The ciliary processes, 
 
THE SENSES 
 
 577 
 
 numbering a hundred or more in the horse, give the part a pleated 
 appearance, and resemble the laminae of the feet. The processes 
 are not all the same length, those on the nasal side being much 
 shorter than elsewhere. Fig. 166 shows the ciliary zone or 
 body in section; it is wedge-shaped, the base being on the 
 circumference of the ring. Fig. 169 shows it viewed from behind. 
 It was Henderson* who first pointed out the advantage of the 
 ciliary processes being short on the nasal side, thereby enabling 
 an animal to see out of the ' tail ' of its eye by permitting the 
 retina to be prolonged forward, by which means the outer 
 boundary of the field of vision is extended. 
 
 Fig. 169. Inner Surface of Anterior Part of Left Eyeball of Horse 
 
 (Equatorial Section) (Sisson-Ellenberger). 
 
 i, Sclerotic; 2, choroid; 3, retina (drawn away from choroid); 4, the ciliary pro- 
 cesses which it will be noted are much shorter on the nasal side; 5, crystalline 
 lens through which the pupil (6) may be seen. 
 
 The use of the ciliary body, together with its processes, is to 
 furnish an immovable diaphragm to the eye, and so complete the 
 dark or vitreous chamber; it also provides the setting into which 
 is fixed the lens, which is adjusted on the ends of the ciliary 
 processes (Fig. 166); further, it secretes the fluid found in the 
 anterior chamber of the eye, and nourishes the vitreous in the 
 dark chamber. By its pleated or laminated structure it gives 
 attachment to the zonule of Zinn. This is a portion of the 
 hyaloid membrane of the vitreous to which the capsule of the 
 lens is attached. The zonule digitates with the processes much 
 
 * ' Note on the Comparative Anatomy of the Ciliary Region,' T. 
 Henderson M.D., British Medical Journal, November 4, 1911. 
 
 37 
 
57» 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 as the laminae of the feet digitate, and the lens is thereby secured 
 in its setting. The ciliary body and processes are highly vascular 
 and deeply pigmented. 
 
 The Iris is the movable diaphragm of the eye placed behind the 
 cornea and in front of the ciliary body and processes (Fig. 166) . 
 It is part of the ciliary body, but a space, known as the posterior 
 chamber, exists between it and the processes; this posterior 
 chamber is a portion of the anterior chamber of the eye, and the 
 aqueous humour has free access to it. The iris in all the domestic 
 herbivora is of a brownish tint — deeply brown in the horse, of a 
 brighter brown in the ox, and brownish-yellow in the sheep. In 
 the dog it is brown or golden-yellow, in the cat greenish or amber, 
 and in the pig a grey -brown or yellow-brown, sometimes bluish. 
 Occasionally the iris of the horse shows partial or complete 
 albinism, as in the so-called ' wall-eye.' In cream-coloured 
 
 horses it is wholly 
 without pigment, and 
 in piebalds the same 
 will be found, unless 
 there are splashes of 
 black on the face near 
 the eye, in which case 
 it may be partly or 
 wholly pigmented. In 
 the foal the iris is grey, 
 the pigment appearing 
 later. 
 
 In the centre of 
 the iris is an opening 
 known as the Pupil. 
 This opening is horizontal in the domestic herbivora and pig, 
 vertical and elliptical in the cat, circular in the dog. The pupil 
 is capable of movement; it contracts in bright light and dilates 
 in low degrees of light; this movement is brought about by two 
 unstriped muscles, a circular and a radiating, each of which 
 has a separate nerve-supply. The circular muscle contracts the 
 pupil, the radiating dilates it. The bleached iris of the horse 
 is shown in Fig. 170; the circular muscle is seen surrounding 
 the pupillary opening; the dilator muscle is on the posterior face. 
 (Nicolas and Gray), and produces the wrinkles and pleats shown 
 in Fig. 170 on the anterior face; the posterior face of the iris is 
 smooth. In a contracted condition the iris is in contact with the 
 lens. As a consequence, it is not placed vertically in the eye, but 
 bulges forward, so that in hardened specimens it is convex on its 
 anterior and concave on its posterior face (Fig. 166). Though 
 we have stated that stimulation of the retina by light is the 
 
 Fig. 170. — Bleached Iris of the Horse. 
 
 , Pupillary opening; 2, corpus nigrum; 3, iris 
 showing wrinkles and pleats in its structure; 
 4, circular muscle surrounding the pupillary 
 opening; 5, Ligamentum Pectinatum. 
 
THE SENSES 579 
 
 natural method by which alterations in the size of the pupil are 
 effected, in the horse this is not strictly true. In direct sunlight 
 the pupil is a mere chink, 'but in ordinary diffused daylight it 
 barely responds, or should it contract it does so but slightly. 
 Even when diffused light is concentrated on the eye by means 
 of a mirror the iris remains practically unchanged; under the 
 influence of artificial light it actually dilates. This permits the 
 fundus of the eye of the horse being examined ophthalmoscopically 
 without the use of atropine. In all herbivora the pupil is relatively 
 sluggish in response to the stimulus of light, the contraction 
 being greater in the exposed than in the opposite eye. In the 
 cat the reaction of the pupil is brisk and sharper than in the dog. 
 Harris* regards the sluggish pupil of herbivora as indicating a 
 low standard of visual acuity, and considers that this is com- 
 pensated by greater keenness of hearing and smell. He points 
 out that they are animals which are hunted instead of hunting, 
 and that hearing and smell with them give earlier indications of 
 approaching danger than sight. The shape of the pupil even 
 is in this connection a matter of great physiological interest. An 
 elongated pupil horizontally placed affords a large visual field, 
 such as is required in single-eye vision by animals which are 
 preyed upon; and, as suggested by Matthiessen, it also affords 
 more acute vision for objects peripherically situated in a horizontal 
 field, a great advantage to the herbivora when exposed to 
 attack. The shape of the cornea in these animals is such that 
 the pupil corresponds to its least curved meridian. Very oblique 
 rays of light can enter the eye with a horizontal pupil, especially 
 from the temporal side — i.e., from behind the animal — and, as 
 we have seen previously, the retina is prolonged on the nasal 
 side in order to receive these oblique rays. We have also seen 
 that the pupil must remain horizontal, no matter what position 
 the head assumes, and have studied the method by which this 
 is effected. 
 
 In the condition of dilatation the pupil of the horse loses its 
 elliptical shape and becomes circular. It dilates after a gallop; 
 it dilates widely after a violent death, contracting in the course 
 of a few hours until it becomes a mere slit. It also dilates under 
 the influence of atropine and cocaine, and contracts under 
 the effect of morphine and eserine. Dilatation of the pupil is 
 spoken of as mydriasis, and contraction as myosis. Drugs which 
 dilate the pupil are described as mydriatics, and their antagonists 
 as myotics. Around the attached margin of the iris— i.e., at the 
 corneo-scleroid junction— a rim of peculiar spongy-looking tissue 
 exists, known as the Ligamentum Pectinatum (see Fig. 170). 
 
 * ' Binocular and Stereoscopic Vision in Man and Other Vertebrates,' 
 W. Harris, M.D., Brain, part iv., I9°4- 
 
5 8o 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 It is, of course, a portion of the iris, but quite distinct in ap- 
 pearance; it is traversed by canals, crevices, and spaces, which 
 lead into the lymphatic system of the eye. The function of 
 this tissue is to carry off the aqueous humour which is always 
 being secreted, and so maintain the normal tension of the 
 eyeball. 
 
 The Nerve Supply of the Iris is through the third cranial 
 nerve, which supplies the circular muscle of the iris, and the 
 sympathetic, which supplies the dilator muscle (Fig. 171). The 
 third nerve, as we have already seen, is intimately connected 
 with the innervation of the voluntary muscles of the eyeball. 
 Through the ciliary ganglion it supplies autonomic fibres to the 
 iris, and its nucleus is linked up with the visual path in the brain. 
 If the ciliary fibres of the third pair be divided, the pupil dilates 
 
 Mia 
 
 n > > >jr 1 ^| Bta'tlt. 
 
 v.onsui.clot Muscle. 
 
 Dilator Mu&cl* 
 
 Jup. letuUal 
 Gcuidlton.. 
 
 tptpatkeuc .Netue. 
 
 Fig 171. — Nerve Supply to Iris. 
 
 under the unbalanced action of the sympathetic. The fibres 
 for, the radiating muscle, derived from the sympathetic, take 
 a very circuitous course. They arise in the mid-brain and travel 
 down the cord, terminating in cells in the lower cervical region. 
 Here fresh neurones arise, which emerge from the spinal cord by 
 the eighth cervical and first three thoracic nerves, and travel up 
 the neck in the cervical sympathetic, connecting with the superior 
 cervical ganglion ; they then pass to the Gasserian ganglion, and 
 by means of the long ciliary nerves in the ophthalmic division of 
 the fifth pair they reach the iris. If the sympathetic supply 
 be . divided, the iris contracts under the unbalanced influence of 
 the third pair. In a normal condition both nerves, through the 
 connections of the retina with the brain, are receiving impulses 
 which neutralise each other, so that the iris is responsive to any 
 
THE SENSES 581 
 
 excess of impulses which disturbs the balance. The balanced 
 condition is an example of a true reflex act. 
 
 In all domesticated animals, but especially well marked in the 
 horse, there exist at the centre of the pupillary edge of the iris one 
 or more black bodies known as Corpora Nigra (Figs. 166, 170). 
 They are largest on the upper edge of the pupil in the horse, though 
 a small one may be seen on the lower edge. Johnson, who studied 
 the comparative anatomy of these bodies, pointed out that in the 
 camel there was an upper and a lower row, which interlocked 
 when the pupil was fully contracted. In the horse, when the 
 pupil is strongly contracted in direct sunlight, these particles of 
 ' soot ' divide the horizontal pupil into an inner and an outer 
 portion. It would appear that this double pupil might cause an 
 imperfect image to be imprinted on the retina, but on subjecting 
 the question to actual experiment with the camera, no broken 
 image was found. The use of these bodies is to absorb rays of 
 light, but their position on the centre of the pupil would not 
 appear to be the most suitable one for this purpose, and they 
 possibly have some other function. 
 
 Lying outside the ligamentum pectinatum and the ciliary body 
 is a narrow rim of tissue known as the Ciliary Muscle (Fig. 166) ; 
 it is of the involuntary type. In all animals it is small as com- 
 pared with the same muscle in man, and its fibres are mainly 
 longitudinal, whereas in man they are both longitudinal and 
 circular. This muscle is connected with the important question 
 of accommodation, or the focussing of the eye for near objects, 
 and will be again referred to when this subject is dealt with. 
 The nerves of the ciliary muscle are derived from the short 
 ciliary branches of the third pair, and issue from the ciliary 
 ganglion. It will be seen that the third pair of nerves is 
 connected with movements of the eyeball, of the iris, and of 
 the ciliary muscle. This is no accidental circumstance, but 
 co-ordinates the direction of the eyes with the closure of the 
 pupil and the contraction of the ciliary muscle; these are simul- 
 taneous movements necessary to the act of accommodation. . In 
 man the constrictor fibres of the iris and the fibres of the ciliary 
 muscle are paralysed by atropine, but in the horse, dog, cat, 
 and other animals, the ciliary muscle is not paralysed by this 
 alkaloid, though it acts, as it does in man, on the constrictor 
 fibres of the iris. 
 
 The Retina is an outgrowth from the brain, placed in the orbit 
 for the purpose of receiving the picture of the outside world. It 
 retains its connection with the brain by means of a large stalk, 
 the optic nerve. It is merely a receptive surface, and constitutes, 
 according to the modern classification of nerves, a receptor organ, 
 which, in point of age, is the oldest but one of the primitive brain. 
 
5«2 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Though it receives the picture it cannot see it; that is effected 
 by a special area of the brain. If this area be damaged, sight 
 is lost, though the retina may be in perfect working order. 
 The optic nerve arises from the external geniculate body, the 
 posterior portion of the optic thalamus, and the anterior corpus 
 quadrigeminum ; by means of the optic tract the fibres proceed 
 from the brain, converging towards the tract from the opposite 
 side, and meeting it in the optic chiasma or commissure (Fig. 172) . 
 At the commissure the majority of the fibres cross, so that right 
 
 brain fibres pass to the 
 left eye, and vice versa ; 
 about one-sixth of the 
 fibres in the horse do not 
 cross, but proceed direct — 
 i.e., right brain to right 
 eye. This crossing of the 
 nerves is a question of 
 physiological interest and 
 clinical importance. In 
 animals with eyes laterally 
 placed — i.e., fishes, am- 
 phibia, reptiles, and birds — ■ 
 there is complete decussa- 
 tion — that is, all the fibres 
 from the right brain pass 
 to the left eye. In all 
 animals higher than these 
 there is partial decussation, 
 as the eyes are not so 
 laterally placed; at first 
 only a small number of 
 fibres pass direct — i.e., right 
 brain to right eye — but as 
 the monkey and man are 
 approached a considerable 
 number of fibres pass direct 
 — about two-fifths in man. Here vision is two-eyed and forward; 
 in a lateral direction it is imperfect. The optic nerves, which 
 gain exit from the cranium en route, proceed from the chiasma 
 to their respective eyes, diverging more and more as they 
 approach the eyeballs; before reaching the sclerotic they make 
 a dip of about half an inch (see p. 575), and penetrate the 
 globe low down and to the temporal side of the posterior pole of 
 the eye. The nerve is constricted as the result of penetrating 
 the sclerotic (Fig. 166), and having reached the choroid it loses 
 its stalk-like character and expands into a large gauze-like veil, 
 
 Fig. 172. — Scheme of the Visual Path 
 (Halliburton, after Schafer). 
 
THE SENSES 
 
 583 
 
 Rods. 
 
 Cones. 
 
 the retina. This change from solid nerve to a flocculent-looking 
 membrane occurs at the optic papilla, which is situated on the 
 choroid (see Plate I., Fig. 4, and Fig. 166). The papilla is about 
 5 mm. in width and 3-5 mm. in depth in the horse, in which 
 animal it is ovoid; in most others it is circular. The optic 
 papilla is found below the tapetum in the pigmented area; it 
 lies so deep in the eye of the horse that it can be seen with 
 the ophthalmoscope only by 
 looking downwards over the 
 lower margin of the pupil. 
 
 The retina consists of 
 many coats, the only one 
 connected with vision being 
 that which, excluding the 
 pigmentary layer, is farthest 
 from the light, and con- 
 sequently nearest to the 
 choroid. This layer is known 
 as that of the rods and cones 
 (see Fig. 173) ; these are the 
 essential visual elements of 
 the retina; whenever they 
 are absent the part is insen- 
 sitive to light, as, for ex- 
 ample, at the optic papilla, 
 which is blind. Each cone 
 is connected with a single 
 nerve-cell, but there may 
 be only one nerve-cell to 
 several rods; the cone is 
 consequently regarded as 
 offering a more direct con- 
 ducting path than the rod. 
 Over the area of most acute 
 vision in man cones only are 
 found — for instance, at the 
 fovea centralis, which is absent in all animals below man and the 
 higher apes, with the exception of certain birds. Here the retina 
 is thinned out until nothing but cones remain. This is in the 
 centre of the field of vision in man; it is a little to the temporal 
 side of the optic disc, and visual acuity diminishes rapidly 
 as the image falls away from the fovea or region of central vision. 
 The field of vision outside the central is known as the peripheral 
 or excentric field; another term for it is the panoramic field. 
 In the lower animals there is no means of knowing with precision 
 which area of the retina is the most sensitive, but it is possible 
 
 Fig. 173. — Diagram of Structure'of 
 Retina (Bowditch, after Cajal). 
 
 A, Layer of rods and cones; B, external 
 nuclear layer; C, external molecular 
 layer; E, internal nuclear layer; .F, in- 
 ternal molecular layer; G, layer of gang- 
 lion cells; H, layer of nerve fibres. 
 
5&4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 to arrive at a conclusion indirectly. Most, of the herbivora raise 
 and draw back the head when staring intently at an object; 
 this is especially well seen in the horse, who appears to draw his 
 body up as if collecting himself for a sudden move, at the same 
 time raising the head high. In this attitude the area of the 
 tapetum may be regarded as that of acute vision, and we may 
 consequently look upon the optical axis of the eye as agreeing 
 with the visual axis. 
 
 It is in the rods and cones that the primary conversion of light 
 vibrations into visual impulses takes place. The rods contain a 
 pigment known as Visual Purple or Rhodopsin, which is readily 
 decomposed by light, especially by the blue and violet rays, 
 by which the pigment is decolourised. The colour is regained 
 in a dim light. The perception of colours, a question which will 
 be dealt with later in considering the theory of vision, is not due 
 to the rods. The cones are regarded as the visual cells in bright 
 light, and provide acute vision; the rods are adapted for dim 
 light, as is proved by their decolourisation by a bright light, 
 and the fact that they are numerous in animals which seek their 
 food by night. It is the tapetum and the rods which enable 
 feline animals to prey by night, and the same structures enable 
 the herbivora to find their way and move freely in what to us is 
 darkness. 
 
 As the rods and cones are nearest to the choroid, while the nerve 
 fibres and cells are nearest to the vitreous, it follows that light 
 has first to traverse the retina to reach the visual layer, and is 
 then reflected by the tapetum; the impulses set up in the rods 
 and cones then travel towards the vitreous in order to reach the 
 nerves which subsequently form the optic. The whole area of 
 the retina is sensitive up to the ciliary processes; in fact, it has 
 been shown that in the horse and other animals the rods and 
 cones are better developed in the temporal peripheral zone than 
 elsewhere, probably, as Nicolas and Gray remark, in connection 
 with binocular vision . There is no tapetum, or rather none exposed 
 in the peripheral zone, it being covered by the dense pigment 
 of the retina; it is obvious that reflection cannot occur here. 
 We may therefore assume that night vision in animals is central, 
 and not peripheral. 
 
 The Lens fits into a circular setting formed by the ciliary 
 processes, and there rests upon the broad edges of the processes 
 (Fig. 166). It is contained in a capsule; the capsule is attached 
 to a suspensory ligament, formed by the hyaloid membrane of 
 the vitreous known as the zonule of Zinn ; this ligament firmly 
 digitates with the ciliary processes, and in hardened specimens, if 
 the lens be removed from its setting, points will be seen around its 
 equator where the zonule is attached. The lens is biconvex, the 
 
THE SENSES 585 
 
 convexity of the posterior face being greater than that of the 
 anterior in herbivora, while the converse is true of felines. In 
 the horse its antero-posterior diameter varies ; we have found it 
 10-5 mm. to 14 mm. (f to f inch). It is made up of many onion- 
 like layers, each having a different refractive index and different 
 curvature. Its mean refractive index in the horse and dog is 
 1-49; in man it is 1-39. Its focal length in the horse is 15-5 
 diopters {%\ inches), while that of the cornea is 12-6 diopters 
 (3^ inches) (Nicolas and Gray), so that the lens of this animal 
 (and the same remark applies to the dog) refracts more than the 
 cornea, which is the reverse of what occurs in man. The lens 
 is an elastic body, as may be seen by puncturing its capsule 
 when the lens becomes herniated; by means of its capsule it 
 is kept in a state of tension. The capsule being attached to the 
 suspensory ligament, and the latter dovetailing with the ciliary 
 processes, the lens is flattened when the capsule is pulled upon, 
 and bulges of its own elasticity when the tension is removed. 
 A further study of the question of the alterations in the shape 
 of the lens will be made when dealing with accommodation. 
 
 The Humours of the Eye are the aqueous in the anterior 
 chamber, and the vitreous in the dark chamber. The aqueous 
 humour is generally regarded as a secretion by the ciliary pro- 
 cesses, some say from the iris; others regard it as a filtrate 
 from the blood. It bathes the tissues in the anterior chamber, 
 and in it are immersed the whole of the iris, the anterior surface 
 of the ciliary processes, the anterior face of the lens, and the 
 posterior face of the cornea. Chemically it is a watery fluid 
 poor in solids, and is in reality lymph. It is carried off as rapidly 
 as formed through the lymphatic channels of the ligamentum 
 pectinatum, which communicate with the veins. The fluid in 
 the anterior chamber is always under pressure equal to 25 mm. 
 of mercury; this maintains the curvature of the cornea. The 
 same pressure which keeps the cornea curved is exerted on the 
 anterior face of the lens and on the two surfaces of the iris. 
 Owing to this pressure, if the cornea be punctured, the fluid 
 spirts out some distance. The cornea now collapses, and a portion 
 of the iris even may protrude, but so rapidly is the fluid secreted 
 that in about twenty-four hours the curvature of the cornea is 
 restored, and in a day or two the puncture is invisible (unless 
 the iris has been detained in the wound) . After death the process 
 of drainage appears to continue, while that of secretion ends, 
 so that the cornea shrinks and the anterior chamber collapses. 
 The lymphatic drainage of the anterior chamber sometimes 
 fails through blocking of the channels. This gives rise in man 
 to excessive tension of the eye [glaucoma], which becomes of 
 stony hardness, and total loss of sight follows unless the tension 
 
586 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 is relieved. The aqueous humour is prevented from exercising 
 any injurious effect on the cornea by the epithelium and lining 
 membrane of the chamber, the membrane of Descemet, which is 
 more glass-like than the cornea itself. The refractive index 
 of the aqueous humour is the same as that of the cornea (i'377). 
 
 The Vitreous Humour is totally different in appearance from 
 the aqueous, being like molten glass. It is enveloped and 
 intersected, in all directions by a membrane, the hyaloid, which 
 prevents it from coming into actual contact with the retina. 
 Anteriorly this membrane is arranged to form the suspensory 
 ligament of the lens, as already described. The vitreous contains 
 much mucin and a small percentage of solids ; its refractive index 
 (1-335) is much the same as that of the aqueous. Its chief function 
 is to maintain the intra-ocular pressure in the dark chamber 
 and keep the delicate concave retina in position. By its pressure 
 on the lens it keeps the suspensory ligament in a condition of 
 tension, its usual state, as it is only for near objects that any 
 relaxation of this tension is required. 
 
 The Visual Areas in the brain must now be considered. 
 In animals possessing the old primitive brain — fishes, for example 
 ■ — the visual path terminates at the mid-brain, as these animals 
 possess no cerebrum. As the scale of animal life is ascended 
 and the cerebrum develops the visual path extends from the 
 mid-brain to the cerebrum, where a visual region arises in the 
 occipital portion. The neurone system is as well marked in 
 the eye as elsewhere (see Fig. 172); the first neurone exists 
 between the retina and mid-brain, the optic fibres being axons 
 of the nerve-cells of the retina. By synapses these are connected 
 with a second neurone, from which issues the occipito-thalamic 
 fibres, running from the mid-brain to the cerebrum (see Fig. 153, 
 p. 523). The visual cortex is situated in the occipital region 
 of the cerebrum, in the area known as the calcarine fissure. 
 Here is found a white line, known as the line of Gennari, which 
 represents a layer of. special nerve-fibres placed in the grey sub- 
 stance of the cortex. This line, which can be readily traced, 
 encloses an area known as the visuo-sensory. The size and 
 complexity of structure of the area increases as the animal scale 
 is ascended. Mott* points out^that the more the animal depends 
 on vision as a directive faculty in its preservation, the more 
 complex the structure of the visual cortex becomes. Thus the 
 pyramidal layer of cells in the visual area forms a deep layer in 
 the cat, less deep in the dog, and this he regards as associated 
 with an increase in the perfection of binocular vision. On to the 
 
 * ' The Progressive Evolution of the Structure and Functions of the 
 Visual Cortex in Mammalia,' by F. W. Mott, M.D., F.R.S., Archives of 
 Neurology, vol. iii. , 1907. 
 
THE SENSES 
 
 587 
 
 * 
 
 
 1 
 
 
 '^ ! 
 
 >,' 
 
 ■7" 
 
 visuo-sensory areas the retinae are projected, not, it will be 
 observed, by retinal fibres, but by those from the mid-brain 
 (Fig. 172), the right occipital lobe receiving the picture from the 
 two right halves of the retina, the left lobe receiving that from 
 the two left halves (Fig. 174). If a visual area be removed 
 experimentally, blindness results in the two halves of the eye 
 which are represented. The visuo-sensory area' receives the 
 picture, but it does not yp 
 
 interpret or elaborate it. ~? / *T 
 
 This is effected in an area 
 immediately outside the 
 visuo-sensory, known as 
 the visuo-psychic, a part 
 possessing special peculi- 
 arities in structure as com- 
 pared with the sensory area, 
 though in animals this is 
 not nearly so well marked 
 as in man. 
 
 We have seen that in 
 animals with binocular 
 vision the right visuo- 
 sensory area receives the 
 visual impressions from the 
 right half of the retina of 
 each eye, the left lobe 
 receiving the impressions 
 from the two left halves. 
 In pure monocular vision 
 the left brain receives the 
 visual impressions from the 
 
 right eye, the right brain RO, LO, Right and left occipital lobes; RE, 
 
 those from the left eye, so 
 that two distinct pictures 
 are formed at one and 
 the same time. This cross- 
 ing of the visual picture 
 is effected at the optic 
 chiasma. With animals — 
 
 for example, the horse — where the vision is commonly monocular, 
 but also frequently binocular, it is doubtful how the visual im- 
 pressions in binocular vision are received. In these cases, as will 
 be shown presently, the two right halves and two left halves of the 
 retina cannot correspond owing to the width between the eyes. 
 
 Accommodation. — All rays of light proceeding from a distant 
 object may be regarded as parallel, and all those proceeding 
 
 Fig. 174. — Diagram s of Relations of 
 Occipital Cortex to the Retina [in 
 Man (Stewart). 
 
 LE, right and left retina; C, optic chiasma; 
 RF, LF, right and left visual field of each 
 eye. The continuous lines passing back 
 from the retin«e to the occipital cortex 
 represent the crossed, the broken lines the 
 uncrossed, fibres of the optic nerves and 
 tracts. The intermediate stations on the 
 visual path shown in Fig. 172 are omitted. 
 
5 88 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 from an object within 6 metres (20 feet) of the eye may be 
 regarded as divergent. A distant object is one situated any- 
 where between 20 feet from the eye and infinity; an object closer 
 than 20 feet to the eye is called near, and this point extends in 
 man up to 10 to 13 centimetres (4 or 5 inches), at which distance 
 no object can any longer be distinctly seen. The nearest distance 
 at which obj&cts can be distinctly seen is called the near point. 
 Parallel rays need no focussing on the retina, but rays from near 
 objects require focussing owing to their divergent nature; it is 
 evident that the nearer the object to the eye the greater the 
 focussing required. This focussing is brought about by a change 
 in the shape of the anterior surface of the lens ; it becomes more 
 convex for near objects, and this increase in convexity is due to 
 
 FAR 
 
 NEAR 
 
 Fig. 175. — Diagram to illustrate Accommodation (Foster, after 
 Helmholtz). 
 
 C.P., Ciliary process; I, iris; Sp.l., suspensory ligament; l.c.m., longitudinal 
 ciliary muscle; c.c.m.. circular ciliary muscle; c.S., canal of Schlemm. The 
 left half represents the shape of the lens for viewing distant objects, and the 
 right half that for viewing near objects. 
 
 the ciliary muscle drawing forward the choroid coat, and with it 
 the ciliary processes. By this means the tension normally 
 exercised through the zonule of Zinn (the suspensory ligament 
 of the lens) is relaxed, and the lens of its own inherent elasticity 
 bulges forward, and so increases the curvature of its anterior 
 face (Fig. 175). A more convex lens is a more convergent one, 
 and its focus is therefore shorter; in this way the images of near 
 objects are brought to a focus on the retina and distinctly seen, 
 whereas if this increase in curvature had not taken place, the 
 image would have been focussed behind the retina. The power 
 the eye possesses of focussing itself is known as the mechanism 
 of accommodation ; the explanation given above is that of Helm- 
 holtz for the human eye, and is the one generally accepted. 
 It is by no means certain that the mechanism of accommodation 
 
THE SENSES 589 
 
 in animals is the same. Their ciliary muscle is insignificant, 
 though this may be due to the fact that the work it is called upon 
 to perform is not of an exacting type. The objects which a 
 horse looks at within 20 feet of the eye are ordinarily not such 
 as to require any great effort in accommodation. In jumping 
 he must accommodate, and his vision must be precise; in the 
 stable objects are very near, and we know that he avoids them, 
 showing that accommodation occurs, but all of these are large 
 objects compared with reading type. He requires no acute 
 vision for food, for he feeds by the sense of smell. It may, 
 therefore, be that the thin rim of ciliary muscle suffices, assuming 
 the Helmholtz theory applies; but we are personally more 
 inclined to think that the lens undergoes no important alteration 
 in curvature, but rather that the distance from the posterior face 
 of the lens to the retina is capable of alteration, and this would 
 be just as effective for the purpose of focussing as an alteration 
 in the thickness of the lens. Such a mechanism may be supposed 
 to exist in the muscles of the eyeball. This takes us back to 
 the theory of accommodation held before the days of T. Young, 
 when it was thought that the straight muscles squeezed the 
 eye about its equator, thereby increasing its antero-posterior 
 diameter, and so carrying the retina backwards. Young an- 
 nounced 120 years ago that the lens was the focussing medium 
 of the eye, and not the ocular muscles. Nevertheless, in animals, 
 especially the horse, the retractor muscle is of extraordinary 
 size, and out of all proportion to the function of occasionally 
 withdrawing the eyeball within its socket. It completely 
 surrounds the globe, and runs a long way forward on its 
 surface; it seems probable that by means of the ocular muscles 
 accommodation is effected. The shape of the eyeball of the 
 horse, as seen in Fig. 166, suggests that under muscular compres- 
 sion it would lend itself to elongation, especially as an alteration 
 of a few millimetres only is required. In fishes it is known 
 that there is a muscular arrangement which pulls the lens towards 
 the retina, and in amphibia and snakes accommodation is brought 
 about by changes in intra-ocular pressure. One other argument 
 against the ciliary muscle theory of the focussing of the eye is 
 that in the lower animals the muscle is not paralysed by atropine. 
 In man the iris dilates and the ciliary muscle is paralysed, so that 
 accommodation for near objects is impossible. In the horse and 
 other animals the iris dilates, but it is impossible to paralyse 
 the accommodation as judged by the fact that the animal is 
 capable of seeing near objects and of avoiding obstructions. 
 This is strong evidence in favour of some method of accom- 
 modation existing in animals below man and the monkey other 
 than the one generally accepted. 
 
59° A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Catoptric Test. — As bearing on the question of accommoda- 
 tion, reference must be made to the reflections which are seen 
 in the eye when a lighted candle, for example, is held up before 
 it. Three reflections of the candle are seen; a very sharp, large, 
 and bright one from the cornea; a second, duller and smaller, 
 which is reflected from the anterior face of the lens ; and a third, 
 very small, brighter than the middle image, and inverted, which 
 is reflected from the posterior face of the lens (Fig. 176); these 
 are known as the Purkinje- Sanson images. 
 
 In a normal eye all three images are distinctly seen, and equally 
 visible at any point on the reflecting surfaces. When the candle 
 is moved from side to side, and up and down, the images follow 
 it, the two upright moving in the same direction as the candle, 
 the inverted image moving in the opposite direction to the candle. 
 This phenomenon has been taken advantage of in determining 
 
 the freedom of the lens from opacity 
 (cataract) ; though superseded by the 
 || greater accuracy of the ophthalmo- 
 II scope, it is still a valuable aid. In 
 A cataract one or more of the reflections 
 become blurred, and sometimes the 
 a □ p blurred image is duplicated. 
 
 The first and second images are 
 Fig. 176.— Diagram of the erect, inasmuch as they are reflected 
 
 Catoptric Test. r . ,1 ,, . , 
 
 from a convex surface; the third 
 A, From the anterior surface i mage i s i nve rted, as it is reflected 
 
 01 the cornea; B, from the r ., ■ , r 
 
 , anterior face of the lens; and trom tn f posterior surface of the 
 
 C, from the posterior face of lens, which, when viewed from the 
 ' h „ e '™' in the diagram, b front is concave . If therefore, 
 
 and C should have been shown ,, , r ,, , 
 
 as nearer together. tne ^ns Of the horse alters 111 
 
 curvature, as does that of man, in 
 accommodating for a near point, the middle image, when 
 accommodating, will come nearer to the corneal image, and 
 become smaller as the lens becomes more convex. It is certain 
 that this does not take place in the horse. 
 
 Physiological Optics — Passage of Light through Lenses. — All rays of 
 light are diverging, but so slight is the divergence of the rays from 
 distant objects that for the purposes of the eye they are regarded 
 as parallel. All rays proceeding from an object situated at from 
 6 metres (20 feet) to infinity from the front of the eye are considered 
 as parallel rays; all rays within 20 feet from the cornea are diverging 
 rays. Obviously the nearer the object to the cornea the greater 
 the divergence, so that there is more divergence in the rays pro- 
 ceeding from a body 1 foot from the eye than in one 10 feet from 
 the eye; conversely, the farther the object is from the eye the less 
 divergent the rays until that point is reached beyond 20 feet at 
 which the rays may be regarded as parallel. 
 
 A convex lens has two curved surfaces, and a line drawn through 
 
THE SENSES 
 
 591 
 
 the centre of these two surfaces is known as the principal axis of 
 the lens (Fig. 177, mm). The essential idea of a double convex 
 lens is that it is thicker at the centre than at the edges. Situated 
 on the principal axis of a biconvex lens at a point in its interior is 
 the optical centre (Fig. 177, O) ; any other straight line passing through 
 the optical centre is termed a secondary axis (Fig. 177, nn). 
 
 When parallel rays of light (Fig. 178) pass through a convex 
 lens, they are refracted and brought to a point / on the opposite side 
 
 Fig. 177. Fig. 178. 
 
 Figures illustrating the Action of Lenses upon Rays of Light 
 passing through them. 
 
 Fig. 177. — Biconvex lens: O, optical centre; mm, chief or principal axis; 
 nn, secondary axis. 
 
 Fig. 178. — Parallel rays, a, are refracted and brought to a focus at/. Divergent 
 rays from/, on passing through a convex lens, are rendered parallel. 
 
 of the lens, known as the principal focus ; the only rays not refracted 
 are those passing through the centre of the lens — viz., those coin- 
 ciding with the principal or secondary axes. The converse of this 
 is also true — viz., divergent rays proceeding from the principal focus 
 of a lens/pass through and are rendered parallel (Fig. 178). 
 
 The distance from O, the optical centre of the lens, to /, its 
 principal focus, is known as the focal length of the lens. If the diver- 
 gent rays, instead of proceeding from the focus of the lens, proceed 
 from a point I beyond the focus (Fig. 179), then the rays, on passing 
 
 Fig. 179. 
 
 Rays of light passing through a convex lens from I at a point beyond 
 the focus /, cross at some point v, and invert the image. 
 
 through the lens, are not rendered parallel, but convergent (as the 
 refractive power is more than sufficient to render them parallel), atld 
 they come to a focus again on the other side of the lens at the point V 
 The distance from the lens at which they come to a focus depends\ 
 upon the distance of the luminous point I from the lens on the op- 
 posite side; thus the nearer the luminous point I to the principal 
 focus /, the farther will the focus on the opposite side recede, and 
 vice versa. The two foci / and v are termed conjugate foci, and they 
 
592 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 bear a definite relationship to each other. If the rays of light proceed 
 from a point L (Fig. 180), which is nearer to the lens than the principal 
 focus F, the lens is unable to refract the rays sufficiently, and they 
 issue from the opposite side divergent, as dd. 
 
 Parallel rays of light passing through a concave lens, instead of 
 being refracted to a focus, are bent and become divergent, so that 
 a concave lens has no real focus ; but if the divergent rays be pro- 
 duced backwards so as to meet on the principal axis of the lens, the 
 point where they meet is called the negative focus of the lens. 
 
 Spherical Aberration. — The rays of light passing through a convex 
 lens are not all equally refracted, those passing through the circum- 
 ference being more bent than those passing near the centre; the 
 result is that the rays do not all meet in the same point, those 
 passing through the circumference of the lens coming to a focus 
 earlier than those passing near the centre. This defect, known as 
 ' spherical aberration,' is remedied in the eye by the introduction 
 of a diaphragm or iris, which prevents some of the rays of light from 
 passing through the circumference of the lens ; spherical aberration 
 
 Fig. 180 
 
 Rays of light from a point L, between the focus F and the lens, diverge 
 when passing through a convex lens. 
 
 is further prevented by the fact that the refractive index of the central 
 part of the lens is greater than that of the circumference. Spherical 
 aberration produces indistinctness of vision by the production of 
 circles of diffusion, caused by those rays which meet too early crossing 
 each other and forming a circle. 
 
 Chromatic Aberration is due to the decomposition of white light 
 into its primary colours by its passing through a prism or a convex 
 lens — i.e., to the formation of a spectrum. The colours of the 
 spectrum are differently refracted,* the red being the least bent, the 
 violet the most; when, therefore, the red is distinctly seen, the eye is 
 not focussed for the violet. There is no compensation in the eye for 
 chromatic aberration. The defect is usually not noticeable, because 
 it is small in amount, and is rapidly corrected by alterations in 
 accommodation. 
 
 Schematic and Reduced Eye. — When a ray of light enters the eye, 
 it has to pass through four surfaces, and, including the air, four media. 
 There are two surfaces to the cornea, anterior and posterior, and 
 two surfaces to the lens, anterior and posterior; each of these 
 surfaces differs from the others in curvature. As media, there are 
 
THE SENSES 
 
 593 
 
 the aqueous and vitreous humours and the crystalline lens ; a further 
 complication is that the last is not of the same refractive index 
 throughout. The formation of an image in such a complex optical 
 system would be difficult to understand, were it not possible to 
 construct theoretically a simplified eye, or, as it is known, a schematic 
 eye. The basis of its construction is that, so long as a complex 
 system has its surfaces and media ' centred ' — that is, symmetrically 
 disposed around the optical axis — it is possible to consider it as 
 consisting of two surfaces and two media — viz., the schematic eye — 
 and even to simplify it still further to one surface and two media — 
 the reduced eye — -the media being air and water. In such a simple 
 optical system it is readily possible to trace the paths taken by the 
 rays of light, and so understand the formation of an image on the 
 retina of the eye. The reduced eye of the horse, ox, and dog will 
 be found described in works on veterinary ophthalmology.* 
 
 Cardinal Points. — The most simple optical system which can be 
 devised has as optic axis (OA, Fig. 181) — viz., a line passing through 
 
 Fig. 181. — The Cardinal Points of a Simple Optical System (Foster). 
 
 OA, Optic axis; apb, a curved spherical surface; », nodal point; F 2 , principal 
 posterior focus; F v principal anterior focus; ef, rays proceeding from F,, 
 rendered parallel to the optic axis; p, the principal point; the rays md, 
 Op, and m'e, pass through the nodal point » and undergo no refraction; 
 the rays cd, parallel to the optic axis, are refracted, and meet at F 2 . 
 
 it's centre perpendicular to its refractive surface (apb). On the optic 
 axis is situated the centre of curvature of the refracting surface ; this 
 centre is known as the nodal point n. All rays of light which strike 
 the refractive surface perpendicularly, such as O, m, pass through 
 the nodal point and are not refracted; all rays of light parallel to 
 the optic axis, such as cd, strike the refractive surface obliquely 
 and are refracted, and the point where they meet is called the 
 principal posterior focus, F 2 . On the optic axis, in front of the re- 
 fractive surface, is •situated a point F ; known as the principal 
 anterior focus ; rays proceeding from this point strike the surface 
 obliquely, and' are so refracted as to be rendered parallel (ef) to the 
 optic axis (OA). To these must be added the principal point p — 
 that is, the point where the refracting surface cuts the optic axis. 
 These various points are known as the cardinal points- of the simple 
 
 * See Nicolas and Gray. 
 
 38 
 
594 A MANUAL OP VETERINARY PHYSIOLOGY 
 
 optical system we have imagined. For a more complex system, 
 such as the eye, even when simplified, there are two nodal points, 
 two principal foci, and two principal points; but with the reduced 
 eye, where there is but one surface and two media, the two nodal 
 points become one, and the two principal points one. In order to 
 be able to calculate the position of the cardinal points of the eye 
 certain data must be known, such as the refractive index of the 
 media, the radius of curvature of each refracting surface, the distance 
 from the cornea to the lens, and the thickness of the latter. * 
 
 Formation of a Retinal Image. — -We are now in a position to 
 understand the formation of an image on the retina. Rays of 
 light entering the eye, as from the arrow XOY (Fig. 182), issue 
 as a pencil of rays from every point of the arrow; the pencil 
 containing the central ray is known as the principal ray. 
 All principal rays, aa' , pass through the nodal point n without 
 
 Fig. 182. — Diagram of the Formation of a Retinal Image (Foster 
 
 u, Principal ray of the pencil of light proceeding from X ; a', principal ray of the 
 pencil of light proceeding from Y ; the principal rays pass through the nodal 
 point n without being refracted; the other rays, be and b'c', are refracted and 
 cross behind the nodal point at the principal posterior point. In this way 
 the arrow XY forms a smaller inverted image of an arrow on the retina YX. 
 OA, optic axis. 
 
 undergoing refraction, while the rays be and b'c' are refracted to 
 a greater or less extent, so that in this way the retinal image 
 becomes inverted, and very much smaller than the object it 
 represents; it is a miniature though perfect representation of 
 the object presented to the eye. The chief refraction in man 
 occurs at the cornea, but in the horse and dog, and probably 
 all domesticated animals, the lens refracts more than the cornea 
 (Matthiessen). A man may have good distant vision without a 
 lens, for, as we have seen, its chief function is connected with ac- 
 commodation, but in the lower animals this is not the case, so 
 far as can be judged by the removal of the lens in cataract. The 
 other media also refract — for instance, the vitreous, but as its 
 index is the same as that of the aqueous, the assistance rendered 
 is not believed to be great . 
 
 * See Nicolas and Gray for an account of the cardinal points of the 
 eyes of animals. 
 
THE SENSES 595 
 
 Though the retinal picture is so completely inverted that the 
 right hand of the object becomes the left of the image, and the 
 top becomes the bottom, yet the mind does not perceive the 
 image as inverted, but mentally refers the picture, not to the 
 retina, but back to the object. In Fig. 182, the angle X«Y 
 is equal to the angle YnX. The angle XwY is spoken of as the 
 Visual Angle, and all objects having the same visual angle form 
 the same sized picture on the retina. By the aid of the visual 
 angle the size of an image on the retina may be calculated, pro- 
 vided the distance of the nodal point from the retina is known. 
 Thus at the distance of a mile, a man 6 feet high is represented 
 on the retina of a horse by an image ^ of an inch in height; 
 in the human eye at the same distance the picture of the man 
 would be TT \j ¥ of an inch, or about the size of a red blood-corpuscle. 
 
 Fig. 1S3. — Direct Method of using the Ophthalmoscope (Stewart). 
 
 Light falling on the perforated concave mirror M passes into the observed eye 
 E'; and, both E' and the observing eye E being supposed emmetropic and 
 unaccommodated, an erect virtual image of the illuminated retina of E' is 
 seen by E. 
 
 The nearer the object the larger the image; taking the 6-foot 
 man again at a distance of 10 yards, his height on the retina of 
 the horse would be i of an inch, whilst on the retina of a man it 
 would be rather over i of an inch. From this it is obvious 
 that the horse with normal sight sees the same image more 
 clearly than a man, for the reason that it produces a larger picture 
 on his retina. 
 
 The Ophthalmoscope. — This instrument is employed in 
 examining the interior of the eye. It consists of a mirror with 
 a hole in its centre, which is applied to the eye of the observer, 
 so that he can see through the hole, and rays from a suitable 
 source of light are reflected from the mirror through the pupil 
 of the eye under examination. These rays are reflected back 
 through the pupil of the observed eye, and entering the hole 
 in the mirror, pass into the eye of the observer (see Fig. 183). 
 
596 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 A brief account of the fundus of the eye when examined b 
 the ophthalmoscope has been given at p. 576. It is a magnifi& 
 image, the refractive media of the eye making it appear abou 
 eight times larger than it actually is. 
 
 Errors of Refraction.— An eye which possesses the powe 
 of seeing an object distinctly from the near point — i.e., a fe-s 
 inches from the eye — to infinity is known as emmetropic ; it i 
 the perfect eye (Fig. 184, 1). All eyes do not possess this range c 
 vision owing to the length of the eyeball. Where the distanc 
 
 Myopia. 
 
 Fig. 184. — Diagram of an Emmetropic, Hypermetropic, and Myopic Eyi 
 to illustrate where the Focal Point exists (Halliburton). 
 
 In 2 the short eyeball causes the focus to form behind the retina; in 3 the lor 
 eyeball causes the rays to come to a focus in front of the retina. 
 
 from the lens to the retina is too .considerable, the eye is myopic 
 the image comes to a focus in front of the retina, and only 
 confuted and blurred image is formed on the screen (Fig. 184, 3 
 Where the distance from the lens to the retina is too short, th 
 retina is in front of the focus formed by the refractive medi; 
 and the picture is not distinctly seen as the focus falls behind th 
 retina (Fig. 184, 2); such an eye is termed hypermetropic. Thes 
 defects are remedied in the human subject by employing a coi 
 cave glass (which scatters rays) for the myopic eye, and s 
 preventing the focus from forming too early; while a conve 
 
THE SENSES 597 
 
 lens is employed for the hypermetropic, as it brings the rays 
 of light to a focus earlier than would otherwise happen. 
 
 There is another defect due to irregularities in the curvatures 
 of the cornea, the curvature of the horizontal meridian not 
 agreeing with that of the vertical. The cornea, in other words, 
 is flatter in one of its meridians than in the other (p. 574), so that 
 the rays of light which pass through one meridian of the eye are 
 brought to a focus before those traversing the other meridian. 
 This condition is known as astigmatism. 
 
 Myopia, hypermetropia, and astigmatism may occur in the eyes 
 of the domesticated animals. The horse, according to the writer's 
 observations,* is commonly myopic and astigmatic, though the 
 degree of error is small. Early work in Germany described him 
 as hypermetropic, but all later researches point to myopia as 
 being very much more frequent than hypermetropia. G. L. 
 Johnsonf regards the myopia of the horse and rabbit as due to 
 domestication, and says the condition is almost unknown in 
 wild animals. The dog is also regarded as myopic, probably 
 for the same reason. The dog certainly does not give one the 
 impression of being myopic, so that it is probable the error is 
 small. Lang and Barrett, in their examination of the refractions 
 of wild animals, found hypermetropia to be frequent, and Johnson 
 found the same condition throughout the wild mammalia. The 
 amount of hypermetropia commonly found by him was under 
 1 diopter (40 inches), which is practically negligible. Emmetropia 
 he met with very rarely, and this agrees with the writer's obser- 
 vations on the horse; similarly, in the horse Johnson found 
 astigmatism to be a common "error; indeed, he says this error 
 occurs universally in the domestic animals, and throughout the 
 ungulata, cetacea, and all mammals below them. 
 
 Monocular and Binocular Vision.— Whether an animal can 
 see an object with both eyes or only with one is, broadly speaking, 
 determined by the position of the eyes in the head. Binocular 
 vision requires that a pair of eyes shall be so placed as to be 
 capable of converging on an object and seeing it as a single, and 
 not as a double one. Eyes situated close together are obviously 
 necessary for this purpose, and true binocular vision is defined 
 as single vision with a pair of eyes, the two eyes acting as one. 
 Eyes situated close together afford acute vision, and are the 
 complement of skilled fore-limb movements; the hand in man, 
 the paw in the feline, are accurately directed by means of 
 binocular vision. Eyes laterally placed in the head — e.g., as 
 in the hare and duck — are evidently intended for one-eyed vision. 
 
 * ' The Refractive Character of the Eyes of Horses,' Proceedings of the 
 Royal Society, No. 334, 1894. 
 t Op. cit. 
 
598 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Each eye can see a distinct picture at opposite poles of the 
 head, and such vision is known as monocular. But though 
 the eyes of an animal may be situated at the sides of the head, 
 it does not follow that it cannot see to the front. The prominence 
 of the eye and shape of the head here play an important part; 
 when a duck swims or waddles towards one, he keeps the head 
 true to the front and sees out of both eyes; the hare can do the 
 same, and certainly can see her pursuer out of ' the tail ' of 
 both eyes. Whether the vision with both eyes is identical in 
 character with binocular vision in man cannot at this point be 
 discussed. An animal with the eyes 
 laterally placed may have these located 
 so far forward that, though vision is 
 usually monocular, it may also be two- 
 eyed. The horse will occur to the reader 
 as belonging to this group, but in this 
 animal there is limited backward vision 
 with both eyes (see p. 574), though by 
 slightly turning the head he can obtain 
 a larger field behind, but only with one 
 eye (Fig. 185). The ox and sheep are 
 unable to see out of the ' tail ' of both 
 eyes simultaneously when the head is 
 erect, as the ears come in the way, 
 but when the head is depressed, as in 
 Fig. 185.— Diagram illus- grazing, they can see behind with both 
 
 TRATING THE EXTENT TO ", ^ .. . . . - . , 
 
 which a Horse can see e y es > though, as in the horse, the field 
 
 behind Him. is limited, for the head is always turned 
 
 With the head straight to the when they wish to obtain a better view. 
 
 front he can see indistinctly The conditions essential to binocular 
 
 e° y U es.° £ By 6 thf leal t * t- vision must now be explained. Each 
 clination of the head, as eye projects into space the image 
 formed on its retina (p. 595). When 
 the projected images are superposed 
 and precisely coincide only one object 
 is seen, and not two. This precise superposition of the images in 
 space occurs when the two images formed on the retina fall 
 on what is known as corresponding points of the retina. To 
 understand what is meant by corresponding points we must turn 
 to the human eye, and in imagination take out the retina of one 
 eye and lay it over its fellow, but not face to face; the left portion 
 of one will now lie exactly over the left portion of the other, the 
 upper and lower parts equally corresponding. A vertical line, vm 
 (see Fig. 186), will now divide them into halves, from which it will 
 be seen that the temporal side, say of the right retina, b'd', corre- 
 sponds to the nasal side of the left retina, bd. In Fig. 186 the 
 
 in Fig. 185, a large visual 
 field behind him may be 
 covered. 
 
THE SENSES 
 
 599 
 
 two circles representing the right and left retina? have been further 
 divided into quadrants, a'V in the right eye corresponding to ah 
 in the left eye, and similarly, for the lower quadrants. It will 
 be observed that the optic nerve, o, is in the left segment of the 
 right eye and in the right segment of the left eye. The area 
 of central vision— i.e., the fovea, /—will in each retina be in the 
 same place. Every rod and cone in the right eye now corresponds 
 
 x 1/ / — " "- \\f* 
 
 ■y° ( n °z' 
 
 Fig. 186. — Diagram illustrating Corresponding Points in the Human 
 Eye (Foster). 
 
 z' x' y' are points in the right eye corresponding to z x y in the left eye; v. I, visual 
 axis. The two figures illustrate the corresponding points of the retina 
 described in the text. 
 
 with every rod and cone in the left eye. If we look at an object 
 a little to the right, as at z, Fig. 186, it will be seen single, 
 for the reason that the image falls on corresponding parts of the 
 retina? — i.e., as z' on the left or nasal side of the visual axis, v.l, 
 of the right eye, and as z on the temporal side of the left eye ; 
 similarly with y; x being in the central line of vision, falls on 
 the fovea in each eye at xx' . 
 
600 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Bearing in mind what has already been said about the decus- 
 sating and direct fibres in the optic nerve (p. 582), the probable 
 explanation of corresponding points in the retina of man is 
 to be found in this arrangement ; the nerve-fibres from the right 
 half of each retina— i.e., the corresponding halves— meet in the 
 optic chiasma and proceed to the right brain, while the fibres in 
 the left halves proceed to the left brain (see p. 587 and Fig. 174). 
 Applying what has been said of the corresponding points of 
 the retina to the case of the hare or the bird, which, besides having 
 
 lateral vision, can also look 
 forward and backwards with 
 a pair of eyes, it is evident 
 to us that binocular vision 
 as above defined does not 
 exist, for the reason that it 
 is impossible for the image to 
 fall on the right halves and 
 left halves of each retina, 
 and be carried to the right 
 and left brain. It falls in 
 these, and in all animals with 
 the eyes wide apart, on the 
 temporal side of each retina — 
 i.e., on opposite instead of 
 on corresponding sides; not 
 only so, but in the case of 
 the bird there are no direct 
 fibres, so that the picture in 
 the right eye can only be 
 seen by the left brain, and 
 vice versa. Is the image in 
 these animals double, or does 
 some different arrangement 
 exist to receive a single image 
 with a pair of eyes where the corresponding points of the retina 
 do not agree ? This question cannot be answered. In the case 
 of the dog and the cat there exists binocular vision, as in man. 
 
 Between the group of animals with the eyes laterally placed 
 and those with them close together we have a third class, of which 
 the horse, ox, and sheep are examples, where the eyes, though 
 laterally placed, are situated so far forward that both may be 
 brought to bear on an object. 
 
 When a horse directs both eyesto the front, as he does when 
 attentively looking at an object, especially if it alarms him 
 (see p. 584), he strongly converges his eyes, producing a double 
 internal squint (Fig. 187). The eyes are rotated inwards and 
 
 Fig. 187. — The Position of the Eyes 
 and Attitude in Binocular Vision. 
 
THE SENSES 
 
 60 1 
 
 slightly upwards by the combined actions of the inferior oblique 
 and internal rectus muscles; the pupils are nearly horizontal, 
 and the pupillary opening brought so far to the front that the 
 inner segments of the cornea and iris completely disappear 
 beneath the inner canthus; the ears are erect and turned 
 forward; the expression and attitude is one of intense attention. 
 On reference to Fig. 188 it will be seen that the width between 
 the eyes prevents an object situated directly to the front being 
 seen as a single object, as it is in man, for the reason that the 
 image falls on the temporal side of each retina — i.e., on opposite, 
 
 Fig. 188. — Diagram showing Horizontal Section of the Head passing 
 through both eyeballs, to illustrate corresponding points in the 
 Retina of the Horse. 
 
 x, x, The frontal bones ; p, p, portion of malar bone entering into the formation 
 of the outer rim of the orbit ; s, the nasal septum. Rays of light proceeding 
 from A are seen by both eyes, being imprinted on the temporal side of each 
 retina at a; rays from B are seen at b in the left eye, but are not seen with 
 the right eye ; in the same way rays from C are imprinted at c in the left eye, 
 but cannot be seen with the right eye. 
 
 instead of on corresponding sides, as in man. There is no reason 
 to think that the animal sees double, as a man under these 
 conditions certainly would, and though we know that the horse 
 has not binocular vision in the sense that this is assigned to 
 man, the dog, and cat, it can with assurance be assumed that 
 binocular vision is not absent, though it is limited in amount. 
 A reference to Fig. 188 shows that objects situated away from 
 the central line of the face can be seen with one eye only. 
 
 Monocular vision is vision with a single eye; it is not regarded as 
 so quick nor so sure as binocular, and is found in animals which 
 
602 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 do not have to depend upon their own exertions, skill, and cunning 
 for obtaining food. It affords a wide field of view, which is 
 termed panoramic. Reference has been made (p. 583) to the 
 fact that panoramic is also another term for peripheral vision 
 in man, or vision occurring away from the macula. A man 
 has panoramic vision extending over a field of at least 90 degrees, 
 and very oblique rays may bring it up to 104 degrees, but as his 
 acuteness of vision is limited to the macula, it is evident that 
 only over a small fraction of the field is he able to see distinctly 
 without moving the eyes. On the other hand, it is thought that 
 in the herbivora acuteness of vision extends all over the large 
 visual field of 180 degrees; such vision is termed periscopic, and 
 indicates that the picture seen by the eye is sharp and distinct, 
 as is a photograph up to the edge of the plate. In this sense the 
 herbivora have both panoramic and periscopic vision. 
 
 There can be no doubt that in the herbivora the retina is 
 sensitive as far forward as the ciliary processes, and this, with 
 a horizontal pupil, allows of extremely oblique rays entering the 
 eye, an impossibility with eyes set close together and a circular 
 pupil. The extremely oblique rays are refracted at the cornea, 
 and, travelling parallel and close to the iris, are then further 
 refracted by the lens, so as to bring them on the retina behind the 
 ciliary processes. It is also likely that the structure of the lens, 
 which, as we have seen, is not homogeneous, but made up 
 of layers of different refractive indices, assists in bringing to a focus 
 on the retina oblique rays which would otherwise be indistinct.* 
 
 An animal employing monocular vision for a near object at 
 its feet is unable to see it without lowering the head and turning 
 it on one side in such a way as to bring the eye opposite to the 
 object. This is well seen in the bird and in the horse. In- 
 cidentally it settles the question of the distance at which a horse 
 views a near object when he can make his own adjustment; 
 it may be taken at about 3 feet. 
 
 Stereoscopic Vision. — Though objects are imprinted flat 
 on the retina, nevertheless they are seen in relief. Vision which 
 furnishes a knowledge of depth — i.e., perspective — is known 
 as stereoscopic. It is acquired by education. Everything 
 appears flat to an infant; in those cases of blindness from 
 birth, where sight has subsequently been obtained by operation, 
 a knowledge of shape has to be obtained by means of the sense 
 of touch. This applies also to the lower animals. The fear of 
 an object may be dispelled by allowing a horse to smell and 
 examine it. The carelessness shown by some in jumping 
 is corrected by a rap on the shins, and future judgment assisted 
 by a fall. The optical condition necessary in the eye for 
 
 * * Sight,' J. Le Conte, LL.D., 1883. 
 
THE SENSES 603 
 
 stereoscopic vision, is that the object shall be seen from two 
 slightly different standpoints, the right eye seeing more of its 
 right side, the left eye more of its left side. When this is 
 obtained, as it is by the interval between the eyes, a single 
 image is secured, and solidity results.* The physiological con- 
 ditions necessary are the same as for binocular vision, the 
 impressions from the left half of each retina being conveyed 
 to the left brain, and from the right half to the right brain. 
 This is effected through the direct fibres (see p. 582), and animals 
 possessing no direct fibres, such as fishes and birds, are said to 
 have no stereoscopic vision. 
 
 The domesticated herbivora have no stereoscopic vision in 
 the sense of the above definition, any more than they have 
 true binocular vision; nevertheless, it is undoubted that they 
 have the power of seeing a single object with a pair of eyes, and 
 of judging the three dimensions in space. Even the loss of one 
 eye does not prevent a horse from being able to jump. It has 
 been suggested that the wide base on which his eyes rest assists 
 stereoscopic vision, and this is probable. 
 
 Recent workf on stereoscopic vision offers the best explanation 
 of it yet put forward. The picture is received in the eye on a 
 concave surface, which in this respect offers a great contrast 
 to the sensitive flat plate of the photographer. This concave 
 surface distorts the image. If a distorted image be examined by 
 reflection in a concave mirror, the eye of the observer is not only 
 unable to see any distortion, but the image is seen in relief. 
 The concave sensitive plate of the eye by distorting an image, 
 renders it undistorted and in relief when projected into space. 
 This appears to explain stereoscopic vision in animals which 
 have no parallel vision. 
 
 Theory of Vision. — The change which enables the vibrating 
 ether to start a nerve impulse is unknown. Cutting or stimu- 
 lating the optic nerve causes no pain, but produces a flash of 
 light. It is known that electric changes occur in the eye not 
 only with flashes of light, but with what has been termed 
 ' flashes of darkness.' If an excised eye be suitably connected to 
 a galvanometer, the eye being kept in the dark, an electrical 
 change occurs when light is transmitted through the pupil, and 
 there is a similar electrical change when the light is withdrawn. 
 The sensitiveness of the eye to these reactions increases by its 
 being kept in the dark. 
 
 A photo-chemical theory of vision has been proposed. It is 
 
 * The student should test this on himself by looking at an object first 
 with one eye and then the other without moving the head, and observing 
 the slightly different pictures formed on the retina of each eye. 
 
 t J. L. Peach, C. R. de la Soc. Biologie, lxxxiii., No, 7, 1920. 
 
6o 4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 suggested that under the influence of light waves chemical 
 changes which give rise to a nerve impulse occur in the rods 
 and cones. The presence of a red pigment in the retina, which 
 becomes bleached by light, lends support to this theory. This 
 pigment, known as visual purple (p. 584), is found in the 
 ■external segments of the rods; the cones contain none. If the 
 pigment be extracted and brought into solution, as it may through 
 the action on it of bile salts, the coloured liquid thus obtained 
 becomes bleached on exposure to light. Advantage is taken 
 of this to obtain in the eye of a frog or rabbit, suitably prepared 
 by being kept in the dark, a retinal photograph or optogram 
 of an object, such as a window. The light bleaches the retinal 
 purple, while the shadow of the framework comes out red. The 
 colour having been discharged from the rods, regeneration occurs 
 through the pigment of the retina; this penetrates between the 
 rods and cones, and in due course the visual purple is restored 
 to the rods. It has also been thought that light influences 
 movement of the cones, producing shortening, whereas in the 
 dark they lengthen. 
 
 It is important to remember that the decomposition of visual 
 purple cannot wholly account for sight, for in man the area of 
 acute vision, the fovea, is entirely without this pigment; it is 
 also absent from the retina of such animals as the hen and pigeon, 
 in both of which vision is acute. 
 
 An eye adapted to light, and therefore not adapted to darkness, 
 is a condition with which all are familiar on passing from a light 
 to a dark room; the vision improves by waiting in the dark, and 
 it can be shown in man that the improvement does not occur 
 over the cone area — -viz., the fovea — but over the peripheral field, 
 where the rods predominate. The explanation of the vision 
 of night-seeing birds and animals probably lies in the function 
 of the purple-coated rods, while acute daylight vision is probably 
 the essential function of the non-pigmented cones. Neither the 
 hen nor the pigeon, the habits of which are diurnal, can be re- 
 garded as possessed of eyes defective in acuteness of vision, yet, 
 as we have seen, neither possesses visual purple. On the other 
 hand, neither possesses vision suitable for night-time. In the 
 bat visual purple has been found; in the owl it exists in abundance. 
 It seems j ustifiable, therefore, to believe that the difference between 
 eyes suited for daylight and those adapted to darkness largely 
 depends on the visual purple, by which the irritability of the 
 rods is increased in dim light, and vision is facilitated in illumina- „. 
 tions of low intensity. ''. 
 
 It is thought that vision results not only from incident, 
 but also from reflected light, thus stimulating the sensitive 
 elements a second time. It is safe to assume that this is 
 
THE SENSES 605 
 
 the part played by the tapetum (p. 576), and that in its area 
 the visual acuity is increased, especially in low degrees of 
 light. 
 
 G. L. Johnson suggests that the tapetum may also be concerned 
 with Colour Vision. He points out that in this structure 
 the reds, yellows, and greens predominate; where blue or violet 
 occurs in the tapetum it is tempered by the overlying retinal 
 pigment, so that the fundus is mauve or greenish-blue. In 
 dim light colours lose their distinction and appear uniform, 
 with the exception of yellow and orange, and nocturnal animals 
 have golden-yellow or golden green fundi. Birds, reptiles, and 
 fishes have red, green, and yellow oil globules in the inner seg- 
 ments of the cones. 
 
 It is thought (Mottram and Edridge Green) that animals 
 have no colour vision beyond red, green, and violet. Brown 
 is a colour largely employed in animal colouration, owing to the 
 ease with which it can be produced in metabolic processes; it is 
 suggested that brown and green are indistinguishable by animals. 
 Yet a horse makes no mistake when shown a bundle of green 
 lucerne ; he at once recognises that it is not hay, and shows his 
 pleasure accordingly ; similarly, he would certainly not mistake 
 a parsnip for a carrot, an article of which he is inordinately fond. 
 There was a well-known funeral contractor in London who at 
 one time kept as many as 700 black Belgian horses in his stud. 
 He found from experience that if he placed a horse of any other 
 colour than black amongst them they became miserable and 
 fretful. The experiment was made many times, but always with 
 the same result.* It is evident that these horses were capable 
 of recognising the difference in colour between brown and black. 
 Why red should be so exciting to a bovine is not known, but his 
 recognition of the colour is undoubted. 
 
 Divergence of the Optic Axis. — Reference has been made at 
 pp. 574 and 597 to divergence of the eyeballs. The position 
 of these in man and the monkey is practically parallel; in the 
 cat the optic axis forms an angle of from 7 to 9 degrees with the 
 middle line of the face; in dogs there is a good deal of variation 
 due to breed. Johnson places it at from 15 to 25 degrees; he 
 also gives for the pig, 33 to 35 degrees; horse, 40 degrees; ox and 
 sheep, 50 to 52 degrees; hare, 85 degrees. We think the angular 
 measurement given for the horse is too small ; 45 degrees is about 
 the angle for binocular vision, but for monocular its position varies 
 between 50 and 60 degrees ; if the eye be turned back to look out 
 of the ' tail,' the angular measurement is about 75 degrees; the 
 post-mortem position is about 65 degrees. 
 
 * ' Horse World of London,' W. J. Gordon, 1893. 
 
606 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Field of Vision.— The angular measurement for man is 
 generally given at 82-5 degrees, but H. Hartridge* allots him 
 104 degrees for each eye. In the hare the field is not less than 
 1 90 degrees for each eye ; the fields overlap both in front and behind ; 
 the same occurs in the bird. This overlapping provides a visual 
 field which completely encircles the head. In the horse the field 
 of vision gives an angular measurement of 90 degrees on either 
 side of the visual axis, so that with successive movements of the 
 eye 180 degrees are covered. A large cornea, horizontal pupil, 
 projecting eyes, and marked divergence of the optic axes secure 
 a large field of vision. 
 
 Pathological. 
 
 Many of the diseases of the eye met with in man are found to 
 occur in the lower animals, though not with the same frequency. 
 They are of especial importance in those required for work where 
 acute vision is necessary. An opacity of the cornea very seriously 
 affects the value of a horse, quite apart from the defect in vision. 
 The structures of the eye are so extremely delicate that very slight 
 pathological change suffices to produce conditions which are in- 
 compatible with normal sight. A horse which is partially blind may 
 be more dangerous for work than if the sight had been totally lost. 
 Injuries to the eyes of horses are frequent, such as lacerated eyelids 
 from projecting nails and hooks in the stable, injuries to the cornea 
 and punctures of the eyeball caused in the hunting-field, and more 
 commonly in the stable. The animal can afford no protection to its 
 eyes other than by the use of the retractor muscle. As might be 
 expected, the most serious injuries to the globe of the eye are pro- 
 duced by the smallest objects. A blow on the eye from a large object 
 may not leave a'mark on the eyeball, while a blow from a stick or 
 the end of a lash may destroy vision. Errors of refraction have 
 been discussed on p. 596, and need not be dealt with further; cor- 
 recting lenses have been employed, but are never likely to prove 
 popular. Shying is not necessarily due to defective vision, though 
 there can be no doubt that both astigmatism and short sight are 
 responsible for many cases. The most destructive eye disease is 
 Specific Ophthalmia of the horse, an attack of which affects almost 
 every tissue in the globe of the eye. Something resembling it has 
 been observed in cattle in Europe and South Africa, but, speaking 
 broadly, there is no eye disease in other animals or in man, which 
 for intensity and destructiveness can approach it. When horses 
 were taken less care of than they are at the present day, and the 
 laws of health were less understood, the disease was rampant 
 throughout Europe. In this country it is now relatively rare, but 
 is still common both on the Continent of Europe and in North 
 America. It was also seriously prevalent among the armies in 
 the field, both during the Great War and the war in South Africa, 
 1899-1902. Operating through a series of attacks, each of which 
 leaves the eye worse than before, it always leads to destruction of 
 one or both eyes. Cataract is the most common internal disease of 
 the eye in horses, and a serious cause of unsoundness. Contrary 
 to what happens in cataract in the human subject, the lens may not 
 
 * Journal of Physiology, vol. liii., 1919. 
 
THE SENSES 607 
 
 become entirely opaque — in fact, total opacity is the least frequent 
 condition met with. Spots of opacity, one or more, sometimes no 
 larger than the head of a pin, and generally situated in the line of 
 vision, represent the majority of cases of cataract. It has been shown 
 by F. Valentin that the lens of the horse contains various lipoids 
 (fat, fatty acids, and cholesterol) in solution, and that these may be 
 deposited as amorphous particles. Cholesterol feeding in the dog 
 produces deposits in various tissues of the eye, especially the cornea. 
 There are some forms of cataract which escape detection when the 
 eye is merely illuminated from the front, and can be seen only when 
 the lens is illuminated from the back by means of the ophthalmoscopic 
 mirror. In fact, no one can, without risk of error, declare an eye 
 free from cataract without using the mirror of the ophthalmoscope. 
 There are many cataracts concealed by the margin of the pupillary 
 opening which become visible only when the eye is examined with 
 this instrument. 
 
 A considerable amount of eye trouble occurs in animals as the 
 result of infection from specific diseases, such as influenza in the horse 
 or distemper in the dog. 
 
 Sympathetic Ophthalmia, so common in man after an injury, 
 is not met with in the lower animals, certainly not in the horse. 
 It is usual to consider the cause of the affection as due to inflam- 
 mation extending along the direct fibres, and so affecting the opposite 
 eye. 
 
Section 2. 
 Smell. 
 
 The Olfactory Organ. — -The nasal chambers are divided by a 
 septum, and each chamber contains the turbinated bones. It 
 has been observed that acuteness of smell is often associated 
 with large and extremely convoluted turbinates. By the 
 position of these bones the nasal passage may be divided into 
 two channels: one which lies next the floor of the chamber, 
 which, from its obvious communication, leads directly to the 
 respiratory passages, and another channel which lies above it 
 and leads to structures situated very high in the face and nose. 
 One is known as the respiratory and the other as the olfactory 
 passage. There are differences in the physical characters of the 
 mucous membrane of the nasal chambers; that of the olfactory 
 region differs from the membrane of the respiratory portion in 
 being thicker and of a yellowish tint, and it is in this membrane 
 that the fibres of the olfactory nerve are distributed. In all the 
 herbivora and many other animals a peculiar organ is found in 
 the nasal chambers known as the organ of Jacobson. In the 
 horse it lies close to the septum, and consists of a tubular carti- 
 laginous channel communicating anteriorly with the nasal 
 chamber by means of an orifice. It is lined by mucous membrane 
 and fibres of the olfactory nerve are distributed to it. It is an 
 organ of smell. Both the respiratory and olfactory portions of the 
 nasal chambers are supplied with sensation by the fifth pair of 
 nerves. In the horse the nasal chambers are of extreme im- 
 portance, inasmuch as he is the only animal we are called upon 
 to deal with which is unable to breathe through the mouth; 
 the majority of animals can breathe through both nose and 
 mouth, but owing to the extreme length of the soft palate in the 
 horse this is under ordinary circumstances impossible. So 
 far as respiration is concerned the question of the nostrils has 
 been dealt with (p. 117), but the arrangement of that portion 
 devoted to the sense of smeU has yet to be considered. 
 
 Olfactory Nervous Mechanism. — -From the olfactory tracts in 
 the brain the olfactory lobes are formed, which in some animals 
 possess a well-marked cavity, in others only a canal. In the 
 cavity some fluid is contained which communicates with the 
 cerebro-spinal, and notably in the horse with that contained in the 
 lateral ventricles . From the large olfactory bulbs non-medullated 
 nerve-fibres are given off, which penetrate the cribriform plate 
 
 608 
 
THE SENSES 609 
 
 of the ethmoid, and ramify over the mucous membrane, covering 
 the upper portion of the septum, the superior turbinated bone, 
 and the upper thirds of the superior and middle meatus. 
 
 These nerve-fibres terminate in olfactory sense organs, which 
 are specialised nerve-cells, from the free end of which projects a 
 tuft of hair-like processes, which are acted upon by olfactory 
 stimuli. These nerves, if traced backwards into the brain, are 
 found to terminate in globular bodies in the olfactory lobes, 
 where they connect with the special cells of this region through 
 the dendrites of the latter. The axons from the new cells are 
 continued into the olfactory tracts of the brain by means of 
 bundles of fibres, some of which terminate in the tract; others 
 cross to the opposite side, to end in the grey matter of the 
 hippocampal lobes; others pass to the hippocampal lobes of 
 the same side. The sense of smell is located in this region, 
 but the connections of the olfactory tract are numerous and 
 incompletely known. The olfactory area is interesting de- 
 velopmentally, as being the first part of the brain to appear, 
 showing the extraordinary importance of the sense of smell in 
 animals very low in the scale. It is the only sensory tract in 
 higher animals which passes directly to the cerebral cortex 
 without going through the thalamus, and the cerebral hemispheres 
 were derived from, and built upon, what in the primitive verte- 
 brate was simply the receptive instrument for smell impressions 
 (G. E. Smith). In animals possessing acute sense of smell the 
 olfactory regions are highly developed (p. 535); in others, like 
 man, where additional protective instincts have subsequently 
 appeared, the sense of smell is not so acute, and the central 
 representation accordingly greatly reduced. In animals where 
 the sense of smell is lost, as in the porpoise and most fishes, 
 the olfactory region is absent, while in others, like the shark, 
 which is guided entirely by this sense, its development is extra- 
 ordinary. In a shark 25 feet long the olfactory nerves spread 
 out over an area of 12 to 13 square feet (Halliburton). 
 
 The Olfactory Stimulus.— Before an odour can affect the 
 olfactory nerves it has to diffuse into the higher cavities of the 
 nasal chambers, and from being gaseous it must become dissolved 
 in the fluid which bathes these surfaces, for a dry olfactory 
 surface is insensible to smell. All odorous substances must be 
 soluble in water and in lipoid, as this material is contained in 
 the olfactory cells. We have no idea of the nature of the 
 particles which constitute an odour, but it is supposed that they 
 act chemically on the hair cells. The odour of a body can be 
 detected with greater accuracy by ' sniffing ' ; by this inspiratory 
 act no time is lost in diffusion occurring between the respiratory 
 and the olfactory region, as the odoriferous particles are forcibly 
 
 39 
 
610 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 drawn upwards. Colin performed a tracheotomy in horses, 
 and tied the trachea above the incision. Under these conditions 
 no air could pass up into the nasal chambers excepting by diffusion . 
 Animals so treated were unable, with their eyes covered, to 
 detect the presence of hay or oats. A stallion, however, under 
 these conditions, was able to recognise a mare, though he failed 
 to recognise food. 
 
 Olfactory Sensations. — -There are certain odours which excite 
 the olfactory organs more readily than others; thus flesh, blood, 
 and offal have remarkably stimulating effects on the carnivora; 
 whilst grass, grain, and vegetable products generally, stimulate 
 the herbivora. The odour of blood or flesh is evidently repulsive 
 to the herbivora, and may even cause nervousness and fright; 
 there are exceptions to this, for horses have been known to eat 
 meat with evident pleasure. The herbivora have a remarkably 
 keen scent; antelopes and deer have the power of detecting the 
 presence of an enemy a considerable distance away. All the 
 herbivora are guided by smell in obtaining their food, for they 
 cannot see what they are eating. 
 
 The acute sense of smell in the herbivora is a protective 
 mechanism. Their sight, comparatively speaking, is not acute, 
 certainly not nearly so keen as that of the animals by whom 
 they are hunted. Horses are quite conscious of the presence of 
 a lion, and exhibit great alarm and anxiety when one is about. 
 The sense of smell is also popularly believed to afford instruction 
 to animals in distinguishing poisonous from non-poisonous 
 plants, and the organ of Jacobson was supposed by Cuvier to 
 afford this protection. Smell, however, plays a very unim- 
 portant role in this connection. Cattle-poisoning as the result 
 of grazing is common all over the world, and especially in South 
 Africa. Experience is here the best master, and survivors 
 may generally be left on a pasturage known to contain poisonous 
 plants, for they have learnt to avoid them. 
 
 It is through the sense of smell that the male is attracted to 
 the female during the ' oestrous ' season, and not only can the 
 odour of a female in this condition be detected at a considerable 
 distance, but the smell is evidently most persistent. By the 
 sense of smell animals have the power of recognising their own 
 offspring ; a cow which has lost her calf will yield milk for weeks 
 to a ' dummy ' clothed in the skin of the dead calf, and she can 
 recognise the difference between her ' dummy ' and that belong- 
 ing to another cow. If the skin of a young animal— a kid, for 
 instance — be dressed with an agent which disguises the body- 
 smell, the mother is unable to recognise her young. The odour 
 of food is readily recognised by the herbivora, though to the 
 human senses all grains are equally free from any smell but 
 
THE SENSES 611 
 
 that of the sack which contains them. Without tasting it, a 
 horse will refuse a grain he is not familiar with. It is possible 
 that everything and everybody has a distinctive odour— at 
 least, it would appear to be so from the remarkable manner in 
 which hounds will follow a scent, or a dog recognise his own 
 master in the dark from amongst a crowd of other persons. 
 In the case of hounds, the amount of odour required to stimulate 
 the olfactory organ must be something too infinitesimal for 
 expression. 
 
 The elevation of the upper lip in the horse is associated with 
 the sense of smell. The stallion, on approaching a mare, ex- 
 hibits this evidence of the pleasurable impression on his olfactory 
 organs, but he does exactly the same after having had a draught 
 of aloes poured down his throat, the bitterness of which he 
 greatly resents. 
 
 The sense of smell rapidly becomes blunted — at any rate, in 
 ourselves; any oppressive odour is always more marked when 
 first detected. The writer has never satisfied himself that 
 animals are conscious of an offensive odour. Garbage is evi- 
 dently not offensive to a dog; horses certainly take no notice 
 of the smell from putrefying bodies, though they refuse to pass 
 them if visible, but that is only because they do not expect to 
 see either men or horses in any other than an upright attitude. 
 
Section 3. 
 Taste. 
 
 The sense of taste is largely, though not quite, dependent upon 
 the sense of smell. There are certain substances which cannot 
 be distinguished when the nose is closed; there are others which 
 can be readily distinguished by the tongue alone. This has led 
 to a classification of taste sensations, of which four kinds exist 
 — viz., sweet, bitter, acid, and salt. Animals are certainly capable 
 of distinguishing all of these. It is probable that each distinct 
 taste affects a particular part of the tongue; in man it has been 
 shown that the back part is 'sensitive to bitter tastes, the tip 
 to sweet and saline tastes, the sides to acid tastes, while the 
 middle portion of the tongue is insensitive to any taste. The 
 flavour of a substance is not obtained by the sense of taste alone, 
 but by the union of the senses of smell and taste. Without 
 smell, taste would be nearly impossible. 
 
 Tongue Papillae. — The papillae of the tongue are spoken of as 
 filiform, fungiform, circumvallate, and foliate. The filiform occur 
 over the upper surface of the organ, and in the ox impart to it 
 its characteristic roughness. The fungiform papillae in the horse 
 exist principally on the lateral parts of the tongue; the vallate 
 are found far back on the dorsum, and are only two or three in 
 number in the horse, twenty or thirty in the ox. The foliate 
 papillae are characteristically present in the horse, forming a 
 large projection in front of the pillars of the soft palate. There 
 are none in the ox, but they are very markedly present in the 
 rabbit. The filiform are in function chiefly tactile, the fungiform, 
 circumvallate, and foliate papillae, are associated with the sense 
 of taste, and in their structures are found the special nerve organs 
 of taste, known as taste buds, bulbs, or taste goblets. These are 
 balloon or barrel shaped bodies, the walls of which are formed of 
 elongated cells resembling the staves of a barrel. This structure 
 is open top and bottom; the nerve-fibrils enter below, while at 
 the outer free end is the gustatory pore, or opening into the 
 interior of the body of the bulb by which fluid finds its way in. 
 The interior of the goblet or bulb is filled with two kinds of cell 
 closely similar to those of the olfactory organ. Of these, some are 
 of a cylindrical shape, and are probably sustentacular. The' 
 others are rod-shaped, and have at their outer ends a hair-like 
 process which projects at the pore. It appears to be essential to 
 taste that fluid should readily find its way into the pore, and as 
 
 612 
 
THE SENSES 613 
 
 a provision to insure this the papilla; containing the buds are 
 situated close to glands. M'Kendrick states that in a single 
 circumvallate papilla of the ox 1,760 taste goblets have been 
 counted. The goblet cells are not strictly limited to the tongue, 
 but have been found in the palate, epiglottis, and even in the 
 larynx. 
 
 Nerves of Taste. — -It will be remembered (p. 555) that the 
 posterior third of the tongue is provided with sensation by the 
 glosso-pharyngeal nerve, the anterior two-thirds of the organ by 
 the lingual branch of the fifth pair. These two nerves, besides 
 endowing the tongue with sensation, are also concerned in carry- 
 ing those impulses connected with the sense of taste. The 
 cortical centre for taste is not definitely known, but is considered 
 to lie close to that of the olfactory sense in the hippocampal lobe. 
 It is believed that the taste-fibres in the lingual branch of the 
 fifth pair are furnished by the chorda tympani of the seventh pair 
 after the latter leave the middle ear. Stimulation of these fibres 
 arouses a sense of taste; this not only places their function 
 beyond doubt, but also proves that, apart from the stimulation 
 at the periphery, there is a specific taste reaction in the brain. 
 The fibres of the fifth pair are not, however, connected with taste 
 bulbs, for none are found over the area of their distribution. The 
 taste bulbs are the essential nerve terminations of the glosso- 
 pharyngeal nerve, and if this nerve be cut the bulbs degenerate. 
 Motor power to the tongue is furnished by the hypoglossal or 
 twelfth pair; section of these nerves prevents an animal from 
 masticating its food, the organ hanging helplessly from the mouth 
 and receiving injury from the incisor teeth. 
 
 It is necessary for the purpose of taste that the substance 
 should be dissolved; this is one of the functions of saliva, and 
 experiments on herbivora show that taste produces an abundant 
 secretion from the submaxillary and sublingual glands, though 
 not from the parotid. 
 
Section 4. 
 The Cutaneous and Internal Sensations. 
 
 The sensations imparted from the surface of the body to the 
 animal are not of one character only; touch is different from 
 heat, cold is something apart from' pain. These four sensations 
 of pressure or touch, heat, cold, and pain, make up the complex, 
 combined or uncombined, which composes the cutaneous senses 
 (p. 528). It is easy to realise the remarkably sensitive nature of 
 the entire surface of the body. There is no part, excepting the 
 horn of the feet or head, which does not give evidence of pain 
 on being cut ; there is no part, including the feet and horns, which 
 is unconscious of touch or pressure. This sensory envelope is 
 in the main protective, but it serves many other functions; some 
 of these we have already studied under Respiration, Secretion, and 
 Animal Heat; some have been referred to under the head of 
 Muscle Tonus, on which more remains to be said. 
 
 There are also afferent impulses proceeding from the skin which 
 indicate to an animal the position of its limbs, which direct its 
 muscular efforts, enable it to judge of weight and resistance, and, 
 in conjunction with afferent impulses from muscles, initiate the 
 changes which lead up to a muscular contraction. With none 
 of these are we at present concerned, but only with those afferent 
 impulses of common sensibility and pain proceeding from the 
 skin to the central sense-organs, where their nature and character 
 can alone be realised and distinguished, though the sensation is 
 referred to the stimulated surface, and not felt in the brain. 
 
 In the matter of sensibility there is a great contrast between 
 the surface of the skin and all that lies beneath it. Under normal 
 conditions muscles, bones, and tendons exhibit no ordinary 
 sensibility; even when aroused by disease they are still devoid 
 of touch, heat, and cold sensations, though pain may be felt 
 acutely. It will be remembered (p. 564) that the thoracic 
 and abdominal viscera are normally devoid of ordinary sensi- 
 bility. 
 
 Experiments made by Head and Rivers have shown that 
 there are two sets of sensory skin-fibres — a deep and a super- 
 ficial. The former, protopathic, convey sensations of pain, and of 
 marked heat and cold; the latter, epicritic, convey sensations of 
 touch, and small differences of heat and cold. Touch sensations 
 are divided into those of tactile localisation and tactile dis- 
 crimination; these are conveyed by separate nerve-fibres — in 
 
 614 
 
THE SENSES 615 
 
 fact, distinct fibres exist for all the cutaneous senses; those con- 
 veying pain do not transmit cold, and those transmitting cold 
 do not convey heat. It is remarkable that touch, heat, cold, 
 and pain senses are distributed over the surface of the skin in 
 spots or dots. There are spots which are responsive to pressure; 
 others to warmth; others, again, to coldness; and on suitable 
 stimulation others^feel pain. Maps showing the distribution of 
 these four surface senses have been prepared in man, in whom 
 alone their differentiation is possible. There is no reason to 
 think that the same principle of distribution does not hold good 
 for the lower animals. 
 
 Temperature Sense.— Cold spots are more widely distributed 
 than warm, which suggests that it is more necessary that the 
 body should be made acquainted with the fact that it is cold 
 than that it is hot. The feeling of warmth or cold depends upon 
 the temperature of the skin, and not upon that of the body. 
 
 Light Pressure Sense. — The distribution of the spots is wider 
 than of those of the temperature sense. The special nerve- 
 endings connected with pressure are found in a ring around the 
 hair follicles, in which position they are obviously most favour- 
 ably situated for stimulation through the hair itself. In the 
 hairless parts of the skin and muzzle special tactile corpuscles 
 are found, and in the horse special nerve-endings exist in the foot 
 associated with tactile sensibility. Tactile sensations play a 
 very important part in the lives of animals. In the lips and 
 muzzle, which correspond to the fingers of the biped, the touch 
 organs proper are located (p. 308); these parts are endowed with 
 exquisite sensibility, which enables the animal to be kept ac- 
 quainted with the nature of its surroundings and the character 
 of its food. The long feelers or hairs growing from the muzzle, 
 face, and brow of the horse are in connection with nerves in the 
 skin, and are valuable for tactile and consequently protective 
 purposes. The ' cat hairs ' on the general surface of the body 
 are doubtless for the same purpose. The tactile sensibility of the 
 foot, by informing the animal of the character of the ground it 
 is travelling over, is useful, though not absolutely essential, in 
 locomotion; nor is the tactile sensibility in the foot of the horse 
 absolutely essential to its safety in progression, as is clearly 
 proved by the results of plantar neurectomy. 
 
 Pain Sense.— This is the most widely distributed of the 
 cutaneous senses. It is distributed in spots probably supplied 
 by special fibres, though no special nerve-endings have been 
 determined. Pain confined to the surface of the body can be 
 readily located, but the localisation of interior pain is difficult; 
 that of colic, for example, is referred to the abdominal wall. It 
 is supposed that the explanation of the difficulty in localising 
 
6i6 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 interior pain is that the segment of the spinal cord supplying the 
 affected organ refers the pain to the skin region of the same spinal 
 segment instead of to the organ (p. 564). Painful sensations are 
 of various characters — -hence such terms as ' stabbing,' ' boring,' 
 ' burning,' ' throbbing,' etc., to express the impression imparted. 
 It is presumed that amongst the lower animals these different 
 qualities of pain exist; it is quite certain, for instance, that the 
 pain exhibited by a horse during an attack of colic is very different 
 from that shown when pus is forming in the foot. Of the nature 
 of pain nothing is known; for its seat see p. 511. 
 
 Muscle, Motorial, or Kinaesthetic Sense. 
 
 The term ' muscle sense,' or deep pressure sense, has been 
 employed to describe several allied, though quite distinct, con- 
 ditions to which the muscles directly or indirectly contribute. 
 It covers such questions as a knowledge of the existence of 
 muscles and of their position, the position of the limbs at rest 
 and in motion, the proper and orderly contraction of muscles, and 
 of the correct grouping of sets of muscles — -viz., muscular 
 co-ordination — both for locomotion and for equilibrium. It 
 goes further : it deals with weight and the resistance offered in 
 muscular contractions, and in conjunction with touch and sight 
 it contributes to the formation of spatial conceptions. To many 
 it has appeared that the grouping of this set of complex functions 
 under the term ' muscle sense ' is liable to prove misleading, for 
 there is no evidence that in all cases the muscles play a pre- 
 dominant part in the various phenomena; in consequence, 
 the term motorial or kinaesthetic sense is considered a more 
 suitable designation. 
 
 Sherrington has shown that muscles receive a rich supply of 
 afferent nerves, one-half to one-third of the nerves in muscle 
 being of this nature. These sensory fibres terminate in special 
 nerve-endings in the muscle or its tendon. These endings are 
 known as muscle spindles (p. 423); they are believed to be 
 activated by variations in the tension of the muscles. The proof 
 that these spindles are sensory in nature is afforded by the fact 
 that when the inferior roots of the spinal cord are divided no 
 degeneration of them occurs. It is through these fibres, which 
 enter the cord by the dorsal roots and travel both to the 
 cerebrum and cerebellum, that the brain is made acquainted 
 with the condition of the muscles. The cerebellum, through its 
 connection with the semicircular canals of the internal ear, 
 enables a judgment to be formed of the position of the body in 
 space, and of its relationships to its surroundings (p. 628) . These 
 are communicated to the cerebrum, from which impulses pass from 
 
, THE SENSES 617 
 
 the centre representing the, muscles to the muscles themselves. 
 It would appear that centres must exist in the cord by which 
 at least part of this work is carried out, for it has been shown 
 (p. 482) that after the cord has been divided in the dog, and 
 sufficient time has elapsed, muscular co-ordination is re-estab- 
 lished, though no communication with the brain is possible. 
 There are other reasons which have also been mentioned which 
 suggest that the cord of the lower animals is capable of automatic 
 activity in the matter of muscular movements, being stimulated 
 by the constant inpouring of sensory impulses through the 
 muscle afferents. 
 
 Deep pressure sensibility, such as- arises from joints, and the 
 sensory impressions arising in the skin and subcutaneous tissues 
 of joints, are intimately concerned in the kinaesthetic sense. It 
 is probable that this deep sensibility is particularly employed 
 in determining weight and resistance, and it would appear to be 
 well developed in draught-horses. The horse employed in 
 ' shunting ' knows in a second the amount of effort required to 
 start truck-loads of varying weight. This information is trans- 
 mitted through the skin under the collar and over the large 
 joints, especially the shoulder, hip, and stifle. 
 
 The importance of ' muscle sense ' in provoking a muscular con- 
 traction is Shown by the pseudo-paralysis which follows division 
 of the sensory nerves (p. 504). This operation causes no loss of 
 power, but it cuts off all afferent impressions from the muscles 
 of the part; the dog drags his legs, the arm of the monkey hangs 
 helplessly, in spite of the fact that in neither case have the motor 
 mechanisms been interfered with. The same is seen in the horse 
 on dividing the superior maxillary branch of the fifth nerve; the 
 animal is unable to use his lips as a prehensile organ, though 
 they possess their full power (see p. 554)- 
 
 Judgment of distance, the relationship of space, and the 
 muscular co-ordination connected with equilibrium, are in- 
 definably mixed up with muscle sense, vision, and the internal 
 ear. The extraordinary judgment shown by horses in jumping, 
 both as to height and distance, is mediated through this channel. 
 Some animals never acquire it, are clumsy, "and make 'but little 
 effort to rise, or, if tired, none whatever; others are willing, 
 but their judgment is defective, and the muscles are directed to 
 perform unnecessarily powerful contractions; a third group, 
 confident in their powers and judgment, and unwilling to do 
 more than is necessary, leave themselves such a fine margin 
 of safety that-if requires a fall to chasten their pride. Muscle 
 sensation is very evident in the tired animal, and fatigue is 
 probably the only condition, apart from lameness, in which ahorse 
 is conscious of possessing limbs., 
 
618 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Muscle sense has been referred to in the chapters on the 
 muscular system (p. 423), nervous system (pp. 491, 504, 517, 
 528), and will be again met with in dealing with locomotion. 
 
 Hunger. 
 
 The sensation of hunger is referred to the stomach. It has 
 been supposed that it is excited by the approximation of the 
 walls of the organ, but animals may exhibit hunger at a time 
 when the stomach contains food — for instance, the horse and the 
 rabbit. Nothing is known of the nervous mechanism connected 
 with this condition. It is one of the senses under the control 
 of a lowly organised part of the brain, for a dog without its 
 cerebrum shows the usual signs, though obviously this hunger 
 cannot be associated with any conscious knowledge of want. 
 The length of time animals will withstand starvation is dealt with 
 at P- 393- A- horse in good condition has been known to live 
 thirty days, provided water be given; but an animal in poor con- 
 dition to start with has died in half that time. The remarkable 
 period during which sheep have lived without food has been 
 mentioned at p. 393, and a similar endurance has been recorded in 
 the pig;* the animal was buried for sixty days in a landslip, and 
 was recovered alive, having lost in that time 12a pounds in 
 weight. It is not known why eating gives practically immediate 
 relief to all the symptoms of hunger, for it is obvious that the 
 food ingested can be of no use until absorption has commenced 
 
 Thirst. 
 
 Very little is known of the subject of thirst — not even why the 
 sensation is referred to the pharynx. It has been supposed that 
 the glosso-pharyngeal nerve has special nerve-endings in the 
 pharynx, which are stimulated when the water content of the 
 body falls below a certain point; for if the palate be moistened 
 thirst is allayed, while filling the stomach with water through a 
 fistula does not at once control the desire for fluid. If a horse 
 be ' sham watered '—viz., with a fistula in the oesophagus — he 
 leaves off drinking, after a few swallows, as if satisfied, and then 
 starts again as the pharynx gets dry (Colin). 
 
 There is a constant loss of water occurring in the body fluid 
 through respiration and the various secretions, such as sweat, 
 urine, milk, and digestive juices. Some of the fluid of the latter 
 may be reabsorbed; especially is this likely to occur in the case of 
 such bulky secretions as saliva in the herbivora. The re- 
 quirements of an animal for water are constant, though varying 
 
 * Martell, Transactions of the Linnean Society, vol. ix. 
 
THE SENSES 619 
 
 with the season and the nature of the food. In summer the loss 
 by sweat in working horses, and the extra transpiration by the 
 air-passages, all have to be made good. Animals receiving dry 
 food require more water than those on green herbage. According 
 to Colin's researches, 4 kilogrammes (8-8 pounds) of hay require 
 16 kilogrammes (35-2 pounds) of saliva, and the fluid for this has 
 to be drawn from the blood. When ' roots ' form a portion of the 
 daily diet, the amount of water consumed becomes considerably 
 reduced. So much is this the case that sheep fed on roots and 
 succulent food are not watered; and the rabbit receiving succulent 
 food does not drink. The assumption that neither of these 
 animals requires water is at once disproved by placing them on 
 a diet of dry food. Purging, haemorrhage, and diabetes, prove 
 severe sources of water loss to the tissues. 
 
 Thirst is badly borne by animals, certainly by the herbivora. 
 With horses the capacity for work falls off considerably, and as 
 the fluid contents of the digestive canal and tissues is drawn upon 
 the abdominal wall contracts, and the animal wears a pinched and 
 anxious look. Under these conditions in war, horses have been 
 known, on obtaining access to water, to drink until they died, 
 presumably from rupture of the intestines. 
 
 The ordinary exhibition of thirst in stabled animals is at once 
 recognised by the trained eye. The horse that pricks his ears and 
 gazes intently when he hears the sound of a bucket, and the 
 peculiar motions of the mouth of the thirsty horse at the sight 
 of water, are indications as unmistakable as speech. 
 
 Deprivation of fluid is productive of intense suffering. Hunger 
 may be taken philosophically, but not thirst; the latter extin- 
 guishes life in a few days, the former in a few weeks. So essential, 
 indeed, is the regular renewal of fluid that the sick animal, 
 though turning from food with disgust, never fails, but in one 
 disease, to consume fluid; during acute abdominal pain a horse 
 will not drink, yet the leathery dryness of the mouth suggests 
 thirst. 
 
Section 5. 
 Hearing. 
 
 The Nature of Sound. — -The vibrations which produce sound 
 are transmitted by air, solids, and liquids in the form of waves. 
 Through air, sound is transmitted by a succession of condensa- 
 tions and rarefactions of the air, due to its elasticity and density. 
 The particles in the air oscillating to and fro are sometimes 
 crowded together, at others widely distributed. The sound- 
 waves travel longitudinally at a relatively slow rate — -about 
 1,200 feet per second; the length of the waves varies from a 
 fraction of an inch to sixty or more feet. The evidence that 
 sound is transmitted by particles in the air is shown by the fact 
 that it cannot be produced in vacuo. 
 
 In comparing one sound with another we are conscious of only 
 three possible differences between them : they may differ in loud- 
 ness, pitch, and quality. Of these, loudness is dependent on the 
 magnitude of the to-and-fro motion of the vibrating particles 
 whose movements transmit the sound; a loud sound means a 
 large wave. Pitch, on the other hand, depends on the frequency 
 of the vibrations, a high note implying rapid vibrations, or a 
 shorter wave-length. Sounds may be simple or compound. 
 The vibrations of a tuning-fork give rise to a typically simple 
 sound of varying loudness or pitch, but possessing little quality. 
 Most vibrating bodies do not give rise merely to such simple 
 vibrations, but set up a variable series of different wave-lengths 
 along with their fundamental simple vibration. Thus, most 
 sounds consist of a fundamental tone, accompanied by more or 
 less of these other tones — the partial tones, overtones, or harmonics, 
 as they are termed. The quality of a sound depends upon these 
 harmonics;. where they are absent the tone is thin, where they 
 are present they give richness, and confer on tone that ' character ' 
 which enables us to recognise one musical instrument from 
 another by the mere sound it emits. Those sounds which are 
 grouped under the general term of ' musical ' result from the 
 regularity of their causative vibrations, and the definiteness in 
 wave-length of the latter. Noise is essentially the result of the 
 absence of this regularity and definiteness. There is usually no 
 difficulty in discriminating noise from musical sounds, but the 
 one may merge into the other, as in the case of the noise of street 
 traffic when near to it, and the musical humming tone it produces 
 when heard from a distance. From observations on the human 
 subject it has been ascertained that the smallest number of 
 vibrations audible is about thirty per second, whilst the average 
 
 620 
 
THE SENSES 621 
 
 human ear can recognise up to 30,000 vibrations per second. It 
 is undoubted that some animals can recognise a smaller number of 
 vibrations than thirty per second. Galton has shown that the cat 
 is capable of recognising sounds inaudible to the human ear. 
 
 External Ear. — The vibrations of sound are collected by the 
 external ear, which is formed of a movable funnel-shaped piece of 
 cartilage of the yellow elastic type. Its mobility lis greatest 
 in animals which are preyed upon, such as the herbivora, and 
 which in consequence require to collect sound-waves from every 
 point. The ears differ in shape in every animal, and there are 
 marked differences among animals of the same species. 
 
 The movements of the ears give evidence of what is passing 
 through the mind of an animal; this observation is as old as Pliny. 
 In the horse, which may be taken as typical, they are turned 
 firmly to the front and closely pricked — i.e., the points approxi- 
 mated—when he is attentive, whether this attention be devoted 
 to something he is alarmed at or pleased with. They are laid 
 back on the poll in sourness of temper, vice, anger, or during 
 rough play unattended by vice. This is a protective mechanism ; 
 when horses fight with their teeth, the neck and ears are the parts 
 which are aimed at; the mane is intended to protect the neck; 
 the ears have to take care of themselves, being kept as far as 
 possible flat on the poll.* The ears are moved rapidly to and 
 fro when the horse is anxious, either from impending danger or 
 other cause; one ear carried forward and the other backward, or 
 both turned backward but kept firmly erect, is considered the 
 sign of a willing determined worker. The carriage of one ear 
 forward and the other backward suggests that the animal is 
 able to appreciate sound from opposite points of the compass. 
 Drooping ears are indicative of exhaustion, and when permanent 
 are associated with age or natural flabbiness. Cattle, sheep, and 
 goats never draw back the ears. It is rather remarkable that the 
 expression of the mind in many animals should be conveyed 
 through opposite poles of the body — i.e., the ears and tail. 
 In dog, cat, and horse movements of these parts tell each its 
 own distinctive story. Wagging the tail, which shows pleasure 
 in the dog, indicates irritation in the cat, and the vice of kicking 
 in the horse. 
 
 The conchial or auricular cartilage forms the funnel which 
 collects the sound-waves; it is pivoted upon another and much 
 smaller cartilage, the annual, over which it slides as a telescope 
 over the joint, while the annual cartilage is similarly fitted over 
 the auditory canal of the petrous temporal bone; a few elastic 
 bands hold these peculiar joints in apposition. The auricular 
 
 * The ears are ako drawn back and laid oa the poll in cases of violent 
 death — for example, by shooting. 
 
622 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 cartilage is largest above, gradually contracting until it approaches 
 the annular, when a sudden reduction in size occurs. The ear 
 is capable of rotation through an angle of 180 degrees, and this 
 remarkable mobility is secured by the cartilaginous joints above 
 described, without the intervention of a synovial apparatus; 
 but at the base of the conchial cartilage is a large pad of fat, 
 which never undergoes absorption, and on this cushion easy 
 rotation is assured. 
 
 -Muscles of External Ear of Horse. (Sisson-Ellenberger- 
 Baum). 
 
 On the right side the superficial muscles and the scutiform cartilage have been 
 removed to display the deeper muscles, a, a', b, c, Various branches of the 
 scutularis muscle; d, e, q,f, anterior auricular group; g, «*, superior auricular 
 muscles; o, v, w, posterior auricular muscles; x, x', deep auricular muscles; 
 y, temporalis muscle; t, scutiform cartilage. 
 
 The skin covering the concha is fine and directly adherent to 
 the upper two-thirds of the underlying cartilage; internally 
 it lines both the conchial and annular cartilages so intimately 
 that it largely assists in their union. The part at the entrance to 
 the ear is provided with long woolly hair to prevent the ingress 
 of foreign bodies; this hair becomes finer as the depth of the 
 tube is approached, and finally is replaced by glands which secrete 
 cerumen or ear-wax. At one point the conchial cartilage is'attached 
 to the guttural pouch; these organs will be dealt, with presently. 
 
THE SENSES 
 
 623 
 
 According to Sisson, there are sixteen extrinsic muscles of the 
 ear, and three intrinsic, the latter being small and unimpor- 
 tant; we shall confine our notice to the extrinsic muscles. In 
 connection with some of these, the anterior group, there 
 exists a most peculiar anatomical arrangement in the form 
 of a floating cartilage, the 
 scutifornt ; from this certain 
 muscles originate, while others 
 hold it in place, and alter 
 its position to agree with 
 the direction in which the 
 group attached to it have 
 to act. The scutiform carti- 
 lage is a concave plate which 
 lies on the temporalis muscle; 
 there is nothing to keep it 
 in position but the muscles 
 which anchor it. It may be 
 pulled inwards, downwards, 
 or outwards, each change in 
 position altering the direction 
 of the pull of the muscles 
 which arise from it in their 
 action on the conchial carti- 
 lage (see Fig. 189). 
 
 The movement of the ex- 
 ternal ear through an angle 
 of 180 degrees indicates that 
 there is a group of rotating 
 muscles; to enable the ears 
 
 to point to the front there is t> External auditory canal; ■*, membrana 
 
 a group of adductor muscles; 
 
 in order that these points may 
 
 approximate other muscles 
 
 are employed; to enable the 
 
 ears to point backwards a 
 
 distinct group of muscles 
 
 comes into play, while other 
 
 muscles pull the ears down 
 
 and lay them parallel to the poll. It is obvious that the 
 
 muscular movements are very complex, necessitating perfect 
 
 co-ordination, fine adjustment, and alterations in the position 
 
 of the floating cartilage, in order to change the direction of 
 
 the pull of those muscles which arise from it. There is no other 
 
 part of the body where a similar physiological arrangement 
 
 exists. 
 
 Fig. 190.' — Diagrammatic Section 
 through the petrous temporal 
 Bone. 
 
 tympani; 3, middle ear, containing the 
 chain of small bones (4) which connect 
 the ear drum with the internal ear; 5, 
 one of the semicircular canals; 6, the 
 vestibule; 7, a portion of the cochlea; 
 8, the Eustachian tube, which enters the 
 middle ear; 9, portion of the guttural 
 pouch, a diverticulum of the Eustachian 
 tube. 
 
6?4 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The Guttural Pouches.— These remarkable air sacs are 
 confined to the equidae. Placed beneath the base of the skull 
 and atlas and above the pharynx, they communicate with the 
 outside air by means of the Eustachian tube (Fig. 190). As the 
 Eustachian tube communicates with the middle ear, -and passes 
 completely through the guttural pouch as an open canal, it is not 
 unreasonable to believe that the pouches are concerned with the 
 sense of hearing. In man acuteness of hearing is enhanced by 
 listening with an open mouth. The fact that the horse cannot 
 breathe through the mouth may explain the presence of these 
 large air sacs for the supply of the needful amount of air to the 
 middle ear. This view is supported by the fact that the pouch 
 
 is attached to the conchial 
 cartilage, and when the ears 
 are carried forward, as in 
 attentive listening, the walls 
 of the guttural pouch are 
 rendered tenser. Air enters 
 the guttural pouch during 
 swallowing, and also during 
 expiration; in the latter 
 respect it resembles the 
 facial sinuses. The use of 
 the Eustachian tube is to 
 convey air to the middle 
 ear, and so equalise the 
 pressure on both sides of 
 the tympanum. 
 
 Structure of the Ear. — At 
 one end of the external audi- 
 tory canal is a piece of mem- 
 brane stretched completely 
 across it, known as the Mem- 
 brana Tympani ; it separates 
 the external from the middle ear (Fig. 191). The Middle Ear is on 
 the inner side of the membrana tympani; it consists of a cavity 
 containing a chain of very small bones, which stretch like a bridge 
 across the space from the membrana tympani to the third or internal 
 ear. The middle ear, like the external, is in communication' with 
 the air by means of the passage known as the Eustachian tube which 
 opens into the pharynx. The membrana tympani has, therefore, 
 air on both sides of it, the object of which is to insure that the 
 atmospheric pressure on both sides is equal, and so permit its free 
 swing. The air ordinarily finds its way into the Eustachian tube 
 during the act of swallowing, and it is conveyed to the guttural 
 pouches by the same channel. The Membrana Tympani is concave 
 towards the external ear; in the middle ear the handle of the malleus 
 is fixed to the central bulging part of it, and as this bone articulates 
 with the incus, and the latter with the stapes, any alteration in the 
 shape of the drumhead, such as is produced by the vibrations of 
 
 Fig. 
 
 191. — Diagrammatic Section of the 
 Horse's Ear. 
 
 , External auditory canal; 2, the membrana 
 tympani ; 3, chain of bones across the 
 middle ear; 4, the Eustachian tube; 5, the 
 internal ear; the number is on the vesti- 
 bule, above which may be seen the semi- 
 circular canals, while below is the cochlea. 
 
THE SENSES 
 
 625 
 
 Fig. 
 
 sound, causes the bridge of bones to move; their movement is 
 further assisted by two small muscles which are attached to them. 
 
 The Internal Ear, known as the labyrinth (Fig. 192), is composed 
 of the semicircular canals, the vestibule, and the cochlea < these are 
 contained in a solid piece of bone in 
 which two small foramina or windows 
 exist, one known as the fenestra ovalis, 
 the other the fenestra rotunda ; the base 
 of the stapes or third bone of the ear 
 is attached to the membrane which 
 covers the fenestra ovalis. 
 
 All three parts of the labyrinth com- 
 municate, but it is quite certain that all 
 three do not take an equally active part 
 in hearing. The cochlea alone is the 
 essential organ of hearing. The whole 
 of the labyrinth is lined by a membrane 
 containing a fluid known as the peri- 
 lymph ; this peri-lymph has free access 
 to all parts of the inner ear. Within this 
 membrane is a membranous labyrinth, 
 the counterpart of the semicircular 
 canals and vestibule, and this also con- 
 tains fluid known as endo-lymph. 
 
 Two windows exist in the bony laby- 
 rinth. The base of the stapes lies over 
 
 one of them, and between the stapes and the peri-lymph is the 
 membrane which lines the internal ear. Every movement of the 
 membrana tympani causes the bony bridge to oscillate, and every 
 oscillation of this thrusts the stapes against the membranous window, 
 and so sets up oscillations in the peri-lymph which are transmitted 
 throughout the internal ear. 
 
 192. — The Labyrinth 
 (Edmunds). 
 
 The semicircular canals are to 
 the right, the cochlea to the 
 left; both windows may be 
 seen, the fenestra rotunda be- 
 ing the lowermost. The groove 
 across the body of the organ 
 lodges the auditory , nerve. 
 The figure is enlarged.' 
 
 « 'iters membrane 
 
 ^anahs cochleae 
 
 -Wembrana 
 te:ctcr'ia 
 
 .Hair cells 
 
 . -Meml r ana 
 m baSilaris 
 
 'i-Rllars of CorH 
 
 Fig. 193. — Diagrammatic Transverse Section of a Turn of the Cochlea. 
 
 The cochlea resembles in appearance the shell of a snail, its interior 
 being divided into three spiral channels, which wind their way from 
 base to apex like a circular staircase. The number of twists in the 
 cochlea is two and a half; the axis around which these wind is 
 composed of soft bone having canals up which the auditory nerve 
 
 40 
 
6z6 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 travels. If a spiral of the cochlea be cut across (Fig. 193), the three 
 canals it contains are seen. These are divided by septa ; one septum, 
 known as the lamina spiralis, separates the upper canal, or scala 
 vestibuli, from the lower one, or scala tympani. The third, or middle 
 canal, is of a triangular shape, and called the cochlear canal ; it 
 contains the essential organs of hearing, and lies between and to the 
 outside of the other two. The roof of the cochlear canal is formed 
 by a piece of tissue known as the membrane of Reissner, while its 
 floor, on which is situated the essential organs of hearing, or organ 
 of Corti, is formed by the membrana basilaris, which connects the 
 outer wall of the cochlea to the lamina spiralis ; a cover to the organ 
 of Corti is formed by the membrana tectoria. The cochlear canal is 
 the continuation of the membranous labyrinth. The upper passage 
 of the cochlea — i.e., the scala vestibuli — is continuous with the 
 lymphatic peri-lymph space of the vestibule, whilst the scala tympani, 
 or lower passage, ends at the base of the cochlea in a blind extremity 
 in which is a, membranous window, the fenestra rotunda, which 
 separates the scala tympani from the cavity of the membrana 
 tympani. The cochlear canal terminates suddenly at the summit of 
 the cochlea, and at this point the two scala?, which in their windings 
 have been decreasing in size from base to apex, meet and communicate 
 by a small opening, the helicotrema, and the fluid of the one is thus 
 in connection with that of the other. 
 
 Organ of Corti. — This consists of a triangular-shaped tunnel 
 (Fig. 193), the base of which rests on the basilar membrane; the 
 tunnel is composed of certain rods arranged side by side, inclined 
 from both sides towards each other, and meeting superiorly like an 
 inverted V. At this point the rods known as the rods or pillars 
 of Corti, fit into each other in a peculiar manner, and by means of a 
 membrane (not shown in the diagram), the membrana reticularis axe 
 connected with the cells containing the terminations of the auditory 
 nerve. Flanking either side of the rods- are certain cells of two 
 distinct kinds ; those nearest to the pillars are somewhat flask-shaped, 
 and, having hairs growing from their summits, are spoken, of as the 
 inner and outer hair cells. The hairs on these cells are stiff and pass 
 into holes in the membrana tectoria. External to the outer hair cells 
 are some tall conical cells known as Hensen's cells. The auditory 
 nerve ascends the axis of the cochlea, giving off branches which in 
 their passage ramify over the lamina spiralis, at the outer edge of 
 which the above-described organ of Corti exists ; having reached this, 
 the fibres lose their medulla, and the naked axis cylinders pass into 
 the cells flanking the triangular tunnel, some fibres crossing the 
 tunnel to reach the cells on the opposite side. How the nerve 
 terminates in the hair cells — for it is to these that/ it is distributed 
 — is unknown, but that the hair cells are the organs of hearing is 
 undoubted; Hensen's cells are probably only of a nutritive nature, 
 and unconnected with auditory impulses. 
 
 Auditory Sensations. — Any analysis of these is hardly 
 necessary in a work- dealing with the lower animals; we have no 
 direct evidence that they understand or appreciate the difference 
 between music and noise; a dog will howl at the one as readily as 
 at the other. At the same time it is certain that animals can 
 learn to recognise sounds and associate them with certain ideas, 
 as, for instance, working instructions for either horse or dog, words 
 
THE SENSES 627 
 
 of command, trumpet calls, or the master's voice. Every dog 
 owner believes that his animal understands all that is said to it ; 
 certainly, in the case of the sheep-dog it is astonishing what he 
 is capable of comprehending. Further, we have undoubted 
 evidence that sounds which are so feeble as not to affect the 
 human ear are readily perceived by some animals, so that the 
 acuteness of their sensations is greater than that of our own. 
 
 When the vibrations of the air molecules reach the membrana 
 tympani, the drum moves in and out, and so sets in motion 
 the chain of bones in the middle ear. This chain acts as a bent 
 lever, whereby its movements, though reduced in amplitude, are 
 rendered more intense, so that the stapes through the fenestra 
 ovalis imparts a push to the peri-lymph of the labyrinth. This 
 impulse is transmitted through the fluid. ojLthe_vestibule, and 
 thence into the scala vestibuli of the cochlea ; it then ascends the 
 spiral staircase and sets in motion the memb rane of.Ejeissn.er, 
 which causes theTy mph in th e cochlea£_canal to vibrate: these 
 vibrations reach the summit of the cochlea and enter the^scala 
 tym pani _th rough the helicotrema. The lymph in this canaTis 
 set in motion, with the result that the basilar membrane on which 
 the org an of Cor ti rests is also affected, and ultimately the 
 vibrations reach the fenestra rotunda, which in turn is pushed 
 out. Every pushjnwards oFTEe - : fenestra_oyalis causes, there- 
 fore, a push ^utwards of the. fenestra rotunda. 
 
 The organ of Corti is suspended in fluid, "and, as we have seen, 
 rests on the basilar membrane; this membrane increases in 
 width from the base to the apex of the cochlea; it is composed 
 of fibres or strings, each string, it is supposed, being connected 
 with a branch of the auditory nerve. According to the theory of 
 Helmholtz, the basilar membrane vibrates sympathetically 
 when the movements of the fluid surrounding it correspond to 
 its own period of vibration, in precisely the same way that a 
 tuning-fork or the strings of a piano vibrate to their own notes 
 
 I from sounds in their vicinity, and to no others. 
 
 As the strings in the basilar membrane get longer and longer 
 from the base to the apex of the cochlea, and as these are some 
 24,000 in number, it is assumed that ample provision is made 
 
 jj for the analysis of sound, from the highest to the lowest notes, 
 each note being picked out by sympathetic vibrations of the 
 
 Is fibres concerned, and then transmitted to the brain by their 
 branches of the auditory nerve. The organ of Corti is responsible 
 for bringing the vibrations of the fibres of the basilar membrane 
 
 \ into communication with the nerve-endings by means of a 
 rocking motion imparted to it by the vibrations of the basilar 
 
 * membrane, the result being that the hair cells are moved from 
 
 .; side to side in the pores in which they lie, and so stimulated. 
 
628 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The most recent theory of hearing is that propounded by 
 Wrightson.* He rejects the theory of Helmholtz with its 
 basilar strings, each having an individual rate of vibration, 
 and states that the basilar membrane vibrates over its entire 
 extent as the result of the sound-waves causing fluctuations 
 in fluid pressure within the ear; these fluctuations produce a 
 rocking movement of the organ of Corti, and consequently to-and- 
 fro movements of the hair cells; the impulses thus set up are 
 then conveyed to the brain for analysis. The differences in the 
 rival theories are that Wrightson believes that the whole of the 
 basilar membrane vibrates — in consequence, every hair cell being 
 stimulated — and that the analysis of sound does not take place 
 in the cochlea, but in the brain. There are, however, difficulties 
 of a physical character connected with this theory which 
 require explanation before that of Helmholtz can be regarded 
 as displaced. 
 
 Semicircular Canals and Vestibule. 
 
 We have seen elsewhere (pp. 515, 549) that the eighth pair of 
 cranial nerves consists of two roots, dorsal and ventral, both dis- 
 tributed to the internal ear, but both are not concerned in the 
 sense of hearing. The dorsal root alone is distributed to the 
 cochlea; the ventral root passes to the vestibule and semicircular 
 canals. In consequence, the roots have been distinguished as the 
 cochlear and vestibular divisions of the auditory nerve. The 
 cochlear division is exclusively devoted to the sense of hearing, 
 the vestibular division to the function of maintaining the equi- 
 librium of the body. The vestibular nerve is connected with the 
 cerebellum, but not directly with the cerebrum. 
 
 The Semicircular Canals are three ring-like structures lodged 
 in the petrous temporal bone; their positions are described as 
 superior, posterior, and external. They are so arranged that 
 their three planes are placed at right angles to each other, two 
 being vertical and one horizontal, thus corresponding to the 
 three dimensions of space (see Fig. 194). Their position in the 
 petrous temporal bone is shown in Fig. 190. 
 
 The semicircular canals, together with the vestibule, constitute 
 what are known as position receptors — that is to say, mechanisms 
 which inform the animal of the position of its body relative 
 to gravity, or of the direction in which its body is moving. 
 They are not the only position receptors ; it is obvious that the 
 eye assists in the perception of direction, while the skin furnishes 
 touch receptors, and there are nerves in joints and spindles in 
 
 * ' The Analytical Mechanism of the Internal Ear,' Sir Thomas Wright- 
 son, 1918. 
 
THE SENSES 
 
 629 
 
 muscles which furnish the muscle sense referred to at pp. 433, 
 491, 517, and 616. 
 
 The bony semicircular canals have within them a membranous 
 counterpart; between the bony and membranous structures 
 is a fluid known as peri-lymph, while within the membranous 
 is a second fluid, known as endo-lymph. The vestibule has 
 within it two membranous sacs, utricle and saccule, external 
 to which is peri-lymph, while the sacs contain endo-lymph. 
 Within each of these sacs is an elevation known as the macula 
 acustica, the seat of the termination of the vestibular nerve. 
 The nerve here terminates in cells known as hair cells, on the 
 free extremities of which are cilia. The cilia of a bundle of 
 such cells form stiff hair-like processes, and in connection with 
 these there are fine solid particles of carbonate of lime known as 
 otoliths. In the ampullae of the semicircular canals are bodies 
 
 Fig. 194. — The Semicircular Canals (Diagrammatic) (Ewald). 
 
 H, Horizontal canal; S, superior vertical canal; P, posterior vertical canal. 
 The two horizontal canals are in the same plane. The plane of the superior 
 vertical canal of one side is parallel to the plane of the posterior vertical canal 
 of the opposite side. 
 
 similar to the macula acusticse in which the fibres of the vestibular 
 nerve terminate, but there are no otoliths. The peculiar nerve 
 terminations briefly described above are bathed in the endo- 
 lymph. Every movement of the head affects the fluid contents 
 of that semicircular canal which corresponds to the plane in which 
 the movement occurs, with the result that the fluid is displaced 
 and causes a deflection of the stiff hair-like nerve-endings; the 
 movement of these results in an impulse which is communicated 
 to the cerebellum. The function of the utricle and saccule 
 appears to be chiefly concerned with slow movements, equilibrium, 
 and the position of the head, especially during the time the body 
 is at rest. In these sacs the otoliths, affected by gravity, 
 mechanically stimulate the nerve-endings. m 
 
 The function of the labyrinth, to quote the words of Sherrington, 
 is to keep the world right side up for the organism by keeping 
 
630 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the organism right side up to the external world. The labyrinth 
 also exercises effects on the voluntary muscles (p. 436), there 
 being a constant outflow of impulses from it which maintain the 
 muscles in a condition of tonus, for it can be shown experiment- 
 ally that unilateral destruction of the semicircular canals causes 
 loss of muscular tone on the same side of the body; in cases of 
 death it causes delay in the appearance of rigor mortis. The semi- 
 circular canals are also connected with the control of the ocular 
 muscles and the maintenance of a horizontal pupil (p. 572). 
 
 Fig. 195. — Dog Twenty-Two Months after Destruction of Right 
 Labyrinth (Wilson and Pike). 
 
 Unilateral destruction of the canals in cats produces a to-and- 
 fro rolling of the eyes, muscular inco-ordination, and rotation 
 of the head. It does not destroy the ' falling reflex ' (see p. 488), 
 though the body generally rotates in a direction away from the 
 lesion. Destruction of the canals in the pigeon produces, among 
 other symptoms, muscular inco-ordination and an inability to fly. 
 
 Unilateral destruction of the labyrinth does not produce in 
 mammals the same degree of muscular inco-ordination as in birds. 
 At first they readily fall towards the injured side and move with 
 difficulty, always in the direction of the side of the lesion, yet 
 the most pronounced symptoms are shown in the head and eyes. 
 The former is turned towards the injured side (Fig. 195), and the 
 eyeballs roll, the direction depending on the canal destroyed. 
 
CHAPTER XVI 
 LOCOMOTOR SYSTEM 
 
 Section i. 
 
 Muscles, Joints, Tendons, Ligaments. 
 
 Muscles and Joints. — The structure of muscle, its innerva- 
 tion, and the phenomenon of a muscular contraction, have 
 been studied. It is here intended to examine the muscles at 
 work, and to ascertain what they are capable of effecting, and 
 how they effect it. Speaking broadly, the skeleton is clothed 
 with muscle; there are exceptions to this, such as the face and 
 the limbs below the knee and hock, where the framework , is 
 uncovered, but generally the bones are clothed, the muscular 
 covering of the limbs becoming progressively lighter from above 
 downwards. The scapula and humerus, the pelvis and femur, 
 are buried beneath a mass of muscle; over the radius and tibia 
 the muscular structure is much less in bulk, and at the knee and 
 hock it suddenly ends. The movements of the limbs of quad- 
 rupeds are of a simple character. Considered only in solipeds, 
 these consist of a to-and-fro pendulum motion, and a limited 
 movement in an outwards and inwards direction. This is in- 
 finitely more simple than in those animals furnished with a paw 
 or hand. The limbs have to be flexed and extended, abducted 
 and adducted, and both in the fore and hind legs these move- 
 ments are carried out entirety by the muscles clothing or attached 
 to the scapula and humerus, pelvis, femur, and tibia.* Speaking 
 broadly, no muscles originate below the elbow or stifle joints. 
 
 * A definition of the following terms employed in limb and joint move- 
 ments may.be desirable: 
 
 Flexion and Extension refers to either joints or limbs. To flex a limb 
 is to bend it ; to extend a limb is to straighten it. Nevertheless, this does 
 not cover the entire ground; the foot, for instance, during the movement 
 of flexion causes the pastern to become extended — viz., straightened. 
 Conversely, when the foot is extended, the pastern is bent or flexed. 
 
 Abduction and Adduction. — These terms apply to limbs only. A limb 
 is abducted when it is carried away from the middle line of the body. 
 Similarly, it is adducted when drawn towards the middle line . The amount 
 
 631 
 
632 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 This being so, it is evident that movements of the upper part 
 of the limbs are automatically followed by movements of the 
 lower parts. 
 
 Flexion and Extension. — It is usual to picture muscles at 
 work from the appearance they present on the dissecting-room 
 table; valuable as this is, it does not tell the whole story. It is 
 only when muscles function that the fundamental principle is 
 grasped that they are really never idle, and that even when not 
 engaged in performing the more active duties for which they 
 exist, they are still part of the animal machine, and engaged in 
 the work of muscle co-operation. Flexion, extension, abduction, 
 and adduction, are not the exclusive functions of individual 
 muscles. A muscle is not, for instance, exclusively engaged as a 
 flexor or extensor; it may be actively employed in flexion, or 
 passively employed in extension; it may be actively engaged in 
 extending one joint, and actively engaged flexing another; it 
 may be actively flexing one joint, and automatically flexing 
 its neighbour; it may be passively employed in extending one 
 joint, and when active act as a flexor of another. Examples 
 of these points will now be given, for if the machine at work is to 
 be correctly visualised, the facts above stated must first be clearly 
 grasped by the mind. 
 
 1. A muscle may be a -flexor when contracting, and an extensor 
 •when not active. A good example of this is the flexors of the 
 fore-limbs, which, when not contracting, are maintaining the 
 elbow- joint firmly extended. Another is the biceps brachii 
 (flexor brachii), which, when actively contracting, flexes the elbow- 
 joint, and when passive is assisting, by means of a strip of tendon 
 to the extensor carpi radialis (extensor metacarpi magnus), in 
 keeping the knee firmly extended. 
 
 2. A muscle may be engaged actively at one and the same moment 
 in extension and flexion. This is seen in the superficial digital 
 flexor (flexor perforatus) of the hind-limb, which in extending 
 the hock- joint flexes the pastern. 
 
 3. A muscle may be actively engaged flexing one joint and 
 
 of abduction and adduction of the limbs is very small ; it reaches its maxi- 
 mum in a ' cow-kicker ' on the one hand, and on the other in a horse 
 that ' brushes ' from defective conformation, but especially in a ' weaver.' 
 
 Rotation is a term limited to joints, and the only joints in the limbs of 
 the horse which can rotate are the shoulders and hip. Rotation of the 
 stifle is produced in the hock and carried out in the hip-joint. 
 
 It is obvious that in all movements of flexion, extension, abduction, 
 adduction, and rotation, the action is not an isolated one ; the movements 
 must work at least in pairs, and sometimes the entire series is employed. 
 Take, for example, a horse ' cow-kicking ' : the limb must first be flexed, 
 then forcibly extended, and at the same time abducted. To admit of 
 this there must be rotation of the hip, and to allow the limb to recover its 
 position adduction must follow. 
 
LOCOMOTOR SYSTEM 633 
 
 automatically flexing its neighbour. The muscles which flex the 
 stifle-joint automatically flex the hip. 
 
 4. A muscle may in flexing one joint flex several others. This 
 is the case in the flexors of the fore-limbs, which flex knee, 
 fetlock, pastern, and foot, at one and the same contraction. 
 
 5. A muscle may be passively engaged extending one joint and 
 actively employed in flexing another. An example of this is the 
 tibialis anterior, which with peroneus tertius (flexor metatarsi) is 
 passively engaged by means of the cord which runs through their 
 substance in maintaining the stifle extended, and when that 
 joint is flexed is actively engaged in flexing the hock. 
 
 6. Two parallel muscles, with a common tendon of origin, may 
 act in opposite senses. This is seen in the tibialis anterior, peroneus 
 tertius, and _ the anterior long digital extensor (flexor metatarsi 
 and extensor pedis); the one extends the foot, the other flexes 
 the hock. 
 
 7. A muscle may both passively and actively be an extensor only. 
 An example of this is the gastrocnemius muscle,- which, whether 
 the foot be on or off the ground, is an extensor of the hock. 
 
 It is possible for a muscle to be so anatomically situated as to 
 appear to act as an extensor of one joint and a flexor of another, 
 yet when the muscle is acting in the living animal it may be found 
 to act only as a flexor, and never as an extensor. Sherrington 
 has shown that this is the case with the semitendinosus muscle 
 of the cat, which is so situated as to act as a flexor of the knee 
 (stifle) and an extensor of the hip-joint; but experimentally it 
 can be shown that the muscle always flexes the knee, and never 
 extends the hip, for the reason that the hip flexor contracts at 
 the same moment, and neutralises its action on that joint. 
 
 It has already been pointed out (pp. 435, 436-, and 491) that 
 even when the muscles of the body are apparently not acting 
 they are in a condition of slight contraction, which is necessary 
 for the maintenance of the standing attitude, and that it is only 
 during the time the animal's weight is actually off its legs that 
 the muscles can be considered to be at rest. 
 
 When a muscle contracts its antagonist is relaxed, and the 
 nervous channel through which this is effected has been described 
 under the head of ' reciprocal innervation ' (see p. 489). Where 
 muscles act on a single joint, one as a flexor and the other as an 
 extensor, the reciprocal nervous mechanism is easily understood, 
 as the two muscles are direct antagonists. But when muscles 
 are not exclusively devoted to one joint — in fact, affect two or 
 more— the nervous mechanism is not so simple, for the reason 
 that flexion at one joint may lead to extension at another. An 
 example of this has already been given; the semitendinosus muscle 
 is permitted to act only as a flexor of the knee (stifle) of the cat, 
 
634 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 • and not as an extensor of the hip, and to secure this the latter 
 joint is fixed by a contraction of its flexor muscles. In such cases 
 the nervous mechanism is identical, not reciprocal, for the flexors 
 of the hip contract in order to prevent extension of this joint, at 
 the same moment as the semitendinosus contracts and flexes 
 the stifle. 
 
 The condition revealed by this experiment is that flexor and 
 extensor muscles, true antagonists, may contract together, 
 co-operate, in order to fix a joint, so that another joint may be 
 more effectively acted upon. Similarly, two extensor muscles 
 may be actively employed some distance from each other in 
 fixing the joint of a limb; they are not true antagonists; they are 
 both extensors, and they act at the same moment; they are 
 spoken of as ' partial antagonists.' The best example of this is 
 afforded by the gastrocnemius muscle, which always acts as an 
 extensor, whether during standing or in active movements, yet 
 its origin from the femur is such that it might flex the stifle. It 
 never flexes the stifle, for the reason that the muscles of the 
 patella keep the stifle extended at the time the gastrocnemius 
 is acting by keeping the hock extended. The patella muscles and 
 gastrocnemius muscles are partial antagonists; they contract 
 together and relax together, and by so doing insure that one joint 
 is kept fixed in order that a muscle crossing it may act better on 
 another joint. The innervation such muscles receive is identical. 
 When muscles have a muscular origin and a tendinous inser- 
 tion, the direction in which their contraction occurs is from 
 muscle to tendon; but when muscles are fleshy throughout, 
 such as the long muscles found in the dorsal region, they may 
 act from either end — for instance, in the group referred to they 
 act from the neck end in kicking, and the croup end in rearing 
 (Figs. 227, 228). 
 
 Tendons and Ligaments. — The chief function of ligaments is 
 to hold bones in position. The greater the necessity for joint 
 freedom, the fewer and simpler the ligaments. The shoulder- 
 joint, for instance, has no special lateral ligaments. The range 
 of motion is too considerable for these, so that tendinous in- 
 sertion of the muscles takes their place. There is very much 
 less motion in the hip, and the ligaments are centrally placed. 
 In the other joints they are laterally situated, but from knee 
 and hock to the foot a ligament seems to have been put in 
 wherever space not otherwise occupied existed. Ligaments are 
 also employed for holding floating bones like the patella and 
 sesamoid in position. They knit together the various segments 
 of the spine, and one of peculiar structure supports the head 
 and neck. Ligaments may also run to tendons to enable the 
 muscle to be partly cut off during the standing attitude. Two 
 
LOCOMOTOR SYSTEM 635 
 
 to the flexors of the lower limb of the horse undoubtedly assist 
 the animal to sleep in the standing position while the muscles- 
 rest. They are known as check ligaments. Another kind of 
 ligament peculiar to quadrupeds is a tendo-ligament— i.e., a 
 muscle with a tendon running completely through its substance 
 from origin to insertion, which, when the muscle is not actively 
 contracting, acts as a ligament, and when it is contracting 
 plays its normal part of tendon . We shall see that this mechanism 
 is employed both in the fore and hind legs for the purpose of 
 maintaining a standing attitude with as little muscular strain as 
 possible. A muscle may furnish a ligament to another muscle 
 in order to afford it support. This is shown in the ligament 
 detached by the biceps brachii {flexor brachii) to the extensor carpi 
 radialis (extensor ntetacarpi magnus). A variation of this is a 
 muscle detaching a ligament to a bone which is considerably out 
 of its path — for instance, the ligament detached by the large 
 biceps femoris (triceps abductor femoris) to the calcis.* 
 
 The elasticity of tendons and ligaments is considered later 
 (P- 653). This subject has proved a fruitful source of difference 
 of opinion, and the matter is by no means agreed upon yet. 
 
 The elasticity of muscle has been dealt with at p. 434, and to 
 form a conception of what occurs to tendons under the influence 
 of strain, the elastic property of muscle must not be forgotten. 
 Nothing could well be more unlike in appearance, texture, and 
 general physical characters, than muscle and tendon. Yet 
 the soft, pulpy, friable material does not tear in the living 
 animal, while the tendon attached to it may be torn apart. 
 The only locomotor muscle in the horse's body which shows a 
 disposition to rupture is the flexor metatarsi, and such a lesion is 
 so rare that many have never seen it. 
 
 Colin tested the breaking-strain of muscles removed from a horse 
 which had just been destroyed. The great extensor of the meta- 
 carpus broke at a tension of 988 kilogrammes (2,173 pounds); the 
 flexor perforans of the fore-limb at 685 kilogrammes (1,057 pounds) ; 
 and the flexor of the arm at 973 kilogrammes (2-140 pounds). In 
 the hind-limb the extensor pedis muscle broke at 415 kilogrammes 
 (913 pounds), the flexor of the metatarsus at 924 kilogrammes 
 (2,032 pounds), and the flexor perforans at 510 kilogrammes (1,122 
 pounds). The extensors of the metatarsus ruptured at 616 kilo- 
 grammes (1,355 pounds), 687 kilogrammes (1,511 pounds), and 
 983 kilogrammes (2,162 pounds) respectively, but the muscles are 
 not mentioned by name. It will be observed that the breaking- 
 strain of the extensors is much higher than that of the flexors, that 
 of the extensor of the metacarpus being nearly one ton. In the 
 
 * Anatomists are responsible for the confusion existing in muscle 
 nomenclature. Where possible, the writer has followed Sisson in modern 
 terminology, inserting the known equivalent in brackets. There are six 
 names for the biceps femoris muscle ! 
 
636 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 hind-limb there is a marked difference between the breaking-strain 
 of the extensor pedis and flexor metatarsi muscles, though the two 
 have a common tendon of origin. Finally, the relatively low break- 
 ing-strain of the flexor muscles is very evident. If the chief flexor 
 of the fore-limb — a muscle which perhaps does half the work of pro- 
 pelling the body in the gallop — breaks at a tension of 1,507 pounds, 
 we may be sure from this that the breaking-strain of its tendon is 
 correspondingly low. As a matter of fact, muscular rupture during 
 life is rare, and it may be urged that such post-mortem observations 
 do not represent what occurs in the living condition. That is 
 certainly true ; nevertheless, it is suggestive that the low breaking- 
 strain of the flexor muscles corresponds to the frequency with which 
 during life the flexor tendons are sprained. 
 
 Levers. — The muscles are attached to bones, and the latter 
 form various angles with each other, which are opened and closed 
 during progression. The mechanical aid introduced in order to 
 effect this is the lever. Levers are of three orders, depending upon 
 the relative positions of the power, fulcrum, and weight. A lever 
 of the first order is mainly a lever of extension, the power being at 
 one end, the weight at the other, the fulcrum between. A good 
 example is the extension of the head on the neck; the weight is the 
 head, the power lies in the neck, the fulcrum is the joint between 
 the head and neck. A lever of the second order is not common in 
 the body. The weight lies in the middle, the fulcrum and power at 
 either end. When the fore-foot is on the ground supporting weight, 
 the foot is the fulcrum, the body through the elbow-joint is the 
 weight, the power lies in the biceps muscle acting on the ulna. A 
 lever of the third order is the lever of speed and flexion. In it the power 
 lies between the fulcrum and weight. In the flexing of the arm 
 the biceps is the power, the fulcrum lies above it in the elbow-joint, 
 the weight lies below — viz., the weight of the limb. This is a lever 
 which sacrifices power for speed ; it istherefore wasteful, but necessary 
 in the limbs. 
 
 Function of Muscles. — The muscles of the skeleton have a two- 
 fold function to perform- — they move the body and they maintain 
 its attitude (p. 435). Accordingly, so long as the animal is not 
 actually lying down, the muscles are in constant action, though 
 the effort required to maintain attitude is less exhausting than 
 that necessary to moving and carrying weight. A standing 
 attitude is maintained by the flexible limbs being rendered rigid, 
 and this is effected by the muscles keeping the joints locked. 
 For this purpose they receive a constant stream of impulses, and 
 if these are withdrawn for ever so brief a period, the body falls 
 under its own gravity. 
 
 It requires a greater muscular effort to maintain a standing 
 attitude in the fore than in the hind legs, owing to the fact that 
 the fore-limbs are not attached to the body by any joint, and 
 are solely dependent on masses of muscle for support. When a 
 horse sleeps in the standing attitude, his muscles not infrequently 
 lose their tonic impulses, and he falls, but always front first, 
 never hind first, and never in a heap. Long before his knees. 
 
LOCOMOTOR SYSTEM 
 
 637 
 
 have reached the ground the muscles have secured control once 
 more, and he seldom falls farther than 
 his fetlocks. 
 
 It seems quite legitimate to regard the 
 muscular union between the thorax and 
 the fore-limb as a joint. There are no 
 bones resting on each other, no synovia; 
 but where the scapula has its largest 
 range of movement there is a remark- 
 able amount of areolar tissue, which 
 renders movement easy. The whole 
 central area beneath the scapula and 
 humerus not occupied by muscular 
 attachment is filled with this easily- 
 moving, apparently gaseously distended, 
 crepitant, areolar tissue, over which the 
 fore-legs glide on the chest wall as freely 
 as if the parts were a large, well- 
 lubricated joint. The animal's body is 
 slung between the fore-legs, and the 
 muscle principally, if not wholly, en- 
 gaged in this important function in the 
 thoracic portion of the serratus magnus. 
 The tendinous cover of this immense 
 muscular plate imparts additional 
 strength, and is present in all muscles 
 where strain is constant or long con- 
 tinued (see p. 424). The serratus magnus, 
 on contracting, also draws the posterior 
 and upper part of the scapula down- 
 wards ; this is antagonised by its cervical 
 portion . This muscle is a good example 
 of a single muscle being so arranged in 
 its fibres that they may act together or 
 in opposition . During the standing atti- 
 tude the fibres of this muscle are acting 
 together, but in opposite directions, in 
 order to maintain the scapula in a fixed 
 position. During locomotion the portions 
 are acting alternately (see Fig. 199). 
 
 The fixation of the scapula is the 
 first essential step towards a standing 
 attitude, and it is secured by the 
 serratus magnus, rhomboideus, and tra- 
 pezius (see Figs. 198, 199). The humerus 
 below has now a firm bed to press against, as the scapula are 
 
 Fig. 196. — Bones of the 
 Fore-Leg of the Horse 
 in the Attitude of 
 Standing. 
 
 The dotted lines show the 
 principal mechanism by 
 which the limb is kept 
 rigid. 
 
 1, i, 1, The great triceps 
 muscle; 2, the flexor of 
 the arm; before insertion 
 it gives off a s'ip of tendon 
 to the extensor of the 
 metacarpus, 3; 4, the ex- 
 tensor tendon of the foot; 
 5 and 6, the flexors of the 
 foot; 7 and 8, the check 
 ligaments; 9, the suspen- 
 sory ligament. 
 
638 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 prevented from being driven through the withers by the serratus. 
 It is essential for the humerus that the shoulder-joint should be 
 fixed and the elbow-j oint locked. The large muscular mass of the 
 triceps, acting from the humerus and scapula on the ulna, is 
 mainly engaged in effecting this (Fig. 196, 1). The more power- 
 fully the triceps contracts, the more firmly is the beak of the 
 ulna forced into the olecranon fossa of the humerus. It is this 
 mechanism which keeps the elbow-joint locked. But what has 
 previously been said about the absence of exclusiveness in 
 muscles being borne in mind, it is evident that the flexors of the 
 fore-leg act passively on the elbow and keep it firmly extended 
 (Fig. 196, 5 and 6), while the biceps brachii (flexor brachii), 
 through its connection with the main extensor muscle of the 
 limb, helps to keep the scapula in position and the shoulder-joint 
 from opening in front (Fig. 196, 3). This is a good example of 
 co-operative antagonism. Here we see the flexor of the arm, 
 extensor of the leg, and extensor of the elbow, all of which are 
 antagonists when actively contracting, working together to 
 one common end. The tendon running throughout the length 
 of the biceps is of invaluable assistance in keeping the shoulder- 
 joint fixed while the limb is in the standing position; it acts, in 
 fact, as a ligament from scapula to radius (Fig. 196, 2). 
 
 The shoulder and elbow being fixed, the next joint, the knee, 
 is far more easily controlled. From the knee to the foot it is only 
 necessary to prevent the joints opening in front in order to main- 
 tain the limb rigid and upright (p. 435), and to insure this each 
 segment of the limb is furnished with an extensor tendon (Fig. 196, 
 3 and 4) . There is a large one to the knee, and another to the pastern 
 and foot, each of which is in turn reinforced for extra strength. 
 The muscles which manipulate this locking apparatus run from 
 the humerus, cross the front of the elbow-j oint, and are dis- 
 tributed segment by segment until the foot is reached. Their 
 efficacy in locking the fore-limb from the elbow to the foot 
 depends upon the elbow-j oint itself being firmly extended and 
 locked. The large knee extensor receives powerful assistance 
 from the biceps brachii in the form of a ligament which runs from 
 the biceps to the extensor (Fig. 196, where 2 and 3 meet). 
 
 The position thus created is as follows : The biceps during the 
 standing attitude, though not out of action, is not engaged in 
 flexing the elbow. It is a flexor muscle which, we have just 
 seen, is passively resisting the triceps, but it does not hesitate to 
 help an extensor, an opponent, during the time it is not other- 
 wise actively employed on its own special duty. This is a good 
 example of the non-party feeling shown by muscles, and their 
 desire for general rather than special utility. 
 
 The weight from the elbow to the foot is carried on the flexor 
 
LOCOMOTOR SYSTEM 
 
 G39 
 
 muscles of the arm, of which there are five, three not proceeding 
 below the knee, to the back of which they obtain attachment, 
 and two which run the entire length from elbow to phalanges 
 
 Extensor carpi obliquus-- 
 
 Olecranon 
 
 '"Ulnar head of deep flexor 
 
 '"Lateral extensor 
 
 -Deep flexor (humeral head) 
 
 Metacarpal tuberosity-- 
 
 Tendon from anterior, 
 to lateral extensor 
 
 External small metacarpal bone---i- T is 
 
 '0 
 
 Branch of suspensory liga* 
 ment to extensor tendon * v /' 
 
 •--Tendon of flexor carpi externus 
 
 —•Accessory carpal bone (trapezium) 
 
 Check ligament 
 
 ■ Suspensory ligament 
 "">£-**" Flexor tendons 
 
 !v,_., Flexor tendons 
 Lateral cartilage 
 
 Fig. 197. — Muscles of Outside of Near Fore-Limb of the Horse 
 (Ellenberger-Baum-Sisson). 
 
 a, Extensor carpi radialis ; g, brachialis ; g' , anterior superficial pectoral ; c, anterior 
 or common digital extensor {extensor pedis)) e, flexor carpi externus. 
 
 All these flexor muscles are characterised by the presence in their 
 substance of a considerable amount of fibrous tissue, and both 
 at the knee and elbow are fused with the fascia of the fore- arm, 
 
640 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 by which means additional support under continuous strain is 
 obtained. But it is by the two long flexors, the superficial and 
 deep digital (flexor perforatus and perforans) (Figs. 196, 5, 6; and 
 197), that the main support to the bony column is given, and this 
 is obtained through the limb bending forward at the fetlock and 
 these two tendons passing beneath and supporting the bent joint. 
 The weight of the body presses the fetlock to the ground. The 
 counteracting force of the flexor tendons sustains that weight, 
 and maintains the fetlock in position. They are not the only 
 mechanism supporting the fetlock, for there is the suspensory 
 ligament, of which later; but the flexors are the chief support, 
 and, if they are divided, the fetlock sinks appreciably closer to 
 the ground. The changes occurring in the fetlock-joint when an 
 animal from injury or other cause is compelled for some time to 
 support its body weight entirely on one limb serves to explain the 
 mechanism of the joint. It is rarely that the muscular portion of 
 the supporting flexors gives way under the strain, but commonly 
 the tendinous portion, and this is well seen at the fetlock-joint. 
 The bursa of the tendons at the part is dry ; the tendons themselves 
 are dry, yellow, and shrunken. The interior of the fetlock- joint 
 reveals nutritive changes, brought about by long-continued com- 
 pression. The articular cartilage has become so thin that the 
 bone has the barest possible covering, the joint is dry and yellow, 
 and so intense has been the pressure that the actual pattern of 
 one joint surface may be imprinted deeply on the adjacent bone, 
 and this imprint is made on the posterior part of the articulation, 
 and not on the anterior; in other words, the pressure falls behind. 
 
 The two long flexors running from elbow to foot are assisted 
 in their duties as weight-bearers by means of so-called check 
 ligaments (Figs. 196, 7, 8; 197). These ligaments are running, 
 not from bone to bone, but from bone to tendon, and make a 
 tendon function as a ligament by cutting off, as it were, the 
 muscular attachment above. The perforatus above the knee 
 and the perforans below it both receive a check ligament, which 
 under the long-continued strain of support enables the flexor 
 muscles to slightly relax and obtain a much-needed rest. 
 
 The mechanisms below the fetlock are a continuation of those 
 above it, the same extensor and flexor tendons being employed 
 from elbow-joint to foot. The slope of the pastern is main- 
 tained by the flexors of the leg, and though a joint exists between 
 the second and third phalangeal bone, its function, when the 
 limb is in the standing attitude, is not in evidence. In looking 
 at the slope of the pastern, the eye always regards it as one bone, 
 instead of two. l"he reason is obvious. There is very little move- 
 ment in the joint between the corona and suffraginis at any time, 
 and there is still less in standing. They are united by a dense 
 
LOCOMOTOR SYSTEM 641 
 
 network of ligamentous material, and, as we shall see later in 
 studying the limb in movement, the chief motion occurring in 
 the pastern bones is between the corona and pedis. 
 
 The fore-limb is kept vertical and the pastern bent by the 
 insertion of the perforans tendon into the pedal bone; every joint 
 from the elbow to the foot is locked by its extensor tendons and 
 supported by its flexors, the fascia of the arm binding up and 
 supporting both groups. 
 
 In the hind-limbs the standing attitude is simpler, owing to 
 the existence of the hip- joints beneath the horizontal spine. 
 The attitude assumed by the legs is the result of flexion in some 
 joints, extension in others. The hip-joint is flexed by the psoas 
 magnus, iliacus, tensor, fascia lata, gluteus superficialis (gluteus 
 extemus), and pectineus muscles. Considering the enormous 
 amount of muscular tissue belonging to the hind-quarters, it is 
 rather remarkable that so little of it is required for hip-flexion, 
 for the above, relatively speaking, are not bulky muscles. The 
 extensors of the hip- joint, on the other hand, are very large — i.e, 
 the insertions of the large biceps femoris, gluteus medius, and 
 semimembranosus (see Pigs. 198, 199, and 200). 
 
 The above groups of muscles fix the head of the femur rigidly 
 so far as to-and-fro and outward movements are concerned, but 
 the joint has also to be supplied with support on its inner side, 
 and this is effected by the great and small adductors and gracilis. 
 The resultant of the pull of these muscles on the hip-joint is the 
 position assumed by the femur in standing, and the angle it 
 forms with the pelvis depends upon whether the limb is bearing 
 weight or resting. The joint below the hip is the stifle, and the 
 part this plays in attitude is considerable, for no flexion of the 
 hind-leg can possibly take place without the concurrence of the 
 stifle. The stifle is flexed by means of the biceps femoris, semi- 
 tendinosus, and gracilis (Figs. 198, 200); it is extended by the 
 rectus femoris and vasti muscles (Fig. 199). Unless these latter 
 concur, flexion of the stifle is impossible. So slight, indeed, is 
 the effort required to keep the stifle rigidly extended that if one 
 hand grasps the muscles inserted into the patella, and presses 
 the bone back on the femur- — the horse having just been destroyed 
 — no ordinary force employed at the foot-end of the limb can 
 flex the hock, and the hock cannot flex without the stifle flexing. 
 
 Tendo-Ligaments of the Thigh. — From the above if is evident 
 that very little muscular effort is required to keep the largest joint 
 in the body fixed during the standing attitude. The stifle is kept 
 extended not only by the muscles inserted into the patella, but 
 also by the contraction of the anterior digital extensor (extensor 
 pedis), peroneus tertius, and tibialis anticus (flexor metatarsi, 
 muscular and tendinous division)— a pair of muscles (Fig. 201) 
 
 41 
 
642 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
LOCOMOTOR SYSTEM 
 
 643 
 
644 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Tail 
 
 Gluteal fa<;< la 
 
 External angle of ilium 
 
 ^'-"■'Superficial gluteus 
 
 Gastrocnemius tendon 
 
 Superficial flexor tendon - 
 
 Deep flexor tendon 
 Superficial tlexor tendon , 
 
 Fig. 200. — Muscles of Off Hind-Limb of Horse (Ellenberger- 
 Baum-Sisson). 
 
 17, Position of tuber ischii ; 0", superficial gluteus ; q, q', q", biceps femoris ; r, sen 
 tendinosiis ; v, semimembranosus ; w, gracilis ; f, gastrocnemius. 
 
 attached to the external condyle of the femur by means of 
 common tendon. The tendon runs from the stifle to the hoc 
 joint, and is a cord of great strength, acting not only as a tend< 
 
LOCOMOTOR SYSTEM 
 
 645 
 
 to the muscles which envelop it, but also as a long straight liga- 
 ment (Fig. 202, 3). Chauveau describes the action of this 
 tendo-ligament as possessing the property of flexing the hock 
 automatically in conformity with flexion of the stifle, but he 
 
 Sartor ins - 
 
 Gracilis 
 
 Tuberosity of tibia'- 
 
 Tibialis anterior.-. 
 
 Internal malleolus. - 
 
 Anterior part of biceps femoris 
 
 ■ Middle part of biceps j'emoris 
 
 -- External condyle of tibia 
 FJH Hi" Lateral extensor muscle 
 
 I HI 
 
 ""Anterior or long extensor 
 
 ~'.fi "" Perotteus terlius 
 -- — Annular ligament 
 
 -■ External malleolus 
 
 Inner tendon of tibialis anterior _ a 
 Metatarsal tendon of tibialis anterior - 
 
 Anterior or long extensor 
 tendon (extensor pedis)- 
 
 fw- --Annular ligament 
 
 ■ Annular ligament 
 
 ■ Tendon of lateral extensor 
 
 Branch of suspensory Hgaircnt ___\ 
 
 Fig. 201.— Muscles of Near Hind-Limb of Horse, Anterior 
 View (Ellenberger-Baum-Sisson). 
 y, Rectus femoris ; 20, patella. 
 
 also regards its other use as that of keeping the stifle-joint 
 extended. It is impossible for the flexor metatarsi to act while 
 the stifle is kept locked, and there can be no doubt that the 
 action of the cord as a long ligament is of great value in relieving 
 
646 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the strain from the flexor metatarsi muscle. That the cord 
 is not absolutely essential to standing may be determined ex- 
 perimentally by dividing it, or clinically by the rupture which 
 occasionally occurs. It is the antagonist of a similar tendo- 
 ligament at the back of the tibia, which runs from the femur 
 to the calcis, the function of which is to oppose hock-flexion 
 (see Fig. 202, 2). 
 
 The dotted lines indicate the 
 powerful mechanisms for 
 maintaining the limb in a 
 rigid condition, ±, 1, The 
 muscles which keep the pa- 
 tella drawn up and the stifle- 
 joint locked; 2, the gastroc- 
 nemius and flexor perforates 
 muscles, both keeping the 
 hock extended in opposition 
 to 3; 3, the flexor of the 
 hock and metatarsus auto- 
 matically affected by 1. In 
 2 and 3 tendinous cords run 
 from end to end, and so act 
 as ligaments, keeping the parts 
 in the relation prescribed by 
 the patella without great 
 muscular effort. 4, The per- 
 foratus below the hock; 
 5, the perforans; 6, the check 
 ligament; 7, the suspensory 
 ligament; 8, the extensor of 
 the foot; 9, the gluteus 
 medius (maximus) keeping the 
 hip-joint extended. 
 
 Fig. 202. — Bones of the Hind-Leg of the Horse in the Attitude of 
 
 Standing. 
 
 The Hock.- — -The muscles which keep the hock extended 
 (Figs. 202, 2; 203) are — (1) The gastrocnemius, running from the 
 femur to the calcis, to which it is attached. It is an active 
 extensor when the foot is off the ground, a passive extensor when 
 the weight is resting on the limb. (2) The superficial digital flexor 
 (flexor perforatus), which runs from the femur to the calcis, to 
 which it is attached, forming a tendinous cup into which the 
 point of the calcis fits, and subsequently running down the back 
 of the limb to be inserted into the second phalanx. This muscle 
 
LOCOMOTOR SYSTEM 
 
 647 
 
 is almost entirely tendinous. It is the tendo-ligament (Fig. 202, 2) 
 antagonistic to the flexor metatarsi (Fig. 202, 3) ' previously 
 described. During the attitude of standing it relieves the con- 
 
 7, Fascia lati 
 
 Patell; 
 «\ External condyle of tibia^. j ; % 
 
 Y 
 
 Crest of tibia ■--■- 
 
 Anterior or long extensor — -- 
 Lateral extensor 
 
 anilcndiymsus 
 
 Biceps femoris 
 
 External head ol 
 gastrocnemius 
 
 Tendon of gastrocnemius 
 Tarsal tendon of 
 biceps femoris 
 
 Deep flexor 
 
 „_ Superficial flexor 
 -'tendon 
 
 Annularligament 
 
 Tendon of anterior extensor 
 
 Tendon of lateral extensor 
 
 Superficial flexor 
 tendon 
 
 Deep flexor tendon 
 
 Suspensory 
 ligament 
 
 Branch of suspensory liga- 
 ment to extensor tendon 
 
 Fig. 203. — Muscles of Lower Part of Thigh, Leg, and Foot of Horse, 
 External View (Ellenberger-Baum-Sisson). 
 
 stant strain which would otherwise be imposed on the gastroc- 
 nemius, and maintains the hock extended. It is not only an 
 extensor of the hock, but one of the flexors of the lower 
 limb. 
 
648 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The Reciprocating Action of Stifle and Hock.— From what 
 has been said, it is evident that flexion and extension of stifle and 
 hock are identical in action. When the stifle is extended, the 
 hock is automatically extended, nor can it under any circum- 
 stances flex without the previous flexion of the stifle. There is 
 no parallel to this in the body. The two joints, though far 
 apart, act as one, and they are locked by the drawing up of the 
 patella, and in no other way (Fig. 202, 1). The so-called disloca- 
 tion of the stifle in the horse is a misnomer. That the patella is 
 capable of being dislocated is beyond doubt, but the ordinary 
 condition described under that term, when the stifle and hock 
 are rigid while the foot is turned back with its wall on the ground, 
 is nothing more than spasm of the muscles which keep the patella 
 drawn up (Fig. 202, 1,1). The moment they relax the previously 
 immovable limb and useless foot have their function restored as 
 if by magic, but are immediately thrown out of gear in the course 
 of a few minutes as a recurrence of the tetanus of the patellar 
 muscles takes place. The fascia of the thigh, like that of the 
 arm, is a most potent factor in giving assistance to the constant 
 strain imposed on the muscles of the limbs during standing. 
 
 Below the hock the hind-limb is arranged like the fore, the deep 
 flexor (perforans) receiving additional support from the ' check 
 ligament,' as in the fore-leg (Fig. 202, 6). 
 
 The natural attitude of standing adopted by the horse is to rest 
 on three legs — one hind and both fore. If he is alert, he stands 
 on all four limbs; but if standing in the ordinary manner, he 
 always rests one hind-leg. He does not remain long in this 
 position without changing to the other. Hour by hour he 
 stands, shifting his weight at intervals from one to the other 
 hind-leg, and resting its fellow by flexing the hock and standing 
 on the toe. He never spares his fore-limbs in this manner in a 
 state of health, but always stands squarely on them. The 
 moment he places one fore- foot in advance of its fellow — •' point- 
 ing,' as it is termed — the experienced eye knows that something 
 is wrong. Many years ago the writer suggested that the periodical 
 resting of the hind-feet was the possible explanation of their 
 freedom from navicular disease. The horse rests the fore-feet 
 only when they are already subjects of this disorder. 
 
 Distribution of the Weight on the Limbs. — The fact that the 
 animal rests the hind-limbs, and never the fore, suggests that 
 the weight carried by the former is greater than that carried by 
 the latter. The reverse, however, is the case. The fore-legs 
 carry from 9 to 19 per cent, more weight than the hind. The 
 weight on the fore-limbs is increased when the head is depressed, 
 and reduced when it is raised. The latter movement displaces 
 the centre of gravity backwards, and throws more weight on the 
 
LOCOMOTOR SYSTEM 6 4 g 
 
 hind-limbs, for the head of the horse is very heavy, being from 
 40 to 50 pounds in weight, more or less, depending on size and 
 breed. 
 
 The Centre of Gravity.— During the standing attitude this 
 lies near to the intersection of a horizontal line below the shoulder- 
 joint by a vertical line passing about 6 inches behind the elbow. 
 During locomotion, jumping, draught, etc., the position is 
 constantly shifting from front to rear and from side to side, 
 depending on the position of the body and the pace. 
 
 Lying Down. — The attitudes assumed by a horse in lying 
 down are quite characteristic. He has only two positions — one 
 flat on his side, with the legs and head fully extended, and the 
 other with the head and spine raised, sitting obliquely on his 
 chest. He cannot sit directly on the sternum, owing to its sharp 
 edge, so the body inclines to one side in order to avoid it. In this 
 attitude of repose both knees and hocks are bent, all four feet 
 are brought under the body and arranged on a plan, which is 
 never departed from. If, for instance, he is sitting on his chest 
 inclined to the near side, the near fore-foot is placed close to the 
 breast-bone, the elbow touching the ground — in fact, the body 
 is resting largely on the near elbow. The near hind-foot is under 
 the abdomen, the outer part of this limb, both hock and shank, 
 resting on the ground. The off fore- foot lies close to the elbow, 
 but outside it. The elbow does not rest on the heel of this foot, 
 as is so frequently taught, and consequently never suffers injury 
 from the heel of the shoe. The off hind-foot is flexed and brought 
 forward towards the elbow, while the point of the hock of this 
 limb rests on the ground. A ' capped elbow ' never occurs from 
 the heel of the shoe, but is due to the underneath elbow coming 
 in contact with the ground. Similarly, a ' capped hock ' may be 
 aggravated or originate from resting on the point of the hock 
 when lying. The horse, as a rule, sleeps fully extended on his 
 side, but, should he sleep in the chest position, he rests the chin 
 on the ground, often everting the lower lip, so that the teeth 
 take the pressure. Cattle and sheep repose on the chest. The 
 flat sternum is perfectly adapted to this, the only position 
 normally adopted. During sleep they turn the head round to 
 one side, the nose being placed towards the flank. 
 
 The Act of Rising is always performed in one way by horses 
 and cattle, and in either animal the methods are reversed. No 
 horse can rise until he has placed his fore-legs out in front. The 
 first effort made is to unbend either knee alternately, and get the 
 feet out in front, resting on the heels. The hind-legs are well 
 flexed, the feet placed under the centre of the belly, and the 
 animal reaches the standing position by means of the hind-legs, 
 with the aid of the fore. The body follows the head, and if the 
 
650 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 head be kept down no horse can rise. All ruminants rise tail 
 foremost. Resting on both knees, they raise the hind-quarters, 
 straighten the limbs, and then rise in front. 
 
 Joints. — Before examining the second function of muscles — 
 viz., the production of locomotion — it is necessary to consider 
 again the question of joints. These have been studied in con- 
 nection with their static function; they must now be looked at 
 from their kinetic aspect. 
 
 Whenever two bones come together a joint is formed and an 
 arrangement provided against the production of friction. When- 
 ever any two surfaces meet, one of which is moving on the other — 
 such as tendons passing through grooves or working over bony 
 projections — similar anti- friction appliances exist. We have 
 even ventured to regard the adjustment of the fore-leg on 
 the chest wall as a joint, although wholly muscular, and have 
 pointed out (p. 637) in what way friction is provided against. 
 It has been shown that the joints of the limbs of the horse are 
 furnished with automatic appliances for maintaining position 
 with the least muscular effort, and enabling the animal to stand 
 continuously for long periods. The features now to be con- 
 sidered are those connected with the movements of joints. 
 
 The Shoulder- J oint is remarkable for the small surface afforded 
 by the scapula, and the large area existing on the humerus. 
 This allows great freedom of movement in both a to-and-fro 
 and a side-to-side direction. There is no bone in the body the 
 movements of which resemble those of the scapula, and there is 
 no joint which resembles that formed by it with the humerus. 
 There are no separate ligaments to the joint; the tendinous 
 insertions of the muscles are the ligaments, and this arrangement 
 would appear to be associated with the immense freedom required 
 in the rolling action of one bone on the other. Both in the fore 
 and hind limbs the two upper bones of the leg are arranged to 
 form an obtuse angle with each other, and this is peculiar to 
 quadrupeds. In the biped the humerus is not bent from the 
 vertical, nor are the femur and tibia. The object of the bending 
 is to insure great rapidity of action and to destroy concussion; 
 there is far less jar in dropping on a limb in which the column 
 of bones is bent towards each other than in a column which is 
 vertical. The scapula and humerus form an angle of about 
 120 degrees. The shoulder- joint is lifted — viz., the fore-limb 
 is advanced — by only one muscle, the mastoido-humeralis (levator 
 humeri) (Fig. 198). Arising from a region as high as the head 
 and atlas, this immense muscle is alone responsible for advancing 
 the limb. The numerous and complex movements occurring 
 throughout the whole length of the leg on the shoulder being 
 raised may here be outlined. The shoulder cannot be advanced 
 
LOCOMOTOR SYSTEM 651 
 
 with the foot on the ground, so the weight must first be transferred 
 to its fellow; the elbow- joint is next unlocked by a relaxation of 
 the extensor muscles, which are inserted into the ulna' the 
 biceps brachn (flexor brachii) next contracts to carry the' arm 
 forward, and by so doing assists to flex the elbow,* the flexor 
 muscles of the back of the limb contract in order to flex the foot 
 and all the joints as high as the knee. The column of bones being 
 
 Mi_ 
 
 Fig. 20j. — The Position of the 
 Column of Bones of the Fore- 
 Leg when the Foot makes Con- 
 tact with the Ground. 
 
 Fig. 205. — The Position of the 
 Column of Bones of the Fore- 
 Leg PREPARATORY TO FlNAL PRO- 
 PULSION. 
 
 broken as described and the whole limb made flexible, it is carried 
 forward, the thoracic portion of the serratus magnus and trapezius 
 pulling one angle of the scapula backwards, and the latissimus 
 dorsi flexing the humerus on the scapula. The limb having been 
 advanced sufficiently, the flexible column of bones is once more 
 
 * At this moment it is lifting a weight of about 17^ pounds {8 kilos) 
 in a riding-horse, this being the weight of the limb from elbow to foot, 
 exclusive of the shoe. 
 
652 
 
 A MANUAL OP VETERINARY PHYSIOLOGY 
 
 made rigid. First the extensors of the carpus and foot act, 
 straighten the limb at the knee, and put the foot in position to 
 receive the weight of the body when it comes to the ground. 
 The effort to advance the leg is trifling, and the foot having 
 made contact, the limb being in the position shown (Fig. 204), 
 the real work now begins, for the body has to pass over the limb, 
 while the leg remains fixed and rigid from elbow to foot. The 
 muscles which pull it over are the latissimus dorsi acting on the 
 humerus, and the large deep pectoral muscle (see Fig. 199), which, 
 
 running forward from the ninth or tenth 
 ribs and sternum beneath the fore-leg 
 to the shoulder-joint, pulls the whole 
 shoulder-joint backwards, and causes 
 the body to rotate over the limb. It 
 is powerfully assisted in this by the 
 cervical serratus and trapezius drawing 
 the scapula forwards and upwards, and 
 by the large head of the triceps pulling 
 on the ulna. At the end of the move- 
 ment the column of bones is in the 
 position shown in Fig. 205, and the final 
 act of the limb is about to occur — viz., 
 its propulsive motion. To effect propul- 
 sion, the limb remains rigid, while the 
 long flexors of the arm straighten the 
 pastern until little more than the toe 
 of the foot is on the ground; then, with 
 a sudden sharp contraction of these 
 muscles, the body is propelled upwards 
 and forwards, the limb being flexed 
 (Fig. 206), carried to the front, unbent, 
 and again placed on the ground. 
 
 The Elbow- J ointhas already occupied 
 attention, more particularly in con- 
 nection with its locking during the 
 attitude of standing. During loco- 
 motion extensive hinge -like motion 
 occurs, which is abnormally exaggerated in high-steppers. The 
 hinge has two condyles and a ridge on the humerus, which fit 
 into a reverse moulding on the radius. The internal condyle is 
 much the larger, and occupies about half the entire articular 
 surface. These condyles, during flexion of the joint, turn the 
 knees outwards. If they throw them out too far, the leg below 
 the knee is thrown in, and so causes ' brushing ' and ' speedy 
 cutting.' This movement is best seen from above when mounted. 
 Though a hinge- joint, the arm appears to describe a semicircle 
 
 Fig. 206. — The Column of 
 Bones forming the Fore- 
 Leg BROKEN UP, THE 
 
 Limb shortened by 
 Flexion at Shoulder, 
 Elbow, Knee, Fetlock, 
 and being carried for- 
 WARDS. 
 
LOCOMOTOR SYSTEM 653 
 
 in the sweep it makes on flexion of the elbow. It is greatly 
 exaggerated in some horses. 
 
 The Knee-joint consists of three main joints and numerous 
 minor ones. The articulatory surface of the radius with the 
 upper row of knee-bones has no parallel in the body; it is a surface 
 both deeply concave and markedly convexo-concave. The con- 
 cavity and convexity on the radius form condyles of which the 
 inner is more curved than the outer. The concavity on the 
 anterior part of the radio-carpal articulation rests on the convex 
 surface of the upper row of knee-bones, and in this way a firmer 
 and closer union between these surfaces is obtained, such as is 
 needed in the upright condition of the limb. But when this 
 joint is flexed, the convex posterior surface of the radius comes 
 forward; this depresses the scaphoid {radio-carpal), and produces 
 between it and the semilunar (intermediate carpal) a very im- 
 portant movement by which the foot is thrown slightly out- 
 wards, probably with the object of enabling it to clear the 
 opposite limb. The upper joint of the knee possesses the largest 
 range of motion, the middle joint very much less, the lower joint 
 hardly doing more than gliding on its fellow. Percivall pointed 
 out many years ago that the trapezoid rests wholly on the head 
 of the inner splint bone, which explaii.;- the frequency with 
 which this bone becomes affected with ' splint.' 
 
 The Fetlock- Joint forms a special variety of articulation, 
 one which is firm and rigid anteriorly, elastic and yielding 
 posteriorly. This is effected by introducing two sesamoid bones 
 into the fetlock articulation, and suspending them between 
 ligaments; the structure by means of which the sesamoids are 
 suspended is the Suspensory Ligament (Fig. 197). This runs 
 from the back of the large metacarpal bone to the fetlock, 
 receiving insertion into the sesamoids. The latter are floating 
 bones, and, speaking more generally, perhaps, than an anatomist 
 would permit, they are embedded in ligamentous material which 
 keeps them in their place. The supsensory ligament supports 
 them above and laterally; the sesamoidal ligaments keep 
 them fixed below. The anterior face of these bones forms the 
 posterior face of the fetlock-joint. 
 
 There is no other joint in the body the mechanism of which has 
 received so much attention as that of the fetlock and suspensory 
 ligament. The rival theorists have been divided on the question 
 of the elasticity of the suspensory ligament. Gamgee (senior) 
 regarded it as absolutely non-elastic; he did not believe that it 
 supported the sesamoids, but regarded it as acting powerfully 
 in flexing the foot (p. 669), being brought into action or set free 
 according to the position of^the limb . 1 1 was he who destroyed the 
 notion onre and for all that the sesamoids moved up and down. 
 
654 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The fetlock- joint furnishes the only example in the body of 
 a bone placed vertically resting on one placed obliquely. The 
 metacarpal rests on an inclined plane in the form of the suffraginis, 
 and but for the sesamoids would slip off at the back. It is one 
 of the functions of the sesamoids and suspensory ligament to 
 keep this bone in its place, and so support the fetlock under 
 stress. 
 
 The sesamoid bones are also intended to furnish the fetlock- 
 joint with a yielding articulation, which receives the weight of 
 the body when the foot comes in contact with the ground. At 
 the moment this occurs the whole pastern from fetlock to foot 
 becomes more oblique, giving the appearance of sinking, which 
 in reality occurs. The sinking, however, is not due to displace- 
 ment of the sesamoids, but to increased obliquity of the pastern 
 under stress, owing to which the suspensory ligament is under 
 great strain. It does not appear to be reasonable to deny it some 
 degree of elasticity — viz., the power to stretch slightly and then 
 return to its original length (p. 662) ; but the stretching is not great, 
 and can be reproduced on a limited scale by lifting up the opposite 
 fore-leg, and observing the tense, sharp lines produced in the 
 suspensory at the moment the fetlock descends, through increased 
 obliquity of the pastern. The evidence that the suspensory 
 supports the fetlock is seen in the effect of dividing it, which 
 brings the fetlock appreciably nearer to the ground, while the 
 evidence that the suspensory ligament possesses only limited 
 elasticity is furnished clinically by the frequency with which 
 it is sprained. If it were highly elastic, a sprain would be a 
 difficult matter. No one, for instance, ever heard of the only 
 elastic ligament in the body being sprained — i.e., the liga- 
 rnentum nucha — but a ligament of this nature in any part of 
 the limbs would be useless. If tendons and ligaments were 
 highly elastic, the efficient action of muscles would be destroyed, 
 for during contraction their tendons would be stretching instead 
 of pulling. 
 
 It is quite easy to show that in the ordinary standing attitude 
 the sesamoid bones, together with the posterior half of the 
 metacarpal articulation, bear the weight of the body, while 
 the anterior surface of the suffraginis and metacarpal takes 
 but a small share. The two movements of the fetlock- joint 
 are flexion and extension. Flexion in every other joint in the 
 body but the fetlock begins by bending, but when the fetlock is 
 to be flexed the pastern has first to become straight before it can 
 be bent backwards. Flexion of this joint is accomplished by 
 the perforans and perforatum, while extension is brought about 
 by the extensor of the foot and suffraginis. The more the foot 
 is extended, the greater the strain on the sesamoids, and the slip 
 
LOCOMOTOR SYSTEM 
 
 °55 
 
 of the suspensory ligament which joins the extensor tendon 
 (Fig. 197) may at this time be associated in drawing the 
 sesamoids closer to their articulating surface, as the fetlock 
 becomes more extended. Or the slip may be considered in the 
 light of an extension of the suspensory ligament, in order to 
 afford it increased surface of attachment. The use of the 
 slip, however, is not very evident, and we have never seen it 
 sprained, which is the best evidence that its share is not very 
 important. 
 
 The suspensory ligament becomes sprained when the flexor 
 muscles tire, for the fetlock- joint is supported . not only by the 
 suspensory ligament, but by both flexor tendons. Division of 
 the perforatus tendon or of the suspensory ligament produces 
 very little effect on the dropping of the joint; division of the 
 
 N°l 
 
 N?a 
 
 N?3. 
 
 Fig. 207. — Effect of Experimental Division of the Suspensory Ligament 
 and Flexor Tendons on the Obliquity of the Pastern (after 
 Lishman). 
 
 1. Division of the suspensory ligament has caused the fetlock to drop a little. ' 
 
 2. Division of the flexor perforatus tendon has caused slight sinking of the 
 
 fetlock. 
 
 3. Division of both flexors, perforatus and perforans tendons, brings the heel of the 
 
 foot to the ground, and causes the toe to turn up. 
 The fetlock-joint does not come to the ground unless all the structures are 
 divided. 
 
 perforans creates evident effect, but it requires division of 
 both flexors to bring the heel of the foot to the ground (see 
 Fig. 207, 3). 
 
 Pastern-Joint. — This joint, formed between the first and 
 second phalangeal bones, allows of limited flexion and extremely 
 limited extension. It is, in fact, specially arranged to act during 
 extension, as if the first and second phalanx were only one bone, 
 and no joint existed between them. This function comes 
 into play when the body rotates over the foot on the ground — 
 i.e., during the period the body is passing from the position in 
 Fig. 204 to that in Fig. 205. When the limb passes the vertical, 
 the two bones, owing to the arrangement of their ligamentous 
 connections, move as one on the pedal articulation; in other 
 
656 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 words, the corona and suffraginis are immovably locked, and 
 for the time being the joint between the fetlock and foot is 
 obliterated. As the locking is entirely ligamentous, the strain 
 must be considerable. When the foot comes to the ground, con- 
 cussion to the pastern joint is saved by the fibrous plate which 
 exists at its posterior part. This plate is on the corona, and is 
 hollowed out for the reception of the rounded surface of the 
 suffraginis. 
 
 " The Pedal-Joint is formed by three bones — the second and 
 third phalangeal and the navicular bone. The chief movements 
 occurring are flexion and extension. This joint is the centre 
 of the entire movement of the leg;- the body rotates over it. 
 Relatively it is a small articulatory surface built up and 
 strengthened by the hoof, the cartilages of the foot, the flexor 
 and extensor tendons of the foot, and the connecting ligaments 
 peculiar to the bones forming the joint. It is a yielding or 
 elastic joint, on account of the introduction of the navicular bone 
 into its posterior part ; this functions as a sesamoid, and by yielding 
 under pressure counteracts shock. The pedal-joint is so deeply 
 buried within the foot, and strengthened by the various structures 
 named, that though it is the centre of movement during the 
 passage of the body over the limb, it is practically immune 
 from the diseases of locomotion, so common in other joints. 
 Not so the joint last considered — the pastern; it is frequently 
 the seat of trouble, probably due to the strain on the ligaments 
 when the bones become locked during extension. 
 
 Joint-Locking. — Repeated attention has been drawn to joint- 
 locking, and the example last given of the mechanism enables 
 a more comprehensive statement to be made. The whole of. the 
 joints in the fore-leg of the horse from knee to coronet are locked 
 during extension. The only joints where movement is occurring 
 are the shoulder, elbow, and the articulation between the corona 
 and pedis. The locking occurs when the limb on the ground is no 
 longer vertical — when, in fact, it is assuming the position shown 
 by the fore-leg in Fig. 205; from that moment every degree of 
 further extension locks knee, fetlock, and pastern the tighter. 
 This renders the limb from the arm to the foot as rigid as if it 
 were composed of one piece of bone. It also explains why the 
 posterior part of the knee-joint is one mass of interlacing liga- 
 mentous structure. 
 
 The Hip- Joint has a wide range of movement in flexion, 
 extension, and rotation; but owing to the position of the round 
 ligament (ligamentum teres), outward movement at right angles 
 to the body is limited. The pubio-femoral ligament, which is 
 confined to the horse, is derived from the prepubic tendon of the 
 abdominal muscles. It is far from clear why a slip of tendon from 
 
LOCOMOTOR SYSTEM 6 57 
 
 the abdominal muscles should require to be inserted into the 
 articulating head of the femur. During inward rotation of the 
 thigh — as, for example, when the foot is on the ground and the 
 body is passing over it— the pubio-femoral ligament is rendered 
 tense (Sisson). When the ligament is long, the stifle turns out- 
 wards, the points of the hock turn in, and the condition known 
 as ' cow-hocks ' is produced. It will be observed that the pro- 
 duction of cow-hocks has nothing to do with any part but the 
 hip-joint. The insertion of the round ligament into the inner 
 side of the head of the femur limits the outward movement of 
 the limb, and is generally considered to explain why a horse so 
 seldom ' cow-kicks.' 
 
 The Stifle-joint. — This is the largest joint in the body, and 
 is built on quite different lines to any other. A large convex 
 surface on the femur is placed resting on a flat surface on the 
 tibia, with absolutely no bony cavity in which to rest or play. 
 Shallow cavities are furnished by two plates of cartilage, which 
 receive the condyles of the femur, and on which they rest quite 
 superficially. It will be observed that, with this exception, no 
 preparation exists on the tibia for the formation of this immense 
 joint. Nothing could be more unlike in appearance than the 
 in-contact ends of femur and tibia. The discs of cartilage, 
 which take the place of bony cups, no doubt allow of con- 
 siderable flexion and freedom of movement, but in a joint the 
 arrangement is unreliable, and consequently the two bones are 
 held together by two crucial ligaments running in opposite 
 directions. These ligaments are somewhat twisted on each other, 
 and when the joint is flexed the posterior ligament is rendered 
 taut. Extension and flexion are the two principal movements of 
 the stifle ; during each of these the discs of cartilage on the tibia 
 slide forward or backward on the bone in conformity with the 
 movement of the femur, the movement of the external condyle 
 and its cartilaginous cup being greater than that of the internal. 
 The above is not the only joint formed by the stifle. In front 
 is the patella, playing over two large convex surfaces, the trochlea? 
 of the femur. The patella is, like the sesamoids, a floating bone; 
 it appears to move up and down, but as it is attached below, 
 this is impossible. It is the trochleae of the femur which move, 
 the large inner one doing most of the work. The muscles 
 inserted into the patella and the muscles which flex the hip-joint 
 are direct antagonists. It is impossible for the hip, or even so 
 remote a joint as the hock, to flex so long as the patella muscles 
 are maintained in a condition of contraction. When they relax, 
 flexion of the hip-joint is permitted by means of the muscles 
 already considered (p. 641), the tibia is drawn up by the biceps 
 femoris and semitendinosus (Figs. 198, 200), the trochleae of the 
 
 42 
 
6 5 8 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 femur appear above the patella, the flexor metatarsi is placed 
 in operation and flexes the hock, and as the stifle becomes 
 more and more flexed, the bulk of the whole joint increases, 
 and is turned outwards from the abdomen, in order to accom- 
 modate its increase in size. This turning outwards of the stifle, 
 a feature seen in any pace, but particularly well marked in the 
 trotting horse, is not due to any mechanism in the stifle, but to 
 the hock-joint. It will be shown presently that the ridges on the 
 
 astragalus, through their action 
 on the tibia, turn the stifle out- 
 wards. 
 
 The Hock-Joint is not only of 
 physiological, but of the greatest 
 clinical interest, owing to its being 
 the frequent seat of lameness. It 
 is a hinge-joint, and a hinge in 
 such a position on the limb must 
 be of great strength to resist the 
 tendency to close under the in- 
 fluence of the body weight. In 
 fact, the hock- joint, unlike the 
 knee, to which it corresponds, is 
 under the constant stress of ex- 
 tension, which is the reason why, 
 in spite of the automatic ap- 
 pliances with which it is furnished 
 for keeping it open, the joint is 
 always being alternately closed 
 for the purpose of rest. The 
 long lever at the back of the 
 joint, in the form of the calcis, is 
 a lever of the first order; it is the lever of power, and the 
 strain on these muscles will be realised when it is remembered 
 that the whole weight of the hind-quarters is supported by 
 the gastrocnemius and flexor perforatus through their inser- 
 tion into the calcis. The strain is largely removed by the in- 
 troduction of a tendo-ligament — viz., a cord running from end 
 to end within the muscle, and forming its two attachments, 
 by which means it is capable of relieving much of the strain 
 from the muscle. The existence of two such cords in the hock 
 is described at p. 645 (see Fig. 202, 2, 3). 
 
 The two movements of the hock are flexion and extension; 
 the extension, which occurs during the attitude of rest, is dealt 
 with at p. 646. The extension of locomotion is carried on by 
 the same mechanism — viz., the gastrocnemius muscle, and the 
 tarsal tendon running from the biceps femoris muscle (biceps 
 
 Fig. 208.- 
 
 -Flexion of the Hind- 
 Limb. 
 
LOCOMOTOR SYSTEM 
 
 659 
 
 abductor femoris, or long vastus) to the calcis (see Fig. 203). 
 It is seldom that the eye can witness extreme extension of the 
 limb— i.e., when the tibia and metatarsus form their nearest 
 approach to a straight line (Fig. 2c 9); the movement is alto- 
 gether too quick during the gallop. Its nearest representation 
 is obtained when a horse ' stretches ' ; the limb is then thrust 
 out behind, rigid and taut, and the hock fully extended. From 
 the position of backward extension the limb is brought forward 
 by flexion of the tibia 
 on the femur, and the 
 femur on the pelvis, by 
 mechanisms already dis- 
 cussed, when flexion of 
 the hock- joint follows 
 (Fig. 2" 8). Thelimbnow 
 comes forward under the 
 body, and the foot, which 
 was previously flexed, is 
 extended, in order to 
 come to the ground heel 
 first or flat, depending 
 on the pace. The leg 
 is then in the position 
 of forward extension 
 (Fig. 210). While the 
 extensor of the foot is 
 producing this action 
 with the assistance of 
 the lateral extensor, the 
 flexor metatarsi (Tibialis 
 anterior) has to relax 
 in order to unbend the 
 hock. The flexor of the 
 hock and the extensor 
 of the foot have a com- 
 mon tendon of origin 
 (Fig. 202), so that a 
 
 remarkable physiological peculiarity now occurs, in that a pair 
 of muscles having a common tendon of origin are enabled to 
 function in a diametrically opposite manner, the extensor of 
 the foot contracting, the flexor of the hock relaxing. The 
 flexor of the metatarsus, besides the common tendon at the 
 femur, has a muscular addition from the tibia (Fig. 201), and 
 it would be quite competent for this muscular portion to relax 
 or contract independently of its neighbour. The difficulty lies 
 in explaining the behaviour of the common tendon to a pair of 
 
 Fig. 209. — Extension Backwards of the 
 Hind-Limb. 
 
66o 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 muscles running parallel to each other and functioning in an 
 opposite sense. 
 
 Chauveau, many years ago, described the tendon of the 
 flexor metatarsi as a conducting cord (Fig. 202, 3), the function 
 of which was to regulate the flexion of the hock by purely auto- 
 matic action. He conceived that this automatic effect was 
 brought about by stifle flexion. There can be no doubt that 
 the position of the cord is greatly altered by flexion of this 
 joint. The more the stifle is flexed, the higher the cord is drawn 
 
 in the direction of the 
 hip; and as it is fixed 
 below to the meta- 
 tarsus, the latter has 
 to follow automatically 
 (Fig. 208). But we 
 cannot concur in think- 
 ing that the conducting 
 cord can effect flexion 
 without muscular aid, 
 and believe that the 
 flexor metatarsi must 
 take an active share, 
 in spite of the difficulty 
 of explaining precisely 
 how this is effected 
 without inducing an- 
 tagonistic action in its 
 neighbour, with which, 
 as we have seen, it 
 possesses a tendon in 
 common. 
 
 The next point to 
 engage attention is the 
 astragalus, the hinge- 
 like ridges of which in 
 the horse are arranged 
 obliquely like the threads of a screw. No animal other than the 
 horse has oblique ridges to the astragalus. It might appear at first 
 sight that their effect on flexion would be to turn the leg out 
 below the hock. But the horse does not carry his legs in this 
 way. When the hock is flexed, the parts below it are carried 
 forward true to the front; if the ridges on the astragalus acted 
 on the lower limb, the legs would be flung outwards. The screw 
 of the astragalus affects the joint above, and not the limb below 
 it. It turns the stifle outwards, producing the stifle action so 
 much admired in trotting horses. When the hock is flexed, the 
 
 F;g. 210. — Extension Forwards of the 
 Hind-Limb. 
 
LOCOMOTOR SYSTEM 66i 
 
 stifle is turned outwards; when it is straightened, the stifle is 
 turned inwards, and in some horses the effect of the screw- 
 turning movement is to cause the foot, when it comes to the 
 ground, to be twisted, and the point of the hock to be thrown 
 outwards. This gives the appearance of a wrench, but is natural 
 to some horses. Animals with a considerable space between 
 the last rib and the stifle-joint do not need to have their stifles 
 turned outwards. In the horse the last rib and stifle are so 
 close together that the latter has to be directed away from the 
 abdominal wall. 
 
 The movement between the small bones in the hock-joint 
 is very slight, and is not a to-and-fro sliding motion, such as 
 might be expected, but partakes of the nature of a rotation 
 or twist on each other. This fact will be understood when it is 
 remembered that the centres of the small bones of the hock are 
 attached to each other, and to the astragalus and metatarsus, 
 by interosseous ligaments centrally placed, which practically 
 prohibit any movement but that of slight oblique twisting. 
 This fact is revealed also by the friction marks in diseased joints. 
 
 The range of motion between the tibia and astragalus is con- 
 siderable, yet it is not fully exercised in all paces. It is only 
 in jumping and the gallop that the astragalus moves from end 
 to end on the tibia. In order to prevent excessive flexion, 
 two stops or buffers exist on the tibia, which come into contact 
 with two rests on the astragalus. The outer stop is the larger 
 of the two, but owing to the obliquity of the face of the tibia, 
 the inner one is the first to make contact. When the hock-joint 
 is fully flexed, only the anterior part of the tibia is in contact with 
 the astragalus; the posterior part is raised from the trochlea, and 
 a space exists between them. The pressure through the hock 
 falls mainly on its anterior part, and on the inner rather than the 
 outer side. This is the region most frequently affected with 
 disease. The tissues at the back of the hock suffer when the 
 fore-hand is raised during jumping, and may also suffer strain 
 when the hind-limbs receive the weight of the body in the 
 gallop; but the chief trouble affecting the hock is one occurring 
 to the bony structures. 
 
 The Functions of the Limbs in Relation to the Causes of Lameness. 
 — The bones forming the flexible columns of the limbs, with their 
 tendons, ligaments, and enveloping tissues, are exposed to strain. 
 It is evident that the strain is greatest during movement, but it 
 requires some little experience with horses to realise the stress 
 to which the parts are subjected even when the animal is doing no 
 work. For instance, laminitis is common when horses are standing 
 in a fixed position, as on board a ship, though nothing more than 
 the weight of the body is being supported. A horse injured in one 
 leg may frequently give way in its fellow throughjthe latter sup- 
 
662 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 porting extra weight, so that, quite apart from work, the legs may- 
 suffer in their effort to support the weight of the body. During work 
 the strain on the limbs is of two kinds — viz., that which arises when 
 the body is propelled from the ground, and that which occurs when 
 the body comes to the ground. One may be spoken of as the strain 
 of propulsion, the other as the strain of impact. Of the two, the strain 
 of propulsion is the greater, and in consequence the fore-limbs which 
 propel the body during the fast paces suffer from injury to a far greater 
 extent than the hind-limbs which receive it. Strain and concussion 
 in the limbs are provided against in several ways. The bony columns 
 of the legs, it will be observed, are broken up, the smallest segments 
 being nearest the ground, the largest away from it. The bones 
 forming the column are frequently arranged at an angle, by which 
 means direct jar is minimised. This is especially the case in the 
 hind-limbs, which receive the weight of the body in all fast paces. 
 Joints of a non-rigid kind are formed in three parts of the limb, all 
 being close together and but a few inches from the ground. One 
 exists within the foot, another at the fetlock, and an imperfect one 
 at the pastern. These offer an elastic rather than a rigid resistance. 
 The muscles, tendons, and ligaments also contribute to this elasticity. 
 Muscles, we have seen, are perfectly elastic; it is a property which 
 belongs to them; neither tendons nor ligaments are regarded as 
 elastic, though more than once attention has been drawn to the fact 
 that there must be some give and take in these tissues, some stretching 
 and recovery. That neither is capable of severe extension is un- 
 doubted, and normally, so long as the muscles maintain their com- 
 plete command of the limb, both tendons and ligaments are in the 
 matter of withstanding strain subordinate to them. When muscles 
 tire and lose their elasticity, their tendinous attachments suffer, 
 and it is almost invariably at this time that sprains of tendons 
 occur. When a horse tires, it is the fore-leg flexors which .sprain, 
 for the reason that the burden of propelling the body falls on the 
 flexor muscles of the fore-limbs; the flexors of the hind-limbs only 
 receive the weight when the body comes to the ground, and, re- 
 latively speaking, rarely suffer. 
 
 The extensor tendons of the limbs never suffer, for the work done 
 by their muscles does not necessitate a powerful contraction such 
 as occurs in the flexors when in the gallop they propel 9 hundred- 
 weight (460 kilogrammes) of material a distance of several feet through 
 the air. This is why the flexor muscles tire in spite of their tendinous 
 intersections, and the moment this occurs the tendons have to take 
 the shock.* One can imagine that for some strides the check liga- 
 ments are useful accessories in helping the tendons to deal with 
 fatigued elongated muscles, but the attempt to propel the body from 
 the ground with tired muscles and a stretched tendon causes the 
 latter to yield, and one or both tendons partly or completely give 
 way either where the subcarpal check ligament joins, or between it 
 and the fetlock. The muscles never rupture, and the tendons rarely 
 yield at any other spot than between the fetlock and the middle of 
 the shank. Both tendons in this region are at their smallest. 
 Sprains of the flexor tendons do not occur at a walk, nor in the 
 stable from slipping, nor on the road from trotting; they occur 
 during the canter and the gallop, for the reason that it is in these 
 
 * We have seen (p. 635) that there is a great difference between the 
 post-mortem breaking-strain of the flexor as compared with the extensor 
 muscles of the fore-limb. 
 
LOCOMOTOR SYSTEM 
 
 663 
 
 two paces only that one fore-leg is called upon to propel the entire 
 body weight .* The suspensory ligament sprains when insufficient 
 support is given by the tendons of tired flexor muscles to the fetlocks 
 and pastern. Other ligaments in the limbs may give way through 
 an actual wrench — i.e., the subcarpal ligament in draught-horses, 
 caused by backing, or while endeavouring to prevent slipping on 
 greasy roads; but here the ligament is caused to act in an unnatural 
 manner, both fore-legs being out in front of the body. Sprains of 
 the connecting ligaments of the pastern-joint — viz., that formed 
 between the suffraginis and corona — and some consider of the hock, 
 may be due to wrenches during work, and form the origin of future 
 disease in these regions. Lateral motion in either of these joints is 
 normally very limited, and wrenches while passing over broken 
 ground must occasionally occur. Notwithstanding, the writer never 
 remembers to have seen a case of lameness, other than arising from 
 external injury, occurring to the pedal joint where the lateral 
 ligaments are well marked. The essential point, however, is that 
 sprains of the flexor tendons or suspensory ligament, due to wrenches, 
 are practically non-existent. 
 
 The causes generally discussed above are mainly acting as the limb 
 leaves the ground; but there are others occurring during impact, 
 producing what is conveniently termed ' concussion.' These jar the 
 foot and the bones at the lower end of the column. Of this the 
 clearest evidence is furnished by fractures of the pastern, which 
 always occur as the foot comes to the ground. When the suffraginis, 
 and rarely the corona, break, the line of fracture is in nearly every 
 case very similar, showing either that a common cause is at work, 
 or that these bones possess inherent lines of weakness. 
 
 We have glanced at two periods during locomotion when injury 
 may occur; there is one other, and that is the time during which 
 the body is rotating over the foot. It has been shown that the 
 fore-leg below the elbow is intended to open and close in one direction 
 only; for instance, the foot may be made to touch the back of 
 the elbow, but it cannot be made to touch the front of the knee. 
 When the body is rotating over the foot, a point is reached where 
 the bones between the elbow and the foot are locked together 
 and rotate as one piece (see p. 656). At this time there is a great 
 strain on the locking arrangement, and considerable compression 
 of the small bones which form the end of the limb. Between the 
 suffraginis and corona the strain would appear to be greatest, on 
 account of the small size and slender nature of the joint, and for 
 the reason that it is placed next to the centre of rotation. Clinically 
 it is recognised that this region is the seat of often incurable lameness. 
 Whether this originates from a sprain of the ligamentous material 
 which forms the ' locking ' apparatus of the pastern, or from injury 
 (concussion) to the bones themselves, is not clear; probably both 
 processes are at work. 
 
 * The flexor tendons give way in the American trotting horse, though 
 not so frequently as in race-horses in this country. It occurs when the 
 animal tires. It has been said above that horses do not sprain their 
 tendons while trotting, but obviously this does not refer to racing. Further, 
 the animal, when match-trotting, does not use his limbs in the same way 
 as in the common trot. On this important point, see p. 671. The writer 
 is indebted to Professor Pierre Fish, New York State Veterinary College, 
 Cornell University, and to Drs. W. Sheppard, M.R.C.V.S., and Grenside, 
 y,S.A., for information regarding lameness affecting the American trotter. 
 
664 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 There are some seats of lameness where apparently specific causes 
 are at work which should not baffle discovery. It is the upper and 
 never the lower articulatory surface of the corona which is involved 
 in ringbone, though these surfaces are only an inch or two apart; 
 it is the under and never the upper surface of the navicular which 
 is affected with caries, though these surfaces are not half an inch 
 from each other. It is the inside and not the outside of the hock- 
 joint which is affected in spavin. The seats of these affections are 
 not matters of accident, but due to definite causes which the writer 
 considers are intimately concerned with the physiology of the parts. 
 
 In concluding this brief outline of the physiological aspect of lame- 
 ness, attention must be drawn to the fact that it is the small and not 
 the large joints which usually suffer; it is not those at some distance 
 above, but those nearest to the ground. It is not the fibrous tissues 
 so frequently as the denser structures. Speaking generally, three- 
 fourths of the cases of lameness occur in the fore-limb, and three- 
 fourths of the seats of lameness in the fore -limb are found within a 
 few inches of the ground. The detection of the seat of lameness will 
 ever be one of the most difficult duties of the veterinary surgeon, and 
 a thorough knowledge of the anatomy and physiology of the limbs 
 is the first step towards forming a sound judgment* 
 
 * The advanced student may profitably consult the following papers by 
 the writer: ' The Pathology of Navicular Disease,' Veterinary Journal, 
 vol. xxii., 1886; ' Some Diseases of the Joints of the Horse,' Journal of 
 Comparative Pathology and Therapeutics, vol. v., 1892; ' Sprains of Tendons 
 and Ligaments,' idem, vol. vii., 1894. 
 
Section 2. 
 Locomotion. 
 
 The movement of the limbs during locomotion is a question 
 both of theoretical interest and practical importance. This is 
 especially true in the case of the horse, owing to the fact that he is 
 one of the few domesticated animals which have to work, and to the 
 frequency with which this results in lameness. There are features 
 connected with lameness which can be explained only when the 
 method by which the limbs are employed during progression is 
 understood. Limb movements in the biped are relatively very 
 simple; in the quadruped there is both simplicity and complexity, 
 the latter owing to the rapidity of movement and our inability to 
 observe four legs at one and the same time. All paces but the simple 
 trot defy accurate visual analysis. It was not until instantaneous 
 photography came to the aid of physiology that the question of the 
 sequence and method of limb combinations during locomotion was 
 finally settled. The name of Muybridge and his co-workers will for 
 ever remain identified with this inquiry. Without instantaneous 
 photography it was impossible to analyse fast paces like the gallop, 
 though it is distinctly remarkable how very close some of the older 
 observers got to the truth. Exactitude, however, was not obtained, 
 nor criticism silenced, until 1878, when the camera, under the 
 direction of Muybridge, settled the question of animal locomotion 
 for ever. 
 
 Step and Stride. — When a man walks, the step is the distance 
 between the two feet measured from toe to toe or other con- 
 venient points in symmetry; two such steps constitute a stride, 
 which is the distance covered by a foot from the time it leaves 
 the ground until it again reaches it. For quadrupedal locomo- 
 tion the same definition holds good, but the introduction of 
 another pair of legs renders it rather more complex. In the 
 quadruped the step is the distance between any two hind-limbs 
 or any two fore-limbs, or any fore and hind limbs employed in 
 moving together. The stride is the distance covered by a fore 
 or hind limb from the time it leaves the ground until it again 
 reaches it. The step and stride can be measured from the 
 impressions left on the ground, though great care is required in 
 order to discriminate carefully between the various feet. In the 
 same horse a step from one hind-foot to its fellow is not always 
 the same length as a step from one fore-foot to its fellow. In 
 the faster paces like the gallop the step taken by the fore-legs 
 is longer than that taken by the hind-legs. In the horse one 
 stride at a walk contains two steps; in any faster pace than 
 the walk the stride embraces a period in the air, during which 
 there are no feet on the ground. In the trot, the legs being used 
 
 665 
 
666 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 in diagonal pairs, there are two steps and two springs to the 
 stride; in the canter and gallop there are two steps and one 
 spring. 
 
 Limb Velocities. — It must not be forgotten that in the slowest 
 paces the legs in motion are moving faster than the horse. Take 
 the case of the walk at five miles an hour : the moving legs have to 
 overtake the body, and in order to do that within the brief period 
 allowed they must move at least double as fast as the horse. In 
 the gallop Muybridge showed that in a horse covering nineteen yards 
 in a second (thirty-nine miles an hour nearly) the advancing limb 
 was brought forward with an occasional velocity of forty yards 
 a second, or nearly eighty-two miles an hour. It is not a matter for 
 surprise that so much doubt and difficulty existed in regard to loco- 
 motion before the introduction of instantaneous photography. 
 
 The ordinary exposures given by Muybridge were the one-thou- 
 sandth part of a second, though much shorter exposures than these 
 were also employed. During the one-thousandth part of a second 
 a horse galloping as above may move forward fa inch, and the 
 moving foot i£ inches, so that shorter exposures had to be em- 
 ployed for high velocities. 
 
 Even when the order and method of moving the limb is known, 
 it is very difficult, excepting in the slowest paces, to catch a glimpse 
 of the real movement in any other pace than the trot. The limbs 
 are as confusing as the spokes of a rotating wheel, while the body 
 of the horse itself helps in no slight way to divert attention. To 
 obviate this, the writer finds that the best method of seeing the limbs 
 working is by looking at them through a narrow slit in a card, so held 
 that only the legs are seen through the slit, the body being 
 cut off. 
 
 The paces commonly employed by the horse are the walk, 
 trot, canter, and gallop. There are others, such as the amble 
 and the rack, neither of which is popular in this country, nor, 
 indeed, is the latter employed for any other purpose than in one 
 form of American racing. In all these paces the limbs are em- 
 ployed in different combinations. In the walk, for instance, a 
 broad base of support is required, so that during a stride for half 
 the time there are two legs and half the time three on the ground 
 at one and the same moment. In the trot for half the time there 
 are two legs on the ground and half the time the body is in the 
 air. The arrangement is different in the canter, and again differ- 
 ent in the gallop. Each pace has its own system of combinations, 
 and when all the possible combinations are classified they amount 
 to fourteen for the four legs . For instance, the number of different 
 ways in which a single limb may be employed is obviously four, 
 the possible combination with two legs is six, and the various 
 combinations with four legs employed in progression is 
 fourteen- 
 
LOCOMOTOR SYSTEM 667 
 
 The Walk. 
 
 This pace is relatively slow; an ordinary horse walks about 
 four miles an hour, and it would seem that, considering each limb 
 can be distinctly observed, the movements would be easy to 
 follow. As a matter of fact, to watch them is bewildering. 
 The walk begins from a fore-leg. If a horse from rest begins 
 to walk, one or the other fore-leg will be found the first to 
 advance; there is no special choice shown as to whether it will 
 be a near or an off leg, though an animal may time after time 
 start with the same limb. This is not the generally accepted 
 opinion outside the veterinary world. Muybridge himself says: 
 
 ' When a horse is standing with the weight of the body equitably 
 distributed over his fore-legs, and under these conditions commences 
 to walk, the initiatory movement will invariably be made with a 
 hind-foot.' 
 
 Borelli, who wrote on animal mechanism towards the end of 
 the seventeenth century, described the walk as beginning with a 
 hind-foot, but this has never been accepted by any veterinary 
 writer. Percivall and Gamgee especially attacked it. Hayes 
 says: ' As a rule a horse begins the walk with a fore-leg.' This 
 suggests he has seen a hind-leg initiate the movement — in fact, 
 he describes the sequence of limb movements which occur when 
 a hind-leg is the first to start. It seems a remarkable fact that, 
 when all the most difficult questions connected with locomotion 
 are settled, the simple one discussed above is not satisfactorily 
 determined. In consequence of the opinion of Muybridge we 
 are unwilling to deny that the movement, though commonly 
 beginning with a fore- limb, may be started by a hind-leg; never- 
 theless, in the experience of the writer and those who have 
 observed for him, the movement invariably begins in front, and 
 never behind, unless the horse is ' reining back.' The first step 
 taken in walking backwards is always with a hind-limb, which 
 appears to support the view that the fist step forwards is always 
 made with a fore-limb. On the other hand, the first step in 
 draught may be taken with either a fore or a hind leg; the former 
 if the draught is easy, the latter when it is heavy. In ' backing ' 
 in draught a hind- foot is first moved if the load be light; if 
 heavy, the animal leans well back and moves a fore-foot first. 
 If the horse's head be placed uphill he starts with a fore-leg, 
 provided the hill is not steep; if steep, he starts with a hind-limb. 
 When the head is downhill the first step is taken with a fore-leg. 
 
 A fore-limb having initiated the movement, the next leg to 
 take part is the diagonal hind; for instance, if the step begins 
 with the off fore, as in Fig. 211 (1), the next leg to move is 
 
668 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the near hind (2). This is followed by the near fore, and lastly 
 by the off hind. There are four distinct movements in each 
 stride of the walk, and as there are two strides to a complete 
 advance of the body — viz., one on the near and one on the off 
 side — there are altogether eight movements. The best method 
 of conveying the eight movements of the walk is by means of 
 a system of notation,* as in Fig. 212. This shows that in the 
 walk the limbs on the ground at any one time are three, followed 
 
 Fig. 211. — The Walk (Ellenberger). 
 
 by two, followed by three, and' so on. In Fig. 211, (1) shows 
 that the support may be by means of one fore and both hind 
 legs, or one fore and one hind diagonal, as in (2), or both fore 
 and one hind, as in (3), or one fore and one hind lateral, as in (4). 
 There may be slight variations in the walk, such, for instance, 
 as in the case of a draught-horse loaded or a horse out grazing; 
 but in the true walk the broad principles above described are 
 never departed from. In Fig. 212 may be seen some of the 
 
 * In the diagrams of notation the narrow end of the figure represents the 
 head; the spots represent the legs on the ground at any one moment; 
 they do not, of course, represent the relative position of these limbs to 
 each other. 
 
LOCOMOTOR SYSTEM 66o 
 
 features of the horse's walk. It will be observed that the three ' 
 limbs followed by two is the characteristic, the three limbs being 
 any three, the two being any two, provided one is a fore and 
 one a hind. The track left by a horse at the walk is shown 
 m Fig. 224. If a track be studied, it will be observed that 
 the impression of the hind-foot is generally in advance of that 
 of its fore-foot, the limbs being in the position of the near legs 
 in Fig. 211 (4). But there is considerable variation; the hind- 
 foot in a good walker will always be placed down in advance 
 of the impression left by the fore-foot. In an average walker 
 the hind-foot impression covers the fore more or less; in ahorse 
 that is a bad walker, or is tired, or taking short steps for any 
 reason, as in heavy draught, the hind-foot impression is behind 
 the^fore. In draught it comes behind the fore if the traction be 
 heavy; in descending a hill, it comes behind the fore if the load 
 
 Fig. 212. — Notation of the Walk, 
 jc, ii, 3, and 4, correspond to similar numbers in Fig. 211. 
 
 is under control, but it falls in front of it if the weight cannot 
 be properly held back. The fore-leg remains on the ground for 
 a longer time than it takes in passing through the air, com- 
 prising the period during which the body is passing over the 
 limbs. The movement in the air of both fore and hind legs is so 
 extremely rapid as almost to defy detection. The snatching up 
 of the foot from the ground is the quickest movement. Muy- 
 bridge, following Gamgee (senior), refers it to the spring or 
 rebound of the suspensory ligament, but it would seem to be 
 due entirely to the flexor muscles. 
 
 The writer caused horses to trace the movement of the leg 
 from the time the foot left the ground until it returned. A pencil 
 was fixed at right angles to the foot, and the horse walked past 
 a piece of prepared canvas, arranged vertically, on which he 
 traced the limb movements. There was a great deal of variation 
 in the character of the curve, depending upon the part of the foot 
 to which the pencil was attached, a toe curve being different from 
 one taken at the quarter. 
 
670 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Fig. 213 shows two curves obtained by this method; they illus- 
 trate the sharp flexion of the foot, followed by a fall, and then a 
 rise as extension occurred. The foot is at its greatest height 
 from the ground in the middle of extension. 
 
 In A the stride was 4 feet 2 inches in length. There is a short 
 upstroke, then a sharp curve upwards as the foot is flexed, followed 
 by a fall; extension of the limb then takes place; this gives the 
 long bold curve, until the ground is again reached. The foot 
 was flexed 2 inches in height, but extension raised it 4 inches 
 from the ground. 
 
 In B the stride is shorter; there are peculiar curves at starting 
 which were very noticeable in many tracings, of which the actual 
 cause is doubtful; it was thought they were due to an inward 
 rotation of the limb. The rise in extension is well marked. 
 
 The curves are not to scale. They are exact reproductions 
 of sketches made at the time (thirty years ago) in the writer's 
 notebook. 
 
 < ■>' 87i° > 
 
 Fig. 213. — The Curve made by the Foot from the time it leaves 
 the Ground until it again makes Contact. 
 
 The pencil was fixed at the quarter of the foot. The arrow 
 shows the direction in which the horse is travelling; the dotted 
 line represents the ground. 
 
 In walking on level ground the majority of horses rarely extend 
 the knee any great distance beyond a vertical line dropped from 
 the point of the shoulder. A sudden movement of the extensors 
 now straightens the limb, and the foot is placed down flat. If 
 the leg is not fully straightened by the extensor muscles, the 
 foot comes to the ground toe first, with the knee slightly bent, 
 and a stumble follows. In heavy draught-work it is no un- 
 common thing to see the toe put down first, but here the condi- 
 tions are very different. Some horses walk with a pair of lateral 
 legs, both off and both near alternately. The camel employs 
 lateral legs as a natural means of walking. All horses in walking 
 nod the head as each fore-limb advances ; it is the only pace in 
 which this occurs. The nod takes place at the moment shown 
 in Fig. 211 (1). 
 
LOCOMOTOR SYSTEM 071 
 
 The walk is a most important pace; few horses are capable 
 of walking well, but it can be greatly improved by education. 
 The ordinary saddle-horse has a stride of from 5| to 6 feet (1-67 
 to 1-83 metres) at a walk. A step made either by a pair of fore 
 or a pair of hind legs varies from 33 inches to 39 inches in length 
 (0-84 to 1 metre). 
 
 The Trot. 
 
 A trot may be described as slow, ordinary, and flying. In 
 the last there is a slight difference in the support given the 
 body, but in the other two the movements of the limbs are 
 identical. The trot is a very simple pace to observe, inasmuch 
 as the legs are worked in pairs, so that instead of four moving 
 in different times, there are two moving together. The two 
 are diagonal legs, one fore and the opposite hind. These thrust 
 the body forwards off the ground, and it is received on their 
 fellows, which repeat the movement. During each thrust there 
 is a period during which the body is in the air and no legs on 
 the ground. In the trot 
 there are two steps to 
 the stride, and two 
 periods in each stride 
 that the horse is in 
 the air; the steps are 
 always with diagonals, 
 as shown in the nota- 
 tion in Fig. 214- In 
 Fig. 21s some of the 
 Characteristic phases . FlG - ^—Notation of the Trot. 
 of the trot may be In (1) the body is being propelled by diagonal legs; 
 .„„„ Tt1 / t n ihp u nr a P (2) the body in transit and ofi the ground; (3) it 
 seen, in (j.) me noibe , g received by oppos it e diagonals, and again 
 is on diagonals; in propelled. 
 (2) he has propelled 
 
 the body forward and off the ground; in (3) the animal has 
 alighted on the opposite pair of diagonals; and the process is 
 then repeated. In America the flying trot has long been culti- 
 vated, with the result that some remarkable velocities have been 
 obtained. The limb movements are not quite the same in the 
 match-trotter as in the ordinary trotter. Muybridge shows it as 
 follows (Fig. 216). The difference between match and ordinary 
 trotting is that the match-trotter does not leave the ground 
 simultaneously with a pair of diagonals, but with one at a time, 
 nor does he arrive on the ground with a pair, but with one at a 
 time. The interval in time between the arrivals is extremely 
 small, but it distinguishes the flying from the ordinary trot. In 
 the ordinary trot, it will be observed, the body is propelled by a 
 
672 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 diagonal fore and a hind leg simultaneously, but in the match- 
 trotter the propelling is done first by a fore and then by a hind. 
 This action of the hind-legs as propellers is important to notice; 
 
 it does not occur in either the canter 
 or the gallop, but appears again in 
 the jump. It will also be noticed 
 that the body, on coming to the 
 ground, is received first by a fore and 
 thenbyahind limb . I n the ordinary 
 trot it is received by fore and hind 
 limbs simultaneously. 
 
 The velocity trotting horses attain 
 is quite remarkable. The historic 
 horse Edgington, the first to be 
 photographed while moving, and 
 whose stride when exhibited on paper 
 revolutionised the public conception 
 of animal locomotion, was taken while 
 trotting the mile in two minutes 
 sixteen and one-fifth seconds.* The 
 length of the stride was 17 feet 
 (5-15 metres), and the space traversed 
 during the time the body was travel- 
 ling through the air with no feet on 
 the ground — viz., the period of 
 suspension — was 5 \ feet (1-65 metres) 
 at each step. This may be compared 
 with the ordinary saddle-horse, which 
 has a stride at the trot of 9 feet more 
 or less. The length of a step taken 
 either by a pair of fore or hind legs 
 rarely reaches 5 feet, and is generally 
 about 4! feet in length. The foot- 
 tracks made on the ground by a 
 horse trotting are shown in Fig. 224. 
 In the trot the impressions of the 
 hind-feet are generally made over 
 those of the fore, but not always, as 
 may be seen in the case of the off fore 
 in the figure . The only real difference 
 between the tracks of the horse 
 trotting and walking lies in the 
 interval between the feet — e.g., in the length of the step 
 and stride. The height of suspension in the air is greatly 
 
 * Higher velocities than the above are common in American racing. 
 The mile has been done in less than two minutes. 
 
 Fig. 215. — The Trot. 
 
 From instantaneous photographs 
 by 0. Anschiitz (Ellenberger). 
 
LOCOMOTOR SYSTEM 673 
 
 exaggerated in the trotting horse. In the ordinary horse 
 it may scarcely be noticed, the feet being lifted only suffi- 
 ciently high to prevent them being dragged over the surface of 
 the ground. The distance covered during suspension is that 
 which separates the fore from the hind legs; if these are between 
 3 and 4 feet apart, that is the distance covered while the animal 
 is in the air. This will be realised from an inspection of Fig.. 215 
 (2), in which the body is shown suspended, having just been 
 propelled by the united actions of the near fore and cff hind. 
 The off fore and near hind are coming to the ground, and we know 
 from our study of the foot-tracks that the latter will be placed 
 over the spot just occupied by the near fore- foot. 
 
 When a horse falls at the trot, he does so either through not 
 flexing his knee sufficiently before bringing the leg forward, or 
 because the extension of the knee is not perfect, and in consequence 
 
 Fig. 216. — -Notation of American Trotting Horse. 
 
 (1) The diagonals are preparing to propel; this is effected first by a fore-leg, one 
 hind being left on the ground (2); the thrust from this lifts the body off the 
 ground (3); it is then received on one fore-leg (4), and then by the diagonal 
 hind (5). From this point the movement is a repetition of (1). 
 
 the limb is unfit to stand weight. The knee should be suffi- 
 ciently but not unduly bent, and the leg brought rapidly forward; 
 the limb is then sharply extended, well braced, and the foot placed 
 firmly on the ground heel first. Regarding the latter point, 
 the question of pace settles whether the heel is distinctly first 
 to make contact. In photographs of the match-trotter the 
 inclination of the toe upwards as the foot comes to the ground 
 is very marked. In slower trotting the foot comes down flat, but 
 always with an inclination for the heel to make the first contact. 
 When a horse is inclined to come on to the toe instead of the heel, 
 which occurs when the muscles tire, when the feet are too long, 
 or when the mobility of the joints is impaired, he at once stumbles. 
 The Amble is the nearest approach to a run; it consists of a 
 short, quick series of steps, in which lateral and diagonal legs 
 are alternately employed, with no period of suspension — e.g.. 
 there is no period during which the body is left without a leg on 
 
 43 
 
674 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the ground. It is best studied from the notation in Fig. 217, which 
 shows that it is executed by a pair of laterals, followed by a single 
 fore lateral, succeeded by a pair of diagonals, followed by a single 
 hind-leg and then a pair of laterals. From the fact that there is 
 
 12 3 4 5 6 7 8 
 
 Fig. 217. — Notation of the Amble. 
 
 never a period during which the body is without support, the 
 pace is an easy one for the horse; and as there is no thrust up- 
 wards into the air, it is an especially easy pace for the rider, who 
 barely moves in his seat. 
 
 In Pacing or Racking the horse, instead of using diagonal 
 
 legs, uses the lateral limbs, so that off fore and off hind are on the 
 
 j 5 ground instead of off fore and 
 
 ' ; near hind (see Fig. 218). An 
 
 animal may ' pace ' both at the 
 
 walk and at the trot, in this 
 
 respect resembling a camel. 
 
 There is no doubt that it is a 
 
 perfectly natural movement for 
 
 some horses ; others are taught it. 
 
 Some extraordinary velocities 
 
 are recorded in America in this 
 
 tic 210. — Notation of the Pace „„;*. t j: j. ai_ 1 
 
 or ' Rack.' S alt - ln last paces the horse 
 
 oscillates from side to side; the 
 
 appearance is very unsightly, and racking is not encouraged in 
 
 preference to trotting. A defective leg which will not stand 
 
 trotting may stand ' racking.' 
 
 The Canter. 
 
 This is a pace entirely different from any previously con- 
 sidered. To understand the canter clearly, it must be remem- 
 bered that the body is propelled forward by one fore-leg, and no 
 matter which of the two it may be, it is received on coming to the 
 ground by the opposite or diagonal hind-leg. There is no ex- 
 ception to this rule, which will be found equally true for the 
 gallop. It is well to get this fact clearly established (see Fig. 220). 
 
LOCOMOTOR SYSTEM 675 
 
 In (1) the animal is propelling the body off the ground with the 
 off fore-limb, and in (3) alights on the near or diagonal hind-leg. 
 The importance of the principle of the employment of diagonal 
 legs will be observed; we have seen it in the walk, it is the essential 
 feature of the whole movement in the trot, and we now meet with 
 it in the canter. The notation of the canter is shown in Fig. 219. 
 It starts from the moment the body is in the air (1), where it has 
 been propelled by the leading fore-leg*— in this case the near. 
 The animal alights on one hind-leg, the off (2), the diagonal to 
 the leading fore-leg; this is followed by the fore-leg on the same 
 side— viz., the off— coming to the ground (3), so that for a very 
 brief period the body in the canter is supported by lateral legs. 
 The next change is that the fellow hind-leg comes to the ground (4), 
 the body now being supported on one fore and both hind limbs; 
 the hind-limb which first received the weight is then withdrawn 
 
 Fig. 219. — Notation of the Canter. 
 
 (5), and the animal is left on diagonals. This is followed by the 
 leading fore-leg coming to the ground (6), so that the body is now 
 supported on both fore-limbs and one hind-limb. The off fore- 
 limb is next withdrawn, and the animal is left on lateral legs (7) ; 
 the near hind is withdrawn, and the horse left on one fore-leg (8), 
 which, as we have seen, is the leading or propelling limb. This 
 sounds very complex, and there is no doubt that the canter 
 is difficult both to analyse and describe. It will be observed 
 that in the seven sequential movements of the canter the body 
 is twice — viz., at the beginning and end of the movement — on 
 one leg; at two periods it is on two lateral legs; at two periods 
 it is on three legs; and at one period on diagonal legs. 
 
 Turning now to actual photographs of these movements, we see 
 in Fig. 220 (1) the propulsion from the ground by means of 
 
 * The leading fore-leg in the canter and gallop is the one which gives 
 the final propulsion; it is the leg which does all the work. When going 
 straight, either fore-leg may lead, and a well-trained horse should be 
 taught to save leg weariness by changing the leading leg. In turning or 
 moving on a circle to the left, the leading leg must always be the left; 
 similarly, in circling to the right, the leading leg must be the right. 
 
676 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the off fore-leg; (2) represents the animal in the air; (3) shows 
 the horse alighting on the near or diagonal hind-leg; (4) exhibits 
 the animal standing on two lateral legs, the off hind, it will be 
 observed, not yet having reached the ground. In (5) the body 
 
 7 8 9 
 
 Fig. 220. — The Phases of the Canter (Anschutz). 
 
 is on three legs, both hind and near fore; in (6) the'animal is on 
 diagonal legs, near fore and off hind only: the toe of the near 
 hind is just leaving the ground. Had the picture been taken 
 the merest fraction of a second later, this foot would have been 
 
LOCOMOTOR SYSTEM 6 77 
 
 seen off the ground. The animal, however, is clearly on diagonal ' 
 legs, for a hind-leg thrust as far back as that shown in (6) is of 
 little support. In the next figure (7) the off fore-leg has come 
 to the ground, and the horse is standing on three legs. Of these, 
 first the near fore (8) is snatched up, leaving the body balanced 
 on lateral legs, followed by the off hind (9), which leaves the 
 animal on one fore-leg, the off, preparatory to the final spring, 
 which, is farther advanced in (1), and actually occurring in (2). 
 
 The canter is an easy pace for the rider, owing to the number 
 of limbs supporting the body in turn — as many, in fact, as in the 
 walk; it is this which gives smoothness to the working. To 
 obtain smoothness the limbs must follow in definite order. 
 A horse that can canter only with the off fore leading is any- 
 thing but comfortable when he tries to lead with the near fore; 
 it is impossible for him to group his legs in the correct order 
 — everything is disjointed owing to want of proper co-ordination. 
 To learn to lead freely with either fore-leg should form an essential 
 part of the training of every saddle-horse. In this way he will 
 change as one leg tires, and so save himself from a sprained 
 limb. 
 
 The essential features of the canter -being a hind-step, a fore- 
 step, and a spring, it is desirable to glance at the value of these 
 in an ordinary horse. The step with a pair either of fore or 
 hind legs varies from 37 to 43 inches in length. The stride varies 
 from 9 feet 8 inches to 11 feet 8 inches. The difference in the 
 length of the stride does not depend so much on variations in 
 the length of the step as in marked variations in the distance 
 of the spring. In a slow canter the spring may be half a foot or 
 even less, whereas in a fast canter it may be 2 feet or more. The 
 tracks left by a horse cantering are shown in Fig. 224. The 
 length of this horse's stride was 10 feet 8 inches, and the spring 
 forward was 6 inches. 
 
 The Gallop. 
 
 Muybridge points out that all animals do not gallop alike — 
 that is to say, by means of identical leg movements — nor, in the 
 case of the dog, does the same animal always gallop in the same 
 way. There are two distinct gallops described by Muybridge: 
 one he calls the transverse, the other the rotatory gallop. In 
 Fig. 221, A, the scheme of a transverse gallop is shown; in B 
 that of a rotatory gallop. The horse always employs the trans- 
 verse, while the dog and many other animals use the rotatory 
 gallop. 
 
 To understand the gallop it is essential to remember that the 
 propulsion or spring is done by one fore-leg, and, as in the canter, 
 the body alights on one hind-leg, which is always the diagonal one 
 
6 7 3 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 to the propeller. The body, having alighted, a step is made 
 with the hind-legs, then a step with the fore-legs, from which 
 it is again propelled. As in the case of the canter, there are two 
 steps and a spring in the stride. There are seven different 
 positions assumed by the legs during the gallop, which may be 
 seen in Fig. 222. Beginning with the body in the air, as in (1), 
 the animal alights on one hind-leg (2), in this case the near, as the 
 
 leading leg was the off. This 
 " B receiving leg is thrust well 
 
 forward under the body, in 
 great contrast to its position 
 in the canter, and for a 
 minute period the animal 
 is standing on one leg; the 
 opposite hind - leg then 
 makes a step and comes 
 to the ground (3) ; the body 
 is now momentarily on two 
 hind - legs. The fore - legs 
 have not yet reached the 
 ground; they are being 
 advanced as far as possible 
 to the front, and to enable them to reach forward the first 
 hind-leg is snatched off the ground, and the animal is again 
 left standing on one hind-limb (4). The fore-legs now begin 
 their descent, and the one diagonal to the hind-limb on the 
 ground now makes contact (5). The horse's body is then sup- 
 ported on one fore and one hind diagonal legs. It is now the 
 
 Fig. 221. — Scheme of the Gallop. 
 
 A, Transverse gallop ; the legs move in the 
 order of the numbers. B, Rotatory gallop. 
 
 1 
 
 6 
 
 8 
 
 Fig. 222. — Notation of the Gallop. 
 
 turn of the fore-legs to make their stride, and to effect this the 
 body rotates over the fore-leg, and in doing so the hind-leg leaves 
 the ground. The animal is once more left on one leg (6). The 
 step is now made, both fore-legs being on the ground (7) ; the first 
 fore to reach the ground is now snatched up, and the horse is 
 left standing on one leg (8), which is the leading one, and within 
 
LOCOMOTOR SYSTEM 679 
 
 the next fraction of a second the body is in the air. In Fig. 223 
 each of the important movements is shown in the horse. (1) is 
 the body in the air; the manner in which the legs are collected 
 under it should be particularly noted; the hind-limb which is 
 about to make contact is thrust as far forward as possible. In 
 (2) this limb has reached the ground; in (3) its fellow has arrived, 
 and the step with the hind-legs been taken; in (4) the big reach 
 forward has occurred, to effect which one hind-leg has been 
 withdrawn; in (5) the body is rotating over one fore-leg in order 
 to make the step with the fore-legs; at (6) this step has nearly 
 been effected; at (7) the step has been taken, and the fellow- 
 fore-leg withdrawn; the body has now rotated over the leading 
 leg at (8), and when this has been completed the spring into the 
 air is made. The gallop is an easier pace to follow than the canter ; 
 there are never more than two legs on the ground at any time, 
 and the greatest part of the work is done by one leg. Briefly, a 
 hind-step with no support in front, a fore-step with no support 
 from behind, and a spring, constitute the essential features of the 
 movement. The only time when the hind-leg gives any assistance 
 to the fore-limbs is when one of the latter makes the forward 
 reach preparatory to taking its step. When a horse gallops, no 
 matter how fast, the fore-feet never extend in front of a vertical 
 dropped from the muzzle. In Fig. 224 is shown the track left 
 on the ground by a galloping horse. Muybridge, working with 
 an average thoroughbred in racing condition, galloping at the 
 rate of thirty-five miles an hour, obtained the following results: 
 Length of stride, 22 feet 10 inches, made up as follows: 
 
 Fore-step - 5 feet o inches. 
 
 Hind-step- • - - 3 ,, 10 ,, 
 
 Interval between hind and fore legs 7 ,, 6 
 
 Spring - 6 ,, 6 ,, 
 
 With the ordinary horse, the strides of which vary from 15 feet 
 to 19 feet, the following results were obtained: 
 
 Fore-step 3 feet 5 inches to 4 feet 3 inches. , 
 
 Hind-step 2 „ n ,, to 3 ,, n ,, 
 
 Spring 2 ,, 11 ,, to 6 ,, 2 „ 
 
 Here, again, it was shown that the difference in the length of 
 the stride is due not so much to difference in the length of the 
 step as in the distance of the spring. The forward reach, such as 
 the horse is making with the off fore in Fig. 223 (3) is of mucn the 
 same length in all ordinary animals— viz., 5 feet 5 inches to 
 6 feet— though occasionally as much as 6 feet 6 inches. In the 
 race-horse above referred to it is 7 feet 6 inches. It will be 
 observed that both in the race-horse and the ordinary horse the 
 
68o A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Fig. 223. — The Phases of the Gallop. 
 After Stanford, Muybridge, and Stillman (' The Horse in Motion'). 
 
LOCOMOTOR SYSTEM 581 
 
 step taken by the fore-legs in the gallop is longer than that taken 
 by the hind-legs. For the velocity of the gallop-see p. 698. 
 
 Differences between the Canter and Gallop. — In the canter it 
 can be seen that the horse makes a big step first with the hind-legs, 
 then with the fore, followed by a spring; he is only once in the air 
 in each stride. In the gallop the same essential features occur; 
 but the canter differs from the gallop in being a slower pace: the 
 hind-leg, in order to receive the weight, is not brought well under 
 the body (see Fig. 220, 3), and in consequence, to enable the hind 
 to complete its step, the lateral fore-leg has to come to its assistance. 
 There is no such support given with a fore-leg in the gallop. In 
 consequence of the slowness of the canter, one hind-leg has now in 
 turn to help support the body while the fore-legs make their step, 
 and this support is not withdrawn until the one fore-leg is ready 
 to make its spring. No such hind support exists in the gallop during 
 the period the fore-legs are making their step. In the gallop there 
 are never three legs on the ground at one and the same time; in 
 the canter this occurs twice in a single revolution. 
 
 The conventional gallop shown in pictures is much more pleasant 
 to look at, but wholly incorrect. F. Galton* observed that the two 
 fore-legs in the photographs of the gallop were extended during one- 
 quarter of a complete motion, and during' another quarter the two 
 hind-legs were similarly extended ; cutting the photographs in halves, 
 and uniting the front half of the former to the hind half of the latter, 
 a fair equivalent of the conventional attitude was obtained. He 
 believed that, owing to the confusion created by the limb move- 
 ments in the gallop, the brain ignores one-half of all it sees, divides 
 the other half into two parts, each alike in one particular, and then 
 combines the two halves ' into a monstrous whole.' 
 
 The Gallop of the Dog is rotatory, as Muybridge terms it (Fig. 221, 
 B), and the greyhound gallops differently to a heavy dog. The latter 
 employs the gallop of the horse, excepting that the animal alights on 
 the lateral and not on the diagonal hind-leg ; the opposite hind-leg then 
 comes to the ground, next the fore-leg of that side, then the opposite 
 fore-limb, followed by the spring. The order is, therefore, as follows, 
 assuming the thrust is given with the left fore-leg: left fore, left 
 hind, right hind, right fore. As in the horse, there is a step with 
 the hind followed by a step with the fore legs, -then a spring. The 
 movement of the limbs of the greyhound also follows in the order: 
 left fore, left hind, right hind, right fore; but there are two periods 
 of propulsion, first with a fore-leg, which sends the body into the 
 air with the fore and hind legs crossed like a pair of scissors, next a 
 thrust with a diagonal hind-leg, which shoots it once more into the 
 air, to alight on a lateral fore-leg. Consequently, in a single stride 
 of the greyhound there are two springs, once from a fore-leg and 
 once from a diagonal hind-leg. 
 
 The Footprints occurring during the various paces of the horse 
 have been previously referred to; they are illustrated in Fig. 224. 
 Before the days of instantaneous photography a great deal of atten- 
 tion was paid to them as a graphic record of locomotion, and nothing, 
 perhaps, better illustrates how differently horses move while per- 
 forming the same pace, or even the same horse at different times. 
 The differences do not, of course, lie in the modes by which the 
 limbs are moved, but in the length of the steps taken and the position 
 
 * ' Memories of My Life,' 1908. 
 
682 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 of the feet relative to the central line of the body. In the walk, for 
 example, the hind-feet may be placed down immediately on top 
 of the impressions of the fore, or they may be placed in front of or 
 half covering the fore-prints. The only real interest now attached 
 to a study of footprints lies in the information they furnish of the 
 extent to which the feet are brought under the middle line of the body 
 as a means of support. This is ascertained by the use of a line so 
 adjusted as to lie midway between the tracks made by the near and 
 off feet. Such a line represents the middle line of the body. It 
 might at first sight be reasonable to suppose that the off feet would 
 fall to the right and the near feet to the left of such a line, and that 
 the distance the impressions were made from the central line would 
 be similar to their distances from a line passing midway between 
 the feet during repose . But this is not so ; a pair of fore-feet 5 inches 
 apart during repose may be shown during locomotion to be brought 
 completely under the middle line of the body, or even to cross it. 
 During the slowest paces the feet are planted very nearly or quite 
 in a line with the centre of the body; during the faster paces they 
 may be brought so far under from either side as to cross the central 
 line. In the gallop the feet are brought well under the central line 
 of the body, forming a very straight track, occasionally broken by 
 their being planted away from the centre, or their sometimes com- 
 pletely crossing the centre to the opposite side of the body. All the 
 features discussed may be seen on inspection of the various foot- 
 tracks shown in Fig. 224. The dotted line represents the middle 
 line of the body. 
 
 Summarising these results, we see that, though in repose the 
 feet may be 4 or 5 inches apart measured from their inside edges, 
 the tracks these feet leave on the ground do not show this interval; 
 on the other hand, they are brought well under the centre of the 
 body, sometimes even crossing it, and the faster the pace, the nearer 
 the footprints approach a straight line. It is obvious that the more 
 the feet are brought under the middle line of the body, the more 
 stable the support afforded, but it is not clear how they are able to 
 avoid striking their fellows. 
 
 The Jump. — It is said that no two horses jump alike, and 
 even that the same horse under identical conditions will jump 
 differently. The differences alluded to originate in the manner 
 in which the hind-legs are disposed preparatory to the body 
 being collected for the spring upwards. Under ordinary circum- 
 stances, as the jump is approached, the animal is steadied in his 
 stride, in order to afford him the needful opportunity of ' col- 
 lecting ' himself and forming his judgment of the obstacle. 
 During the steadying period he is getting ready to raise the fore- 
 hand ; this he could not properly effect without a modification of 
 the pace; he is also estimating at what point he should take 
 off. This has to be learnt by experience; everything depends 
 upon it. In Fig. 226 (1) and (2), the horse is seen steadying him- 
 self, bringing the hind-legs well under the body, and raising the 
 fore-hand. In (3) the final upward push has been given by a 
 fore-leg, and in this process not only the fore-legs, but the muscles 
 of the back and loins, are playing a most important part. It is 
 
nh n 
 
 NF-0 
 
 n oh 
 
 l)0F 
 
 NHfi 
 
 Walk 
 
 LOCOMOTOR SVSTEM 
 
 in 
 
 NH 
 
 OH 
 W OF 
 
 
 NF 
 
 ft OH 
 
 n 
 
 OH 
 
 NH/ 
 
 
 
 NF 
 
 o 
 
 OF 
 
 "in 
 
 OH 
 
 683 
 
 ~10 or 
 
 NF, 
 
 o 
 
 OH 
 
 NH jj) 
 
 Sfn'iu'Sj 
 
 "10 
 
 OF 
 
 Trot Canter Gallop 
 
 Fig. 224. — Foot-Tracks at Various Paces. 
 
 A stride is indicated by the line at the toe of identical feet. These tracks were 
 furnished by the same animal. 
 
68 4 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 impossible for a horse to rear even to the partial extent required 
 for an ordinary jump if the back is weak. In (3) the obstacle is 
 being faced, the animal is lifting the fore-hand and bending the 
 knees, the hind-legs being well under the body, in order to support 
 the weight. The next important point is the continued bending 
 of the knees in (4) and the straightening or extension of the 
 hind-legs. The moment the fore-feet are above the level of the 
 
 Fig. 225. 
 
 -A Jump without an Obstacle by a Highly Trained 
 
 Animal. 
 
 obstacle, propulsion forward with the hind-legs begins; first 
 both hind-legs straighten themselves, extending to the utmost, 
 while the limb nearest the jump is left to give the final push off 
 (5) and (6). It is now the turn of the hind-legs to become flexed, 
 the hocks bending as much as the knees had previously done, 
 while at the same time the fore-legs now extend, with the object 
 of making contact with the ground. This contact must be made 
 with a firm, straight leg, one (6) followed by its fellow (7), which 
 in (6) is seen placed out in advance. It is advanced in order that 
 the first fore-leg to make contact (6) may be carried forward out 
 
7 * 
 
 Fig. 226.— The Phases of the Jump (Anschutz). 
 
686 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 of the way (7), for it is occupying the place where the hind-feet 
 will alight. These now come down, first one, then its fellow, 
 but before the second hind-leg reaches the ground the fore-leg has 
 already pushed off (8). It will be observed that the jump re- 
 solves itself into a partial rear, a doubling up of the fore-limbs 
 and powerful extension of the hind, followed by the upward push, 
 the extreme flexion of the hocks, the marked extension of the 
 fore-legs, and the reception of the weight of the body on first 
 one and then the other fore-leg. Should the animal fail to alight 
 fair and true on his fore-feet he falls, generally pitching on to his 
 head. The jump reverses the use of the limbs in ordinary loco- 
 motion; there the thrusting is done by the fore-legs, and the 
 weight of the falling body is received on the hind; in the jump the 
 hind-legs do the thrusting and the fore-legs receive the weight of 
 the falling body. 
 
 A record jump is believed to be that of ' Confidence ' at 
 Olympia in 1912, when 7 feet 5| inches were cleared. A jump 
 of 25 feet without an obstacle has been recorded. A brook 
 of 19 to 21 feet in breadth has been known to work havoc with 
 such a hunt as the Pytchley. Fig. 225 shows the attitude of 
 a highly trained horse in making a jump without an obstacle. 
 
 In Kicking with both hind-legs (Fig. 227) the head is depressed, 
 a powerful contraction of the muscles of the quarters and back, 
 A A, throws the croup upwards, the hips are flexed through B B, 
 the stifles by the contraction of C C, and both legs are violently 
 extended through D D. No estimate of the force employed in 
 kicking can be made. A shell is not productive of more damage 
 than that caused by a determined kicker. It is no figurative 
 expression to say that under suitable conditions the muscles 
 may he heard to contract. Kicking may be practised either 
 with both hind-legs backwards, or with one hind-leg backwards, 
 forwards, or outwards. The two latter methods are very 
 dangerous; fortunately, few horses can effect them. Owing to 
 the presence of the pubo-femoral ligament, a horse can kick 
 outwards only with difficulty. It is known as ' cow-kicking,' but 
 this term may also be applied to the forward kick. Striking 
 with the fore-feet and cow-kicking are not common among nor 
 characteristic of British horses; they are methods of attack 
 particularly employed against man. 
 
 It is rather remarkable, however, that our horses should be 
 so much more given to kicking among themselves than those of 
 Continental nations. 
 
 In Rearing (Fig. 228), the fore-hand is raised by the centre 
 of gravity being thrown back, and by the body being pushed 
 upwards by the fore-limbs, and at the same time raised by a 
 contraction of the long muscles of the back (A B) which run from 
 
LOCOMOTOR SYSTEM 
 
 687 
 
 croup to neck, attached throughout their whole length to the 
 vertebrae (Jig. 229)- The muscles of the hind-limbs keep the 
 
 Fig. 227. — Kicking (after Peyremol). 
 
 Fig. 228. — Rearing (after Peyremol. 
 
 stifle-joint closed, and the hock-joint open, without which 
 provision the extra weight would cause the animal to sit down. 
 The position in Fig. 228 is one of perfect stability. More 
 
688 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
LOCOMOTOR SYSTEM 
 
 689 
 
 powerful contractions of the muscles inserted into the cervical 
 vertebrae will pull the body farther back, and if it passes outside 
 the base formed by the hind-feet, the animal comes com- 
 pletely over, either on to its side, or frequently on to the 
 occiput when rearing through temper. A fall on the occiput 
 commonly results in a fracture of the base of the skull. 
 
 In Buck - Jumping 
 (Fig. 230) the animal 
 springs bodily and sud- 
 denly off the ground, the 
 head being depressed 
 between the fore - legs 
 and the back violently 
 arched. In this action 
 the psoas muscles play an 
 important part by bend- 
 ing the hind - quarters 
 inwards, but they cannot 
 by themselves produce 
 the arched condition of 
 spine, which largely de- 
 pends, apart from the 
 upward spring, upon the 
 ability to get the head 
 down, the neck bent, and the abdominal muscles firmly con- 
 tracted. Much the same configuration of spine occurs when 
 horses are cast for operation, and in practice this is controlled 
 by keeping the head and neck straight, and the tail from being 
 forcibly drawn in (see p. 446). In buck-jumping it is the spring 
 which displaces the rider. 
 
 Fig. 230. — Buck-Jumping (after Peyremol) 
 
 44 
 
SECTION 3. 
 
 Work. 
 
 The Amount of Work expected from horses is a question which 
 has hitherto been greatly lacking in exact expression. That many 
 animals are worked beyond their powers is undoubted, the evidence 
 being the short duration of their useful life. In days gone by, when 
 the horse was the only means of transport, there was no attempt made 
 to save him, especially in public vehicles ; his life in the early stage- 
 coach was three years. The stages were from fourteen to twenty 
 miles in length, but experience and economy brought them down 
 to ten miles, with a rest on the fourth day. Youatt* tells us that 
 towards the end of the eighteenth century, during election-time, 
 bets were made as to which express would kill the greatest number 
 of horses. | In his day animals in the public omnibuses only lasted 
 five or six months. Taplin, writing in 1793, says that the rage for 
 expeditious travelling was leading to the destruction of thousands 
 of horses. In an otherwise admirable book on horse management, 
 published in 1805,}: it is stated that a horse should travel thirty to 
 forty miles on end without drawing rein at from eight to ten miles 
 an hour; this is described as ordinary work ! Severe or, as it is 
 termed, ' extraordinary ' work was represented by a gallop of twenty 
 miles, or trotting sixteen miles in the hour. W. Ward,§ id. m itj6, 
 advised that a distance of forty-five to fifty miles a day in "three 
 stages should not be exceeded. He says he knew of people doing 
 sixty, seventy, and eighty miles a day, but it required a very good 
 horse, and could only be done for one or two days. J. Lawrence, 
 writing in 1809, || considered that- fifty to sixty miles a day, between 
 7 a.m. and 5 p.m., could be done in stages of twenty miles, trotting 
 and cantering alternately. In a journey lasting weeks and months 
 together, he regarded twenty to thirty-five miles a day as suitable. 
 The ordinary hack of that period is described as travelling forty or 
 fifty miles a day at seven or eight miles an hour, and it is stated that 
 with a suitable weight they may do this for two or three successive 
 days. The best hack did ten or eleven miles an hour, and anything 
 over sixty miles a day was considered ' a severe trespass on their 
 powers.' These facts are mentioned in order to explain that the 
 general feeling during the eighteenth century was in the direction 
 of excessive work, the racing especially being of a most punishing 
 character. When Watt, who presumably knew nothing of horses, 
 was determining the working power of his engine, he adopted a 
 standard known as ' horse power,' which has ever since been mis- 
 leading. He found that a horse could raise a weight of 150 pounds 
 passed over a pulley at a rate of 220 feet a minute (equal to 2^ miles 
 an hour) : 150 x 220 = 33,000 pounds lifted 1 foot high per minute, 
 
 * ' The Obligation and Extent of Humanity to Brutes,' 1839. 
 
 f In the Gentleman's Magazine for 1825 it is stated that a stage-coach- 
 man admitted to having killed fifty horses in one year from overdriving. 
 
 J ' Analysis of Horsemanship,' Adams. 
 
 § A ' New Treatise on the Method of Breeding, Breaking, and Training 
 Horses.' 
 
 11 ' History and Delineation of the Horse.' 
 
 11 690 
 
LOCOMOTOR SYSTEM 691 
 
 or 33,000 foot-pounds per minute. From that time, on the basis 
 of Watt's original excessive estimate, the most extravagant demands 
 have been made on the strength of horses, for which engineers and 
 mathematicians, who alone have studied the question, have been 
 responsible. A horse can, of course, raise 150 pounds at the velocity 
 mentioned, but the practical question is for how long ? He could 
 do it for three and a half hours and not be overworked, but eight 
 actual working hours are expected from him, and the standard then 
 becomes excessive. Horses ploughing frequently exercise a force 
 of 150 pounds, but here the pace is the saving factor. 
 
 The question of condition or fitness for work is discussed at p. 447. 
 Mechanical daily work is the product of three quantities — the 
 effort, the rate, and the number of working hours. The difficulty lies 
 in determining the effort ; there is no means of ascertaining this with 
 any degree of precision in riding-horses, but with draught animals 
 by means of a dynamometer it can be registered with sufficient 
 accuracy. It is evident, however, that the force of traction depends 
 upon the weight of the load, the nature of the roads, state of the 
 weather, and whether the roads be hilly or level. There is no more 
 variable factor, and one aspect of this is well illustrated in the 
 following table, which shows the effort required to pull 1 ton 
 over roads of different natures, the surface in all cases being level : 
 
 Force of Tra.tion 
 per Ton. 
 
 Smooth surface 31 lbs. 
 
 Paved 45 ,, 
 
 Macadam 44 to 67 lbs. 
 
 Gravel 150 lbs. 
 
 On arable land 200 ,, 
 
 In practice, the force of traction on a level macadamised road is 
 regarded as equal to one-thirtieth of the load. 
 
 There are other features, apart from the nature of the road, which 
 influence the question of effort in draught — for instance, the height 
 of the wheels, the width of the tyre, the presence or absence of 
 springs. These points are only referred to; they belong to the 
 realm of mechanics, but their physiological bearing in increasing 
 or reducing the effort required is well known. Experience goes to 
 show that eight hours' work, at a walking pace of three miles per 
 hour, with a load drawn without difficulty though with effort, con- 
 stitutes a working day. If the load or pace be increased, the period 
 of labour, must be reduced. 
 
 The amount of work performed is spoken of as foot-tons or kilo- 
 grammeters — viz., so many tons raised 1 foot or so many kilo- 
 grammes raised 1 meter (p. 438). The published tables of work by 
 Redtenbacher, Rankine, Morin, etc., are all too high. These place 
 the normal daily work of a horse at from 6,200 to 6,700 foot-tons 
 (1,926,544 to 2,071,104 kilogrammeters) . Kellner and Wolff, in 
 their experiments on nutrition in horses, caused 2,154,000 kilo- 
 grammeters of work (7,000 foot- tons) to be performed daily on a 
 circular dynamometer,* but the writer believes that the following 
 more closely approximates to what should be demanded for regular 
 daily work: 
 
 A moderate day's work = 3,000 foot-tons. 
 A hard day's work = 4,ooo foot-tons. 
 ^ A severe day's work = 5,000 foot-tons. 
 
 * This is the amount Waft's horse would have performed in eight hours. 
 
692 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 As a means of conveying to the mind the value of quantities which 
 cannot be visualised, the following table has been compiled ; the work 
 is equivalent to 3, 000 foot-tons, it being assumed that the weight of the 
 animal, together with the weight carried, is equal to 1,000 pounds: 
 Walking at 3 miles per hour for 8-7 hours. 
 
 4 .. .. 5-3 ,. 
 
 5 .. .. 3'7 ,< 
 Trotting 8 ,, ,, 1-5 
 
 It must not be forgotten that in addition to the effort required 
 to carry or drag a load there is another source of expenditure — 
 i.e., the force required to move the animal's own body weight through 
 space. Many observations have been made to ascertain what this 
 amounts to, but no definite statement can be made, as the question 
 of velocity comes in. It takes more to move the body at a fast than 
 at a slow pace. At the average walk the effort amounts to about 
 one-twentieth of the animal's body weight for every foot of ground 
 passed over — say an effort of 50 pounds; it has, however, been 
 placed much lower. 
 
 It is a very difficult matter to obtain exact information regarding 
 the work performed by horses. Fortunately, a good deal of precise 
 information was collected many years ago in connection with the 
 wear and tear of animals, at a time when road-building in this 
 country was being scientifically studied, and still later when the 
 running of coaches had, in consequence of the improvements in the 
 roads and the general improvement in the care and management of 
 horses, become an art of national importance. Information was thus 
 obtained on the force of traction over different gradients, and the 
 amount of work expected from the horses. The following table is 
 by Stewart;* it shows the daily distance travelled, the velocities, 
 the weight of the load, and number of working hours injScotland : 
 
 Carriers (Two Wheels). 
 
 Velocity in 
 
 Miles per 
 
 Hour. 
 
 Daily 
 
 Distance in 
 
 Miles. 
 
 Gross 
 
 Weight of 
 
 Load. 
 
 Weight 
 
 pulled per 
 
 Horse. 
 
 N umber of 
 
 Working 
 
 Hours 
 
 Daily. 
 
 Number of 
 Working 
 
 Days 
 Weekly. 
 
 Mileage 
 Weekly. 
 
 2| 
 
 3 
 
 22 
 
 27 
 
 Cwt. 
 26 
 
 34 
 
 Cwt. 
 13 
 
 8 
 9 
 
 6 
 4 
 
 132 
 
 108 
 
 In the last observation it will be observed that the increase in pace, 
 in distance travelled, and in load, leads to a reduction in the number 
 of working days. 
 
 Waggons (Four Wheels). 
 
 Velocity in 
 
 Miles per 
 
 Hour. 
 
 Daily 
 
 Distance in 
 
 Miles. 
 
 Gross 
 
 Weight of 
 
 Load. 
 
 Weight 
 
 pulled per 
 
 Horse. 
 
 Number of 
 
 Working 
 
 Hours 
 
 Daily. 
 
 Number of 
 Working 
 
 Days 
 Weekly. 
 
 Mileage 
 Weekly. 
 
 2 
 
 4 
 
 22 
 22 
 
 Cwt. 
 60 
 40 
 
 Cwt. 
 
 3° 
 20 
 
 II 
 
 5i 
 
 6 
 6 
 
 132 
 132 
 
 * ' Stable Economy.' 18-jo. 
 
LOCOMOTOR SYSTEM 
 
 693 
 
 In the second tab'e the pace is increased, the load reduced, and 
 the working hours diminished by half. 
 
 Stage-Coach. 
 
 Velocity in 
 
 Miles per 
 
 Hour. 
 
 Daily 
 Distance in 
 
 Miles. 
 
 Gross 
 
 Weight of 
 
 Load. 
 
 Weight 
 
 pulled per 
 
 Horse. 
 
 Number of 
 
 Working 
 
 Hours 
 
 Daily. 
 
 Number of 
 Working 
 
 Days 
 Weekly. 
 
 Mileage 
 Weekly. 
 
 8 
 9 
 
 16 
 16 
 
 Cwt. 
 
 32 
 46 
 
 Cwt. 
 16 
 
 "'5 
 
 2 
 
 If 
 
 5 
 
 3 
 
 80 
 48 
 
 Mail-Coach. 
 
 Velocity in 
 
 Miles per 
 
 Hour. 
 
 Daily 
 
 Distance in 
 
 Miles. 
 
 Gross 
 
 Weight of 
 
 Load. 
 
 Weight 
 
 pulled per 
 
 Horse. 
 
 Number of 
 Working 
 Minutes 
 Daily. 
 
 Number of 
 Working 
 
 Days 
 Weekly. 
 
 Mileage 
 Weekly. 
 
 9} 
 10 
 11 
 
 9 
 
 8 
 
 Cwt. 
 35 
 3° 
 30 
 
 Cwt. 
 
 8-7 
 75 
 75 
 
 57 
 
 51 
 
 44 
 
 7 
 
 7 
 
 7 
 
 63 
 
 59i 
 56 
 
 In'ithe aboveTtable a small increase in pace necessitates a reduction 
 in ..<the load^ancf. distance, travelled, and in the duration of the period 
 of work. 
 
 y- The draught employed in the stage-coach in the example given 
 was 62J pounds per horse on the level, which in the mail-coach was 
 reduced to 40 pounds per horse. It is manifest that with an increase 
 in velocity something must yield — either the effort must be reduced 
 or the duration of labour, or both. It has been shown by actual 
 experiment what amounts of increased effort are required to pull a 
 definite weight (1 ton) on a good road at varying velocities and on 
 different gradients.* In the following table only one gradient, a 
 very easy one, is taken for the purpose of illustration: 
 
 
 Four Miles 
 an Hour. 
 
 Six Miles 
 an Hour. 
 
 Eight Miles 
 an Hour. 
 
 Ten Miles 
 an Hour. 
 
 Level 
 1 in 64 
 
 Lbs. 
 
 76 
 
 IO9 
 
 Lbs. 
 80 
 
 115 
 
 Lbs. 
 
 85 
 I20 
 
 Lbs. 
 
 91 
 
 126 
 
 This shows the increase in force required, even on a good road, 
 for every increase of velocity, and the extra effort needed where a 
 load has not only to be drawn, but lifted through a height, as it has 
 to be in the case of uphill work. 
 
 The Physiology of Draught. — The horse, with its horizontal spine, 
 was never intended to carry weight; its muscular power is best 
 exerted in hauling, owing to the size of the base on which the animal 
 stands and the immense weight which can be thrown forward beyond 
 
 * ' A Treatise on Roads,' Parnell, 1838. 
 
694 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the base. The horse in draught allows his weight to act as a 
 falling body, but it would be of no mechanical value if it fell verti- 
 cally; it has to fall obliquely, and to secure this the feet are employed 
 as a means of purchase, while the body is kept on the move. 
 
 In ordinary draught-work simple pressure against the collar 
 suffices to move the load, but where the latter is considerable the 
 animal gets closer to the ground, bending both knees and hocks 
 in order to obtain a foothold, and pressing forward from the hind- 
 feet especially on starting. In starting a heavy load the diaphragm 
 is fixed, the abdominal muscles tense, those of the hind-quarters 
 so shortened that the skin over them is thrown into ridges; the 
 muscle tensing the fascia of the thigh stands out like a cord, and if 
 the animal is ascending a hill, the toes of the fore-feet are turned in, 
 perhaps in order to increase the size of the chest by turning out the 
 elbows. In heavy draught body weight tells, and in well-made 
 horses body weight and height go together. The increase in body 
 weight for increase in height may generally be considered to be 
 i hundredweight for every 4 inches (1 hand) ; so of two horses, one 
 of 15 and the other of 16 hands, all other things being equal, the 
 latter should be able to exert a force in draught roughly 112 pounds 
 greater than the former. Neither height nor weight govern the 
 indefinable quality of spirit and determination. Of two horses of 
 equal weight and similar build, one may be invaluable, the other 
 worthless, as a worker (see footnote, p. 438). 
 
 The strain in heavy draught is not confined to the limbs; it is 
 impossible for a horse to start a weight unless the muscles of the 
 back and loins are fit, and it is equally impossible for him to stop 
 it unless these parts are in sound working order. In heavy draught- 
 work on the level it may sometimes be noticed that the toes of the 
 fore-feet are the first to make contact with the ground. This is due 
 to the animal's being unable to extend his shoulders properly owing 
 to the load. Though the horse is designed to. drag weight, he 
 drags it well only on a level. Uphill his power rapidly diminishes, 
 owing to the effort required to lift both the load and his own body 
 weight as well as to drag them (p. 440). If, for example, he hauls 
 1 ton on the level with an expenditure of 60 pounds, he will require 
 to exert a force of 223 pounds to ascend a hill of 1 in 14, and this 
 second rate of effort is made up of 156 pounds to counteract the 
 gravity of the load, and 67 pounds that of his own body weight. 
 So badly, indeed, does a horse climb a hill with a load, that Desagu- 
 liers* stated that, if the hill be steep, three men, each carrying 
 100 pounds, will ascend it faster than a horse dragging 300 pounds. 
 A horse drawing a load on a good hard road with a rise of, say, 
 1 in 20, is really lifting one-twentieth of the load, besides over- 
 coming the resistance. 
 
 A perfectly level road is not the most favourable for work; this 
 was proved to be the case during the old coaching days. Mail-coach 
 contractors found that horses lasted longer on roads with gentle 
 undulations, and this was rightly explained as due to fresh muscles 
 being brought into play and others eased off (see p. 441). The most 
 favourable gradient is 1 in 40, or an angle of a little over 1 degree. 
 This is also the steepest declivity down which a vehicle will not roll of 
 its own weight, and is known as the angle of repose. The steepest 
 hill down which a horse can be driven at a fast trot without danger 
 is a smooth road of 1 in 35, equal to 150 feet in a mile, or an angle 
 
 * ' A Course of Experimental Philosophy,' 1763. 
 
LOCOMOTOR SYSTEM 695 
 
 of about 1 -5 degrees. A road of 1 in 25, a little over 2 degrees, is 
 an easy hill to climb; one of 1 in 20, nearly 3 degrees, takes three 
 times as much effort as on a level. A rise of 1 in 15, nearly 4 degrees, 
 is steep, and 1 in 12, rather more than 4-5 degrees, is a severe hill. 
 The question of severity depends upon the length of the hill and 
 weigh c of the load; one of 1 in 9, a little over 6 degrees, may be 
 ascended by a single horse with a light load. A hill of 16 degrees, 
 nearly 1 in 3, is impossible for any form of draught or for any- 
 thing but a led riding-horse; it is a dangerous hill for a man to 
 descend mounted. 
 
 In the descent of a hill the force of gravity acts with, instead of 
 against the horse, but where the inclination is so steep that a marked 
 effort has to be made to hold back the body and load, no benefit 
 is obtained; for example, a gradient of between 5 and 6 degrees 
 requires as much effort to descend as work on a level. 
 
 Strength of Horses. — A comparison between the strength of a horse 
 and that of a man has frequently been made, and the amount of 
 a horse's strength variously stated at the strength of from five to 
 seven men. We have found that most horses weighing about \ ton 
 will just pull seven men along, but in the case of a notoriously 
 powerful draught-horse weighing 14 hundredweight, his strength was 
 equal to thirteen men, whom he dragged very slowly along. This, 
 however, is most exceptional. The greatest force a horse can exert 
 for a few seconds in a steady pull is known as the limit of his strength ; 
 no horse can possibly exert in draught a force equal to that of his 
 own body weight. The writer tested the question on the dynamo- 
 meter, and found that horses pulled, according to their spirit, from 
 65 to 78 per cent, of the body weight.* The grouping obtained was 
 as follows : 
 
 Excellent pullers, 78-5 per cent, of their body weight. 
 Good ,, 77-6 
 
 Fair ,, 70-6 
 
 Bad ,, 65-6 
 
 The only horse he met with which pulled 88 per cent, of his own 
 body weight was the animal mentioned above as moving thirteen 
 men. 
 
 The Weight a Horse should Draw is a question frequently asked. 
 Other things being equal, an animal's power of traction varies 
 directly as his body weight. An attempt has been made by the 
 writer, working on these lines, to ascertain, in the regular draught- 
 work performed generally throughout the country, what proportion 
 the total load bears to the horse's body weight. In this way it is 
 hoped data have been obtained which will enable the question of 
 overloading to be dealt with on a sound basis. 
 
 The mean of 650 observations in England was that the horse 
 dragged a load equivalent to 2-53 times his body weight. From 
 the nature of the inquiry it was impossible to avoid favourable cases 
 being selected, but subsequent investigations, where this was speci- 
 ally avoided, show that for work outside towns and cities this ratio 
 may be accepted. In cities, where the roads are good and friction 
 reduced to a minimum, the ratio of load to body weight may be 
 raised to 1 to 3-5. In Lancashire the mean ratio is 1 to 3-5, but in 
 individual cases it may be much higher, even 1 to 5-5. These repre- 
 
 * ' The Maximum Muscular Effort of tjie Horse,' Journal of Physiology, 
 vol. xix., 1896. 
 
696 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 sent heavy loads pulled short distances by specially heavy horses 
 and for a small number of hours only, whereas the ratio of 3-5 
 is for eight working hours a day on good roads, though moder- 
 ately hilly. 
 
 Eighty years ago Stewart put the load for slow draught at 22 to 
 30 hundredweight, cart included. A large contractor at Manchester, 
 in his evidence before the Strathnairn Committee fifty years ago, 
 stated that nothing beyond 25 hundredweight for ordinary draught- 
 work (exclusive of the cart) should be placed behind any horse, 
 no matter whether the roads be good or bad. This is in the ratio of 
 1 to 3-4, and supports the correctness of this proportion. For light 
 draught a load of 15 hundredweight, and for heavy draught one of 
 from 20 to 25 hundredweight, in both cases exclusive of the vehicle, 
 appear to be average amounts. The vehicles are heavy: one of 
 two wheels weighs from 7 to 11 hundredweight, while a four-wheel 
 waggon weighs about 15 to 16 hundredweight. 
 
 The following tables present a physiological statement of the 
 question of draught for an ordinary working day of eight hours : 
 
 Roads outside Cities and Towns. 
 
 Ratio of 
 
 Walking pace 
 
 Trotting/^ wl ? eel f 
 6 (four wheels 
 
 Body Weight to Load. 
 
 Roads in Cities and Towns. 
 
 Walking pace 
 
 (two wheels 
 I four wheels 
 
 Trotting/! 
 
 I 
 
 to 
 
 2-5 
 
 I 
 
 to 
 
 2-0 
 
 I 
 
 to 
 
 2-3 
 
 
 Ratio of 
 
 y • 
 
 Weight to Load. 
 
 1 
 
 to 
 
 3-5 
 
 1 
 
 to 
 
 2-2 
 
 1 
 
 to 
 
 2-5 
 
 For loads in excess of the above, the hours of labour must be 
 reduced, and horses of sufficient substance employed: 
 
 Minimum Eody 
 Weight of Hose. 
 
 Where the ratio exceeds 3-5, but does not exceed 4-0, 15 cwt. o qrs. 
 4'° .. .. 4 - 5. 16 ,, o ,, 
 
 4\5 .. .. 5 - o, l6 .. 2 .. 
 
 5-0 ,, ,, 5-5, 17 ,, 2 „ 
 
 For as long as observations on draught have been made, the fact 
 has been noted that two horses working side by side do not pull 
 the sum of their individual efforts. Three horses working abreast 
 give almost the same power as four horses working in pairs. This 
 has been attributed to such causes as not working in exact rhythm, 
 and, if in teams, to loss arising from being too far from their work. 
 These are the chief sources of loss, but there is another, not, perhaps, 
 so obvious. Horses are very human in some of their failings, and 
 one is an inclination in double harness or in team not to take their 
 full share. The question cannot be further considered here, but 
 carriers and contractors have a special scale of work for pairs and 
 teams, which is less than what each individual horse is capable of 
 drawing. 
 
LOCOMOTOR SYSTEM 697 
 
 The following table shows the actual loads drawn in the past by- 
 horses in the streets of London. The load includes the weight of the 
 vehicle. The authority is Gordon.* 
 
 Brewers' team, each horse draws 2 § tons. 
 
 Heavy railway van, each horse draws 3J tons. 
 
 Tramway, each horse drew 2 J tons for a distance of 13 miles. 
 
 Omnibus, each horse drew i£ tons for a distance of 12 miles. 
 
 Water-cart horse draws 4 tons. 
 
 Coal -waggon, each horse draws 3 tons. 
 
 Vestry-cart horse draws i\ tons. 
 
 Weight a Horse should Carry. — This question is one especially 
 affecting the vital interests of mounted troops; there is a great 
 difference between the total weight and the effective weight an animal 
 can carry. As in the case of draught, the question of weight is largely 
 influenced by that of pace. Generally speaking, the weight an animal 
 can carry bears some proportion to its own body weight, but this 
 rule appears not to apply to the diminutive breeds of horses. The 
 Burmese pony carries a soldier in campaign kit whose feet are within 
 a few inches of the ground. A Korean pony does his thirty miles 
 a day in a roadless hilly country, carrying 160 to 200 pounds. 
 It was stated in 1807'f that a Shetland pony would carry a 12-stone 
 man forty miles in one day with ease. Lawrence, in 1810, % stated 
 there was a country postman, working between Glasgow and Edin- 
 burgh, and riding 16 stone, who carried his ' northern pony' in his 
 arms to avoid paying toll. The South African farmer, standing 
 over 6 feet in height, rides a lightly-built pony remarkable distances, 
 either at a canter or walk. Our own Exmoor and Dartmoor ponies 
 afford similar instances of endurance. Facts somewhat similar to 
 these suggest that as the horse became bigger his spine became less 
 fit to carry weight. At no time is a horizontal spine a weight-bearer. 
 In proportion to his weight a man can carry far more than an 
 ordinary horse, for the reason that he carries it on a vertical spine. 
 The weight carried by pack-horses, at the time when these furnished 
 the only means of communication in this island, was from 16 to 
 19 stones (102 to 121 kilogrammes) at their own pace. DesaguliersJ 
 said that in his day (1763) the pack-horses used by the fellmonger 
 and skinner carried heavier weights than any others, and were 
 sometimes loaded with 32 to 40 stones (204 to 254 kilogrammes), 
 carried at a very slow pace. If what we are told respecting the 
 weight-carrying power of the Cleveland pack-horse can be believed, 
 their capabilities exceeded those of any other breed. Culley|| (1794) 
 is responsible for the statement, that three Cleveland mares each 
 carried 50 stones (318 kilogrammes) sixty miles in twenty- four hours, 
 and did it four times a week. YouattlT states that mill-horses have 
 carried 65 stones (4I3'6 kilogrammes) two or three miles. One 
 would hesitate to accept the correctness of these loads, were it not 
 that extraordinary weights maybe carried by perfectly trained men. 
 A Kashmir coolie will carry 3 hundredweight on his back up a steep 
 hill, while the limit of strength of a Turkish or Egyptian porter does 
 not appear to be reached either by a ship's boat or a large office safe, 
 carried on the back and supported by a band around the forehead. 
 An ordinary man will carry for long distances a third of his body 
 
 * ' The Horse World of London,' W. J. Gordon, 1893. 
 
 f ' The Complete Farmer.' \ ' Treatise on Horses.' 
 
 J Op. cit. || ' Observations on Live Stock.' If ' The Horse.' 
 
698 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 weight, and for short distances more than his body weight. The 
 writer has shown that the effective weight a horse in good condition 
 will carry for long distances lies between one-sixth to one-fifth of the 
 animal's own body weight.* The mean weight of a cavalry horse 
 is 1,100 pounds (500 kilogrammes), so that the effective weight he is 
 capable of carrying may be taken at 14 stones (83-2 kilogrammes). 
 Cavalry horses are called upon to carry as much as 20 stones, or 
 280 pounds (127-2 kilogrammes), roughly, one-quarter of the body 
 weight, an amount greatly in excess of their strength. 
 
 Apart from the features touched on above, the main physiological 
 fact connected with weight-carrying in horses is that the muscles 
 of the back and loins must be conditioned for the work. An animal 
 in the hardest condition for draught purposes is quite unfit to carry 
 a man; the muscles of the back and those beneath the loins are 
 unable to carry weight unless the animal is properly conditioned 
 beforehand. And this holds good for any class of work which neces- 
 sitates the employment of a fresh and unskilled group of muscles. 
 
 The Speed of Horses — The Gallop. — The fastest pace at which a 
 horse has been known to gallop is at the rate of 37-69 miles an hour. 
 The animal was Salvator, who in 1890 was galloped on a straight 
 course against time. Flying Childers, the fastest horse known in 
 this country, galloped 3J miles at a velocity of 34J miles an hour. 
 In another trial of over 4 miles the speed attained was an average 
 of 33-62 miles per hour. Flying Childers has been credited with 
 galloping a mile in a minute, but this is impossible. The mean 
 velocity for the Derby during a period of ten years was 33-54 miles 
 per hour, the distance being \\ miles. For the Lincolnshire Handi- 
 cap, the distance of which is a mile, the average speed for ten years 
 was 33-84 miles per hour. The ordinary gallop varies from 12 to 
 14 miles an hour. The length of the stride is from 15 to 19 feet for 
 the average horse, for the race-horse considerably more (see p. 679). 
 Eclipse and Flying Childers were credited with a stride of 25 feet. 
 
 The Canter varies greatly in velocity, depending, as we have 
 seen, on the length of the spring. The stride may be from 9^- feet 
 in a slow, to 11 J feet in a fast, canter (iee p. 677). 
 
 The Trot is generally performed at a pace of 7J to 8J miles per 
 hour, the length of the stride being between 7 and 10 feet (see p. 672) . 
 When trotting matches were popular in this country, some remark- 
 able feats of endurance were obtained. In 1780! a mare trotted 16 
 miles in 58^ minutes, carrying 12 stone. She repeated the per- 
 formance when eighteen years old. In 1785 Archer trotted 16 miles 
 in 54J minutes, but died the next day. In 1800 Phenomenon trotted 
 17 miles in 56 minutes, and in the same month repeated the distance 
 in 53 minutes. In America, where trotting has reached a degree of 
 excellence unknown elsewhere, some astonishing velocities have 
 been obtained, the mile being covered in 1 minute 58J seconds, or 
 30-4 miles per hour. 
 
 The Walk of an ordinary horse is a mile every 15 minutes, the 
 length of the stride being from 5 feet 6 inches to 6 feet (see p. 671). 
 
 Endurance Tests. — Some years ago long rides were inaugurated 
 on the Continent with the object, it was stated, of encouraging the 
 development of the horse for long-distance work in war. These rides 
 are practically physiological experiments, and are consequently of 
 
 * ' The Weight of a Horse and its Weight-carrying Power,' Journal of 
 Comparative Pathology and Therapeutics, vol. xi„ No. 4, 1899. 
 t J. Lawrence, op. cit, 
 
LOCOMOTOR SYSTEM 699 
 
 interest. The first was from Brussels to Ostend, a distance of 
 82 miles. The winner covered the distance in seven hours. Of 
 61 horses entering, 17 died, and 17 got no farther than the sixty- 
 second mile. In subsequent rides the principle of racing was 
 abolished, and the essential factors governing the results were the 
 maintenance of condition and fitness. Two of these rides may be 
 briefly referred to. One year the distance was 250 miles. This was 
 covered in 50 hours, the last 53 miles being completed at i2£ miles 
 an hour. Another year the distance was 93 miles, which was 
 covered in io£ hours. Out of 47 starters only 24 came in, and but 
 15 of these were qualified. Tests such as these are no indication of 
 the endurance of the average horse, for in all these trials the animals 
 are the pick of many thousands, and, this notwithstanding, we have 
 seen that 50 per cent, fell out in the last trial above recorded. 
 
 Two features in these tests need noting. One of these is expressed 
 in an aphorism as old as horses themselves. It is the physio- 
 logical truth that ' it is the pace which kills.' The other raises 
 the question of whether there is any deterioration in the stamina 
 of horses. Are they as capable of withstanding fatigue as the 
 horse of, say, two hundred years ago ? A good deal of evidence 
 might be brought forward, including that mentioned on p. 690, 
 to show that the same powers of endurance are not exhibited, and 
 the explanation would appear to be that with an increase in body 
 height there is a falling off in stamina. The standard of height 
 in horses generally has been raised in order to obtain greater 
 speed in the case of the race-horse, and in other breeds to meet 
 the demands of fashion. But with this increase in stature there 
 certainly appears to be a reduction in stamina. The cavalry of 
 Frederick the Great astonished Europe, and what they did con- 
 stitutes an object-lesson for cavalry for all time. But in order to 
 get his results the same class of horse must be employed. The 
 standard of height for cavalry since his day has been raised 4 inches 
 and over. 
 
CHAPTER XVII 
 
 THE FOOT 
 
 Horns, nails, claws, and hoofs represent a modified form of 
 epithelium, and the tissues from which these are secreted 
 correspond to the deep layers of the skin. The other horny 
 structures found connected with the skin of the horse — i.e., the 
 chestnuts and ergots — have a similar origin . Considered by itself, 
 a piece of horn, a claw, or nail, would not appear to possess any 
 special physiology; but when the nail surrounds and completely 
 encloses the end of the limb, as it does in the ungulata, a special 
 physiology arises, and this reaches its highest development in 
 the solid-footed horse. The foot in this animal has become a 
 highly specialised structure, for the purposes of resisting wear i 
 and tear, of supporting the weight of th e body, and of saving the 
 foot and limb from concussion. If it were merely a block of 
 horn on which the horse stood, it would offer nothing of special 
 interest. 
 
 A foot consists of three feet, ' each enclosed within another. 
 Externally there is the hoof or horn-foot; within this is a com- 
 plete counterpart in fibrous and vascular tissue known as the 
 sensitive foot, and the latter is moulded upon a bony structure 
 which in appearance resembles a miniature foot. It is usual, 
 however, to regard the foot as being divided into two portions, 
 an insensitive and sensitive, or an external and internal, the 
 latter term connoting both the vascular and the bony foot. 
 The internal and external feet are exact counterparts of each 
 other, the former being on a smaller scale, to enable it to fit into 
 the external foot in the same way as a finger fits into a glove. 
 
 The external and internal feet are not independent of each 
 other — one is the complement of the other. The external does 
 not produce itself; it is dependent entirely on the internal for 
 every horn cell it possesses. The internal foot cannot tolerate 
 pressure, not even touch; it is dependent on the external foot, 
 for the circumstance that, in spite of its highly sensitive 
 nature, it is throughout the life of the animal enabled to be 
 kept within § to \ inch of the ground without causing pain. 
 
 700 
 
THE FOOT 
 
 701 
 
 Growth and protection are not the only features in the foot. 
 No one can listen to the thuds of the galloping horse, or the 
 clatter of one trotting, without realising that every time the feet 
 come to the ground there must be considerable concussion unless 
 mechanisms existed for its prevention ; and no thoughtful person 
 will have any difficulty in realising that the horse's foot repre- 
 sents a remarkably small pedestal for such a bulky body to be 
 carried upon. These are the essential features to which attention 
 must be directed in the physiology of the horse's foot, and must, 
 from their paramount importance, be considered in detail. In 
 order to elucidate them, 
 it is desirable to glance at 
 some of the structural 
 features of the parts con- 
 cerned. 
 
 Bones of the Foot. — The 
 core of the foot consists of 
 bone, around which all the 
 other structures are moulded. 
 The bone is not one solid 
 piece, such as it might be 
 imagined would be necessary 
 in such a position, but, on 
 the other hand, consists of 
 three pieces. One of these 
 is the pedal bone, which is 
 a miniature foot in shape 
 (Fig. 231), while its sub 
 stance is porous to such 
 an extent as to resemble 
 pumice-stone in appearance 
 and density. A second bone, 
 the navicular, is very small, 
 of peculiar shape, dense in 
 structure, rests slightly on 
 ■ the pedal bone, and is mainly held in position by ligamentous 
 tissue. The third bone belongs partly to the foot and partly to the 
 limb. One might suppose that the pedal bone should occupy the 
 whole of the interior of the hoof, as high as the coronary edge and as 
 far back as the heels, but this is not so. It only occupies a portion of 
 the internal foot (Fig. 232), mainly situated towards the anterior 
 and lateral parts ; the posterior part of the foot contains very little 
 pedal bone, but the deficiency is made up by the introduction of 
 two large plates of cartilage attached to it, over which the structures 
 are reflected and moulded as on the bone itself. This singular 
 deficiency of bone, in a part where one might be led to regard 
 its existence as a necessity, and the presence of large carti- 
 laginous plates to take its place, are due to the lateral movements 
 the foot has to perform, and which could not be carried out if the 
 bone were proportioned relatively to the structure within which 
 it fits. The pedal bone is not placed parallel to the ground, but 
 fits within the hoof, with its toe slightly lower than its heels (see 
 Figs. 219 and 237). 
 
 Fig. 231. — Pedal Bone and Corona 
 (Modified from Leisering). 
 The dotted line through the latter indicates 
 the portion buried within the foot, or hidden 
 by the soft structures at the back. 
 
7 02 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The Foot-Joint. — Three bones form the foot-joint (Fig. 233). The 
 question naturally arises why the joint is not composed of two bones 
 instead of three, and what advantage is gained by the introduction of 
 a small dense bone, such as the navicular, into the articulation. The 
 articulation furnished by the pedis is much smaller than that pro- 
 vided by the corona, but by the introduction of the navicular, the 
 pedis plus navicular surface is nearly, but not quite, equal to the 
 corona surface. One use of the navicular bone is to increase the 
 articular surface of the pedis. But it is conceivable that this small 
 articular surface of the pedis might have been increased in some 
 other way than by the introduction of a distinct bone and other 
 complicated apparatus, and it is evident that the value of the 
 navicular articulation does not depend entirely on the fact that it 
 
 Fig. 232. — The Wall partly removed in order to show the Position of — 
 (1) The Pedal Bone; (2) the Extent of the Lateral Cartilage. 
 
 The dotted line through the latter indicates the portion within and without the 
 hoof. In the figure the cartilage has curled in and shrunk a little from 
 exposure. It will be noted that the pedal bone is lower in front than behind. 
 
 increases the size of the joint, but that it supplies what elsewhere 
 h as been spoken of as a yielding artic ulation (see pp. 654 and 656). 
 The use of this yielding articulation is to save direct concussion. 
 During locomotion, when the foot comes to the ground, the weight 
 through the corona falls in the first instance largely on the navicular, 
 which under its influence yields slightly in a downward direction; 
 from the navicular the weight is transferred almost entirely to the 
 pedis, which also yields slightly under its influence, and in this way 
 direct concussion to the joint is prevented. 
 
 The Navicular Bone and Bursa. — The navicular would be of very 
 little use for the above purpose, if it depended on being kept in 
 position solely by the delicate ligaments which have origin from it. 
 Its chief support is the broad expansion of the perforans tendon 
 which passes beneath it ; between the tendon and the bone the most 
 
THE FOOT 
 
 703 
 
 intimate fitting occurs, and a synovial apparatus exists to save 
 friction. It is probable that the perforans tendon and the inferior 
 face of the navicular are more closely adapted to each other than 
 any articulation in the body, excepting some of those found in the 
 knee and hock joints. B riefly , then, the small dense navicular bone 
 i§_enabled_to form a yielding articulation in the loot, owing to the 
 manner in which" it is supported in posi tion by the powerful per- 
 forans tendon. It might be urged on purely theoretical grounds 
 that a small bone thus placed in the foot would be very liable to 
 damage, and such is clinically true. There is no intention here 
 to touch on the subject of navicular disease, excepting in so far as 
 it helps to elucidate the physiology of the part, but it is permissible 
 to regard the lesions of navicular disease in the light of physiological 
 
 Fig. 
 
 -Longitudinal Section of the Foot. 
 
 The corona; 2, the pedis; 3, the navicular; 4, the horn wall; 5, the horn sole; 
 6, 6, the foot-pad; 7, 7, the plantar cushion; 8, the perforans tendon passing 
 under the navicular bone, to be inserted in pedis; 9, the wall-secreting 
 substance; 10, the extensor pedis tendon; n, junction of wall and sole, the 
 ' white line.' 
 
 experiments, and to learn from them how intimately the freedom 
 and elasticity of a horse's action depend upon the navicular bone, 
 and how the stilty, pottering, shuffling gait arises in the animal when 
 this bone is no longer capable of performing its functions properly. 
 The very close support afforded to the navicular by the perforans 
 tendon may possibly be a cause of disease, for the conclusion has 
 been forced on the writer that, under the influence of the weight of 
 the animal, and the counteracting influence of the perforans tendon, 
 the navicular bone must be exposed to considerable compression 
 (see Fig. 233). This compression exists, not only during locomotion, 
 but also during standing. The only complete rest from compression 
 which the navicular bone of the fore-limb obtains is while the 
 animal is lying down. Those of the hind-limbs are relieved from 
 pressure every time the horse rests the leg by flexing the hock, 
 and no case of navicular disease in the hind-feet has ever been 
 known (p. 648). v. 
 
704 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The navicular bone does not exercise any pulley function in con- 
 nection with the perforans tendon, such as has been usually de- 
 scribed — that is, if by the use of the term ' pulley ' it is intended 
 to convey the impression that some mechanical advantage is ob- 
 tained. It is true that by passing beneath the navicular bone the 
 direction of the pull of the tendon is altered, but no mechanical 
 advantage is thereby derived. The perforans tendon at its insertion 
 spreads out fan-shaped, and is attached over a considerable semi- 
 lunar surface of the pedal bone ; so extensive is this attachment that 
 it is erroneous to believe the tendon plays over the navicular bone . 
 It is a fact that movement occurs between the tendon and the 
 bone, but the tendon is passive, while the yielding of the navicular 
 bone under the influence of the body weight is the active agent. 
 It is interesting to observe the direction in which the largest amount 
 of friction occurs between these two surfaces". Reasoning from the 
 position of the parts, one would think it occurs at the moment 
 the foot comes to the ground ; but if the eroded tendon of navicular 
 disease be examined, it will be observed that the fibres are all stripped 
 upwards, and rarely or never downwards. This_ points to the 
 greatest friction occurring,, _not when the bone yields under the 
 weightj.but when it returns to its place.jas the body, under the 
 influence of the flexor tendon, passes over the foot. ThlFTifequerlcy 
 with which the middle of the ridge of the navicular bone, and the 
 area on either side of it, are affected with disease points to this part 
 as being the seat of the largest amount of pressure. 
 
 Lateral Cartilages. — Attached to each side of the heel of the pedal 
 bone is a large curved plate of cartilage, in parts fibrous, in others 
 hyaline in nature. So extensive is this plate that it reaches high 
 above the margin of the hoof — i.e., outside the foot in an upward 
 direction as far forward as the coronet and as far back as the heel 
 (Fig. 232). There is no other structure in the body with which this 
 arrangement can be compared : a bone possessed of two large car- 
 tilaginous wings is a something peculiar to the foot. The use of 
 these cartilages is intimately connected with the main principles of 
 the physiology of the foot, to be dealt with later. 
 
 Plantar Cushion. — Placed between the two plates of cartilage just 
 spoken of is a large somewhat pyramidal-shaped body known as the 
 plantar cushion (Figs. 233, 7, 7; 234). In appearance it resembles a 
 fibro-fatty mass, is composed of interlacing bands, is pale yellow in 
 colour, almost destitute of bloodvessels, firm to the touch, yet 
 yielding in its nature. 
 
 Mettam* has shown that, though the plantar cushion to the naked 
 eye is fibro-fatty, the microscopical characters show it to be mainly 
 tendinous in structure, the fibres being disposed in bundles running 
 in different directions ; uniting the bundles is a connective tissue, in 
 the meshes of which fat is found in islets, and not abundantly, as one 
 of the older names of this body implied. f There is only a small 
 amount of elastic tissue present. The position of the plantar cushion 
 in the foot is at the posterior half, between the cartilages, rising up 
 above the level of the hoof and filling in completely the hollow of the 
 heel. Its inferior face is V-shaped (Fig. 234), and a complete counter- 
 
 * ' The Development and Histology of the Hoof,' etc., by Professor A E 
 Mettam, B.Sc, M.R.C.V.S., Veterinarian, 1896. 
 
 t Nevertheless, W. C. Spooner, of Southampton, writing in 1840 
 (' Treatise on the Foot and Leg of the Horse '), said analysis showed that 
 the plantar cushion contained no fat in its composition. 
 
THE FOOT 
 
 7°5 
 
 part of the horn-cushion or foot-pad above which it lies. Its position 
 is shown in section in Fig. 233, 7, 7, and the manner in which it pro- 
 tects the navicular bursa and tendon. The cushion is softer pos- 
 teriorly than anteriorly, where at its apex it is dense and fibrous. 
 The cushion does not secrete horn, but its surface is covered by a 
 delicate papillated membrane which secretes the horn of the foot- 
 pad. 
 
 | Sweat-glands exist in the plantar cushion, situated in and on the 
 side of its central depression. The glands, as figured by Franck, are 
 shown in Fig. 235. A more modern description is given by Mettam. 
 They are large, coiled, single or multiple glands opening on the sur- 
 
 Fig. 234. — The Plantar Surface of the Internal Foot. 
 
 , 3, The sensitive foot-pad, or plantar cushion, 1 being the bulbs, 2 the cleft; 
 
 4, the termination of the wall-secreting body of the heel, where it blends 
 with the plantar cushion; the numerous papil'as on its surface can be seen; 
 
 5, the vascular or sensitive sole, covered by papilhe; 6, terminal ends of 
 the sensitive lamina?, which may be seen around the entire rim; it is at this 
 point where each lamina terminates in four or five papillae; 7, the laminae 
 of the wall inflected at the heel, and here forming the sensitive ' bars.' 
 
 face of the horn in the so-called ' cleft ' of the frog by means of 
 tubes, which take very spiral courses. They secrete an unctuous 
 fluid, which helps to maintain the horn in a pliable condition. 
 
 The Corium of the foot completely covers the whole of the pedal 
 bone, plantar cushion, and a large surface of the lateral cartilages. 
 This tissue has received various names — viz., from its colour, the 
 vascular foot; from its appearance, the fleshy; from its character, 
 the velvety foot ; whilst from one of its functions it has been termed 
 the horn-secreting foot. 
 
 45 
 
■job 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The Vascular Wall or Laminal Tissue (Figs. 236, 4; 237, 6) is com- 
 posed of corium arranged in the form of a number of leaves lying side 
 by side, which run from the coronet downwards and forwards to the 
 edge of 'the wall. In number there are about 500 or 600 ; they invest 
 the entire wall of the pedal bone and the greater part of the lateral 
 cartilages, their extreme vascularity giving the appearance of a 
 thin layer of muscle. The leaves at the toe are longer than those at 
 the heel, where they are very short and turned in under the foot, 
 running forwards beneath it to form the part known as the sensitive 
 bars (Fig. 234, 7). . 
 
 If a single leaf, say at the toe, be removed and examined, it is 
 
 Fig. 235. — The Sweat Glands of the Plantar Cushion (Franck). 
 d, d, The glands, the corkscrew-like ducts of which (e, e, b) pass out through the 
 horn of the foot-pad, opening at /, /, on to the surface of the foot at the cleft. 
 At c is the deep-seated portion of the horn of the foot-pad, where it grows 
 from the papilla? of the corium of the plantar cushion; g, g, are horn fibres seen 
 in longitudinal section. 
 
 found to commence immediately under the thick cornice-like 
 structure known as the coronary or wall-secreting body, and to 
 be most firmly attached to the pedal bone ; in fact, so intimate is 
 the attachment that it is almost impossible to remove this tissue 
 cleanly from the bone. The leaves under the coronet are very short 
 from front to rear (depth), but as they proceed from the coronet 
 towards the ground they rapidly increase in depth, and attain 
 their full depth about \ inch or so from their origin. The edge of 
 the leaf is not regular, but denticulated, and at its inferior part 
 each leaf terminates in five or six papillae. The leaf is extremely 
 vascular — in fact, quite scarlet in colour — the effect over the whole 
 mass of leaves being very striking. If the tissue be examined 
 microscopically, it is found that part of its substance is devoted to 
 
THE FOOT 
 
 707 
 
 leaf-formation, whilst the remainder is a sublaminal tissue, the 
 function of which is to secure the laminae firmly to the wall of the 
 pedal bone. This sublaminal tissue has been described by Moeller* 
 as consisting of two layers ; the one nearest the bone is designated 
 the stratum periostale, and acts as the periosteum of the bone 
 (Fig. 238, b). Outside this is a layer of fibrous connective tissue 
 and elastic fibres, arranged in bundles, crossing and forming net- 
 works; this layer is extremely vascular, and has been designated 
 the stratum vasculosum. External to this layer are the laminae, 
 
 Fig. 236.- 
 
 -The Hoof removed, and the Vascular Wall seen from the 
 Front. 
 
 1, Groove between the skin and wall-secreting body from which the periople grows ; 
 2, the wall-secreting body; the rough surface is due to papillae and small 
 adherent fragments of horn; 3, the beginning of the sensitive laminae; 
 4, the lamina?; 5, their papillated ends. 
 
 formed of elastic and connective-tissue fibres, as in the previous 
 layer, only the network is much finer. The laminae contain numerous 
 bloodvessels and nerves. 
 
 The microscopical appearance of a horizontal section of the vas- 
 cular laminae is shown in Fig. 238, d, from Moeller. Each sensitive 
 lamina is not smooth, as its naked-eye appearance indicates, but 
 denticulated, each tooth-like depression representing secondary 
 laminae, or laminellae (Fig. 238, e), first fully described by Fleming. t 
 The number of these in a lamina depends on the depth of the primary 
 
 * Veterinary Journal, vol. v., 1877, p. 114. 
 
 ■j" In 1840 W. C. Spooner, op. cit., wrote: ' The inner edges of the laminfe 
 appear fimbriated, like the edge of a fine-tooth comb.' 
 
yo 8 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 lamina, but they may be from 60 to 120 in number. It is quite 
 common to find some of the secondary lamina? bifurcate. The 
 appearance presented on horizontal section is very characteristic, 
 and has been aptly likened by Chauveau to a feather, the barb of 
 which is represented by a lamina and the barbules by the secondary 
 lamina? (see Fig. 238). The function of the secondary laminae has 
 been a fruitful source of discussion. 
 
 The Origin of the Horn Laminae. — No one doubts that the wall 
 grows from the coronet, but great controversy has taken place over 
 the origin of the horn lamina?, some saying they grow like the wall 
 from a part of the coronary cushion, and others affirming that they 
 obtain their origin from the sensitive lamina?. If we were to judge 
 solely by the result of pathological processes, we should say the 
 sensitive secreted the horn lamina?; but Moeller* points out that the 
 
 Fig, 237. — The Hoof removed, and the Vascular Wall seen from the 
 
 Side. 
 
 I, The7periople"'groove ; 2 and 3, the wall-secreting body; 4, the bulb of the 
 plantar cushion, richly covered with papilla?, and running into 3, with which 
 it joins; 5 and 6, the sensitive laminae; 7, increase in size of the periople 
 groove at the heels, where a soft horn is formed, which plasters over the 
 junction of the various foot tissues which here meet. Note that the plantar 
 cushion is below the level of the heel of the pedal bone and lateral cartilage. 
 
 sensitive and insensitive lamina? are never in actual contact, and that 
 between them are placed the secondary lamina? of both varieties 
 (Fig. 239) . Therefore he argues that the vascular cannot secrete the 
 horn lamina?, but that the secondary vascular secrete the secondary 
 horn lamina?. If a portion of wall be removed experimentally and the 
 vascular lam'na? exposed, in the course of a short time the part be- 
 comes covered with a layer of horn, with lamina? on its inner surface, 
 and this has been used as a strong argument in favour of the secre- 
 tion of horn lamina? from sensitive lamina?; but the horn which is 
 thus secreted is derived from the secondary vascular lamina?, and 
 no one contends that these secrete the primary horn lamina?. The 
 following explanation appears to be the correct one : The hornlamina? 
 are_secreted from the lower edge of the coronary body; here proto- 
 plasmic cells arc poured out between the papilla?; these cells are 
 
 * Op. cit. 
 
 
THE FOOT 
 
 709 
 
 carried down with the wall, being pressed into and moulded between 
 the sensitive leaves, thus becoming horn laminae, the exact counter- 
 part in shape of the mould in which 
 they are made. All this occurs in 
 the region marked 3, Fig. 236. As 
 the wall grows down the horn-leaves 
 are carried with it, so that there 
 is a perpetual movement occurring 
 between the slowly travelling in- 
 sensitive and the fixed vascular 
 laminas. The rate of this movement 
 is probably about 0-0125 inch in 
 twenty-four hours, on the assump- 
 tion that the wali_ grows % inch in 
 the m onth . DTJfmgthe time the 
 
 lorny are gliding between the sensi- 
 tive leaves, the vascular lamellae 
 furnish them with horny lamellae; 
 when the wall reaches the sole, the 
 horn lamellae are left behind, while 
 
 Fig. 238. — Horizontal Section 
 of the Hoof and Vascular 
 Tissues at the Anterior 
 Part of the Horse's Foot 
 (Moeller). 
 
 a, Bony tissue of the os pedis; b, 
 stratum periostale; c, stratum 
 vasculosum; d, sensitive laminas; 
 e, secondary lamina;, or lamella; ; 
 /, primary horn laminae; g, wall 
 of the hoof, with its horn fibres. 
 
 Fig. 239. — Horizontal Section of 
 Portion- of a Pair of Lamina: 
 (1) Sensitive, (2) Horny, Each 
 with their Lamellae. 
 
 The laminae and lamellae have been 
 pulled apart; the protoplasmic cells 
 which separate the lamellae of each 
 type are partly represented. The 
 figure shows that these cells come 
 between both primary and secon- 
 dary laminae of each series, which 
 are therefore never in actual contact. 
 It is from these protoplasmic cells 
 that the horn lamellae are secreted. 
 
 the primary laminae emerge with the wall destitute of these 
 structures . This statement does not explain away all the difficulties 
 which could be raised, but suffices for practical purposes. 
 
710 A MANUAL OF VETEklNARY PHYSIOLOGY 
 
 The Wall-secreting or Coronary Body is a thick, half-round, cornice' 
 shaped welt of material situated above the laminae (Figs. 236, 2; 
 2 37> 2 . 3) I it h as received several names, the most rational being 
 that based on its function as the structure which secretes the 
 hoof wall. Externally this body is covered by a highly vascular 
 membrane possessing long papillae, which are readily seen by im- 
 mersing the foot in water, while on section it is found to be fibro- 
 fatty in appearance, and consists of a coarse network of elastic and 
 fibrous tissue. It is this latter which forms the main substance of 
 the welt, which projects like a big rim from the sensitive foot. The 
 basement substance takes no part in the secretion of horn; the 
 papillated membrane which covers it alone carries out this function, 
 while the use of the welt is to provide the secreting membrane with 
 a sufficiently firm and extensive surface. The waljUsecreting 
 substance extends all round the coronet from heel to heel, where it 
 joins the plantar cushion (Fig. 234, 4). On its superior margin is 
 ajwrrow_groove (Fig. "236, 1), which is the dividing -line between 
 sEn and hoof, and from which a~peculiar horn known _as~the 
 periople is secreted. ~"This~horn "cements over tHe yunction~between 
 hair and hoof (Fig. 241, X). On its lower margin the coronary 
 substance fuses with fibres from the vascular laminae. The entire 
 body fits into a half-round groove in the wall, and the papillae on 
 its surface are lodged in canals formed in the horn. Beneath the 
 coronary welt is a well-developed subcutis, which unites it to the 
 tissues covering the corona and lateral cartilages. The vascular 
 papillated membrane covering the coronary substance is frequently 
 irregularly pigmented corresponding to the colour of the horn wall. 
 
 The Vascular Sole (Fig. 234, 5) is scarlet in colour, and covered by 
 long papilla? which are lodged in the depressions in the horn sole. In 
 each papilla an artery and one or more veins may be found. The 
 corium covering the plantar cushion is similarly arranged, the papillae 
 being lodged in canals in the foot-pad or horn frog. 
 
 The Blood-Supply to the foot is exceedingly rich. With the excep- 
 tion of the interna] organs, there is no part of the body so vascular, 
 and the horse has more blood in his feet than in his brain. Mention 
 has already been made of the scarlet appearance presented by the 
 laminae, the vascular sole, and the tissue covering the plantar cushion. 
 The pumice-stone-like character of the pedal bone is for the purpose 
 of affording passage to the innumerable vessels which are passing 
 from the interior of the bone in an outward direction to reach the 
 vascular tissues; in fact, no description or drawing can adequately 
 convey an idea, of the appearance of vascularity presented by the 
 foot. The veins are large and numerous (Fig. 240), and are not 
 provided with valves; some pass through the substance of the 
 lateral cartilage, and a large plexus exists both outside and inside 
 the cartilage. The relation of these vessels to the lateral cartilages 
 and the absence of valves are matters which will be considered 
 later. 
 
 The Hoof, or • itive loot, is moulded over the sensitive struc- 
 
 tures in such a "'•>■ is to cover them completely, and form in horn 
 a perfect countermart of the sensitive foot. The hoof is composed 
 of a wall, with its inflections the bars, a sole, and a foot-pad (frog) ; 
 each of these )e considered separately. 
 
 The Wall i. ^;trt of the hoof which can be seen when the foot 
 
 is on the ground; its division into toe, quarters, and heels is for con- 
 venience of description, as no natural division exists. On the 
 exterior of the wall, at the meeting of the hair and hoof, is a rim 
 
THE FOOT 
 
 7" 
 
 of peculiar non-pigmented horn, previously spoken of as the periople. 
 Under natural conditions it is grey in colour, soft to the touch, 
 and it cements the skin to the hoof. Its non-pigmented condition 
 is rendered evident only when the foot has been soaked in water 
 or poulticed; the cells then swell, and a white curdy rim occurs all 
 round the top of the wall (see Fig. 241.x). It is wider at the bulbs of 
 the heel, where it cements over not only the union of skin with foot- 
 pad, but the meeting-place of the wall and foot-pad. The periople 
 provides the wall with an extremely thin covering, resembling a 
 delicate coat of varnish, which is intended to prevent undue 
 evaporation from the horn beneath. 
 
 The Colour of the horn of the hoof is commonly described as black 
 or white; to be strictly accurate, it is neither. The so-called black 
 horn consists of various tints of slate colour; the white is pale 
 yellow or buff. It is, however, convenient to speak of black and 
 
 Fig. 240. — The Venous System of the Horse's Foot (Storch). 
 
 white feet. In dealing with the skin (p. 315), the colour of feet was 
 touched upon, and it was made clear that it depended upon the 
 colour of the hair of the coronet. If this is white, there is no pigment 
 in the skin, and consequently none in the horn; if it is partly black 
 and white, the foot is striped accordingly. If 
 other colour than white, the feet are dark; bu 
 mixed with white, the feet are striped. It mi b 
 a grey horse would have white feet, but a grey 
 horse, and a grey with grey legs has dark fe- 
 white legs has white feet. The number of wh. 
 horses is very small.* The physiological importance of non-pig- 
 mented horn is its weakness, brittleness, am" ■ growth, as com- 
 pared with the pigmented variety. A white foot constitutes local 
 albinism (see p. 316). 
 
 The wall is thickest and longest at the toe fluaaest and shortest 
 
 * I am indebted to the late Captain Martin Millar, A V.C., for exact 
 information respecting 500 grey horses: 57-3 per cent, had black feet, 
 34 per cent, parti-coloured feet, and 8-7 per cent, white feet. He s\sp 
 observed that a dark spot on a white coronet frequently colours the hoof 
 out of all proportion to its size. 
 
 coronet is of any 
 
 f these colours are 
 
 u » imagined that 
 
 ,io3 is not a white 
 
 but a grey with 
 
 l_ feet among grey 
 
712 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 at the heel. A gradual decrease in thickness occurs from front 
 to-rear (Fig. 242) ; but if a section of the wall be made in the direction 
 of its fibres, it will be found that whatever the thickness may be 
 at that particular part, it is maintained from the coronet to the 
 ground surface. The greater thickness of the wall at the toe and 
 quarters as compared with that of the heels is connected with the 
 wear and tear of the hoof, and the movements which the latter under- 
 goes under the influence of the body weight. If the wall were as 
 thick at the heels as at the toe it would be a rigid box; it is, how- 
 ever, a yielding box, and the yielding occurs in the region of the thin 
 wall of the heels. The reason why the wall is thick at the toe is 
 that it is here the greatest friction and strain occurs. The wall 
 at the heels is suddenly inflected. (Fig. 242, 5), running under the 
 foot in a forward direction for a short distance, and forming an 
 
 fSr '■ J HRF- >' 
 
 
 
 
 
 
 r 
 
 
 
 ^t^s^- 
 
 lk-^ at 
 
 Fig. 241. — The External Foot or Hoof. 
 
 The fibrous appearance of the wall may be seen, also the fieriopte marked X; 
 the hair of the edge of the coronet is clipped away to show this band of white 
 horn, which for the purpose of the photograph was swollen by immersion in 
 water. 
 
 acute angle with the wall. This inflected portion of the wall is called 
 the Bars (Fig. 242, 6), and in the gap formed between the two bars is 
 lodged the foot-pad. Thus the wall is an incomplete circle of horn, 
 the circle being broken at the posterior part of the foot, and the piece 
 of wall which might have completed the circle is sharplj' bent on itself 
 and caused to run in practically the opposite direction. When this 
 arrangement is considered, it is easy to see the advantages gained. 
 The foot is not a rigid body, but a yielding one ; and it would be 
 difficult to understand how any lateral movement could take place had 
 the wall been a complete circle. From their position the bars afford 
 additional strength as weight-bearers, for they represent the wall 
 carried within the foot; they also prevent any rupture occurring 
 between the wall and foot-pad during the lateral movements of the 
 foot. 
 
 The hind-feet differ from the fore in shape, being more upright 
 and narrower. They do less work, for they arc exposed to less 
 concussion, while, from the natural attitude of the horse at rest 
 (p. 648), they are alternately relieved of weight. 
 
THE FOOT 
 
 7*3 
 
 On the inside of the hoof-wall a very complex arrangement 
 presents itself. At the upper edge, corresponding to the coronet, 
 is a deep semicircular groove, in which is lodged the thick welt of 
 tissue previously described as the wall-secreting substance. Covering 
 the entire surface of this groove are innumerable pin-point holes, 
 into which the papillse projecting from the ' substance ' are lodged. 
 The jthickness 1 of : the wall at any one place corresponds to the size 
 of the coronary substance; the wider it is, the larger the area it 
 alJOTdFto the horn-secreting membrane covering it. There are very 
 marked'differences in the thickness of the wall. In some horses it is 
 very thick, in others, especially in ' white ' feet, it is extremely thin. 
 
 Fig. 242. — The Hoof seen from its^Ground Surface. 
 
 1, 2, 3, 8, The foot-pad, 2 being the cleft; 4, the wall; 5, its inflection at the 
 heels to form 6, the 'bars'; 7, the white line; 9, the sole. The 'flakes 
 of sole present in an unmutilated foot are seen. 
 
 This is one reason why white feet are objected to. Feet possessing 
 a thin wall grow very slowly, and are always a trouble in shoeing; 
 "sometimes it is impossible to obtain a fresh hold for the nail without 
 driving the latter high, an objectionable practice which ruins the 
 wall for subsequent shoeing. Slowly growing feet are at once recog- 
 nised by the numerous nail-holes they contain. The effect of driving 
 a nail into the wall is to cause the horn below the ' clench ' to become 
 dry and brittle, owing to the loss of moisture occasioned by the horn 
 tubes being cut across. 
 
 Horn Laminae. — On the inside of the wall of the hoof a number of 
 leaves of horn are. found arranged side by side, running all round the 
 
7M a manual of veterinary physiology 
 
 foot from heel to heel, and extending from coronet to ground surface. 
 It is easy to see that they correspond in size, direction, and length 
 to the vascular or sensitive laminas previously described, and, like 
 them, they possess secondary horn lamincs or lamella (Fig. 239, 2). 
 These insensitive and sensitive laminae fit into each other by the 
 process of dovetailing, which results in extraordinary strength being 
 obtained. So powerful is the union that, in endeavouring to separate 
 them, the vascular laminas will often tear from the pedal bone rather 
 than rupture the dovetail. In this way the most intimate and perfect 
 union between the vascular and the hoof wall is brought about, and, 
 in addition, other advantages are obtained which will be dealt with 
 shortly. The horn laminas, as their name implies, are composed of 
 horn, but the secondary laminas which invest them are composed of 
 cells which are a something between horn and epithelium — i.e., the 
 cells have not undergone a true horny conversion, but remain proto- 
 plasmic in nature ; this is recognised from the fact that they readily 
 stain with carmine, whereas horn does not. It will be remembered 
 that though the sensitive and insensitive laminas dovetail yet ihey 
 are never in actual contact, for between them are the lamellae, both 
 sensitive and insensitive. The sensitive lamellae look towards the 
 horn wall, the insensitive lamellae point in the direction of the pedal 
 bone, so that the dovetailing of the lamellae is not necessarily 
 effected at right angles to the primary laminae (Fig. 238). The 
 origin of the horn laminae has been considered at p. 708. 
 
 Horn laminas are found on the bars, for these, though situated 
 under the foot, are a part of the weight-bearing wall, and possess all 
 its essential structural elements. 
 
 The Sole of every normal foot is concave inferiorly (Figs. 242, 243), 
 that of the hind-feet being more concave than that of the fore. This 
 concavity agrees with the concavity of the solar surface of the pedal 
 bone, which in itself is ample evidence that the general surface of the 
 sole is not intended to bear weight, though the portion in contact 
 with the wall is a weight -bearing surface (Fig. 244, 6) . Soles vary in 
 thickness, some being rigid and firm, others thin and yielding; the 
 sole cannot be too thick. Those shown in Figs. 242, 243, and 244, 
 are excellent specimens of a good sole. The growth of the sole 
 is peculiar; in exactly the same way as was noticed in the wall, the 
 papillae from the vascular sole fit into pin-point holes in the horn- 
 sole, and horn is developed around them. But here the resemblance 
 ends; while the horn of the wall is capable of growing to almost 
 any length, until, in fact, it curls at the toe like a ram's horn, the 
 horn of the sole can only grow a very short distance before the fibres 
 break off, and scales or flakes of horn are the result; these either 
 fall out or are pulled out. In other words, the foot determines for 
 itself how thick the sole shall be, and, without any assistance, the 
 fibres break off when the proper thickness has been attained, and 
 allow the part to drop out. This shelling out of the sole, which 
 can be seen in Fig 242, is advantageous in the shod foot, inasmuch 
 as the part, not being exposed to friction, cannot wear away. In 
 the unshod foot friction maintains the wall and bars at their proper 
 length (see p. 736, last sentence) so that no natural breaking off of 
 horn fibres occurs. 
 
 The union between the vascular and the insensitive sole is brought 
 about by the papillas on the surface of the former. The extra- 
 ordinary length and number of these can only be appreciated by 
 examining the parts under water. 
 The sole and wall are united in a distinctive minner, and the unipn 
 
IMe Foot 
 
 715 
 
 is indicated by a white line (Fig. 242, 7) which runs around the entire 
 junction of sole and wall. The white line represents the layer of the 
 
 Fig. 243. — A Vertical Section through the Foot, One-Ouarter of its 
 Distance from the Toe to the Bulbs of the Heel (Fig. 242, 8). 
 
 1, The pedal bone; 2, the horn wall; 3, the sensitive lamina?; 4, the sensitive 
 sole; 5, the, horn sole; 6, the junction between wall and sole; 7, the margin 
 of sole capable of bearing weight. 
 
 Fig 244. — A Vertical Section through the Foot, One-Half the Distance 
 
 BETWEEN THE TOE AND THE BULBS OF THE HEEL. 
 
 i, The lateral cartilage; 2, the pedal bone; 3, the sensitive lamina?; 4, anterior 
 portion of foot-pad; 5, the sole; 6, the weight-bearing portion of the sole. 
 
 wall next to the insensitive lamina? (Fig. 245, 6); within this come 
 the ends of the lamina? themselves, and between these and the 
 sole a layer of much softer yellowish plastic horn (Fig. 243, 6). 
 This cement substance is secreted by the four or five papilla? found 
 
?i 6 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 on the extremity of the sensitive laminae, shown diagrammatically 
 and microscopically in Fig. 246. The soft cement substance, besides 
 ensuring the union of wall and sole, also admits of slight yielding 
 of the sole, to which reference will be made later. A microscopical 
 examination of the union shows the horn laminae digitating with 
 the sole : the laminae are now wavy, and no longer in possession of 
 secondary laminae (Fig. 246, B). The horn between the laminae is 
 arranged in concentric layers, being formed by the four or five 
 secreting papillae on the ends of the sensitive laminae. 
 
 The Foot-pad, or ' frog,' as it is vulgarly known, is a pyramidal- 
 shaped piece of horn, accurately moulded over the plantar cushion, 
 
 Fig. 245. — Horizontal Section across the Foot, One Inch above and 
 Parallel to the Ground. 
 
 1, The wall; 2, its inflections at the heels; 3, the bars and convexity of the sole 
 caught in the section; 4, part of the pedal bone; 5, portion of the plantar 
 cushion; 6, the white inner layer of horn between the sensitive lamin* and 
 the outer wall. 
 
 and filling up the space left by the inflection of the wall at the 
 posterior part of the foot (Fig. 242, 1, 2, 3, 8). In the foot-pad we 
 meet for the first time a peculiar soft elastic horn, possessing some- 
 thing of the character and appearance of india-rubber. The horn 
 fibres of this structure are wavy, arranged in strata which run at 
 right angles to each other, and confer on the part its fibrous character 
 and elasticity. The horn of the pad contains much more moisture than 
 ^that of any other part of the foot, and it is the moisture which, aided 
 by the secretion of the sweat glands of the plantar cushion, confers 
 on it its peculiar soft pliable condition. The foot-pad grows from 
 the vascular membrane covering the plantar cushion. The over- 
 growth of horn is provided against by a method which is a combina- 
 
THE FOOT 
 
 717 
 
 tion of those found in the wall and sole— viz., it is cast off after 
 growing to a certain thickness, while the part next the ground is worn 
 away by friction. In consequence, owing to its rubber-like nature 
 rags of horn along the edges of the foot-pad are a common and natural 
 condition. » 
 
 A transverse section of the foot-pad at its posterior part resembles 
 in appearance the letter W. The two lower points are the sides of 
 the pad; the space between them is the so-called ' cleft ' or central 
 depression (see Fig. 247, 3). Above the cleft on that side of the pad 
 next to the sensitive foot is a projection of horn known as the 
 ' frog-stay, or ' peak.' 
 
 The function which A. 
 
 has been assigned to 
 this peak is to prevent 
 the parts becoming 
 displaced ; but its posi- 
 tion, shape, and con- 
 nection suggest that 
 it acts the part of a 
 wedge, being forced 
 upwards under pres- 
 sure when the foot 
 comes to the ground, 
 and it may thus exert 
 a central pressure on 
 the plantar cushion 
 and assist in the ex- 
 pansion of the foot. 
 But it appears, how- 
 ever, to be more valu- 
 able as a means of 
 stimulating the nerve - 
 endings in the plantar 
 cushion, which are es- 
 pecially abundant in 
 this region, and of so 
 acting the part of a 
 touch organ. X- 
 
 The Structure of 
 Horn. — The horn of 
 the foot consists of 
 epithelial cells which 
 have undergone com- 
 pression and keratini- 
 sation, by which latter 
 
 Fig. 246.- 
 
 -The Junction of the Horn Tissues 
 of the Wall and Sole. 
 
 A, Diagram of the wall, horn lamina?, and sole, 
 seen vertically. Between each horn lamina may 
 be seen the foramina, into which the papilla? 
 on the terminal end of the vascular lamina? fit. 
 Note the rounded termination of the horn lamina?. 
 
 B, Microscopical appearance of a horizontal section 
 of the junction of wall and sole ; the horn lamina? 
 are wavy, devoid of secondary lamina?, and be- 
 tween each may be seen the papilla? in section. 
 On the right of the figure the horn tubes of 
 the sole are shown. 
 
 process they become 
 hard and tough. It is possible to have horn in the foot which is 
 not keratinised, and the two kinds are very readily distinguished by 
 the process of staining. The double stain picro-carmine has a selec- 
 tive affinity for each kind of horny tissue ; the carmine picks out the 
 protoplasmic and non-corneous cells and stains them red, whilst 
 the picric acid stains a yellow colour all tissue which has undergone 
 the process of keratinisation. By means of this stain it is easy to 
 determine the character of the horn under examination. 
 
 The ultimate horn cell is a very thin, spindle-shaped, oblong, or 
 irregular body (Fig. 248), containing granular matter, a nucleus, and 
 frequently pigment. In all cases the cells are united at their edges 
 
71* 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 and sides by a cement substance. By horn being acted upon by 
 caustic alkalies, the cells are in the first instance rendered clear; 
 they then gradually dissolve, are converted into a gelatinous mass, 
 and finally disappear. The highly alkaline nature of decomposing 
 urine, owing to the presence of free ammonia, being borne in mind, 
 
 Fig. 247- — Vertical Section through the Foot at a PointjTwo-Thirds 
 the Distance from the Toe to the Bulbs of the Heel. 
 
 I, The lateral cartilages; 2, heel of the pedal bone; 3, the foot-pad; 4, the sensitive 
 lamina?; 5, the ' bars.' 
 
 F~ig. 248. — Horizontal Section of the Horn of the Wall, Highly 
 Magnified. 
 
 Horn tube, a canal containing cellular elements; b, the tubular horn — that is, 
 the horn secreted from the papilla?, forming an oval or circular nest of cells 
 around the canal; c, the intertubular horn. 
 
 the practical application of this fact in the care and management of 
 the feet is very obvious. 
 
 if a portion of horn be examined microscopically, it is found 
 that the compressed epithelial structure is tunnelled in such a way 
 as to form canals or tubes, or, at any rate, to form a structure 
 which is tube-like in nature (Fig. 248). These tubes exist wherever 
 the growing surface is invested with papilla?, so that where the 
 
THE FOOT 
 
 719 
 
 papillae are numerous the tubes are numerous, where they are absent 
 the tubes are absent. The only horny stru ctures not secreted from a 
 Pa pillated surface are the horn larmrT^ ancT consequently inTnese 
 there arenohorn tubes ; everywhere else the horn is found to possess 
 a more or less tubulaFstructure. The method of tube formation is 
 simple. The papillae growing from the various secreting surfaces are 
 lodged in canals in the horn. As the horn grows down from the 
 surface which secretes it, the canal enclosing the papilla gradually 
 slides off, but throughout the length of the horn a tubular appearance 
 
 
 b a, b, c, The outer, middle, and 
 inner portions of the wall, 
 showing the canal system with 
 the tubular and intertubular 
 horn; d, the horn laminae 
 bearing on their sides the 
 lamella?, shown black: there 
 are sometimes a few short 
 lamina? to be seen — one is 
 shown in the figure; e, the 
 sub'aminal tissue, from which 
 the sensitive laminae may be 
 seen dovetailed between the 
 horn laminas, and from the 
 sides of which the sensitive 
 lamellae grow. 
 
 Fig. 249.- 
 
 -Horizontal Section of the Horn and Vascular Wall 
 of the Horse's Foot. Low Magnification. 
 
 indicates where the papilla was at one time lodged, and the cells of 
 these tubes, from their reaction with carmine, prove themselves to be 
 different to true horny structure. 
 
 The horn which is secreted in the foot is therefore formed (1) from I 
 papillae found on the secreting surface, and (2) from the spaces 
 between the papillae. The papillae form tubular horn, the spaces ' 
 between them form intertubular horn (Fig. 248, b and c), and this 
 is arranged in an oval or concentric manner around the canals (Figs. 
 249 and 250), the cells composing it being so placed" that their edges 
 are towards the papillae. There is, however, a layer of cells which 
 actually forms the wall of the canal, and these are arranged with 
 their sides next it; or, to put it another way, they stand on their 
 
720 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 edges. In the deep layer of the wall the papillse produce a much 
 greater secretion, and here the circular or oval masses of cells invest- 
 ing the canal are more prominent; and, further, unlike those in the 
 anterior and middle parts of the wall, they need no reagent to 
 demonstrate their cellular nature (Fig. 249, c) . lis. section of wall be 
 stained with picro-carmine, only the canal contents of the external 
 and middle wall stain with carmine; all the remaining substance 
 takes up the picric acid. In the deep wall this is different; here the 
 whole of the cellular material secreted by the papillas is stained 
 red, showing that these cells are protoplasmic rather than horny, 
 and partly accounting for the fact that this deep horn is always 
 softer than the middle or external horn of the wall. 
 
 If a vertical section of horn be made, the canals are now seen 
 divided in their length (Fig. 250). Though spoken of as canals or 
 tubes, they are really not empty, but throughout their entire length 
 contain cells which are protoplasmic in nature. These, owing to the 
 manner in which they reflect light, give to the part a beaded ap- 
 pearance. The cells contained within the canal are secreted by the 
 apex of the papilla ; they do not fill up the entire lumen of the canal. 
 
 Fig. 250. — Microscopical Structure of Horn : Vertical Section of the 
 Wall. Low Magnification (after Lungwitz). 
 
 Note the different size of canals ; those on the right are nearest the laminae ; those 
 on the left are towards the outside wall ; they are smaller and more numerous 
 than those deeper seated; d is a portion of a horn lamina. 
 
 The use of the canal system in horn is for the purpose of irrigation ; 
 the horn must be supplied with moisture; most of this is obtained 
 through these imperfect canals, the soft protoplasmic canal-wall 
 readily admitting transudation. It is not intended to assert that 
 anything like a fluid is circulating along- the tubes, but moisture 
 certainly does find its way down, and is readily imbibed by the 
 surrounding cells. There is no doubt that in the intertubular horn 
 moisture also passes from the secreting surface from cell to cell, and 
 in this way is transmitted throughout the length of the foot. 
 Finally, it can be absorbed from without. Constant evaporation is 
 taking place from the foot, and the loss is made good in the manner 
 indicated. If the invisible moisture which is always escaping from 
 the foot be hindered in its evaporation, the horn becomes sodden, 
 crumbles away, and resembles a grey cheese-like mass. This ex- 
 periment can be readily performed on the sole and foot-pad by 
 accurately moulding to their surface a sheet of gutta-percha and 
 leaving it there. The practical lesson is obvious: no impervious 
 material should be applied to the foot as a protection, or, if used, 
 it should be ventilated. 
 
THE FOOT ?2I 
 
 Use of the Moisture in Horn. — The amount of moisture contained 
 in horn is something considerable, and the rate at which it evaporates 
 is remarkable.* If parings of the foot-pad be enclosed in a bottle, in a 
 short time the interior will become bedewed with moisture. The 
 use of the moisture is to maintain the elasticity of the foot, and to 
 prevent the part from becoming brittle. The agents whose function 
 it is to prevent the too rapid evaporation of moisture from the wall 
 are the thin, varnish-like layer which covers the hoof, and the 
 natural hardness of the external fibres of the wall. Horn con- 
 taining but little moisture is in an abnormal condition; it is rigid 
 and brittle; nails driven into the part cause it to crack, and the 
 elasticity, on which the natural shape and usefulness of the foot so 
 largely depends, becomes impaired, or even destroyed. A museum 
 specimen of a foot illustrates these facts very clearly; in its dried 
 condition it is so brittle that, if dropped, it will frequently fracture 
 like a piece of glass; but if such a foot be placed in water for a few 
 days, it comes out as fresh and elastic as though it had just been 
 removed from the body. All that' the horn has done is to imbibe 
 water, and the previously brittle substance now becomes yielding 
 and elastic. The entire_p_hysiology of the horse's foot is centred 
 around this question of the moisture contained in horn. The pres- 
 ence ofrmoisture confers elasticity, and elasticity of the hoof prevents 
 its fracture under the pounding effects of concussion during work. 
 
 Chemistry of Horn. — An analysis of the horn of the foot has given 
 the writer the following results :-f 
 
 Wall. Sole. Foot-Pad. 
 
 Water - 24-735 37-065 42-54 
 
 Organic matter 74'74° 62-600 57 - 27 
 
 Salts 0-525 0-335 o - i9 
 
 100-000 100-000 100-00 
 
 The pad contains the largest amount of moisture, and the wall the 
 least. The salts are small in amount, and consist principally of 
 those of sodium, magnesium, iron, and silica, in the form of chlorides, 
 sulphates, and phosphates. Keratin, a substance which replaces 
 the protoplasm originally existing in the cells, is a protein-like body 
 found in hair, nails, and even, in a modified form, in the nervous 
 system; it consists of carbon 51-41, hydrogen 6-96, nitrogen 17-46, 
 oxygen 19-49, and sulphur 4-23, per cent. The sulphur is loosely 
 combined, and it is this, in combination with hydrogen, which causes 
 the offensive odour which arises from diseased feet, sulphuretted 
 hydrogen and mercaptan being formed. Keratin is a very insoluble 
 substance, but is dissolved by strong or by boiling acids and by 
 alkalies. With sulphuric acid it yields leucine and tyrosine, and on 
 incineration gives off the odour peculiar to burnt horn, together 
 with a gas (cyanogen ?) which burns with a peach-coloured flame. 
 
 Provisions for Elasticity and Toughness. — From what has 
 previously been said, it can be seen that it is the wall of the foot 
 
 * James Clark (' Observations on the Shoeing of Horses ') described the 
 moisture in the foot in 1782. He speaks of the insensible perspiration 
 exuding, and states that if a newly-pared piece of frog be held up to the 
 light of the sun, vapour may be seen arising from it. 
 
 •(• ' Chemistry of the Hoof of the Horse,' Veterinary Journal, vol. xxv., 
 p. 313, 1887, 
 
 46 
 
72J A MANUAL OF VETERINARY PHYSIOLOGY 
 
 which supports the horse's weight. On examining the wall, we 
 find iFis thickest at the toe7 thinner at the quarters, and thinnest 
 at the heels (Fig. 253); it is thickest at the toe owing to the 
 functions performed by this part, leading to excessive wear 
 and tear. As the pad and posterior part of the foot are the first 
 to make contact with the ground (at any rate, in all fast paces), 
 so the toe is the last part to leave it, the final propulsion to the 
 body being given by it, as we have seen in studying locomotion. 
 The object of the wall becoming thin towards the posterior part 
 of the foot is to allow of the elastic movement which has yet 
 to be described. Two physical conditions have to be pro- 
 vided for in the wall — namely, elasticity-gj the posterior part 
 and toughness of the anterior- portion. The first is furnished by 
 the wall being" thinner at the heels than elsewhere; but besides 
 being thinner, the wall of the heel contains more moisture than 
 the wall of the toe, and this moisture insures its elasticity. 
 The younger the horn — viz., the nearer to the coronet, at which 
 it exists — the more moisture it contains; the farther away from 
 the coronet, the less moisture it possesses, and the tougher and 
 more resisting the horn. Toughness of the anterior part must 
 now be considered. 
 
 The wall grows evenly from the coronet all the way round; 
 if it grows -| inch in the month at the toe, it grows the same 
 length at the quarters, and the same at the heels. The anterior 
 part of the wall is longer than the posterior, therefore the anterior 
 is tougher than the posterior, for the reason that the horn is 
 much older at the extremity of the toe than it is at the heel, 
 and being farther away from the coronet, it contains less moisture. 
 The wall at the heel is some months younger than that at the 
 toe; it is thinner and contains more moisture, therefore it is 
 more elastic, but not so tough. The age of the wall is an impor- 
 tant factor in the wear of the foot. If it takes from nine'to twelve 
 months for it to grow from the coronet to the toe, the piece 
 of wall at/', Fig. 251, is, say, twelve months old, whilst that at 
 a' is less than six months old. The horn of the quarter is older 
 than the horn of the heel, and the horn of the toe older than 
 that of the quarter. This provision admits in the unshod foot 
 of considerable friction occurring at the toe without producing 
 undue wear, for the part is hard and tough, while the younger 
 and moister horn at the posterior part of the foot allows of 
 elasticity. In this way the ground surface of the foot is provided 
 with the hardest horn where friction is greatest. In theory, no 
 fraction of an inch of the ground surface of the foot, from toe to 
 heel, is of quite the same age. 
 
 The toe of the wall appears to grow faster than either the 
 quarters or the heels, but this is more imaginary than real; it 
 
THE FOOT 
 
 723 
 
 is the tendency of the foot to grow forward as well as downward 
 which produces the illusion. That the foot grows forward may 
 readily be determined by experiment, for if a cut be made in the 
 wall at the coronet, say an inch or so from the heels, it will in 
 course of time be carried some considerable distance towards the 
 toe; the exact distance can be determined by observing the 
 obliquity of the horn fibres. 
 
 How the Weight is carried by the Foot.— It is universally 
 recognised that the weight of the body is supported by the 
 union of the insensitive with the sensitive laminae. That the 
 enormous weight of the horse's body should be carried upon — 
 or, rather, slung upon — thin delicate strips of sensitive material 
 on the one hand, and correspondingly delicate strips of horn 
 
 ■H 
 
 Fig. 251. — Diagram illustrating the Age of the Wall. 
 
 a, b, c, d, n, f, are arcs drawn round the hoof parallel to the coronet ; in this way 
 it is ascertained that the age of the wall at a is the same as that of the heel at 
 a' ; the age of the wall at d corresponds to the age of the quarter at d'. 
 Every portion of the ground surface of the wall is of a different age, being 
 oldest and hardest at /, /', and youngest and most elastic at a, a'. 
 
 on the other, is perhaps the most remarkable feature in the 
 physiology of the foot. This union is so firm that it is a matter 
 of extreme difficulty to separate the two surfaces, even by 
 mechanical means. In a single foot the weight is carried on 600 
 or more primary laminae, assisted by 72,000 or more secondary 
 laminae. Those laminae situated at the anterior part of the foot 
 are exposed to more strain than those posteriorly placed, for 
 the reason that, during progression, the final propulsion of the 
 body comes entirely on* them; they are also longer, and have 
 no plantar cushion or foot-pad to assist them, as the shorter, 
 posteriorly placed laminae have. The latter have their strength 
 increased by the direction in which the weight of the body 
 comes upon them. Instead of bearing it on the length of the 
 
724 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 laminae, as at the toe, they carry it on the side in such a manner 
 that the work of one lamina at the toe is shared by several at 
 the quarter. 
 
 It will be remembered that the lamina at the anterior and 
 on part of the lateral face of the foot are mainly attached to bone, 
 but on the remaining lateral face and at the posterior part of 
 the foot they are attached to stout cartilage; if a line be drawn 
 through the foot at the junction of these structures (Fig. 252), this 
 feature will be demonstrated; part of the laminal attachment 
 is cartilaginous and part osseous, the cartilaginous portion being 
 situated where elasticity is required — viz., on the posterior face of 
 the wall. The line between hair and hoof shown in the figure 
 indicates that part of the lateral cartilage is within and part 
 
 Fig. 252. — The Rigid and Yielding Portion of the Foot. 
 
 1, i, The line of the lateral cartilage, ascertained by passing pins through the 
 laminae into the junction of the cartilage with the bone; 2, 2, the portion 
 of the vascular wall, coronary body, and skin covering the lateral cartilage. 
 The part of the lateral cartilage extending above the hoof has its upper 
 edge outlined, but the dotted line is too straight (see Fig. 232). 
 
 outside the hoof. One function of the laterarcartilages~of the 
 foot is to afford a movable wall-attachment to the sensitive 
 laminae, and thereby to enable them to be carried outwards 
 during expansion. A knowledge of the relation of the posterior 
 laminae to the lateral cartilage explains the cause of lameness in 
 ' side-bone ' — viz., the squeezing of the sensitive structures be- 
 tween the wall on the one hand and the ossifying cartilage on the 
 other. 
 
 The folding up of the horny and vascular leaves in the foot, 
 in the manner previously described, has another function besides 
 that of merely supporting the weight and rendering the union 
 firm. Reference has previously been made to the remarkably 
 small size of the horse's foot in proportion to the size of his body. 
 
THE FOOT 725 
 
 In a comparison of the horse's foot with the human foot, the ad- 
 vantage, so far as size is concerned, in the majority of cases lies 
 on the side of the biped. The most interesting fact which 
 physiology has to demonstrate is that, though the foot presents a 
 small surface, in reality it encloses a vast area, due to the 
 anatomical arrangement of the laminae. It is clear that by the 
 process of folding up material within, the surface of the foot is 
 considerably increased. In other words, by this arrangement 
 the foot has been kept within small proportions without affecting 
 its strength. A book, say of 600 pages, may, by placing one 
 leaf on the other, be made to occupy a bulk represented by a 
 few inches; but if each page be laid out separately on the ground, 
 and made to touch the others, the surface covered will be con- 
 siderable. This is exactly what occurs in the foot ; the insensitive 
 and sensitive leaves by their singular arrangement increase the 
 surface of the foot, and yet keep it within reasonable limits. 
 Bracy Clark, one hundred years ago, was the first to recognise 
 this provision, and had a calculation made as to the increased 
 surface afforded, which was considered to be equal to i| square 
 feet. Moeller* has estimated that it is equivalent to 8 square 
 feet, whilst Gader's estimatef is iof square feet. For safety 
 Moeller's number is adopted. The bearing surface afforded by 
 each foot is equivalent to 8 square feet, giving a total area of 
 32 square feet. It is evident that, as feet vary greatly in size, 
 this surface must accordingly be greater or less. 
 
 The physiological function of the leaves of the foot is demon- 
 strated by pathological observation. Inflammation of the 
 laminae, apart from septic or intestinal poisons, occurs either 
 through severe work or through an animal standing too long 
 in one position; in either case the parts get strained. The prac- 
 tical value of exercising horses which from any cause have to 
 stand for a length of time is well known; exercise overcomes the 
 tendency of the laminae to congestion from continual strain, and 
 the feet not only become cool, but the animal may continue 
 standing for a considerable time if exercised daily. The treat- 
 ment of laminitis by exercise possesses a sound physiological 
 basis.J If any doubt exists as to the function of the laminae in 
 supporting the weight of the horse's body, it is only necessary 
 to look at the processes which occur in them as the result of 
 disease. Laminitis is often attended by separation of the 
 laminae, when, the horse's weight being no longer properly sup- 
 ported, the pedal bone under the influence of the body weight is 
 actually forced through the sole of the foot. 
 
 * Op. cit. 
 
 t Goubaux and Barrier, ' Exterior of the Horse ' (translation). 
 
 X This treatment has been known for at least 1,000 years. 
 
? 2 6 A MANUAL OP VETERINARY PHYSIOLOGY 
 
 The Use of the Bars. — As we have previously mentioned, the 
 portion of the wall known as the ' bars ' runs forward under 
 the foot instead of completing the circle of the wall. The 
 object of this non-completion of the ring the wall originally 
 gave promise of forming, is to allow of expansion of the foot by 
 making room for the elastic posterior foot — viz., the plantar 
 cushion -and foot-pad. The explanation why the wall turns in 
 instead of ending abruptly is that it thus affords a solid bearing 
 to the posterior part of the foot, gives additional strengt'h, and 
 secures a more intimate union with the sole. The bars, being 
 part of the wall, are intended to bear weight, and in conse- 
 quence in the foot of the wild horse and zebra they present the 
 most extraordinary development. 
 
 The Use of the Sole is quite clear: it is to afford protection to 
 the sensitive parts above. Its normally concave shape (Figs. 243 
 and 244) proves that it is not intended to bear on the ground 
 over its general surface, and the acute lameness which results 
 from a stone in the foot gives further proof, if any were required, 
 of its indifferent weight-supporting properties; that margin, 
 however, in contact with the wall can bear weight, as there is 
 no sensitive part immediately above it (see Fig. 244, 6). Under 
 the influence of the body weight the sole becomes slightly natter, 
 especially that portion of it situated posteriorly in the horns of 
 the crescent (see p. 716). When the expansion of the foot is 
 studied, the object of this flattening will be more apparent. 
 \ The Use of the Foot-Pad. — This is one of the chief anti-con- 
 cussion mechanisms in the foot; it exists to prevent jar, and it 
 does so by receiving, in conjunction with the posterior wall, the 
 impact of the foot coming to the ground; the shock is imparted 
 to the plantar cushion, and through the lateral cartilages to the 
 Wall of the foot, which bulges or, as it is termed, expands (see 
 Fig. 253). In breaking the jar (not only to the foot, but to the 
 whole limb), it is assisted by its elastic, rubber-like nature. 
 For its perfectly healthy condition the foot-pad needs contact 
 with the ground; it is strange that two structures situated side 
 by side — viz., the sole and the pad — should be so opposed in this 
 respect . If the foot-pad be kept off the ground, the part atrophies, 
 the heels contract, the foot is rendered smaller, and the pad be- 
 comes diseased. This wasted condition of the pad and narrow 
 foot may be remedied by pressure, but that pressure must be 
 ground pressure. It is possible by means of a bar-shoe to throw 
 considerable pressure on the pad and heels, but the foot still con- 
 tracts; it is only when the pad is bearing on the ground that it 
 continues in a healthy condition, and retains its normal size. 
 Foot-pad pressure is, therefore, one of the rules in shoeing if the 
 part is to be able to exercise its natural functions. In thinking 
 
THE FOOT 727 
 
 of the foot-pad as a buffer, sight must not be lost of the fact that 
 it affords protection to the joints and flexor tendon of the foot, 
 matters of vital importance. The position of these relative to 
 the foot-pad is seen in Fig. 233. 
 
 Use of the Lateral Cartilages.— Those functions of the lateral 
 cartilages which have already been referred to may be sum- 
 marised as follows : These structures form an elastic wall to the 
 
 Fig. 253. — Horizontal Section across the Structures of the Foot 
 Parallel to the Ground and Two Inches above it. 
 
 The plantar cushion, a, i, practically occupies half the foot, and is lodged between 
 2, 2, the lateral cartilages: these may be seen extending forward and attached 
 to the wing of the pedal bone at 6; they end posteriorly at 3, fusing with the 
 plantar cushion; 4, cut surface of perforans tendon; 5, cut surface of the 
 navicular bone; 7, the pedal bone; 8, the white line, extending around the 
 wall from heel to heel; 9, the wall: note the difference in thickness between 
 the toe and the heel; 10, the vascular laminae. 
 
 sensitive foot, and a surface of attachment for the vascular 
 laminae; they also admit of increase in width occurring at the 
 posterior part of the foot without destroying the union of the 
 two sets of leaves. Further, by their connection with the vascular 
 system of the foot, their elastic movements materially assist the 
 circulation. The_ primary us e of the.lateral cartilages is to. render 
 the int ernal foot elast ic, and to admit of its chan ges in shape 
 which occurs under thelnfluence of the weightof the body. The 
 
728 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 alteration in the shape of the foot is brought about by pressure 
 on the pad, which widens and in consequence presses on the bars, 
 The pressure received by the pad is also transmitted to the plantar 
 cushion, which likewise flattens and spreads under pressure. 
 Both of these force the cartilages slightly outwards. When the 
 posterior wall recoils, the cartilages are carried back to their 
 original positions . Should this elastic cartilage under pathological 
 conditions become converted into bone, its functions are destroyed 
 and lameness may occur. It has been demonstrated by the writer 
 that by surgical interference the hoof can be made permanently 
 wider, and thereby rendered capable of accommodating lateral 
 cartilages which have undergone an increase in size as the resrd>> 
 of ossification.* 
 
 Anti-Concussion Mechanism. — The special physiology of the 
 foot is a consideration of the factors whereby the parts are saved 
 from concussion, in spite of wear and tear, batter and jar. The 
 weight carried on each fore-foot when the horse is standing is 
 rather more than one-quarter the weight of the body; during 
 locomotion the -amount varies from half the weight in the trot 
 to the entire weight in certain stages of the canter and gallop. 
 The mechanisms which exist in the foot to save concussion are 
 intended not only for the protection of the foot, but also to save 
 the limb, and they may be tabulated as follows: 
 
 i. The yielding articulation in the pedal joint. 
 
 2. The increase in the width of the foot when the heels come 
 to the ground, known as expansion. 
 
 3. The elastic foot-pad. 
 
 • 4. The slight descent of the pedal bone, and with it the 
 sole. 
 
 Expansion is a term warranted by custom, though perhaps not 
 free from objection. It indicates the fact that when the weight 
 comes on the heels, the foot becomes wider opposite to them 
 (Fig. 254). The increase in the width of the foot is due to a 
 temporary alteration in the shape of certain of its structures. 
 As a matter of fact, an increase in the width is not the only 
 change which occurs; it can be shown that the heels at the 
 coronary edge sink closer to the ground, while the coronary edge 
 of the wall in line with the toe of the foot retracts, or travels 
 backwards and downwards (Fig. 255, A). In all fast paces, 
 when the foot comes to the ground, the posterior wall and foot- 
 pad first receive the weight. Under the influence of the body 
 weight the foot-pad, as we have seen, is compressed and becomes 
 wider; the plantar cushion with which it is closely in contact is 
 also compressed and becomes wider. The effect of this increase 
 
 * ' Operation for the Cure of Lameness arising from Side-Bone,' Veterin- 
 ary Journal, vol. xxv., p. 393, 1887. 
 
tup poot 
 
 729 
 
 in width is that the foot-pad presses on the bars, while the plantar 
 cushion presses on the cartilages, both of which, yielding laterally, 
 force outwards the wall at the heels (Figs. 254 and 255, B). When 
 the weight is taken off the foot, the heels return to their original 
 positions, and the foot becomes narrowed. The increase in width 
 which the foot undergoes is very small indeed; this is probably 
 the reason why for years its existence has been disputed, 
 especially in this country.* The employment of delicate 
 apparatus such as those used by Lungwitzt and by the 
 writer j (Fig. 256), and experiments 
 upon feet which have not been 
 mutilated in shoeing, have placed 
 the subject beyond all doubt. 
 
 The area over which the wall 
 expands can be seen in Fig. 255, 
 A; the shaded portion of the heel 
 represents the part which yields 
 laterally. At times expansion is 
 registered at the coronet, and little 
 or none on the ground surface, 
 but as a rule the amount ob- 
 tained at the coronet can also 
 be obtained near the ground. As 
 to the amount of expansion no 
 definite statement can be made; 
 it is small and is influenced by the 
 shape of the foot ; horses with low 
 heels and full, well-developed foot- 
 pads register a larger amount than 
 those in which the heels are high 
 and rigid. The measurements ob- 
 tained by the writer with very 
 delicate apparatus were smaller than those obtained in Germany 
 by Lungwitz. On an average 'there was obtained, by simply 
 lifting up one fore-foot, and so causing the horse to throw 
 double weight on the other limb, an expansion of ^l_ of an 
 inch for one-half of the foot, or J-g- of an inch total increase 
 
 * Nevertheless, Bracy Clark demonstrated it over one hundred years 
 ago. He says: 'The term " elasticity," however, by its exercise and 
 use will explain, like the principles of gravitation in the hands of the 
 astronomer, nearly everything that before was dark and obscure in the 
 arts of the foot.' W. C. Spooner (' Treatise on the Foot and Leg of 
 the Horse ') stated in 1840 that the expansion of the foot was equal to 
 T V of an inch. i f . 
 
 f The Journal of Comparative Pathology and Therapeutics, vol. iv., p. 3, 
 1891. 
 
 % ' The Apparatus Employed in Inquiring into the Physiology of the 
 Horse's Foot,' Veterinary Record, vol. iv., p. 263, 1891. 
 
 Fig. 254. — Diagram to illustrate 
 the Expansion of the Foot 
 (Lungwitz). 
 
 The unbroken outline illustrates the 
 shape of the foot at rest; the 
 dotted outline shows the portion 
 of the foot which has yielded 
 laterally under the influence of 
 the body weight. 
 
7^o 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 in width. Naturally, during locomotion a greater expansion 
 than this occurs. The question may be asked, What advantage 
 
 A B 
 
 Fig. 255. — Diagrams to show the Area over which the Wall expands, and 
 to illustrate the Retreat of the Anterior Coronary Edge of the 
 Hoof, and the Sinking of the Heels (Lungwitz). 
 
 A, The unbroken outline shows the shape of the foot with no weight on it; the 
 dotted outline illustrates the retreat of the coronary edge in front and sinking 
 of the heels. The shaded part illustrates the area which expands. 
 In this figure the hoof is looked at from above; the unbroken outline is the 
 coronary edge from heel to heel. The dotted line shows the change in shape 
 it undergoes under the influence of the weight of the body. 
 
 B 
 
 ^^^^^ 
 
 Fig. 256. — Apparatus for Measuring Expansion of the Foot. 
 
 The gauge is adjusted to different levels of the wall by means of a block; it 
 registers from -j-J-j- to ^ T inch. 
 
 can be gained by such a small increase in the width of the 
 foot ? Small as the increase is, it still makes all the difference 
 
THE FOOT 731 
 
 between a yielding and an unyielding block of horn being 
 brought to the ground; it 'gives' instead of offering re- 
 sistance, and this ' give ' is sufficient to prevent the hoof 
 from being fractured, while the pad which has largely caused 
 the expansion has acted as a buffer and assisted to destroy 
 concussion. 
 
 There is no point in the physiology of the foot which has given 
 rise to greater diversity of opinion than the question of ' expan- 
 sion ' ; modern investigations completely support the views of 
 the earliest observers. The retraction of the coronary edge 
 of the foot in front and the sinking behind are accompanied by 
 a tense condition of these parts which, since the days of Bracy 
 Clark, have been regarded in the light of an elastic ring or support 
 to the pedal joint. The tense condition is due to the change in 
 the shape of the coronary edge, but whether this is capable of 
 affording support is not evident. 
 
 In addition to the changes in the coronary edge of the foot 
 during the period of expansion, another condition is present — 
 namely, compression of the wall 
 under the influence of the body 
 weight, which produces a diminu- 
 tion in its height. This can be 
 roughly demonstrated in the follow- 
 ing manner: If a portion of the 
 wall at the heel be cut away so as 
 
 just to clear the shoe when the FlG - 257.— Experiment for As- 
 
 , .. ■ r , , -i •, -11 i r i CERTAINING THE COMPRESSION 
 
 latter is fitted, it will be found on 0F THE wall. 
 placing weight on the limb, by 
 
 lifting up the opposite fore-foot, that the wall has descended 
 sufficiently to touch the shoe. The experiment may be rendered 
 less free from objection by removing a piece of the wall, as in 
 Fig. 257. If this is made sufficiently large to admit a penny 
 when the foot is off the ground, the coin cannot be introduced 
 into the slot when the weight is placed on the foot. The only 
 explanation which can be afforded is that given above — viz., the 
 wall has undergone sufficient compression to allow the part which 
 was originally clear of the shoe to come in contact with it, and 
 to do this it must have diminished in height. 
 
 The Descent of the Pedal Bone is the last factor employed in 
 saving concussion. The existence of this has been as strenuously 
 denied as the expansion of the wall, but there is, however, no 
 difficulty in "demonstrating it ; the value of such a movement is 
 undoubted. Concussion to the sensitive foot is prevented by 
 a slight up-and-down play between the sublaminal tissue and 
 the pedal bone; as the weight comes on to the foot the pedal 
 bone descends slightly, to rise again when the weight is taken 
 
732 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 off the limb.* As the pedal bone descends, the sole on which it 
 is resting also descends slightly and comes nearer to the ground 
 (p. 726) ; this is one reason why the sole is concave instead of 
 flat. The soft horn uniting the sole and wall (p. 716) specially 
 provides for the slight descent of the sole. The descent of the 
 internal foot saves concussion, in the same way that it is easier 
 to catch a cricket-ball with a retreating movement of the hand 
 than by rigid opposition. Its movement also facilitates the 
 circulation. The descent of the pedal bone is a most important 
 physiological factor, and one of the safeguards of the sensitive 
 foot. 
 
 Vascular Mechanism. — Situated as the foot is farthest from 
 the heart, it is natural to inquire how it is that the blood is able 
 to circulate through it so thoroughly, and whether some means 
 are at hand for assisting the force of the heart in facilitating the 
 circulation. Though the contraction of the left ventricle is 
 sufficient under ordinary circumstances to bring the blood back 
 to the right side of the heart, it is doubtful whether it would 
 be wholly sufficient to empty the foot of blood and keep the 
 considerable plexus of veins full. This plexus is shown in 
 Fig. 240 (p. 711), which is a reproduction from a photograph of 
 a corrosion injection. f The figure conveys very accurately an 
 idea of the remarkable venous arrangement of the foot. The 
 venous circulation is assisted by two movements in the foot — ■ 
 viz., the expansion and recoil of the outer foot, and the descent 
 and elevation of the inner foot. There is no difficulty in seeing 
 the movement imparted by locomotion to a column of fluid 
 circulating in these parts, for if a plantar vein be divided and 
 the horse made to walk, every time the foot comes to the ground 
 the blood spurts out from the vein as if from an artery; when 
 the foot is taken off the ground, the stream of blood becomes greatly 
 reduced. A perfect pumping action is in this way produced. 
 The mechanism can also be demonstrated on the dead limb 
 by placing a manometer tube filled with water in each plantar- 
 vein, and then pressing downward on the limb, thus roughly 
 imitating the weight coming on to the leg. The water rises in 
 the manometer tubes with every compression of the foot, and 
 falls during the period of no pressure, a period corresponding 
 in the living animal to that during which the foot is off the 
 ground. 
 
 * Eighty years ago W. C. Spooner wrote: ' On trying the elasticity of 
 the membrane which connects the laminas to the bone, I found, somewhat 
 to my surprise, that it yielded considerably when pulled downwards and 
 very slightly when pulled upwards ' (op. cit.). 
 
 t The figure appeared in an article by Dr. C. Storch, of Vienna, on 
 ' The Venous System of the Horse's Foot,' Oesterreichischen Monatsschrift 
 fur Thierheilkunde, 1893. 
 
THE FOOT 733 
 
 We must accept, therefore, as a proved fact that the venous 
 circulation is largely facilitated by the expansion and contraction 
 of the posterior part of the foot; during expansion the blood 
 is being driven upwards, and during recoil it is aspirated by 
 the veins. Indeed, so perfect is this mechanism that there are 
 no valves in the veins of the foot, and none are found nearer 
 than the middle of the pastern. To assist the circulation to the 
 utmost, the large venous trunks at the postero-lateral part of 
 the foot are in close connection with the lateral cartilages, and 
 some of the vessels even pass through their substance. 
 
 A summar y of the physiological features counteracting con- 
 cussion and facilitating circulation may be stated as follows: 
 \ When the weight comes on to the, foot, if is received by a 
 yielding foot articulation, an elastic wall, an india-rubber-like 
 pad, and through these by the plantar cushion. The elastic 
 posterior wall is pressed outwards by the compressed pad and 
 plantar cushion, and it expands slightly from the ground surface 
 to the coronet. At the moment of expansion the bulbs of the 
 heel of the foot at the coronary edge sink under the body weight 
 and come nearer the ground, and as a result of this the anterior 
 coronary edge retracts. The pedal bone descends slightly 
 through its elastic connection with the sensitive laminae, and 
 presses the sole down with it, while the wall of the foot slightly 
 diminishes in height owing to the compression to which it is 
 subjected. Under these conditions the blood-pressure in the 
 veins of the foot rises, and the vessels are emptied. When the 
 weight is removed from the foot the bloodvessels fill, the pad and 
 posterior walls recoil, the bulbs of the heel rise, and the foot 
 becomes narrower from side to side ; at the same time the anterior 
 edge of the coronet goes forward, and the pedal bone and sole 
 ascend. 
 
 The Nervous Supply of- the foot was mainly worked out 
 by Mettam.* Tactile sensibility had long been known to 
 exist, but the nature of the end organs concerned was unknown 
 He has shown that these partake of the character of Pacinian 
 corpuscles, arranged sometimes singly, sometimes in groups, and 
 with other positional modifications, which has enabled four 
 distinct methods of distribution in the plantar cushion to be 
 recognised. Pacinian corpuscles have also been recognised in 
 the vascular laminae and in the skin of the coronet and heel. 
 Valuable as this nerve-supply is in keeping the animal informed 
 of the nature of the ground travelled over, yet it is not essential 
 to progress or safety. All sensory impulses may be cut off from 
 the feet without interfering with the safety of the animal (p. 615). 
 The inner foot is acutely sensitive to touch, especially the vascular 
 
 * Op cit. 
 
734 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 tunics. The plantar cushion is not nearly so sensitive^, although 
 Mettam has shown that it is liberally supplied with touch organs. 
 
 Physiological Shoeing. — It is impossible to conclude this chapter 
 on the foot without some mention of what may be termed ' physio- 
 logical shoeing.' By bearing in mind the functions of the various 
 parts of the foot, we can confine the evils connected with shoeing 
 within comparatively narrow limits. The following rules form the 
 basis of physiological shoeing : 
 
 i. The reduction of the wall to its proper proportions, such as 
 would have occurred through friction had no shoe been worn. 
 
 2. Fitting the shoe accurately to the outline of the foot, and not 
 rasping away the exterior of the crust to fit the shoe , since this not only 
 renders the horn brittle, but caiises much loss of bearing surface. 
 
 3. The exterior of the wall should be left intact. The practice of 
 rasping the wall for the sake of appearance destroys the horn, and 
 allows of such considerable evaporation from the surface of the foot 
 that the part becomes brittle. 
 
 4. The sole should not be touched with the knife; it cannot be 
 too thick, as it is there for the purpose of protection. 
 
 5. The bars should not be cut away; they are part of the wall, 
 and intended to carry weight. The shoe should rest on them. 
 
 6. The foot-pad should not be cut, but left to attain its full growth. 
 No foot-pad can perform its functions unless on a level with the 
 ground surface of the shoe. 
 
 7. The pattern of shoe is immaterial so long as it has a true and 
 level bearing, and rests on both the wall and bars. The simpler 
 the pattern of shoe, the better. Its weight should be regulated by 
 the class of horse, and calculated to last a month. 
 
 8. A shoe can be fitted cold, but it is not so satisfactory as hot 
 fitting, which insures perfect adaptation between the shoe and the 
 foot. The heat employed in fitting should not scorch the sole ; the 
 shoe should be at a ' black ' heat, and the wall must not be roasted. 
 
 9. The shoe should be secured with as few nails as its size admits. 
 No more nails should be used than are absolutely necessary, as 
 nails destroy the horn; further, they should not be driven higher 
 than needful, for high nailing is ruinous to feet. 
 
 10. The ' clips ' should be laid evenly, and not driven violently 
 into the wall. 
 
 Such, briefly, are the conditions which fulfil physiological shoeing. 
 The vicious and senseless practice of cutting away the horn of the 
 foot-pad and sole, and of thinning the wall by rasping it, are the 
 abuses of shoeing, and are capable of control. The real physiological 
 evil in shoeing, and one which cannot be. remedied, is the driving 
 of nails into the foot (see p. 735, Evulsion). 
 
 Pathological. 
 
 The diseases affecting the foot of the horse are numerous, and as 
 a rule serious; they may be connected with the vascular supply, 
 with the bony foundation, with the cartilages and structures around 
 the coronet, with the joint, or with the external cover. Laminitis and 
 Navicular Disease have been previously touched upon in their physio- 
 logical bearing. Ossified Cartilages have been referred to in connection 
 with the lateral cartilages. Fistula of the Coronet is serious, owing 
 to its proximity to the pedal joint, and its affecting non-vascular 
 
THE FOOT 735 
 
 structures like the lateral cartilage ; the process, is destructive and 
 repair slow. Suppuration around the coronet is a formidable affair; 
 the parts allow of very little swelling, are rigid and unyielding, 
 and burrowing of pus occurs such as is hardly met with elsewhere! 
 It is the dread in all foot injuries, and is due to the manner in 
 which the parts are confined within an unyielding box. There 
 are few diseases more painful, and few of which the surgeon has 
 more genuine dread. Suppuration may follow any injury, from an 
 injury in shoeing to a tread on the coronet. Injuries in Shoeing are 
 generally caused by a heel nail, occasionally by the foot being over- 
 reduced. A shoeing injury causes intense pain, and, if severe, is 
 followed by considerable destruction within the foot. A frequent 
 injury in the angle between the bar and wall is caused by the heel of 
 the shoe, and known as Corn; it is due to sole pressure, which in 
 this region especially cannot be tolerated. It is a bruise of the 
 part which is very persistent, and frequently permanent. It has a 
 physiological basis; the foot grows forward (p. 723); it carries the 
 shoe forward, and so takes it off the bars. The heel of the shoe 
 then beds itself into the angle formed by the sole between the wall 
 and bar. Bruise of the Sole may occur at any other part of the 
 sole, but is never followed by permanent results, as is a corn. The 
 sole cannot withstand pressure, especially that caused by a stone 
 becoming wedged between the foot-pad and the shoe. Arthritis 
 from penetrating wounds of the foot is common, but the pedal joint 
 does not suffer from locomotive arthritis, as does, for instance, the 
 joint immediately above it, and many others in the body. Nor do 
 the foot-joint or tendons suffer from sprains, though, under the influ- 
 ence of erosion after neurectomy, the tendon in navicular disease 
 may be worn through and snap, or even the bone 'fracture. Frac- 
 ture of the Pedis, in spite of its porous nature, is very rare. When it 
 does occur, the bone rarely unites, though the parts are contained in a 
 permanent splint, which, theoretically, should lead to perfect union. 
 The fact, however, is that, under the influence of the body weight, 
 the fragments are always being forced apart. Nutritive changes 
 occur in the foot after neurectomy, and the hoof may come off, or 
 so-called gelatinous degeneration of the foot and limb occur as high 
 as where the nejrves are divided. Evulsion of the healthy foot may 
 occur as the result of an accident, the foot being caught, for instance, 
 in railway-points, and the animal, in its struggles, pulling the inside 
 foot out of its cover. In these cases the nails in the wall have such 
 a powerful hold that, rather than allow the shoe to tear away, the 
 hoof is pulled off — an object-lesson in the security afforded by nails, 
 and the hopelessness of ever attaching the shoe by any other means 
 so simple and effective. 
 
 A weak wall in the fore-foot is liable to fracture during dry and 
 hot weather ; this is known as Sand-crack, and its physiology will be 
 understood when the necessity for moisture in the horn is borne 
 n mind (p. 721). The crack always begins at the coronet; it opens 
 and closes in accordance with the expansion and recoil of the foot, 
 and demonstrates these movements. Sand-crack in a hind-foot is 
 a totally different matter; it occurs at the toe, extends from bottom 
 to top, and is due to violence, especially in heavy draught. Con- 
 traction of the foot was the great bugbear in days gone by. It was 
 regarded as a disease; in the present day it is looked upon as a 
 symptom. A foot will contract if it is rested, as in navicular disease, 
 and the cause of this must be evident from what has been said in 
 
736 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the previous pages. If the foot-pad be cut away in shoeing and no 
 pressure given it, contraction occurs as a result, in consequence of 
 loss of function of the food-pad and plantar cushion. Inflamma- 
 tion of the glands of the plantar cushion leads to an offensive dis- 
 charge known as Thrush, which erodes what is left of the horn of the 
 foot-pad. This condition is aggravated by dirt, and is therefore 
 common in the hind-feet, but its chief cause is a want of proper 
 foot-pad pressure. The horn of the sole and foot-pad, from causes 
 which are not clearly known, but are probably microbic, takes on an 
 unhealthy, cheese-like, and offensive condition, due to disease of the 
 horn-secreting membrane of these parts. It is a most intractable 
 disease, known as Canker, and is frequently associated with defec- 
 tive hygienic care. The layer of white horn at the toe between the 
 insensitive laminae and the outside wall is liable to a curious dis- 
 integration known as Seedy Toe, which, by extension, may excavate 
 the wall nearly as high as the coronet. It is a slow process, and 
 recovery is tedious. The hoof frequently shares in any general 
 disturbance of the system. There is a tropical form of skin disease 
 in which the hoofs frequently indicate by the sdaly condition of the 
 wall that the horn-secreting substance — a modified skin — is sharing 
 in the general disorder. Similarly, the growth of the feet may 
 from constitutional causes be temporarily inhibited, and then start 
 again with renewed vigour, every increase in the production of the 
 wall being marked by a ring which extends all round the hoof. 
 Horses exposed to standing in wet places like marshes have an 
 impetus given to the growth of the wall, resulting in rings on the feet. 
 The above indicates the numerous diseases or injuries to which 
 the foot is liable. The foot has a special pathology, as well as a 
 special physiology. It is the most common seat of incurable lame- 
 ness, and has always been so since the horse was domesticated. 
 ' No foot, no horse,' is as old as the days of Xenophon. This horse- 
 master tells us how to keep the horn of the feet of cavalry horses, 
 hard — a very necessary matter at a time when shoes were unknown. 
 It is a remarkable fact that the horn of unshod feet is infinitely harder 
 than that of horses wearing shoes. It may, indeed, be so hard as 
 to resist the entry of a nail. 
 
CHAPTER XVIII 
 GENERATION AND DEVELOPMENT* 
 
 Section i. 
 
 The differences in anatomical arrangement of the generative organs 
 of the various domesticated animals may be briefly noted. They 
 are very remarkable, and afford an object-lesson in the methods 
 by which identical physiological results may be obtained by very 
 diverse anatomical structures. 
 
 The horse is the only domesticated animal which in the general 
 arrangement of the generative organs resembles man. The penis 
 is provided with a large amount of erectile tissue, but the glans 
 differs; there is a fossa around the opening of the urethra, and the 
 latter projects by means of a short tube. The corona, also, during 
 the orgasm swells enormously, resembling, as described by Chauveau, 
 the rose of a watering-can. The penis of the horse is capable of 
 being retracted within its sheath by means of an unstriped muscle 
 running from the coccygeal vertebra as far forward as the glans. 
 The retractor penis muscle is found not only in the horse, but in 
 the ox, dog, and cat. 
 
 In the ox the testicles are relatively much larger than in the 
 horse; they are also elongated, their long axes being vertical, whereas 
 in the horse they are horizontal. The penis possesses very little 
 erectile tissue, a fibrous core largely replacing the usual vascular 
 tissue. In consequence, the organ is always rigid, and increases 
 very little in size during erection. The penis is also doubled on 
 itself, producing an S-shaped flexure effected by muscular agency. 
 This flexure is obliterated during erection. The glans is small, 
 pointed, and twisted, and in the twist the urethra opens by an 
 extremely small side outlet. The prepuce, which internally is very 
 narrow, runs much farther forward than in the horse ; it has a pair 
 of muscles to draw it forward over the glans and a pair to draw it 
 back; both of these muscles are absent in the horse. 
 
 In the sheep the structures are much the same as in the ox, with 
 the exception that the urethra terminates in a narrow spiral process, 
 about i£ inches in length, which comes off from the left side of the 
 
 * In the preparation of this chapter several members of the profession 
 have been good enough to afford me information on subjects which have 
 not come under my personal observation. Some names are noted in the 
 text, but I am also indebted to Lieut. -Colonel Head, Major J. Peddiei 
 Messrs. H. Gray, H. Begg, R. J. Hickes, F. T. Harvey, P. Wilson, and 
 J. Brown (of Invergordon). 
 
 737 47 
 
738 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 body of the glans. This appendage contains erectile tissue, and 
 both it and the penis are rendered rigid by comprising in their 
 structure a fibrous cord, which runs on either side of the urethral 
 canal throughout the entire length of the organ. 
 
 In the pig the testicles are placed in the perineal region, the tail of 
 the epididymis being very large. As in the ox, the penis possesses 
 a sigmoid flexure; it has also a fibrous core and very little erectile 
 tissue. The organ possesses no glans; it terminates in a corkscrew- 
 like appendage of erectile tissue, in which the urethra opens by a 
 slit. There are no muscles to the prepuce. Within the prepuce 
 there is a diverticulum containing an offensively smelling mixture 
 of decomposing urine, worn-out epithelium, and sebaceous material. 
 
 In the dog the testicles are placed between the inguinal and peri- 
 neal regions, the long axis is oblique, and the epididymis large. This 
 animal, as in carnivora generally, has a compound penis, the posterior 
 part composed of erectile tissue, the anterior part formed of a bone 
 covered with mucous membrane which forms a long, narrow, pointed 
 glans, which in the dog is of considerable length, and extends over the 
 entire length of the os penis (Sisson). Behind the elongated glans 
 is a bulb. The bulb consists of erectile tissue, which, during inter- 
 course, becomes enormously swollen; until the swelling subsides 
 this bulb causes the organ to be imprisoned in the vagina. 
 
 In the cat the penis in a flaccid state turns backwards, as in the 
 camel; in the erect condition it points forward. It possesses a 
 small bone at the anterior part, and the membrane covering the 
 glans is studded with erectile papillae. 
 
 The accessory generative organs of the male also present differences. 
 In the horse the vesiculce seminales are not only a receptacle for 
 seminal fluid, but are also glandular. In the ox they are wholly 
 glandular. In the pig, also, they are glandular and very large; 
 in the dog and cat they are absent. 
 
 In the horse the prostate gland connects with the urethra by a 
 large number of openings ; in the ox the gland is so small as to be 
 frequently unobserved; it is also small in the pig, but large in the 
 dog. 
 
 The bulbo-urethral (Cowper's) glands are well marked in the horse, 
 and possess several excretory ducts; they are much smaller in the 
 ox, and each possesses a single duct; in the pig they are very large, 
 with a single excretory duct for each gland. In the dog they are 
 absent. 
 
 There is much less to be said about the difference in the organs 
 of generation in the females of different species. In all cases the 
 uterus is two-horned. It is in one horn, and to a limited extent 
 in the body of the uterus, that the foetus is contained. In the case 
 of twins one may be contained in each horn. In animals such as 
 the pig, dog, and cat, both horns are occupied by foetuses. The 
 non-gravid uterus occupies the dorsal position in the abdominal 
 cavity. On the other hand, the gravid uterus lies on the floor of 
 the abdomen. 
 
 The uterus undergoes a great increase in length in consequence 
 of pregnancy. In the mare, from about 16 inches in length it 
 becomes 2^ to 3 feet when gravid; the empty uterus has a thick 
 mucous membrane containing glands. The urethra is short and 
 very distensible, a practical point in the extraction for calculus. 
 
 The ovaries of the cow are smaller than those of the mare, and 
 the fimbriae not so extensive. The body of the uterus is very 
 
GENERATION AND DEVELOPMENT 739 
 
 short, the horns very long and spiral in their course; the muscular 
 coat is thicker than in the mare; the mucous coat, especially of the 
 horns, in both cow and sheep is studded with cotyledons, which 
 during pregnancy become greatly enlarged; the cervix is very 
 thick and rigid, the mucous membrane lining it being thrown into 
 folds. In the pig the horns of the uterus are very long, repeatedly 
 -flexed on themselves, and possessed of great movement owing to 
 the length of the broad ligaments. There is a long neck to the 
 uterus, this and the vagina directly connecting without the inter- 
 vention of an os uteri. 
 
 In the dog the ovaries and fimbriated extremities of the Fallopian 
 tubes lie in a bursa. The horns of the uterus are long, narrow, and 
 straight. In a gravid condition the horns are dilated for each 
 foetus with intervening constrictions, and the same remark applies 
 to the pig and cat. 
 
 Descent of Testicles. — The testicles are intra-abdominal during 
 the greater part of foetal life; they gradually descend from the 
 vicinity of the kidneys. A pouch of peritoneum grows down into 
 the inguinal canal, carrying with it fibres of the abdominal muscles, 
 which subsequently form the cremaster. Extending from the 
 epididymis of the foetal testicle to the bottom of what will become 
 the scrotum is a cord known as the gubernaculum testis. It is 
 this which, by gradually shortening its length, guides the testicle 
 through the inguinal canal into. its final position. 
 
 The testicles are normally in the scrotum of the foal at birth; 
 they are sometimes temporarily withdrawn. Failure to descend 
 is termed ' cryptorchidism.' Animals so affected may not be 
 fertile, but inmost other respects give every evidence of sex charac- 
 teristics, and as a rule are most troublesome to deal with. In 
 some animals— eig'., the elephant and whale — the testicles never 
 leave the abdominal cavity; in others they appear during the sexual 
 season, and are withdrawn at its close, or the : process of withdrawal 
 may be voluntary. The horse is capable of exercising considerable 
 retractile power owing to the size of the cremaster muscle. 
 
 Spermatic Fluid. — The portion of the testicle which secretes 
 the semen is the seminiferous tubules ; these are lined by several 
 layers of cells which by the process of division result in the 
 formation of spermatozoa. The first step is the production of 
 spermatogonia ; from these, by division, are derived cells, known 
 as spermatocytes, and from these in turn are formed spermatids, 
 from which the spermatozoa immediately originate. During 
 this process of cell division the head and subsequently the tail 
 of the spermatozoon is formed. Masses of spermatozoa, with 
 their heads to the wall of the tube and their tails to its central 
 canal, may be seen in streams or clumps. Their development 
 is assisted by other cells, known as those of Sertoli, a secretion 
 from which is believed to furnish the spermatozoa with nourish- 
 ment, and eventually enables them to turn in the lumen of 
 the tube. During the process of development from sperma- 
 togonium to spermatozoon certain changes occur in the number 
 of chromosomes; in the spermatid there is one-half of those 
 
740 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 found in the spermatocyte. The number of chromosomes in the 
 body cells of a given species is definite (see p. 753); the reduction 
 to one-half in the case of the spermatozoon (and also of the 
 ovum, see p. 755) is preparatory to the union of these two cells; 
 this union restores the full number of chromosomes to the body 
 cells of the animal which results from their union. Sperma- 
 togenesis is associated with the production of nucleic acid, which 
 is probably the material on which the cell proliferation depends. 
 This nucleic acid is derived from lipoid granules furnished by 
 the adrenal cortex. The store of lipoid in the testicle is con- 
 tained in the cells of Sertoli. 
 
 The spermatozoa, having been formed, proceed along the ducts 
 of the testicle and subsequently reach the epididymis. The 
 epididymis is a secreting surface as well as a receptacle and 
 conduit. It does not secrete semen, but it contributes to the 
 production of a hormone, for animals in which a portion of the 
 epididymis has been preserved in castration exhibit strongly 
 marked male characteristics, even to the thickened crest, un- 
 certain temper, and proud bearing. In the pig, an animal 
 possessing a large epididymis, Iwanow has described energetic 
 secretory activity as occurring here. 
 
 The tubules in the testicle are connected by an intertubular 
 structure which contains the interstitial cells of Leydig ; these 
 cells provide the internal secretion of the testicle on which the 
 secondary male characteristics depend for development, such 
 as growth of mane, richness of coat colour, and, in ruminants, 
 the growth of horns. 
 
 In all the higher animals the spermatozoa are tadpole-like 
 bodies, propelling themselves by the flagellum or tail after leaving 
 the testicle, but not, it is said, during the time they are within 
 that organ. In the epididymis there is activity; here, as we have 
 seen, the semen is to some extent stored, and during this time 
 it is believed to undergo a process of ' ripening ' under the 
 influence of the secretion of this organ. This is supported by 
 the undoubted fact that the seminal fluid becomes progressively 
 poorer in fecundating properties and in number of spermatozoa 
 in consequence of frequent coitus, there being insufficient time 
 for ripening in the epididymis. The vitality of the spermatozoa 
 inside the body of the female may be considerable; in the bat 
 they remain alive for months, and then impregnate an ovum; 
 in the hen all eggs are fertilised which are laid during three 
 weeks succeeding the last act of copulation. Their period of 
 life in the uterus of the pig is said to be brief. Lewis* 
 states out of twenty-five sows the spermatozoa were alive 
 
 * Oklahoma Experiment Station Bulletin, 96. Quoted by F. B. Mumford, 
 ' The Breeding of Animals, ' New York, 1919. 
 
GENERATION AND DEVELOPMENT 741 
 
 in three cases after twenty hours, and in two cases at forty 
 hours, and in all the other animals they were dead at the end 
 of sixteen hours. On the other hand, living spermatozoa have 
 been found in the uterus of the dog eight days after coition. 
 Outside the body the vitality is a question of temperature and 
 daylight. At a temperature of 13 to 21° C. 25 per cent, of the 
 sperm cells of a stallion were alive at the end of six and a half 
 hours (Lewis). Heape,* on the other hand, found in examining 
 the fluid of a dog sent him by post that fully half of the sper- 
 matozoa were alive eighteen hours after being obtained. By 
 keeping the fluid of the horse warm and in the dark it is said 
 that it will retain its vitality for twenty hours; the addition of 
 5-2 per cent, solution of dextrose will keep it active for thirty to 
 seventy hours (Sato). These facts have a practical application 
 in view of the extension of artificial insemination. 
 
 The amount of seminal fluid ejaculated by different animals 
 is dealt with at p. 759. 
 
 Spermatozoa are produced in enormous numbers; each cubic 
 centimetre of human seminal fluid contains from 60,000,000 to 
 70,000,000 of cells, in the dog 45,000,000, in a vigorous stallion 
 42,800,000 (Lewis), but in the same animal, after very moderate 
 stud demands lasting nine days (one service daily), the numbers 
 had fallen to 7,430,000 per cubic centimetre. 
 
 The seminal fluid of animals differs in its physical characters. f 
 All contain protein. That of the horse is rich in mucin and 
 slightly gelatinous, but rapidly liquefies ; it has a yellow, turbid 
 appearance, and on being passed through the centrifuge this 
 turbidity is not entirely removed, while the colour is unaffected; 
 it has no smell other than that of urine; the proportion of fluid 
 to spermatozoa is high. The sperm of the bull resembles that 
 of the horse; in the ram the seminal fluid is an opaque, white 
 liquid free from odour; in the dog it is turbid, greyish- white in 
 colour, and free from viscosity or odour. 
 
 The Function of the Accessory Glands. — Spermatozoa find 
 their way into the seminal vesicles, but whether these organs are 
 mainly designed as receptacles is another point. The vesicle 
 is undoubtedly glandular in structure, especially in the ox, and 
 there can be no question that it adds a secretion of its own to 
 the semen. This serves to dilute the semen, and may assist 
 in maintaining the motility of the spermatozoa; but a gland 
 measuring some five inches in length 'and two inches in width 
 and thickness, as it does'in the ox, is no doubt intended for other 
 purposes, even though the dog and the cat are able to do without 
 
 * 'The Physiology of Reproduction,' F. H. A. Marshall, D.Sc, 1910. 
 t This account is ta^en from E. Iwanow, Comptes Rendus, vol. lxxx., 
 
 I9 r 7- 
 
742 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 it. In castrated animals the glandular epithelium atrophies. 
 In geldings Sisson records that he has found dilatation of the 
 vesicles, which contained as much as a quart of thick, amber- 
 coloured fluid. Between the seminal vesicles and the epididymis 
 is the vas deferens ; the mechanism by which the semen passes 
 into the seminal vesicles is by no means clear. In the horse 
 and ox the vas deferens and ejaculatory duct join and open by 
 a common orifice. Sometimes in the horse these vesicles open 
 into the urethra separately from the ejaculatory duct. In cases 
 where the tubes unite the passage of semen from vas deferens 
 to vesicula seminalis may be carried out by a closure of the 
 ejaculatory duct, a reflux into the vesiculae then occurring, but 
 when each opens separately into the urethra, the flow of semen 
 from vas to receptacle is difficult to explain. 
 
 On the vas deferens are certain swellings known as ampulla ; 
 they are well marked in the horse and ox. It is in the ampullae 
 that the semen is stored while undergoing the process of ' ripen- 
 ing ' ; doubtless there is a secretion added to it from the wall of 
 the ampulla itself, for the thickened secretion found in the 
 epididymis becomes considerably diluted by remaining in the 
 ampullae. 
 
 The large seminal ejaculation of the horse and ox is largely 
 stored up in the epididymis and vas deferens, and to a limited 
 extent in the vesiculae seminales; in the dog it is wholly held 
 between the vas deferens and epididymis. 
 
 The secretion of the other accessory glands — i.e., the urethral, 
 Cowper's— and the prostate, remains to be considered. Iwanow 
 suggests that the urethral glands furnish the first fluid in ejacu- 
 lation, a clear liquid possessing bactericidal properties, for, as 
 he says, it may be kept in unsterilized tubes for eight months 
 without becoming turbid. He thinks its function is to wash 
 and disinfect the urethral canal. The secretions of the urethral 
 and bulbo-urethral glands and the prostate are added to the 
 seminal ejaculation in the prostatic portion of the urethra. 
 Cowper's secretion probably acts like that of the urethral glands; 
 that of the prostate excites energetic movements of the sper- 
 matozoa. In the dog this gland furnishes the remarkable third 
 ejaculation discovered by Iwanow which has yet to be dealt with. 
 The prostatic secretion is more intimately connected with the 
 sexual organs than those of the urethral and Cowper's glands. 
 After castration the prostate atrophies, the other glands ap- 
 parently do not. In ruminants the' prostate plays a very 
 unimportant part, the gland being insignificant in size. 
 
 The Ovum. — The main function of the ovary is to contain the 
 ova and favour their development. It does not secrete them; 
 the ova are laid down with the other cells of the body very early 
 
GENERATION AND DEVELOPMENT 743 
 
 in the life of the embryo, a portion of material being set aside 
 for their special development, as apart from the cells required for 
 the construction of the other portions of the body. It is a very 
 remarkable fact that within a few hours of the impregnation 
 of the ovum, one of the first acts in the cellular scheme is to 
 provide for the reproduction of the future animal, the prospective 
 parent of which at this stage is a mere mulberry-like mass of cells. 
 The germinal cells, as these are called, as distinct from the body 
 or somatic cells, subsequently form an epithelial layer, which 
 grows into the body of the future ovary as a long cylinder of 
 cells; these are eventually cut off, and remain in the ovarian 
 structure. At birth the ovaries contain some thousands of eggs, 
 some of which in due course, as the period of puberty arrives, 
 undergo further and more active changes. At this time the 
 most advanced cells are enclosed in a follicle containing fluid, 
 the Graafian follicle. Lining the follicle is a layer of cells known 
 as the membrana granulosa ; at one part these are heaped up, 
 known as the discus proligerus and within this mass the ovum 
 lies buried. The Graafian follicle makes its way to the surface 
 of the ovary, being enveloped in a two-layered cover formed from 
 the stroma of the organ, known as the theca folliculi. The cells 
 of the inner layer develop the yellow pigment, lutein, and sub- 
 sequently, on the rupture of the Graafian follicle and the discharge 
 of the ovum, secrete a yellowish pigment, which stains the cells 
 filling up the gap left by the discharged ovum. As a result of 
 the rupture of the Graafian follicle, a rent is made in the ovary. 
 This wound generally fills with blood from the opened vessels, 
 and for some time afterwards appears as a pigmented spot. If 
 pregnancy has not supervened, it undergoes a retrogressive 
 metamorphosis and soon disappears. If, however, the ovum is 
 fecundated, the corpus luteum, as this pigmented spot is termed, 
 continues to grow, and may be observed in the ovary even near 
 ' term.' 
 
 The Corpus Luteum of the pregnant animal is very much 
 larger than that of the non-pregnant, and it appears to be con- 
 clusively settled that the existence of this yellow tissue in the 
 ovary is not merely for the purpose of filling up a cavity in its 
 structure: the yellow body is a ductless glan d, which^on_preg 
 aancy _becoin £S-aa-actiy - e secreting j igent, producing 
 substance (hormo ne) b y whi ch the ovum is _ anch oje ' 
 
 Jol^^literasJIind^lts^ouris hment an^Tl?eve Topjnent_^ssisted,. 
 ThiiTductless gland iF functional until about the "middle _ of 
 pregnancy, when it is no longer a necessary factor in the fixation 
 of the embryo, and consequently degenerates. That the corpus 
 luteum takes little or no share in the production of seasonal 
 gexual exciternent appears quite clear; as we shah see, this is 
 
744 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the function of the stroma of the ovary, which pours an internal 
 secretion into the blood, and so brings about menstruation (in 
 primates) and cestrus. It has been stated that towards the end 
 of pregnancy the corpus luteum produces a second hormone which 
 helps to bring about involution of the uterus after parturition. 
 
 The mature ovum is very small ( T l,j- to T £ ff of an inch), yet 
 large enough to be seen by the naked eye. It is therefore one 
 of the largest cells in the mammalian body, though infinitely 
 smaller than the eggs of birds and reptiles. There is a mammal, 
 the ' duck-billed platypus ' (Omithorhynchus) , which produces 
 eggs the size of a hazel-nut, and the ancestors of all mammals 
 had eggs probably as large. The greater size of the eggs of 
 reptiles and birds is due to the quantity of yolk, or deutoplasm, 
 they contain, which in mammals is very small in amount, as the 
 embryo of the latter is dependent upon the yolk-sac for nourish- 
 ment only for a very brief period. 
 
 The ovum is a typical cell; it is spherical, more or less trans- 
 lucent, and contained within a membrane, the zona pellucida. 
 The contents, or protoplasm, of the cell consist of fatty and 
 albuminous granules, known as yolk spherules, and lying in the 
 protoplasm is a nucleus, containing one or more nucleoli. The 
 nucleus is spoken of as the germinal vesicle, the nucleolus as the 
 germinal spot. One more body is found in the protoplasm, an 
 attraction sphere, or centrosome. This latter is extremely small, 
 but its functions are of the utmost importance. 
 
 When the rupture of the Graafian follicle occurs, the ovum is 
 flushed out, and at the same moment, according to Hensen, 
 the fimbriated extremity of the Fallopian tube becoming erect, 
 grasps the ovary, and thus the escaping ovum is received into 
 its ' duct.' Probably the converging furrows found on the 
 plicated extremity of the Fallopian tube may assist in directing 
 the ovum to the ostium aodominale. If by chance the ovum be 
 not caught and carried away to the uterus as described, it may 
 fall into the peritoneal cavity and perish ; or if it has been already 
 fertilised, abdominal fcetation may occur, the peritoneum acting 
 as a matrix in which the embryo may develop. The method by 
 which the ovum gains the Fallopian tube is not, however, settled. 
 There is some evidence to show that it may be discharged into 
 the abdominal cavity, and make its way into the Fallopian tube. 
 The experimental introduction of small objects into the pelvic 
 cavity has resulted in these being taken up by the tubes. In 
 animals which have had one ovary removed the embrj-o has 
 been found developed in the horn of the uterus on that side, 
 though the ovum was derived from the opposite ovary. Even 
 the ovary of one side and the uterine horn of the opposite side 
 have b ee n remoyed without interfering with conception; this 
 
GENERATION AND DEVELOPMENT 745 
 
 indicates that the ovum may travel across the peritoneal cavity 
 to the opposite Fallopian tube, and if, as Ewart states, the horse 
 embryo is generally contained in the right horn, this must be 
 of frequent occurrence in equines. Notwithstanding, the simple 
 direct method of Fallopian tube grasping its own ovary appears 
 the most reasonable explanation. In the dog special provision 
 exists for the ovum to pass in this manner direct from the ovary 
 into the Fallopian tube, for, as we have seen, the ovary and 
 fimbria are in a capsule. 
 
 An ovum may fail to ripen and be discharged from the ovary ; 
 in this case it undergoes degeneration and atrophies. It is 
 believed that this is liable to occur in underfed and in very fat 
 animals, and Ewart draws attention to its occurrence in mares 
 turned out during the winter. Freedom from sexual intercourse 
 may also lead to degeneration of the ova, and probably explains 
 the difficulty in getting old mares to breed. 
 
 Effect of Removal of Testicles and Ovaries. — -In the chapter 
 dealing with the endocrine glands reference has been made at 
 p. 307 to the internal secretions furnished by the genital glands 
 of both sexes; we are now in a position to consider in greater 
 detail the internal secretions of the testicle and ovary. 
 
 If the genital glands are removed during early life or prior to 
 puberty, profound changes occur in the organism. The secondary 
 sexual characters are lost, the horse fails to develop a crest to 
 his neck or the voice of the stallion (p. 154), his proud bearing 
 and aggressive nature disappear, he becomes humble and sub- 
 missive, his coat colour is never as rich as in the uncastrated 
 animal, and a pure black horse becomes an impure black ; pigment 
 production, though not withheld, is modified. The ox lacks the 
 massiveness, voice, and ferocity of the bull, and the flesh not only 
 becomes tender, but is rendered palatable ; there is also a disposi- 
 tion to accumulate fat (p. 379), a condition equally well marked 
 in the sheep and pig, but absent in the horse, probably as the 
 result of work. The change in the taste of the flesh of the sheep 
 and pig is as well marked as in the ox. Such are the general 
 changes resulting from the removal of the testicles. There are 
 local changes; for instance, in the stag, if castration be performed 
 in early life, the antlers never grow; if performed after the antlers 
 are grown, these are shed from the seventh to the ninth day" 
 after operation, fourteen days being the longest time they 
 remain.* If the epididymis be left, its presence modifies the 
 growth of the next pair of antlers. We have already seen that 
 the epididymis is a secreting gland, and noted its influence in the 
 partial castration of horses (p. 740). If castration of the stag 
 be practised on one side only, it is said— but the matter requires 
 * JI, I^eney, M.R.C.y.S. CommuniQstfed, 
 
746 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 confirmation — that only the antler on that side is shed and fails 
 to be reproduced. 
 
 Though castration has no effect on the development of hollow 
 horns, there is one ruminant, the prong-buck, that periodically 
 sheds its hollow horns, and the effects of castration in this animal 
 is to alter their direction of growth. 
 
 In horned sheep the effect of castration is to arrest the develop- 
 ment of the horns, and they approximate to the type produced 
 by the female (Marshall). Cocks converted into capons when 
 young do not develop such full male plumage, and the combs 
 and wattles are more like those of the hen. Crowing does not 
 occur. If extracts of the testicle of the pig be injected the 
 colour of the comb and gills is restored, crowing takes place, and 
 the sexual instincts are re-established. When the injections are 
 stopped the animal returns to its condition of capon (Pezard).* 
 
 Cats operated'upon when very young have heads which are 
 indistinguishable from that of the female; the tissues which 
 give the head of the male cat its massive appearance are lost, 
 and this may occur even when the operation is performed late 
 in life (Leeney). 
 
 The removal of the ovaries brings about in women an artificial 
 menopause; menstruation ceases, and there are characteristic 
 alterations in features and body shape. There is not in the 
 domesticated animals anything corresponding to the ' change 
 in life ' which occurs in women ; animals may breed when ex- 
 tremely old, but the effect of removal of the ovaries is certainly 
 to prevent the appearance of the cestral period, and ill-tempered 
 mares have been benefited by the operation. It does not affect 
 the secretion of the mammary gland in the cow, for animals in 
 France are frequently ' spayed ' when in full milk in order to 
 keep them in this condition. In the sixteenth and seventeenth 
 centuries in this country fillies were frequently operated upon 
 within a few days of birth; there were complaints that this 
 prevented their growth and development, which probably led 
 to the abolition of this barbarous custom. The only animals 
 regularly submitted to ovariotomy are pigs; it promotes early 
 fattening and renders the flesh palatable. 
 
 Sexual Inversion. — It has been observed that secondary male 
 characteristics follow the menopause in women, and in certain 
 animals similar results have been observed. The cat ov'ario- 
 tomised in early life may develop a head of the male type; 
 pullets from which the ' clutch ' has been removed sometimes 
 put on male plumage and spurs begin to grow; diseases of the 
 ovary in hens or pheasants may lead to their crowing (Leeney). 
 Testicular and ovarian grafts have been inserted into castrated 
 * Physiological Abstracts, vol. iii., 7. 
 
GENERATION AND DEVELOPMENT 747 
 
 animals of both sexes with a certain amount of success. They, 
 of course, act only by producing the internal secretion of these 
 glands. If ovaries are grafted into young male rats after cas- 
 tration and testes into female rats after ovariotomy, the male 
 shows motherly instincts in the protection and nursing of the 
 young, and the female exhibits male instincts (C. R. Moore).* 
 
 The Sexual Season of female animals is a subject which has 
 received exact expression from the pen of Heape, j whose com- 
 munication, quoted below, we have followed in treating of this 
 subject. Heape divides female mammals into two classes — 
 Moncestrous, or those which have one cestrous period during 
 the complete cycle, and Polycestrous, or those having a series of 
 cestrous cycles during the cestrous season. The first phase of 
 generative activity at the beginning of a sexual season is known 
 as Prooestrum, or the procestrous period ; it corresponds to the 
 period ' coming on heat,' or ' coming in season,' and in the 
 human female to the period of menstruation. The period lasts 
 a variable time in different animals, and is succeeded by the 
 period of desire, or CEstrus. It is only during this period that 
 sexual intercourse is permitted, or that fruitful coition is possible. 
 If conception does not occur or is prevented, cestrus is followed 
 by Metcestrum, or the metcestrous period, during which sexual 
 activity passes away, and is succeeded by a period of complete 
 rest or freedom from sexual excitement, known as Ancestrum. 
 The a ncestrou a. period may, last tw o, th ree, eleven, or jnore 
 m onths. depending nn the species . 
 
 The sexual cycle is not always as above described; there are 
 animals in which metcestrum is not followed by a period of 
 complete rest. Jjut by a short quiesce nce, know n as. D icestrum, 
 which lasts, a certain_rru mb_er of day s and is then foUpwecT by 
 a ne w prooe strum, cestrus, metcestrum, and dioestrum. Among 
 moncestrous mammals is the wolf, which in "trie wild state has 
 onryone sexual season in the year. Another is the dog, though 
 in this case the sexual season recurs during the year, but 
 the periods in each case are quite distinct, and followed by 
 complete rest, which is the essentially distinguishing feature. 
 Among polycestrou s mammals are the mare, cow, sheep, pig, 
 and all ofthese duraig~aTpcition_o£the year exhibit a serigTof 
 dicestrous cycles (in the absence of pregnancy), TcTtowed. by 
 a ncestrum until _the J Jlgid-^ear. The number of annual sexual 
 cycles which any given species passesthroughjs vastly influenced 
 by domestication .^Trobably in all primitive species-one sexual 
 season yearly was the rule. Domestication alters this. The 
 
 * Physiological Abstracts, vol. iv., 3. 
 
 •f ' The Sexual Season of Mammals,' etc., by W. Heape, M.A., Quarterly 
 Journal of Microscopical Science, vol. xliv., p. 1, 190J. 
 
748 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 abundant food supply renders the struggle for existence no 
 longer acute, the dread of being preyed upon by the enemies 
 peculiar to each species is removed, and one of the responses 
 to these altered conditions is a greater desire to multiply, for 
 the reason that the energies previously expended in the struggle 
 for life are turned into a fresh channel. The cat in a wild state 
 has one sexual period a year; the domestic variety has_three 
 or, four. Thewild dog and wolf breed once annually, in captivity 
 twice annually. The lioness in a wild state has probably but a 
 siiigirbfeeding season ; in captivity the oestrous period may recur 
 three or four times a year. Bears in a wild state have a single 
 breeding season, in captivity more than one. The wild otter 
 has a single season, but in a state of captivity she comes ' in 
 season' every month (Marshall and Jolly). So far, in fact, as 
 evidence is available, a single sexual season for animals in a state 
 of freedom appears to be the natural condition, -fwl yasstruw . 
 being an acquired character. The frequency of oestrus under 
 domestication is essentially influenced by food, temperature, 
 and envirojjjnent. 
 
 The complete oestrous cycle in the dog* under domestication 
 is six months. Every six months, in spring and autumn, the 
 majority of dogs come ' on heat,' though there are many excep- 
 tions to this rule, some of the smaller breeds of dogs having a 
 three and four heat period in the year. The period of prooestrum 
 lasts from s even to ten da ys, and oestrus lasts anot her week. 
 
 In the mare the complete oestrous cycle, with" its dicestrous 
 intervals, may last for months, in the majority of mares from 
 February to June or July; and unless the mare is rendered 
 pregnant, the dicestrous periods last twenty-one days, and are 
 followed by prooestrum, oestrus, etc., as previously described, 
 though the time-duration of these is irregular, generally brief, 
 and always uncertain. For instance, the exact period at which 
 the mare is ripe to receive the male may only be a matter of a 
 few hours, whereas she may be several days in a highly unsettled 
 sexual condition. The mare is in a condition of oestrus on the 
 seventh to tenth day after foaling; in thoroughbred mares it 
 may be the sixth. At this period, though still nursing, she 
 desires intercourse, and in this respect differs from the nursing 
 cow and sow. If she does not conceive, the period of dioestrum 
 is twenty-one days, and is followed by oestrus, the returning 
 heat usually lasting longer by two or three days than the original 
 'heat.' 
 
 The cow under domestication will breed at any time of the 
 
 * ' Contribution to the Physiology of Mammalian Reproduction.' 
 Part I.: 'The (Estrous Cycle in the Dog,' by F. H. A. Marshall and 
 W. A. Jolly, Phil. Trans., B, vol. cxcviii., 1905. 
 
GENERATION AND DEVELOPMENT 749 
 
 % 
 year. She ordinarily takes the bull six weeks or two months 
 after calving, but it is unusual for her to accept the bull while 
 suckling her calf. If the latter be removed or weaned, she 
 shows signs of oestrus six or seven days later, the duration of 
 which may be twelve hours. The period of dicestrum is twenty- 
 one days, at the end of which time both cows and heifers 
 exhibit oestrus. This cycle continues until they are settled 
 in calf. 
 
 With sheep* oestrus may only last one or two days, or it may 
 pass away very quickly, the dicestrum which follows lasting 
 from thirteen to eighteen days. The number of recurrent periods 
 in any one cycle in the sheep has been observed to depend upon 
 breed; two, three, or four recurrent periods have been noted. 
 There are some breeds of sheep which may produce two sets of 
 lambs in one year. The period of cestrus may be induced almost 
 at any time in the late summer and autumn by the introduction 
 of the ram to the ewes (Goodall). 
 
 The sow takes the boar about a week after she has weaned 
 her litter, or about eight weeks after farrowing. The period of 
 cestrus lasts about two days, the dicestrous period twenty-one 
 days. 
 
 The only known animal which in a wild state exhibits a con- 
 tinuous series of dicestrous cycles is the monkey, but even in 
 this case the season is limited at which conception is possible 
 (Heape). 
 
 By systematically preventing animals from breeding, the 
 sexual season may be interfered with to the extent of complete 
 cessation (Heape). Certainly the mare used late in life for 
 breeding purposes often proves barren. Yet there are mares 
 which, though deprived of the services of the male, never lose 
 their desire, and may for the greater part of their lives be a 
 source of danger from sexual excitement. 
 
 Cause of Procestrum and (Estrus. — The oestrous period may 
 appear in the dog after a portion of the spinal cord has been 
 excised, proving that it, is a process quite independent of the 
 nervous system, that it may exist in the absence of conscious- 
 ness on the part of the animal, and that its production is under 
 no central control. Furthermore, an animal so operated upon 
 may become pregnant and be delivered in the ordinary way, 
 though quite unconscious of the process. Heat and menstrua- 
 tion are produced as the result of an internal secretion derived 
 from the stroma of the ovary. It is possible with some animals — 
 for example, sheep — to hurry forward the oestrous cycle by liberal 
 feeding; similarly, it may be retarded or the cycle shortened 
 
 * ' The CEstrous Cycle and Function of the Corpus Luteum in the 
 Sheep,' by F. H. A. Marshall, B.A., Phil. Trans., B, vol. cxcvi., 1903. 
 
750 A MANUAL OP VETERINARY PHYSIOLOGY 
 
 (mare) by poverty of keep, but these are merely features of 
 general metabolism; £hejreal factorjs the chemical secretion of 
 v the ovary. 
 
 The~E'xternal Signs of Procestrum in all domesticated animals 
 are a swelling_o f the vulv a, more or less pronounced, and a slight 
 flow of m ucus, which may be blood-stained. This is the only 
 approach to the condition of menstruation in women. Theje is 
 exciteiaent. The mare may refuse to work, squeals and kicks 
 when approached, elevates and protrudes the clitoris, and 
 micturates frequently, the material being very mucoid. The 
 cow bellows, is_excited, and moun ts its fel lows. Sheep become 
 restless and follow the ram. The dog is"playful, excited, and 
 desires the attention arid, companionship of the males of her 
 own species. In all animals it is only during the actual period 
 of oestrus or desire that copulation is permitted, and in all 
 polycestrous domestic animals this period is variable in duration ; 
 it may be a day or two, or a few hours only. 
 
 Changes in the Uterus during Sexual Cycle. — During procestrum 
 there is injectio n and swelling of the m ucous membrane of the 
 uterus, followed by capillary haemorrhage, ex travasation of bl ood, 
 aricHater, in the human subject, destruction of the _muco sa'"and 
 ep itheliu m. In the woman, haemorrhage occurs into the interior 
 of the uterus, and the ut erine .glands pou r out their secretion; 
 in due course the mucosa and epithelium are repaired and the 
 haemorrhage ceases. In the herbivora there is no destruction of 
 the mucosa, and but slight destruction of the epithelial layer; 
 capillary haemorrhage of the cotyledons occurs in ruminants, 
 but of a limited character; the mucous gland's of the uterus and 
 vagina pour out a secretion, and it is this, together with the 
 capillary haemorrhage, which gives rise to the slight discharge 
 observable. In sheep of more than a year old the mucous 
 membrane of the uterus during the period of ' rest ' may be 
 observed to be black with pigment, the result of extravasated 
 blood. In carnivora more extensive changes occur than in 
 herbivora, but not nearly so destructive as those taking place 
 in women and the monkey. 
 
 As a result of these changes the uterus is prepared for the 
 arrival of a fecundated ovum, procestrum passes away, and is 
 followed by oestrus. 
 
 Rutting.— When male animals suffer from a periodic sexual 
 excitement, it is known as rutting. This term should be con- 
 fined entirely to a male sexual season, such as is experienced 
 by the camel, stag, elephant, and ostrich. In the rutting stag 
 the neck becomes enormously_swollen (Leeney), the elephant 
 experiences a discharge from the temporal gland, the camel eats 
 little, and in consequence becomes ' tucked up ' ; he also protrudes 
 
GENERATION AND DEVELOPMENT 751 
 
 from the mouth a disgusting-looking bladder formed by the 
 soft palate; the ostrich becomes red in the legs. All these 
 animals are at this time dangerous to approach, being invariably 
 violent and aggressive. 
 
 During the period of ' rut ' mature spermatozoa are produced, 
 and probably at no other time. In insectivora and rodents 
 there is a growth of the generative organs, especially the testes, 
 which, if intra-abdominal, may shift position, and appear ex- 
 ternally. In the hedgehog the seminal vesicles and prostate 
 gland assume remarkable proportions, shrinking to normal when 
 the period of excitement has passed away; in the mole the 
 testicles become sixty-four times larger than during the resting 
 period; the preputial glands of many animals emit a penetrating 
 odour of mask. ' Rutting ' is due to an in ternal secretion of the 
 testicle elaboratei_bj?Jheintexsiitial-ceJls. 
 
 A male generative cycleTcToes not occur in domesticated animals. 
 Nevertheless, both the dog and cat give evidence of what might 
 be regarded as ' rutting ' when aware of the existence of an 
 cestrous female. 
 
 Desire in all male animals is due to a hormone produced by 
 the interstitial cells of the testicle (Leydig's). These cells are 
 also present in the epididymis, and account for the sexual charac- 
 teristics present in animals in which the epididymis has not been 
 removed in the operation of castration. They are likewise the 
 cause of the desire met with in animals in which the testicles 
 have not descended, though such cases are frequently sterile 
 owing to the absence of spermatozoa. If the testicles be ex- 
 posed to X rays the spermatic cells are destroyed, but not the 
 interstitial cells of Ley dig. The animal consequently does not 
 lose desire, but is rendered sterile. 
 
 The Cell. — It will help to a clearer understanding of the structure 
 and development of the ovum if the features of body cells in general 
 are considered. 
 
 There are two distinct classes of cell in the body. The more 
 numerous tissue cells, known as somatic " r had$L T?( c are responsible 
 for the structure of every tissue in the body, from the hardest to 
 the softest. Another and much smaller set are the germ cell s, which 
 are concerned solely in the reproduction of the individual. The 
 somatic cells are enormously active during embryonic life, and 
 subsequently during the process of growth. When this is com- 
 pleted, they settle down to normal activity, which consists in 
 growth by the process of division; by this process of multiplying 
 they are capable of repairing or replacing the worn-out cells of the 
 body. The living animal is merely a gigantic collection of cells, 
 each having an individual existence. As the cells die or are worn 
 out, their places are taken by others, and this process is occurring 
 during the whole life of the animal. Within this minute speck of 
 material is contained all the elements of life, and, small as it is, 
 it contains all that is necessary for multiplying and producing its 
 
752 
 
 A MANUAL OP VETERINARY PHYSIOLOGY 
 
 kind, in this respect differing in no essential particular from the 
 larger organism of which it is only a microscopic atom. 
 
 All cells, somatic or germ, consist of cell-contents and cell-wall. 
 The contents are colloidal in character, structureless, and are 
 
 Fig. 258.— Diagram illustrating the Process of Cell Division 
 (after Boveri). 
 
 . generally described as protoplasm or cytoplasm. Within the cell 
 and lying in the cytoplasm is the nucleus. The nucleus consists of 
 nuclear membrane and contents, but the latter are not structure 
 less. The nucleus of all cells is an important feature without 
 which the cell could not be reproduced. The reproductive material 
 
GENERATION AND DEVELOPMENT 753 
 
 in the nucleus consists of a network of threads (Fig. 258, a), which, 
 owing to their affinity for staining material, are known as chromatin. 
 Within the threads are refractile bodies. In addition to the nucleus, 
 the parent cell contains a spherical area of protoplasm, known as 
 the centrosome, a body of the utmost importance. The centrosome 
 is surrounded by radiating fibrils, which give it the appearance of 
 a star (Fig. 258, a), and the area surrounding it is spoken of as the 
 attraction sphere. The reproduction of cells is brought about by 
 indirect division, or mitosis, and the change is initiated in the 
 nucleus, or in the case of the centrosome by division of the body. 
 The chromatin threads of which the nucleus consists form larger 
 threads by the process of fusion until the network is lost, and there 
 only remains a collection of bodies resembling bent or curved rods 
 (Fig. 258, b, c). These are the chromosomes, and the number present 
 in each cell of the body is always definite for the particular species, 
 but is not the same in all species of animals. There are, for example, 
 twenty-four chromosomes in the mature somatic cells of man, the 
 mouse, trout, salamander, and lily; in some of the threadworms 
 there are two; in some of the Crustacea as many as 168. The 
 number of chromosomes in the horse is thirty -six plus one accessory; 
 in the mule fifty plus one accessory; in the ass there are probably 
 sixty-five;* in the cow sixteen. No matter what somatic cell is 
 examined, the number is invariable for any given animal. The 
 importance attached to the number, which is always even for every 
 animal and plant sexually produced, will shortly be evident. The 
 chromosomes themselves are of extraordinary interest. 
 
 The first change preparatory to the division of any cell takes 
 place, as has been stated, in the nucleus, and while this is occurring 
 the centrosome has divided into two, passing to opposite sides 
 of the nucleus (Fig. 258, b, c, d) ; these, with their star-like fibrils, 
 are known as astrospheres. The chromosomes being formed, the 
 cell wall of the nucleus disappears, and so liberates them (Fig. 258 d) ; 
 they are then arranged between the two poles formed by the astro- 
 spheres, placing themselves end to end in Indian file jquatorially 
 (Fig. 258, e), and, having done this, they split lon gitud inally, and 
 each forms two (Fig. 258, /). Each split chromosome is attracted 
 towartlsTEe astrosphere nearest to it, so that an equal number of 
 chromosomes pass to each of these bodies. The cytoplasm of the 
 cell has hitherto been an inactive witness of this remarkable 
 phenomenon. It now, however, divides into two portions (Fig. 258, 
 h, i, j), each engulfing an astrosphere with its attendant chromo- 
 somes, and in this way two cells are made out of one, which resemble 
 the parent in every respect, excepting that they contain only half 
 the amount of chromatin, though the full number of chromosomes 
 is present. The subsequent history of the new cell is simple. The 
 chromosomes come together, form a reticulum, surround them- 
 selves with a membrane constituting the cell wall of the nucleus, 
 and increase the amount of chromatin up to that originally existing 
 in the parent cell (Fig. 258, ;'). Every body cell contains, therefore, 
 a double set of chromosomes; each time a cell divides half of the 
 chromosomes pass to the new cell, and in course of time the divided 
 cells again form a double set. 
 
 Every animal originates from a cell which differs in no essential 
 
 * * Causes of Sterility in the Mule, ' J . E. Wodsedalek, Biological Bulletin, 
 vol. xxx., 1916. The accessory chromosome referred to in the text is 
 explained at p. 781. 
 
754 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 particulars from the one just described. The process of multiplica- 
 tion by which cells form a lion or a mouse, an elephant or mole, a 
 philosopher or an idiot, or a humble intestinal worm, are m all 
 cases identical. In the development of the ovum the stages through 
 which the higher animals pass before they become higher may, as 
 it were, be witnessed or readily visualised. There is nothing in 
 the early development of the ovum which sharply defines man 
 formation from dog, cat, or pig formation; each goes through the 
 same phases up to a certain point, and then comes the parting of 
 the ways. The unity of type of the embryo in its early stages is 
 succeeded by sharp differentiation; this is even more remarkable 
 than the incomprehensible likeness of the starting-points. 
 
 How does a cell work ? It respires in the sense that it takes 
 up oxygen and gives up carbon dioxide. It feeds, for the particles 
 of food in the amoeba may be seen passing into the interior, where 
 they are digested. No rupture of the cell wall occurs in consequence 
 of this ingestion of food material; the substances pass through and 
 the gap closes behind them. The cell moves, for the movements 
 may be seen; those connected with the white blood-corpuscles and 
 amoeba have been described and figured at p. 16. The movements 
 are effected by protrusion of protoplasm, the cell wall being pushed 
 out as the result of change either of the fluid pressure within the 
 cell or of the fluid medium in which all body cells are bathed. 
 
 The cell wall is permeable to certain solutions, but not to others. 
 The conception of the cell wall is that it is finely perforated and 
 permits the passage of molecules which are small enough to pass 
 through, those too large being held back. This passage through 
 a cell wall is produced by differences of molecular concentration 
 known as osmosis, to which subject reference has been made at 
 pp. 277, 278. At p. 279 will be found a reference to the influence 
 of electrolytic action in living cells, a subject intimately concerned 
 with that of osmosis. 
 
 It has been mentioned above that the cell cannot be reproduced 
 without a nucleus. At p. 471 evidence of this fact has been adduced. 
 
 It is impossible in a work of this kind to enter more deeply into 
 the physiology of the cell. It is obvious that a knowledge of the 
 cell is intimately connected with the nature of life. All the physio- 
 logical processes of the body which have been considered in previous 
 chapters are carried out by the cells, and the methods by which 
 they work are chemical, physical, and biological problems of great 
 complexity, which, if fully understood, would explain the enigma 
 of life. 
 
 Maturation of the Ovum. — The maturation of the ovum is 
 not the same as its fertilisation. Maturation is concerned 
 with the production of a perfect from an immature ovum, of a 
 mature from an embryonic cell. This process is effected on the 
 lines above indicated. It has been stated that the cells in the 
 ovary at birth are imperfect ; their number is laid down, and this 
 we have seen amounts to several thousands. They are com- 
 pleted as required, and are only matured some months or years 
 after birth, depending on the species of animal. It is obvious 
 that only a few are matured at one time, and, again, the species 
 of animal determines whether they are matured, as in the woman 
 
GENERATION AND DEVELOPMENT 755 
 
 at short regular intervals, or at relatively long intervals. The 
 periodical development of a mature egg or eggs constitutes the 
 essential feature of maturation. 
 
 The primitive ovum takes its first step towards maturation 
 by the process of cell division. Two cells unequal in size result : 
 one is the ovum, the other the first polar body. The number 
 of chromosomes in the immature ovum was originally, in the 
 case of the mare, thirty-six; as the result of the formation' of 
 the first polar body, these thirty-six fuse and form eighteen. 
 Before the first polar body is cut adrift the eighteen chromosomes 
 split, so that eighteen go to the polar body and eighteen remain 
 with the ovum. Though these two cells contain only half the 
 number of chromosomes contained by a somatic cell, yet each 
 is a whole chromosome, and not a half, as in the division of 
 somatic cells. The ovum now divides a second time, and 
 extrudes a second polar body. This time the eighteen chromo- 
 somes either split or divide, thus forming thirty-six; half pass 
 to the second polar body, and half remain with the parent cell, 
 the ovum. So that the formation of the two polar bodies has 
 caused the ovum to suffer a loss in the number of chromosomes, 
 and it is finally left with only half its original number; but its 
 cytoplasm has undergone no practical loss. The polar bodies 
 are regarded as abortive ova; they die, and are not concerned 
 in the subsequent changes. The important point is that the 
 mature ovum is left with only half the number of chromosomes 
 normal to the cells of the species. One further change in the 
 process of maturation has also taken place, and that is the 
 centrosome of the cell is lost. These remarkable changes pave 
 the way to an understanding of what follows, should im- 
 pregnation of the ovum occur. 
 
 An ovum without a centrosome and with only half the normal 
 number of chromosomes has, in order to be fertilised, to be 
 furnished with a centrosome and its full number of chromosomes. 
 It is the function of the spermatozoon to supply these (see p. 740). 
 It is evident that the two sets of chromosomes described at 
 p. 753 as existing in the body cells of animals are derived from 
 its parents, one set from the sire and one set from the dam. 
 The chromosomes are believed to convey the hereditary charac- 
 ters of the transmitters, but there are arguments to the contrary 
 (see, however, p. 799). 
 
 Ovulation. — This occurs at the moment the Graafian vesicle 
 ruptures and the ovum is ejected. In some animals, as the 
 rabbit and ferret, it can occur only as the result of coition, the 
 presence of spermatozoa in the uterus being essential to the act. 
 In'others — and they represent the majority — such as the mare, 
 ass, cow, sheep, pig, and dog, ovulation occurs during oestrus— 
 
756 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 in the case of the mare, frequently towards the end of the oestrus 
 period (Ewart) — so that the act of copulation is not necessary 
 to extrusion, and in such animals artificial insemination is there- 
 fore possible. The period of oestrus is not necessarily identical 
 with the period of ovulation; cestrus may occur without ovula- 
 tion, and ovulation may occur without oestrus. Ewart says 
 the mare may mature and discharge one or more eggs after she 
 has become impregnated ; this is known to occur in other animals, 
 including man, and accounts for superfcetation, or the produc- 
 tion of a second conception in an animal already pregnant 
 (see p. 783). The number of ova which may be extruded during 
 one sexual period is not know'n with any degree of certainty; 
 in the case of the cat and dog there is evidence of several being 
 ejected, for each foetus represents a separate egg; the number 
 of eggs extruded in such cases is in excess of the number im- 
 pregnated. The mare, which probably produces only one egg 
 at a time, and with whom twin births are very rare, is believed 
 by Ewart to shed about ten or twenty ova annually. Whether 
 an equal number is discharged by each ovary is unknown. 
 Probably one ovum for the mare, cow, ass, deer, elephant, or 
 monkey, at each oestrus period is the rule, though two may be 
 discharged. The sheep probably discharges one to four; the 
 dog, wolf, and cat five to six; the pig ten, or even fifteen. The 
 egg or eggs, having been extruded, find their way into the Fal- 
 lopian tubes in a manner already described (p. 744). 
 
 The Period of Puberty, or the age at which an animal becomes 
 capable of procreation, has been put at one and a half years 
 for the horse, eight to twelve months for bovines, and six to 
 eight months for the sheep, pig, and dog. The full development 
 of the reproductive system is greatly influenced by the question 
 of nutrition, well-fed animals arriving at puberty earlier than 
 those which have been underfed. There is, however, a great 
 difference between capability and fitness for procreation. 
 
 The advent of puberty is marked by the production of the 
 secondary sexual characters. The animals lose their awkward- 
 ness, the outline of the frame becomes more consolidated and 
 in greater unison. The organs of generation are in a state of 
 activity. In the male the neck becomes thick and curved, the 
 voice deepens, and the whole appearance denotes life and vigour. 
 In both the stallion and bull the temper is usually irritable and 
 uncertain, and often extremely vicious. These changes occur 
 gradually, and are fully established at maturity. In the horse 
 maturity is commonly believed to be attained at five years of 
 age; it is probably attained nearer six, and some have thought 
 even later than this. The ass, according to Crisp, is mature 
 at five years old; the ox, sheep, and goat at four; the pig at three. 
 
GENERATION AND DEVELOPMENT 757 
 
 The age at which procreation ceases is not known. Mares 
 have produced foals at twenty-eight, thirty-two, and thirty- 
 eight years of age,* and it is certain that some good stallions 
 have been advanced in years. There is not present in the horse, 
 ruminant, or carnivora any ' change in life ' such as women, and 
 men in a less marked degree, experience as the result of age. 
 
 The Act of Erection is a vascular phenomenon produced by 
 an engorgement of the erectile tissue of the penis with blood. 
 This engorgement is brought about by stimulation of the nervi 
 erigentes, which are derived from the autonomic system, and 
 arise from the sacral portion of the cord. These nerves furnish 
 dilator fibres to the vessels of the penis, and under their influence 
 the cavernous spaces of the erectile tissue become gorged with 
 blood under pressure. The nervi erigentes act reflexly through 
 an erection centre in the cord, while the erection centre is under 
 the influence of higher centres in the brain. Erection and 
 ejaculation in the dog may be produced by stimulation of a 
 definite area of the cortex of the cerebrum; they may also be 
 produced without the knowledge of the animal — -for example, 
 after section of the spinal cord in the lumbar region. The 
 sensory nerves in the penis, by which erotic sensations are 
 carried, are the pudic; if the pudic nerve be cut erection is 
 impossible; if the central cut end be stimulated it leads to 
 ejaculation. 
 
 The first portion of the penis which receives the excess of 
 blood in erection is the corpus cavernosum ; the spongiosum and 
 glans are not fully erect in the stallion until the penis is within 
 the vagina. The erector penis muscle compresses the penis 
 against the pelvis, and by thus blocking the return of blood 
 assists in maintaining erection. Before ejaculation in this animal 
 the glans swells enormously, resembling the mouth of a trumpet, 
 apparently in order to cover or grasp the os uteri. After inter- 
 course the organ is withdrawn into its sheath by the contraction 
 of a pair of unstriated muscles, known as the retractor penis. 
 Though the organ in the horse assumes such considerable pro- 
 portions, in the bull this is not so. The peculiar penis in this 
 animal undergoes very little vascular engorgement ; it is already 
 firm (see p. 737). In the act of erection the retractor penis 
 
 * In a paper on ' The Growth and Maturity of Animals,' by Dr. E. Crisp, 
 referred to above, the writer states he knew a Suffolk cart-mare that bred 
 a foal at the age -of thirty-nine (Veterinary Review and Stockowners' 
 Journal, March, 1865). The ' Tartar Mare ' was considered to be thirty- 
 four to thirty-six years of age when she bred ' Queen Mab.' ' Driver,' an 
 Australian imported mare, bred ' Moss Rose ' at thirty-three (Standard, 
 December 25, 1893). 
 
 Mr. F. T. Harvey, F.R.C.V.S., informs the writer that he has known 
 a cow breed long after the loss of all her incisors as the result of age. 
 
'758 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 muscle is inhibited, and the S-shaped curve of the penis removed; 
 by this means the organ gains considerably in length. At the 
 same time the retractor muscles of the sheath draw back the 
 prepuce. In the ram, erection and elongation are carried out 
 as in the bull, the vermiform appendix (see p. 737) undergoing 
 peculiar gyrations before penetration. It is believed by many 
 that the vermiform appendix is essential for successful impreg- 
 nation, for if it be removed, it is said, the animal proves sterile; 
 this is certainly not invariable.* In the dog the increase in 
 the size of the penis is mainly at its posterior part, at the 
 bulbous swelling there observable, the bone in the penis of this 
 animal facilitating its introduction into the vagina. 
 
 Sexual Intercourse. — Copulation is not necessary in all 
 animals, nor indeed in any. What is required is merely an 
 interchange of elements from the nuclei of two different kinds 
 of cells. To this last statement a slight exception might be 
 taken, because there is a condition — parthenogenesis — where the 
 access of a second element is not required, but this method of 
 development is unknown in the higher animals. The act of 
 intercourse is of short duration in the majority of animals, 
 excepting the dog and pig. The friction of the male organ 
 leads to ejaculation, a process confined to the male. 
 
 In a vigorous stallion the period of intercourse is from five 
 to ten seconds; when exhausted, from fifteen to twenty seconds. 
 For the bull and ram intercourse is very brief' — one to two seconds. 
 In the pig and dog it is prolonged, the boar occupying about 
 ten minutes. In the case of the dog the actual period of excite- 
 ment to ejaculation is brief — about four seconds— but sixteen 
 or seventeen minutes may elapse before the sexes are parted, 
 owing to the incarceration of the bulbous swelling by con- 
 traction of the sphincter muscle of the vagina. The object of 
 this will be explained presently. 
 
 During coitus the clitoris of the female is erect and the 
 sphincter vagina muscle stimulated to contraction; the vessels 
 of the vulva are dilated, and the uterus is believed to become 
 
 * Mr. Hugh Begg, F.R.C.V.S., has been good enough to make inquiries 
 for me on this point from breeders and others. Mr. M. G. Hamilton, of 
 Cobbinshaw, Lanarkshire, informs him that a famous black-faced ram, 
 ' Radium,' lost the entire appendix through sloughing, owing to an opera- 
 tion for the relief of an impacted calculus. This animal, when there was 
 ' not a vestige ' of the appendix left, was a, sure stock-getter for the 
 next five years, and three of his sons took prizes at the Highland and 
 Agriculture Show at Aberdeen in 1908. Mr. Hamilton adds that since 
 then he has used another ram without the appendix with equally good 
 results, and knows of other stockmen with similar experience. 
 
 Mr. P. Wilson, M.R.C.V.S., of Lanark, says he has removed the appendix, 
 and the animals have subsequently been successfully used foi stud pur- 
 poses. 
 
GENERATION AND DEVELOPMENT 759 
 
 erect and undergo a series of peristaltic contractions, the os 
 dipping down into the pool of seminal fluid and probably sucking 
 it in by aspiration. These movements have actually been 
 observed in the rabbit. The glands of the vaginal canal also 
 pour out a secretion. 
 
 Ejaculation is a complex reflex act in which the seminal 
 tubes of the testicle, the epididymis, vas deferens, and accessory 
 glands are all stimulated by a definite series of muscular con- 
 tractions to press out their contents, the bulbo-cavernosus and 
 ischio-cavernosus muscles by rhythmical contractions forcing 
 the fluid along the urethral canal. This act is accompanied by 
 up and down movements of, the tail in the horse and by alternating 
 movements of the hind-feet in the dog. It is only recently 
 that anything like precise information of the details of ejacu- 
 lation has been obtained through the researches of E. Iwanow,* 
 whose work we will now follow. 
 
 In the horse the semen is ejaculated by an intermittent jet; 
 the first portion consists of a little clear fluid from the urethral 
 glands, which it is thought is intended for the purpose of washing 
 out and disinfecting the urethra (see p. 742); the next secretion 
 is that produced by the glands of Cowper ; then follows the true 
 seminal fluid, which is diluted in the prostatic portion of the 
 urethra by the secretion of the prostate (see p. 742); and, finally, 
 a rather gelatinous and thickish mass of material, believed to 
 be derived from the vesiculas seminales, and containing few, if 
 any, spermatozoa (p. 742). 
 
 An emission in the horse is from 50 to 100 c.c. (1-75 to 3-5 
 ounces), though Iwanow states he has measured emissions of 
 300 c.c. 
 
 In the bull the amount ejaculated is about 100 c.c. (3-5 ounces), 
 and in. the ram from 2 to 5 c.c. (34 to 84 minims). Iwanow 
 gives very detailed information in respect of the dog. In this 
 animal there are three stages of ejaculation. The first lasts 
 from 6 to 26 seconds, followed by an interval of from 8 to 
 26 seconds; then succeeds the second ejaculation, which lasts 
 10 to 44 seconds; this is followed by a third ejaculation, which 
 occupies from 4 to 16 minutes or longer. The amount of fluid 
 ejaculated at the first and second evacuation is in each case 
 from 1 to 2 c.c. (17 to 34 minims); the third ejaculation is much 
 larger, and amounts to 10, 15, 20,. or even 29 c.c. (0-35, 0-5, 0-7 
 to 1 ounce). 
 
 This amount of fluid, which obviously must vary depending 
 upon the size of the animal, is largely due to the activity of the 
 prostate; the other accessory glands take very little share, and 
 there are no vesiculae in the dog; nor is the seminal fluid derived 
 
 * Op. cit. 
 
760 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 directly from the testicle, but from the reserves in the epi- 
 didymis and the ampullae of the vas deferens. Hitherto it has 
 been impossible to explain why the canine species should be 
 locked together; , we now know that it is due to the prolonged 
 character of the ejaculatory reflex. 
 
 Probably in no animal is "the whole of the seminal fluid retained. 
 In the case of the mare there are expulsive efforts, apparently 
 not under the control of the will, though every effort is made to 
 prevent them occurring. As far back as the seventeenth century 
 the method was to give a gallop immediately the stallion had 
 descended; the Arabs and other nations have employed water 
 thrown on the hind-quarters of the mare. It is quite clear that 
 with such a large ejaculation as the stallion possesses there must 
 be an outflow of fluid, the greater part of which, as we have seen, 
 is sterile. In the case of the cow there is a slight loss, but not 
 invariably so. What occurs at the height of the orgasm is by 
 no means clear. The stallion has a projecting urethra and a 
 trumpet-shaped glans which is quite capable of embracing the 
 os, while the projecting urethra enters it or is applied closely 
 to it. Whether some sperm is actually injected into the uterus, 
 as we believe, can only be settled by experimental inquiry. 
 The bull, ram, and boar are also provided with means of effect- 
 ing a direct entry into the uterus. The vermiform appendix of 
 the ram may actually enter the os. but at present nothing 
 definite is known. In the case of the dog the penile bone and 
 the prolonged period of vascular engorgement are eminently 
 favourable for the passage of fluid into the uterus, but again 
 the point is not clearly known. In both the dog and rabbit 
 spermatozoa have been found in the uterus immediately after 
 coition, which is suggestive of direct entry. In all animals the 
 aspiratory action of the uterus must also be taken into account 
 in deciding this question. Artificial insemination, now so 
 widely practised, depends very largely for its success on the 
 introduction of the seminal fluid into the uterine cavity. It 
 is well known, however, that in intercourse conception has 
 followed when the semen has penetrated no farther than the 
 inside of the vulva. As we have already seen, spermatozoa 
 may remain alive for many hours outside the body if kept under 
 suitable conditions of warmth and darkness (see p. 741); while 
 in the passages of the female the precise limit of their vitality 
 is unknown; a fortnight is probably the maximum period. 
 F. B. Sumner* is of opinion that in the case of mice the sperma- 
 tozoa may remain alive for weeks after their reception into the 
 uterus or Fallopian tube, and it is known that in the bat they 
 may remain alive for some months after coitus. 
 
 * Biological Bulletin (U.S.A.), vol. xxx, 
 
GENERATION AND DEVELOPMENT 761 
 
 Fertilisation. — The activity of the spermatozoa is the means 
 by which fertilisation is effected; they swim or move by a wave- 
 like motion of the tail; the head remains motionless. Iwanow 
 tells us that there are 841 millions in the seminal evacuation 
 of the dog, but as only one is required for a single ovum there 
 is an enormous waste of fertilising material, and this occurs 
 both in animals and plants. The vast majority of the sperms 
 must therefore die, and the seat of their disintegration must be 
 throughout the length of the uterine horns and Fallopian tubes. 
 Guided by-some extraordinary instinct, if the term may be 
 employed, the spermatozoa have only one objective, and that 
 is to reach the Fallopian tubes. The presence of cilia in these 
 offers no obstacle, though they create a current in the opposite 
 direction to that in which the spermatozoa are travelling; in fact, 
 it is said that an opposition current renders their movements 
 more vigorous. In due course they reach the tubes, and in 
 the case of the rabbit have been found at the upper end of the 
 Fallopian tube nine or ten hours after coition. The periods in 
 other animals are unknown ; they are probably , variable, but 
 under no circumstances would it appear to be longer than a 
 day or two. 
 
 Fertilisation of the Ovum. — Somewhere in the Fallopian 
 tube, though occasionally on the surface of the ovary, the meet- 
 ing between the sperm and germ cell occurs; what is known 
 of the mechanism of the passage of the ovum from ovary to 
 Fallopian tube is described at p. 744. Owing to the enormous 
 number of sperm cells existing even in a droplet of the secretion, 
 it is easy to understand why the ovum cannot escape coming in 
 contact with them. A single spermatozoon penetrates the wall 
 of the ovum, and enters the cytoplasm. Others may attempt 
 to follow, but the surface of the egg at once becomes impervious 
 to further attacks. The spermatozoon, having got inside the 
 egg, loses its tail, which is no longer required. The essential 
 portions of the organism are the head and middle piece. The 
 former is the cell with its nucleus, which contains the all-im- 
 portant chromosomes ; the middle piece contains the centrpsome, 
 which, it will be remembered, was lost to the mature ovum. 
 There are now two nuclei within the egg — one the male, the other 
 the female pronucleus. These meet, fuse, and, under the direc- 
 tion of the centrosome, the process of cell division immediately 
 follows. The union of these two nuclei is a matter of extra- 
 ordinary importance. It will be remembered that by a process 
 of reduction the chromosomes in the ovum and spermatozoon 
 were reduced in each case to one-half. As the result of the 
 union of the nuclei, a segmentation nucleus is formed. From 
 this segmentation nucleus the future animal is formed. The 
 
762 A MANUAL OR VETERINARY PHYSIOLOGY 
 
 segmentation nucleus divides, the chromosomes it contains 
 split, so that each division of it contains the number of chromo- 
 somes normal to the species, the essential feature being that 
 each parent is represented in each division of the cell and in 
 all subsequent subdivisions. It necessarily follows that not only 
 are the parents represented, but in a less degree the ancestors 
 on both sides (see p. 799). 
 
 Fig. 259.— Section of a Rabbit's Ovum at the Close of Segmentation. 
 II., III., IV., Stages_in the Formation of the Blastodermic Vesicle 
 (E. von Beneden). 
 
 Z.R., Zona radiata; Ex.L., external layer of cells; I.M., inner mass of cells- 
 I.L.M., inner lenticular mass of cells; s.c, segmentation cavity. 
 
 It will be observed that the spermatozoon and ovum are the 
 complements of each other. The ovum contains an abundance 
 of cytoplasm and a nucleus, but no centrosome; the spermatozoon 
 contains no cytoplasm, but both a nucleus and a centrosome- 
 the characteristic common to the two is that they both contain 
 the same number of chromosomes. There is a great deal of 
 experimental work which tends to show that if to the ovum a 
 
GENERATION AND DEVELOPMENT 763 
 
 centrosome be supplied, or if to the spermatozoon some cyto- 
 plasm be given, development will occur without the aid of 
 spermatozoon or ovum.* Loeb has shown that by physico- 
 chemical means the eggs of the sea-urchin may be made to 
 develop into larva:, though normally a spermatozoon is required 
 for this purpose. 
 
 When segmentation of the ovum is completed, as indicated 
 in Fig. 259, I., a sphere is formed (Fig. 259, IV.), consisting of 
 
 Fig. 260. — Diagram of a Longitudinal Section of a Mammalian Ovum 
 
 AFTER THE COMPLETION OF THE AMNION (ShARPEY-SCHAFEr). 
 
 an outer cellular layer, formed of single cells {Ex.L.) enclosing 
 a cavity known as the segmentation cavity (s.c). The sphere is 
 called the blastodermic vesicle. It contains broken-down yolk, and 
 at one part some smaller cells (I.L.M.), which probably spread 
 over the inner wall of the sphere and form an inner cellular layer. 
 The outer cellular layer (Ex.L.) is the epiblast ; the inner cellular 
 layer (I.M., II.) is the hypoblast. While the latter is spreading 
 
 * There are some eggs which normally develop without the intervention 
 of the male element; this is known as pathogenesis. 
 
764 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 over the inner wall, a white disc is developed at one point of the 
 vesicle; this disc is the germinal area. In this area the embryo 
 develops. A development of the cells of the epiblast leads to 
 the formation of a groove in the area, known as the primitive 
 groove , the sides of the groove grow up, meet, and enclose a 
 space — -the neural arch — and from this arises the cerebro-spinal 
 system. A third layer of cells now develops between the epiblast 
 and hypoblast, and is spoken of as the mesoblast. 
 
 The mesoblast furnishes a layer of cells under the neural canal, 
 the notochord (Fig. 260); from these the bodies of the vertebrae 
 develop. The mesoblast splits: the outer layer of it unites 
 with the epiblast, and so forms the somatopleure , the inner 
 unites with the hypoblast, and forms the splanchnopleure. 
 As the result of the splitting, a cavity is formed between the 
 layers, known as the pleuro-peritoneal space, or ccelom. This is 
 the body cavity of the future embryo. From the three layers 
 of cells developed around the ovum the following tissues of the 
 embryo are formed: 
 
 From the Epiblast : 
 
 The whole of the nervous system, both cerebro-spinal and 
 
 sympathetic. 
 The epithelial sensory end-organs of the nerves of special 
 
 sense, and the crystalline lens. 
 The epidermis and its appendages — hair, hoof, claws, horns. 
 The epithelium of all glands opening on the surface of the 
 
 skin, including mammary, sweat, and sebaceous. 
 The epithelium of the mouth" (not the tongue) and glands 
 
 opening into it, the enamel of the teeth, and the epithelium 
 
 of the anus. 
 The epithelium of the nasal passages and facial sinuses. 
 
 From the Mesoblast : 
 
 The bones, cartilages, and connective tissues of the entire 
 
 body. 
 The whole of the muscles, skeletal and visceral. 
 The entire vascular and lymphatic system — blood-corpuscles, 
 
 spleen, and serous membranes. 
 The generative organs and generative elements. 
 The urinary organs. 
 
 From the Hypoblast : 
 
 The epithelium of the alimentary canal and the ducts of all 
 
 glands opening into it. 
 The epithelium of the respiratory organs. 
 The cells of the liver and pancreas. 
 The epithelium of the bladder, ureters, thyroid body, and 
 
 part of thymus gland. 
 
 During the growth of the mesoblast the embryo, which is 
 developing in the germinal area, is gradually being lifted off the 
 blastodermic vesicle by the formation of a sulcus or depression 
 
GENERATION AND DEVELOPMENT 765 
 
 which extends around it. The embryo at this period possesses 
 within it two tubes — one dorsal (the neural canal), the other 
 ventral (the alimentary canal). The latter opens in front and 
 communicates with the yolk sac (Fig. 260), the earliest source 
 of nourishment to the embryo. 
 
 It is not our intention to carry the development of the ovum 
 beyond the above point. It is, however, most essential to 
 understand in what way the ovum behaves on entering the 
 uterus, how it is nourished before and after the circulation is 
 established, and the nature and function of the various mem- 
 branes within which it is contained. 
 
 Implantation of the Ovum. — At every monthly period in the human 
 female the mucous lining of the uterus undergoes certain changes, 
 which result in the formation of a membrane known as the decidua ; 
 this is in shape a counterpart of the interior of the uterus. The 
 membrane is shed during menstruation; if the woman becomes 
 pregnant the decidua is not exfoliated, but undergoes further 
 development in connection with the ovum. The latter on its 
 arrival in the uterus becomes embedded in the folds of the mucous 
 membrane, which grow up around and anchors it to the wall of the 
 uterus. That portion of the mucous membrane which grows over 
 and envelopes the ovum is known as the decidua reflexa, that which 
 lines the interior of the uterus is known as the decidua vera. 
 
 In that portion of the human decidua between the ovum and the 
 uterine wall known as the decidua reflexa or basalts is formed the 
 placenta, a cake-like mass of vascular tissue in which the interchange 
 of blood between the mother and foetus occurs. The placenta 
 normally forms at the upper part of the body of the uterus, so that 
 the ovum hangs from the roof. The reflexa lying between the 
 ovum and uterus consists of two layers: that nearest the ovum is 
 a compact layer, that next the uterus is a spongy layer, the spongy 
 appearance being due to the dilated uterine glands. From the 
 surface of the ovum, processes (chorionic villi) grow into the com- 
 pact layer and then into the spongy layer, where in due course 
 they meet with the maternal blood, in which they are bathed. The 
 villi accordingly bring about tissue destruction, either by phago- 
 cytosis or enzymic action, and this erosion is effected by a special 
 layer of cells belonging to the embryo known as the irophoblast. 
 The function of the trophoblast in all animals is to effect connection 
 between the mother and the embryo. 
 
 While this connection is being effected the .embryo is growing 
 and being nourished by means which need not detain us ; blood is 
 being formed in it which leaves the body in a structure known as 
 the connecting stalk ; it is the growth of these vessels (villi) through 
 the layer of .the decidua basalis into the excavated blood-sinuses in 
 the uterus of the mother, which culminate in the placenta. 
 
 The Placenta. — We need not follow the formation of the 
 human placenta any further, but what has been said will enable 
 the arrangement in animals to be better understood. 
 
 No domesticated animal, with the exception of the dog and 
 cat, has anything resembling a decidua, and even in these the 
 
766 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 resulting placenta is quite different from that of man. Con- 
 sequently, a primary classification of animals has been into 
 deciduate and indeciduate, but these terms are no longer retained 
 for morphological purposes, as the chief interest is centred, not 
 in the decidua, but in the group of cells described above as 
 trophoblastic, the function of which in all placental mammals 
 is to establish a nutrient connection between the embryo and 
 the parent. This group of cells belongs to the embryo. Where 
 there is a decidua they work as above described; where there is 
 no decidua they work direct into the various pits and crypts 
 in the mucous membrane of the uterus, but do not destroy the 
 mucous membrane; therefore they do not create blood-sinuses 
 in the walls of the uterus, and the foetal villi are not bathed in 
 maternal blood, but merely brought sufficiently close to the 
 vessels of the mother to obtain the necessary exchange of blood, 
 gases, and nutriment. 
 
 If this sketch of deciduous and indeciduous mammals is 
 understood, the first step is taken towards the comprehension 
 of a very complex subject, many of the details of which still 
 await settlement. One further point in the comparison must, 
 however, be mentioned. In man the blood which finds ^ts way 
 into the chorionic villi of the embryo and establishes the placenta 
 is carried by the ' connecting stalk ' ; whereas in all domesticated 
 animals the placenta is vascularised by the allantoic sac, a large 
 and important structure, which in man is of extreme insignifi- 
 cance. 
 
 Even the deciduate dog and cat have the placenta vascularised 
 from the allantois. 
 
 Types of Placenta. — It- is convenient here to draw attention 
 to the appearance presented by the placenta in various animals. 
 In man it is a circular, flat cake, about 6 inches in diameter and 
 i inch in thickness, located in the body of the uterus as far 
 away from the internal os as possible. The reason is obvious; 
 if near the os, the placental connection with the child is destroyed 
 in the early stages of labour, and, further, the woman frequently 
 bleeds to death. This condition is known as -placenta prcevia, 
 and is impossible in the domesticated animals other than the 
 cow, in which a cotyledon has been known to lie near the os. 
 
 In the dog and cat the placenta consists of straps or bands 
 of villi surrounding the elongated ovum, the straps being placed 
 well apart. This is a zonary placenta. 
 
 In the ox and sheep the placenta consists of sixty or more 
 vascular knobs {cotyledons) scattered over the enveloping mem- 
 brane of the foetus, and directly attached to a similar number 
 of cotyledons on the mucous membrane of the uterus of the 
 mother. This is a cotyledonary or multiple placenta. 
 
GENERATION AND DEVELOPMENT 767 
 
 In the horse and pig the placenta consists of one large vascular 
 sheet completely encircling the foetus, and consequently inti- 
 mately in contact with the mucous membrane of the entire 
 uterine horn or horns in which development occurs. This is 
 known as a diffuse placenta, and is the most primitive variety 
 known. 
 
 The vascular or semi -vascular membrane of the above types, 
 the villi of which, by union with the uterine wall, form the 
 placenta, is known as the chorion. 
 
 Nutrition of the Embryo. — The fertilised egg does not pass 
 rapidly through the Fallopian tubes to the uterus. In the sheep 
 the period occupied is four to five days; no definite periods have 
 been fixed for the other domesticated animals, but the time is 
 known to be comparatively long in the dog and cat. Probably 
 in all the ovum has reached the uterus in the course of eight 
 days. In the tube it is still surrounded by the zona pellucida 
 (p. 744) and a sticky gelatinous layer (the prochorion). The 
 latter furnishes nutriment until better arrangements can he 
 made. On its arrival in the uterus the bloodvessels and 
 lymphatics of this organ dilate, the mucous membranes become 
 oedematous, and a glandular secretion is poured out for the 
 purpose of nutrition. The egg is not at once fixed to the 
 wall of the uterus in domesticated animals; it has a period 
 of freedom which varies in the different species. In the sheep, 
 for instance, it is free until the seventeenth day, and its 
 attachment to the mucosa is completed by the thirtieth day 
 (Assheton). During this time, however, the development of 
 the ovum is not delayed. The prochorion ruptures at the 
 ninth day; the trophoblast is formed, and eventually comes 
 into contact with the wall of the uterus; the germinal vesicle 
 elongates and fills the uterine horn, or both horns if only one 
 embryo is present (Marshall). The yolk sac (yet to be described) 
 is formed, and the allantois takes shape and grows rapidly, 
 furnishing the blood which forms the villi seen on the twenty- 
 eighth day on the external covering of the embryo. These 
 villi fit into crypts in the cotyledons. By the forty-fourth 
 day the foetal cotyledons are scattered over the whole surface 
 of the embryonic covering, and by the seventy- eighth day the 
 general character of the placenta is established (Assheton). In 
 the pig the impregnated ova are spherical until the tenth day; 
 then they elongate, and by the tenth day fill the horns of the 
 uterus, which is contracted between each ovum. In the mare 
 no details are known of the early behaviour of the ovum ; accord- 
 ing to Ewart, it is usually located in the right horn. For the 
 nutrition of the horse embryo from the fourth to the ninth week 
 see p. 775, where it is inserted as a matter of convenience in con- 
 
768 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 sidering the question of the death of the embryo. In the dog 
 and cat, owing to the thickness of the gelatinous layer (pro- 
 chorion) the ovum does not become adherent to the uterus until 
 the twentieth day in the dog, and the twelfth day in the cat. 
 No matter what delay occurs in attachment, the development 
 of the ovum continues to progress, its nutrition being furnished 
 by the fluid found in the uterus, and presently by the tropho- 
 blast and yolk sac. 
 
 The trophoblast, a layer of cells of extraordinary importance, 
 is formed on the outer enveloping membrane of the ovum; its 
 function, as previously indicated (p. 766), is to form a vascular 
 connection with the uterine wall, and so furnish nutriment to the 
 developing egg. It takes no part whatever in the formation of 
 the embryo or its membranes. In animals possessing no decidua 
 (herbivora, etc.) it first becomes adherent to, and then eats its 
 way into, the mucosa, probably by phagocytosis. The epithelium 
 and glandular structure are reduced to a debris, and on this the 
 trophoblast lives, ingesting the blood-cells and leucocytes. In 
 cotyledonary placentae it also attacks these enlargements ; there 
 being no glandular substance within them, their destruction is 
 less; but there occurs effusion of blood, which the trophoblast 
 utilises, while at the same time it is preparing the cotyledons for 
 the reception of the villi which are presently pushed out, and 
 gain attachment to them. Not only does the trophoblast take 
 up broken-down tissue cells and liberated blood-cells, but also 
 uterine milk furnished by the glands of the uterus, and this in 
 ruminants is the most important source of nourishment taken 
 up by the trophoblast. In the deciduata (carnivora), as in the 
 indeciduata, the mucous membrane of the uterus is previously 
 prepared for the ovum; the epithelium is swollen, the glands 
 and crypts enlarged, the spongy and compact layers (p. 765) 
 are formed in the region where the trophoblast will unite. The 
 trophoblast, followed by villi pushed out by the developing 
 embryo, penetrates to the capillary layer of the uterus, leading 
 to a disappearance of the maternal tissues less the capillary 
 vessels, which are left as a skeleton, into and between which 
 the villi ramify. The spongy layer is finally reached and con- 
 verted into labyrinths, and here the maternal and fcetal blood 
 meet, separated only by the endothelium of the maternal and 
 fcetal capillaries. Haemorrhages occur in places. The site of these 
 may be seen in the dog and cat in the region known in the former 
 as the ' green border ' and ' green pockets ' of the placenta 
 (Marshall). Here the trophoblast finds abundant nourishment 
 to send back to the embryo through the villi in the form of 
 broken-down tissue and cells, which are dealt with by phago- 
 cytosis. It will be observed that in the haemorrhage areas the 
 
GENERATION AND DEVELOPMENT 7 6g 
 
 trophoblast is in direct contact with effused maternal blood. 
 So good is the blood-supply that the glandular secretion from 
 the uterus, so well marked in herbivora, plays a very unimpor- 
 tant part in the nutrition of the embryo of carnivora. 
 
 The Yolk Sac or Umbilical Vesicle. — While the vascular 
 connection above described is being effected, the nutrition of 
 the embryo is to an extent looked after by the yolk sac. In 
 birds this contains yolk; in the large majority of mammals it 
 does not, but merely an albuminous fluid which is very soon 
 exhausted. The yolk sac is one of the vestiges of an early 
 ancestor, and is functionally active but for a few days until the 
 circulation with the mother is established. In the sheep, pig, 
 and cow it is of use until the twentieth day of pregnancy; in 
 the mare it persists to about the end of the eighth week. In the 
 dog it is large, and, according to some, persists to birth, though 
 not functional during the whole of the time. Broadly speaking, 
 it functions for about three weeks, but may persist longer, 
 during which time it is unimportant. It is derived from a portion 
 of the ovum which is not set apart' for the development of the 
 embryo, and is attached to the latter at the umbilicus through 
 the medium of a stalk which subsequently becomes the umbilical 
 cord; to the mother it is attached by the layer -of cells so fre- 
 quently spoken of as" the trophoblast. It contains in its sub- 
 stance some granules; its walls are vascular,' 'and it supplies 
 nourishment to the embryo additional to the material which the 
 trophoblast takes up from the fluid in the uterine cavity, and 
 the material which it extracts from the uterine wall during the 
 formation of the placenta. As the placental circulation becomes 
 more thoroughly established the yolk sac shrinks, and finally 
 forms a part of the umbilical cord. Figures of the yolk sac 
 in the bird and the embryo of the horse at different stages of 
 development are seen in Figs. 260, 261, 264 and 265. 
 
 Uterine Milk. — If the placenta be separated from the wall 
 of the uterus, especially in ruminants in the region of the cotyle- 
 dons, a white or pinkish-white fluid may be expressed, cream- 
 like in consistency, containing protein and fat in considerable 
 quantities, and a small proportion of ash. This is the uterine 
 milk, believed to be peculiar to the ruminant. We have already 
 seen that a fluid secretion from the uterine glands is poured out 
 in greater or less quantities into the uterus of all animals, though 
 it is small in amount in the carnivora; in none does it present 
 the creamy appearance seen in cattle and sheep. It is perhaps 
 permissible to refer to any uterine secretion intended for the 
 nutrition of the embryo as uterine milk, though it is not the 
 same in composition in all species. In that of ruminants rod- 
 like crystals abound which were supposed at one time to be 
 
 49 
 
770 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 crystals of protein, but the latest view is that they consist of cal- 
 cium oxalate. In all species of animals the cellular elements of 
 the uterine secretion are abundant, derived from the epithelium 
 of the uterus, its gland ducts, and the extravasation of blood- 
 cells. It is apparent that the composition of uterine milk is 
 perfectly adapted for the phagocytic activities of the trophoblast, 
 and later of the placenta, which transfer the nutritive material 
 to the blood of the embryo. Inasmuch as the cotyledons do not 
 contain glandular tissue, it has been urged that the fluid part of 
 the secretion cannot occur from these proj ections, but only from 
 the area between the cotyledons. Assheton has shown that in the 
 sheep the glandular secretion diminishes towards the end of 
 pregnancy, in consequence of the destruction of the uterine 
 
 cLZlantois 
 
 asruiconj 
 
 — - airspace. 
 
 yolk-sizc 
 
 ste& 
 
 Fig. 261. — Hen's Egg at the Ninth Day of Incubation (Ewart, after • 
 Milnes Marshall). 
 
 glands, but that at the upper end of each uterine horn the 
 mucous membrane furnishes an active secretion from its epi- 
 thelial layer. 
 
 Membranes of the Embryo.— We shall first consider the 
 bird, and by so doing prepare the way for a better understanding 
 of the mammal. 
 
 If the egg of the hen be examined after incubating nine days, 
 the appearance seen in Fig. 261 presents itself. A chick in an 
 advanced stage of development is bound within a thin tough 
 skin containing fluid; this water-jacket is known as the amnion 
 and its use is to prevent jar when the egg is moved. The supply 
 of food required by the embryo chick during development is 
 contained in the yolk sac ; to this food-supply the embryo is 
 connected by a stalk through which the nourishment enters 
 
GENERATION AND DEVELOPMENT 77 i 
 
 its body. The walls of the yolk sac are vascular and connected 
 with the vessels of the embryo. It is through the medium of 
 the vascular wall that the altered yolk is taken up. The chick 
 has another foetal appendage known as the allantois ; it grows 
 out from the body, being connected to the embryo by means of 
 a stalk, and forms a vascular sac through which blood from the 
 chick's body circulates. The allantois in the chick is a breathing 
 organ; the air enters through the pores of the shell, and the 
 
 Fig. 262. — Diagram of the Foital Envelopes of a Five Months Horse 
 Embryo (Bonnet). 
 
 blood takes up oxygen from the air surrounding the allantoic 
 sac; an air space also exists at the end of the shell. 
 
 Amnion. — Precisely as does the chick, the mammal possesses 
 an Amnion, derived from a portion of the embryo, which grows 
 completely around its body (Fig. 260), forming a sac containing 
 fluid, the function of which is to act as a water-bed to the foetus 
 and save concussion. In the horse it is .fully formed at the 
 twenty-eighth day. In ruminants the amnion contains a number 
 of white patches, particularly on the cord; these amniotic pl'ates 
 
772 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 c 
 
 C 
 
 are rich in glycogen, so that their function may be connectec 
 with the nutrition of the foetus. Their period of functiona 
 activity is probably only temporary, as they undergo atrophy 
 
GENERATION AND DEVELOPMENT 773 
 
 after the sixth month. Liquor amnii is an alkaline fluid, yellow- 
 ish-red during the early days of gestation, but reddish towards 
 the end of it, probably owing to discoloration with foetal feces 
 or meconium. The fluid contains protein, urea, sugar, lactic 
 acid, keratin, and some salts. Besides these, there are in hoofed 
 animals portions of hoof, and in all animals epithelium and hair 
 derived from the foetus. The source of the fluid is transudation 
 from both mother and foetus; indigo blue injected into the veins 
 of the mother stains the amniotic fluid, but not the foetal tissues. 
 During parturition the fluid helps to dilate the os, and lubricates 
 the maternal passage. 
 
 Fig. 264. — Semi-diagrammatic Representation of a Four Weeks Horse 
 Embryo and its Fojtal Appendages, Natural Size (Ewart). 
 
 am., The amnion; y.s., the yolk sac, which is vascular (v) as far as the circular 
 bloodvessel (si.), and crowded with granules; a, b, c, the trophoblast; all., the 
 allantois; -d, sac containing the embryo; t.g., the girdle; v', vessels in wall of 
 allantois. The embryo measures nearly § inch in length, and is curved so 
 that the tail lies under the head. The limbs are represented by lobes; the 
 rudiments of the limb skeleton have not yet- appeared. 
 
 The Allantois grows out from the body of the embryo at the 
 future umbilicus (Fig. 260). The part within the body forms 
 the bladder, that outside it forms a sac which in the mare 
 completely envelops the amnion (Fig. 262) on the one surface, 
 and on the other lines the chorion. As it is a complete 
 replica in shape of the chorion, it possesses a body and two 
 horns. In ruminants it is an elongated sac which does not 
 completely line the chorion, though it projects into both horns 
 (Fig. 263). The bladder and the cavity of the allantois are 
 connected by a funnel-shaped canal in the umbilical cord, 
 known as the urackus. The remains of this may be seen 
 in the adult as a scar on the fundus of the organ. The fluid 
 
774 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 found in the allantois is derived from the foetal urine. In the 
 first instance it is colourless or turbid, later on it becomes brown 
 in tint. This fluid contains urea and a substance known as 
 allantoine; also protein, sugar (levulose), lactic acid, and certain 
 salts. The allantois is the organ of respiration, and to an extent 
 of nutrition. During early foetal life the vascular wall of the 
 allantois is able to bring the blood of the embryo sufficiently 
 near to that of the uterus to cause an exchange of gases to be 
 effected. Later it furnishes the villi which penetrate into the 
 walls of the uterus, and vascularises the placenta in all domesti- 
 cated animals. In the mare the function of the allantoic fluid 
 is to provide a second water-jacket for the foetus. At parturition 
 it acts hydrostatically by dilating the os uteri and vaginal 
 passage. 
 
 In the allantoic fluid of the mare, or attached to the wall of 
 the sac, are certain peculiar masses termed hippomanes ; these 
 are doughy, oval masses contained in a capsule, yellowish-brown 
 in colour, and about 2 or 3 inches in length. Their origin 
 and use are quite unknown. It is commonly believed that these 
 bodies are found in the foal's mouth at birth, but this is a fallacy, 
 for they belong to the allantoic sac, whereas the foetus lives in 
 the amnion.* Hippomanes have also been observed in the 
 cow.t 
 
 The Chorion, or outer covering of the foetus, envelops the two 
 sacs previously mentioned. In shape it is an exact replica of the 
 uterus, and consists of a body and two horns. In the equine it is 
 closely adherent to the uterine wall through the medium of the 
 minute villi which stud its outer surface; its inner surface is lined 
 by the allantois. In ruminants also it consists of a body and 
 two horns, but otherwise differs greatly in appearance from that 
 of the horse, as it is vascular only in the region of the cotyledons. 
 In the pig and dog the chorion is an elongated sac without body 
 or horns. Through the umbilical cord it forms the vascular 
 connection between the foetus and the mother, while the villi 
 on its surface, furnished by the allantois growing through 
 (Fig. 265), project themselves into the mucous membrane of the 
 uterus, and are thus brought close to the maternal vessels. The 
 chorion is the vascular covering through which the blood from 
 the foetus is conveyed to the mother for the purpose of getting 
 rid of its carbon dioxide, and receiving in its place oxygen and 
 nutriment. The villi in the horse are not more than one-eighth 
 
 * Mr. F. T. Harvey informs me he has frequently felt in the mouth of 
 the foal before delivery and has never found hippomanes within it. 
 
 f It is a curious fact that even at the present day, in some country 
 districts, hippomanes are sought for in virtue of the properties they have 
 been supposed to possess from time immemorial — viz., for use as love 
 philtres, 
 
GENERATION AND DEVELOPMENT 775 
 
 of an inch in length (Ewart), and consequently the chorion is 
 readily detachable from the interior of the uterus at birth. The 
 villi of the cotyledons in ruminants are also not difficult to 
 separate. 
 
 Death of the Embryo. — An impregnated ovum which has 
 become linked up with the mother in the manner described in 
 the previous pages does not always necessarily live through 
 the whole process of development. It may die from failure of 
 nutrition, the causes of which are various and some not clearly 
 understood. In the pig, for example, living and atrophied 
 foetuses may be found side by side in the 'uterus (Hammond).* 
 In the mare death of the embryo is more serious, especially in 
 the case of thoroughbred stock, whose fertility is extremely low, 
 and where the date of foaling is all-important. ' Breaking 
 service,' as it is known in the language of the stud,- or detach- 
 ment of the embryo, is common from the sixth to the ninth 
 week, and has formed the subject of special inquiry by Ewart. f 
 He has shown that the embryo at four weeks old is held in 
 position on the uterine wall by the layer of trophoblast cells 
 (Fig. 264, a, b, c) strengthened by a ring of tissue (an.) which 
 crops up around them. The ovum is suspended in the uterine 
 horn upside down by the stalk of the yolk sac to which the 
 trophoblast cells are attached. This position, which is also the 
 normal one in man, would appear to invite detachment. By 
 the fifth week a girdle (Fig. 264, t.g.), peculiar to the horse only, 
 has* appeared around the equator of the ovum, and is adherent 
 to the uterine wall. It is an additional means of security, and 
 is now most useful, for the trophoblast area is suffering a 
 reduction in the size of its attachment. At the sixth week the 
 weight of the embryo is twice that at the fifth week; the 
 trophoblast adhesion (Fig. 265, a-c) continues to get smaller, 
 and the girdle, (t.g.) has moved from the equator to the pole— 
 i.e., nearer to the trophoblast. In other words, the attachment 
 is less secure ; nutrition also is failing, as shown by the reduction 
 in the size of surface which unites the yolk sac to the uterus, and 
 the chances of the ovum being cast off should the mare be ' upset ' 
 from any cause (such as recurrence of oestrus), are considerable. 
 Ewart regards the end of the sixth week as the period at which 
 the primitive ancestors of the horse were born. During the 
 seventh week the function of the yolk sac is almost exhausted, 
 and the allantois is not yet fully developed, though three times 
 
 * Journal of Agricultural Science, vol. vi., part iii., 1914. 
 
 t ' A Critical Period in the Development of the Horse, ' by Professor Cossar 
 Ewart M.D., F.R.S., 1897. This paper should be read by all interested in 
 breeding and especially by the veterinary profession. It is through this 
 communication that our knowledge of the horse embryo from the fourth 
 to the ninth week of development is so complete. 
 
776 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 larger than it was during the preceding week. The girdle, 
 however, is stronger. By the middle of the eighth week the 
 allantois has put forth the villi which eventually form the 
 chorion, and these are lodged in pits in the uterine mucous 
 membrane; if the development of these fail from any cause, 
 the embryo is doomed. When the embryo has reached the 
 ninth week the placental circulation is established and it is 
 now safe. 
 
 The Umbilical Cord.— This is the channel by which the blood 
 is passed from foetus to mother and from mother to foetus. It 
 contains two arteries and a vein; in ruminants two veins. In 
 
 Fig. 265- 
 
 -A Seven Weeks Horse Embryo, 
 (Ewart). 
 
 Half Natural Size 
 
 all., Allantois, which has greatly increased in size; am., amnion; c.v., non- vascular 
 villi (coelomic) lying between the allantois and the yolk sac, not hitherto 
 found in any mammal, and function unknown ; y.s., yolk sac; a-c, the tropho- 
 blast; v', villi of allantois preparatory to growing through the wall and 
 forming t.t., the chorion; t.g., the girdle now moved to the pole; s.t., circular 
 bloodvessel; all. 2 , allantois in connection with amnion; all. 3 , all.*, part of 
 allantois in connection with the yolk sac. 
 
 its substance is enclosed the now unnecessary yolk sac; the 
 cord also contains the urachus, the tube leading from the fcetal 
 bladder to the allantois. On reference to Fig. 262 it will be seen 
 that in the horse there is an amniotic and allantoic portion of the 
 cord. The amniotic portion is by far the longest; it lies slack in 
 the amniotic cavity, and presents the appearance of a twisted 
 rope. The cause of the twisting is unknown. The allantoic 
 portion is shorter, less twisted, and terminates in the chorion in 
 the upper part of the wall of the uterus between the two horns. 
 The entire cord is covered by a layer of connective tissue known 
 as Wharton's jelly. 
 
GENERATION AND DEVELOPMENT 777 
 
 In ruminants, owing to the elongated character of the allan- 
 tois, the umbilical cord passes to the chorion direct after 
 traversing the amnion, so that there is only an amniotic portion 
 (Fig. 263). 
 
 Foetal Circulation. — With the formation of the foetal circula- 
 tion the blood follows a course altogether different to that in 
 the vascular area of early embryonic life. JjnpureJsloocLthat 
 Jias-circula-ted through the tis sues o f the developing young is 
 _bjaugh±Jj[iJhej3lj^nta by tjhe umbilical arteries these acting 
 to_J±£JLqetus as the pulmonary arteries to the adult^ After an 
 interchange of~gases — ancr~a~ renewal ~of~ food supply, the blood 
 is carried back to the foetus by means of the umbilical vein, 
 or veins, found in the cord. The vein enters the body at the 
 jay^Lci umbilicus, passes forward along the floor of theabdomen, 
 reaches the falcifornTTigament of the liver, travels along the 
 iree_edge- of that structureT and empties itself intothe" portal 
 ^_srgijL~ After birth the remains of the umbilical vein are found 
 as a thickening at the free edge of the falciform ligament, and 
 form the round ligament of the liver. In ruminants the umbilical 
 veins are two in number, but they unite to form a single vessel 
 on entering the body. The vessel thus formed passes along 
 the abdominal' floor towards the falciform ligament to occupy 
 the same position as in other animals, but before reaching it, 
 it detaches a large branch— -ihejluctus venosus (Fig. 266, d.v.) — 
 which_p_asse.s_forwards to join ffie~po¥ferior vena cava. After 
 the blood has circulated in the liveTTFleaves by the hepatic 
 trunks, and is poured into the posterior vena cava, where it 
 meets with the blood in that vessel, and is thus conducted to 
 the heart. Thus in the horse the whole of the fcetal blood 
 passes through the liver before reaching the heart by the posterior 
 vena cava. In ruminants and the dog, part of the blood passes 
 through the liver, subsequently gaining the posterior vena cava, 
 and part goes direct to the posterior vena cava through the 
 ductus venosus.- 
 
 In the fcetal heart the cavities of the right and left auricles 
 are in communication by means of a foramen, the foramen ovale. 
 This opening in many animals is provided with a valve — the 
 Eustachian — that stretches from the mouth of the posterior vena 
 cava to the annulus or thickened border of the foramen ovale; 
 the valve is absent from the hearts of the fcetal horse and pig. 
 The function of this valve is to direct the blood-stream into the 
 left auricle; from there it passes into the left ventricle, and 
 thence into the aorta. The greater portion is driven into the 
 vessels that supply the head, neck, and fore-limbs (anterior 
 aorta and branches), and is conveyed to the head and anterior 
 portion of the body; the remainder passes backwards in the 
 
77 8 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 •Jfilk 
 
 j||P|L anl.a. 
 
 « mJtfn ^ikj»- - <*■* 
 
 pv n: 1 1 . m* p-a. 
 
 r.a.- I ^MlPli? l '"" 
 
 f°- mt . '-' - :: "P- v 
 
 Kh 'TWfS*- 9 ™ 
 
 d.v. i[./,5-.lK.\l.T 
 
 nit. \ , ► dir - 
 
 ' •"<■■ .jio.a. 
 
 — -.V.V.' 
 
 MW\/ 
 
 P..Y.C. 
 
 .u.c. 
 
 Fig. 266. — Diagram of the Fcetal Circulation (Ellenberger). 
 
 u.v., Umbilical vein; d.v., ductus venosus; pt.v., portal vein; /., liver, shown in 
 outline; v.h., hepatic veins; p.v.c, posterior vena cava; r.a., right auricle; 
 f.o., foramen ovale; r.v., right ventricle; p. a., pulmonary artery; p.v., pul- 
 monary veins; d.a., ductus arteriosus; I. a., left auricle; l.v., left ventricle; 
 »., the aorta; a. a.., arch of aorta; ant. a., anterior aorta; i.v., innominate veins; 
 a.v.c, anterior vena cava; po.a., posterior aorta; i.a., iliac artery; h. a., hypo- 
 gastric artery; ».«., umbilical arteries; i.ve., iliac veins; h.v., hypogastric 
 veins; u.c, umbilical cord. 
 
 The diagram actually represents the fcetal circulation in ruminants; to make it 
 applicable to the horse the ductus arteriosus (d.v.) must be supposed to be 
 removed; the whole of the blood then traverses the liver by the union of 
 the umbilical vein (u.v.) with the portal vein (pt.v.). The arrows indicate 
 the course taken by the blood. Observe that the stream entering the right 
 auricle divides, part passing into the right ventricle, and part into the left 
 auricle through the foremen ovale {f.o.). 
 
GENERATION AND DEVELOPMENT 779 
 
 posterior aorta. The head, it will be noticed, receives almost 
 pure blood. After the fluid has circulated in this part of the 
 body, it is returned to the right auricle of the heart by the 
 -anterioLJten^^ava. From the right auricle it passes to the 
 right ventricle, and from this cavity it is pumped into the pul- 
 monary artery, The lungs, however, are not functional, and 
 are more or less solid organs; consequently, they are not yet 
 prepared to receive the blood. This must therefore take another 
 course than through the lungs. The course is provided by the 
 ductus art eriosu s (Fig. 266, d.a.), a short vessel uniting the 
 pulmonary artery to the aorta, and thus bringing their lumina 
 into communication. By this conduit the blood enters the 
 posterior aorta, and is conveyed to the hinder parts of the body 
 and to the placenta. 
 
 The allantoic or umbilical arteries (Fig. 266, u.a.) convey the 
 blood from the foetus to the placenta. These arteries are branches 
 of the internal pudics, or of the parent vessels, the internal iliacs, 
 and during intra-uterine life they are larger than the parent 
 vessels. Soon after birth, however, their walls become thickened, 
 their lumina are lost, and they become impervious to the passage 
 of blood. In the adult they are recognised as the thickened cords 
 found in the lateral ligaments of the bladder. The ductus arterio- 
 sus just prior to birth has a lumen easily receiving an ordinary 
 cedar pencil, but it steadily diminishes until, at about a month 
 after birth, it is no greater than the diameter of a knitting-needle. 
 It is probable that little blood passes this way after birth, but 
 the exact period of total occlusion is unknown. Similarly, the 
 foramen ovale is blocked up by the development of a membrane, 
 which may be pulled out with the forceps shortly after birth, 
 and then resembles in shape an old-fashioned lace nightcap or 
 cowl. When undisturbed, it lies in a heap, filling up the 
 foramen. 
 
 The short cuts in the foetal circulation — viz., the ductus 
 venosus, ductus arteriosus, and foramen ovale — exist mainly with 
 the object of insuring that the purest blood reaches those organs 
 which require it most. The heart, head, and fore-limbs receive 
 blood which is much purer than the blood circulating through 
 the hind-limbs and abdominal viscera, for the brain must be 
 well fed. 
 
 Foetal and Maternal Nutrition.— This subject is very imperfectly 
 understood ; we do not know with anything like precision what the 
 mother loses during pregnancy, nor the form in which it is lost. 
 This is due to the great difficulties connected with such an inquiry, 
 and probably to the. fact that the nutrition of the foetus is not the 
 same in all animals. The mother's blood does not circulate through 
 the foetus, yet the mother supplies, by the methods outlined in the 
 
780 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 previous pages, all that is required for the process of development, 
 growth, and the purification of the blood. But the foetus is not 
 without its share in the matter. It is furnished with its protein, 
 fat, carbohydrate, and salts, but the conversion of these into body- 
 tissue is the work of the foetus. The nutriment may be received in 
 a ready-made form, but there is a good deal of evidence to show that 
 the foetus is not always provided with the fully formed material, but 
 has to elaborate it in its own tissues. Nutrition in the early stages, 
 before the final circulation is in working, order, is carried out in the 
 trophoblast and by absorption through the outer layer of the 
 embryo. The materials available through the trophoblast are the 
 cellular elements of the mother's blood, masses of protoplasm, and 
 in some animals (carnivora) actual tissue from the walls of the 
 uterus. The nutriment absorbed by the outer covering of the 
 embryo is derived from the uterine secretions of the uterine glands, 
 especially in herbivora, and from the blood extravasations into the 
 mucous membrane. 
 
 When the placental connection is established, the work of taking 
 up protein is easier; in fact, in the sheep nucleo-protein is formed 
 in the cotyledons; the placenta also furnishes glycogen in the 
 case of the carnivora, but not in ruminants; it also furnishes 
 fat, the placenta of the herbivora being especially rich in this 
 substance, while an abundance is obtained through the uterine 
 milk. 
 
 The foetal tissues contain glycogen. The carbohydrate meta- 
 bolism is very active in view of the fact that the energy it yields 
 is utilised in the building up of the embryo. The mucin-holding 
 tissues, so evident in the fcetus, contain a carbohydrate group which 
 may be derived from glycogen. It is not clear in what way glucose 
 is transferred from the mother to the fcetus ; it is supposed to pass 
 by diffusion, but this is probably not correct, as the allantoic fluid 
 of the ruminant and the blood-serum of the foetus contain lasvulose. 
 The glycogenic functions of the liver are probably established very 
 early in foetal life. The secretion of bile begins early, and is dis- 
 charged into the intestines as meconium. The liver, indeed, in the 
 foetus is an extremely active organ, as may be judged from its size, 
 and it is accordingly abundantly supplied with blood. 
 
 The supply of salts is obscure ; the whole question requires working 
 out, but the drain on the mother is no doubt considerable. Iron 
 obtained from broken-down haemoglobin is stored in the liver of 
 the foetus for the synthesis of haemoglobin, and, according to Bunge, 
 in extra-uterine life to supply the deficiency of iron in the mother's 
 milk. 
 
 The placenta is an efficient filter for many pathological substances, 
 though not for all. The tubercular mother does not convey tuber- 
 culosis to the foetus, though the syphilitic mother can infect her 
 infant. Colouring matter may not be arrested by the placenta. 
 The bones of the foetus are stained if madder be administered to 
 the mother. 
 
 The metabolism of the fully developed foetus may be low, the 
 oxygen content of its arterial blood being below that of the mother's ■ 
 this is true for the sheep, and probably all herbivora, though not 
 so for all animals. In the sheep the umbilical vein of the foetus 
 may contain 6-3 per cent, oxygen, while the arterial blood of the 
 mother contains 20 per cent. 
 
 There is one feature in the matter of intra -uterine nutrition which 
 
GENERATION AND DEVELOPMENT 781 
 
 is unquestionable. It is beyond doubt, both from direct experi- 
 ment on animals and the practical results obtained in breeding, 
 that if the mother during pregnancy is insufficiently nourished, the 
 offspring is stunted in its growth. The question of growth has 
 been touched on elsewhere (p. 376). The main feature of intra- 
 uterine growth is its enormous energy during the early stages of 
 gestation, and its sudden fall when all the tissues and membranes 
 are laid down and the final system of blood-supply fixed. It is 
 said that in the rabbit between the ninth and fifteenth day 704 per 
 cent, is added to the weight daily, and for the fifteenth to the 
 twentieth day only 212 per cent. Ewart tells us also that the 
 eight weeks' embryo of the horse is twice the length and four times 
 the weight of the seven weeks' embryo. It is evident that if 
 this rate of growth were maintained throughout the whole of 
 . intra-uterine life, the foetus would exceed the size and weight of 
 its dam. 
 
 All animals within the uterus are carnivorous. 
 
 Causation of Sex. — There are many theories accounting for the 
 determination of sex, some of which have been handed down from 
 antiquity, such, for instance, as the right and left ovary or the right 
 and left testicle furnishing sex of a definite type. The theory of 
 the mean vigour of the male and female cells at the time of con- 
 ception has found a good deal of support, a low mean vigour, it is 
 said, leading to male and a high vigour to female progeny. Quite 
 recently it has been stated that in his day's work a vigorous 
 stallion produces male progeny at the beginning of the day and 
 female progeny as his ardour becomes exhausted. No attention 
 need be paid to this statement; it is refuted by the practical results 
 of breeding. 
 
 In all animals an examination of the birth-rate shows that the two 
 sexes are produced in approximately equal proportions. 
 
 It was Heape who first authoritatively stated that there was no 
 such thing as a purely male or purely female animal, but that each 
 sex contained a dominant and recessive character, excepting herma- 
 phrodites, in which both sexes were equally represented. As evidence 
 of this, he drew attention to the assumption of male characteristics 
 in old females and of female characteristics in old males of the 
 human species — facts apparent to all. The same may be observed 
 in the lower animals; attention has been drawn at p. 746 to the 
 effects of castration in young cats of both sexes in the production 
 of a skull of male type in the female, and of female type in the 
 male; to the effect of caponing in the production of female plumage, 
 and in the absence of crowing; capons have even incubated eggs 
 and reared the resulting progeny. Interference with the ovaries in 
 birds has led to the production of male characteristics, and there is 
 much evidence of a similar kind in all animals. In all such cases it 
 is evident that the recessive sex asserts itself when the dominant 
 sex becomes weakened. Every animal, therefore, contains in its 
 cells the factors for its own sex and also those for the opposite 
 sex. 
 
 Everything points to ova and spermatozoa being sexual; that is 
 to say, there are male and female ova, male and female spermatozoa. 
 Microscopic differences in the number of chromosomes in sperma- 
 tozoa give additional support to this, for it is known that there 
 are spermatozoa containing the full number for the species and 
 others from the same animal containing one extra, or accessory, as 
 
7 8j a manual of veterinary physiology 
 
 it is termed. This has been shown to be true for the horse and 
 other animals by Wodsedalek (see p. 753). Spermatozoa containing 
 the ' accessory ' chromosome are, according to this authority, 
 female-producing; he further states that the spermatic secretion 
 contains male and female producing spermatozoa in equal numbers. 
 It would therefore appear evident that the sex of the offspring is 
 fixed at the time of fertilisation, and no subsequent influence can 
 alter it; the ovum in which one sex is dominant must be fertilised 
 by a spermatozoon in which the opposite sex is dominant ; whether 
 the sex be determined by the ovum or the spermatozoon depends 
 upon which is the more powerful of the two. 
 
 Heape has shown in his study of the ovary of the rabbit* that 
 ova may degenerate, and that one of the chief causes of degenera- 
 tion is imperfect nutrition (see pp. 745 and 775); in this way, it is 
 thought, a selective action effects a variation in the proportion of 
 the sexes of the ova produced. Where no such selective action 
 occurs in the ovary the proportion of the sexes in ovarian ova pro- 
 duced is governed by the laws of heredity. 
 
 That one parent may be prepotent* over the other in the produc- 
 tion of sex is a matter of everyday observation, and this prepotency 
 may be shown in the female even when the sires are different. It 
 is well known, for instance, that a cow may produce offspring 
 always of one sex, in spite of the fact that each year she may be 
 mated with a different sire. ' 
 
 Twin and Multiple Births. — We have already seen at p. 756 the 
 number of ova shed at an oestrus period. Multiple births are the 
 rule in the pig, dog, and cat. Twin births are frequent in sheep, 
 but the normal number of progeny produced by the mare and cow 
 is one. Twin births in the mare are about eight per thousand in 
 thoroughbred stock. In cattle (especially Shorthorns) they are more 
 frequent. Human twins may be formed from a single egg (true or 
 identical twins), or from two eggs. If from a single egg, the 
 amnion is common to the two; there is a single chorion, but two 
 umbilical cords. If from two eggs the membranes are not only 
 distinct, but the placenta is double. In man, twins formed from a 
 single egg are invariably of the same sex; if resulting from the 
 impregnation of two eggs they may be of the same or of opposite 
 sexes. The monstrosities met with in obstetrical practice "are 
 commonly found in identical twins. There is a form of twin birth 
 in cattle which is of exceptional interest: the sexes of the twins 
 are opposite; the male is potent, the female generally sterile. The 
 sterility is due to an undeveloped condition of the internal 
 generative organs; the external organs appearing normal. Such 
 a heifer-calf is known as a free-martin. The problem of the de- 
 velopment of this imperfect animal has occupied the attention of 
 morphologists for over one hundred and fifty years, and the question 
 is not yet settled. 
 
 Berry Hart concluded that the two embryos arose from a single 
 egg, and that in consequence the genital cells of a single ovum were 
 insufficient to furnish each animal with a complete generative 
 apparatus. As a consequence, one of the twins (the male) is per- 
 fectly developed, the other imperfectly. Hart pointed out that 
 
 * 'Ovulation and Degeneration of Ova in the Rabbit,' Proceedings of 
 the Royal Society, B, vol. lxxvi., 1905. 
 
 f In Mendelfan phraseology the expression ' dominant ' is employed. 
 
GENERATION AND DEVELOPMENT 783 
 
 the imperfectly developed animal, though apparently a female, 
 was in reality an imperfectly developed male. It is not an her- 
 maphrodite ; such a creature must be in possession of both male and 
 female sexual organs, and produce both ova and spermatozoa.* 
 
 The question has been examined recently by F. Lillie and Catherine 
 Chapin.j and the conclusions arrived at were that the twins arose 
 from different eggs, one from each ovary, and that the sex of the 
 free-martin is female, and not male. It is pointed out that cattle 
 twins have a common chorion and complete vascular anastomosis 
 between the pair, with the result that intermixture of the blood of 
 the two foetuses results. Owing to this vascular connection the sex 
 hormones of the male embryo enter the circulation of the female 
 twin and inhibit the development of the female organs. In those 
 rare cases where the female is found fertile, the two chorions are 
 unfused and the twin circulations are distinct. 
 
 Superfecundation and Superfcetation. — Superfecundation is a 
 term applied to indicate that two eggs have been impregnated at 
 different periods at a short interval — for example, the classical case 
 of the mare which produced twins, a horse and a mule foal, as the 
 result of coitus with first a stallion and then a donkey. This case is 
 not unique. J Superfcetation is the impregnation of a second egg at 
 a period remote from the first act of fertilisation. This is not un- 
 known in the cat, but does not appear to have been met with in 
 the larger domesticated animals. It has recently been urged by 
 Sumner, § as the result of observations on mice, that spermatozoa 
 may retain their fertilising power for days or weeks after reception 
 into the uterus or Fallopian tubes, and that a later copulation with 
 a different partner may coincide with a conception in which the 
 earlier insemination is really the effective one. Sumner suggests 
 that some alleged instances of telegony may thus be explained. 
 
 The Duration of Pregnancy appears to be based on no fixed 
 law. Judging from the length of time the elephant is in gesta- 
 tion, it might appear that body size had an influence, but against 
 this is the fact that the ass carries her young longer than the 
 horse, while, whether it be a toy terrier or a Newfoundland, a 
 dog goes from sixty to sixty-six days. It certainly does appear 
 that among animals of the same species breed has an influence 
 in the matter; different herds of cows vary from, 277 to 288 
 days, Merino sheep average 150 days, Southdowns 144 days. 
 It is not clear why, of two rodents, the guinea-pig should require 
 a period of gestation twice as long as the rabbit. 
 
 The only point which appears fixed is that animals born in 
 a stage of development when they can immediately follow their 
 mothers (herbivora) have a longer period of gestation than those 
 born helpless, as in the case of the carnivora. 
 
 * Hermaphroditism may be produced experimentally in rats by im- 
 planting ovary into testis, and by transplanting ovary and testis into 
 young castrated animals (see also p. 747)- 
 
 f Journal of Experimental Zoology, vol. xxiii., 1917- 
 
 J See Fleming, Textbook of Veterinary Obstetrics. 
 
 § Biological Bulletin, vol. xxx., 1916. 
 
784 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The following are average periods of gestation: 
 
 Elephant 20 months. 
 
 Mare* n months, and liable to vary- 
 within relatively wide limits. 
 
 Assf 358 to 385 days. 
 
 Zebra - 358 to 385 days (Ewart). 
 
 Cow J 40 weeks.' 
 
 Sheep 21 weeks (average). 
 
 Camel 45 weeks. 
 
 Pig 16 weeks. 
 
 Dog 60 to 66 days. 
 
 Cat 63 days. 
 
 Rabbit - 32 days. 
 
 Guinea-pig - 63-days. 
 
 Parturition. — The cause of parturition is unknown. The 
 foetus having reached its full stage of development, changes of 
 an obscure nature take place which lead to its expulsion. During 
 uterine life the equine foetus is lying on its back on the floor 
 of the mother's abdomen, with its chin on its chest, the fore- 
 legs bent at the knee, and the hind-legs in the right horn 
 (Fig. 267). Preparatory to birth the foetus changes position and 
 turns on its side, so as to assume, first a lateral position (Fig. 268), 
 and lastly an upright one (Fig. 269), by which the foetal 
 and maternal spines are brought nearer together. To assume 
 this position the foetus has had to make a complete revolution ; 
 it is now brought with the muzzle and fore-legs in the direction 
 of the pelvis (Fig. 269), and dilatation of the passage follows. 
 In the cow the foetus lies on its back on the floor of the abdomen 
 as in the mare, but somewhat crooked — viz., the head inclining 
 
 towards one side, and the hind extremities towards the other ; in 
 \ 
 
 * Tessier, in a memoir on the period of gestation presented in 1825 to 
 the Royal Academy of Science in Paris, stated that in 582 mares, after 
 one sexual congress, the shortest period of gestation was 287 days, the 
 longest, 419 days, a difference of 132 days. In 312 mares which visited 
 the horse several times, the shortest period of gestation, calculated from 
 the last cover, was 290 days, the longest 377 days, a difference of 87 days. 
 
 L. Dupas, The Revue VUhinaive Militaire, June, 191 3, records the 
 gestation period of the mares in a breeding depot. He places it at from 
 eleven to eleven and a half months, the total number of observations being 
 415- 
 
 20-96 per cent, of the total births occurred between the 331st and 340th day. 
 33'°i ..' .. .. .. .. 34ist „ 350th „ ' 
 
 2 3-37 .. .. .■ - .. 35ist ,, 360th ,, 
 
 9-63 „ ., .. ,. ,, 361st ,, 370th ,, 
 
 But for these precise figures, we should have thought it unlikely that 
 over 9 per cent, of the births occurred as late as a year after conception. 
 
 •f Tessier gives the period of two asses, one being 12 months 20 days 
 the other 13 months 1 day; and these agree very closely with Sutherland's 
 observations (Veterinary Record, October 13, 1894), quoted in the text 
 above. 
 
 J Of 1,131 cows, the average period observed by Tessier was 240 days- 
 the longest was 231 days [Veterinarian, vol. i., 1828). 
 
GENERATION AND DEVELOPMENT 
 
 785 
 
 all other respects its position resembles that of the foetus of the 
 mare. The alteration in the position of the foetus does not 
 
 Fig. 267. — The Position occupied by the Equine Pcetus during 
 Intra-Uterine Life (Franck). 
 
 occur through its own movements, but by the contraction of 
 the uterus; on the other hand, the stretching of the limbs is the 
 
 Fig. 268. — The First Stage in the Revolution of the Fcetus : Lateral 
 Position. #The Os is dilated by the Membranes which have not yet 
 ruptured (Franck). 
 
 a, The allantois; 6, the amnion. 
 
 result of foetal movement.* There can be little doubt that the 
 revolution of the foetus prior to birth is the explanation of the 
 complete torsion of the neck of the uterus and vagina which is 
 sometimes found in both the cow and mare. 
 
 * This description of the change in the position of the f cetus preparatory 
 to birth is taken from Ellenberger's ' Physiologie ' (after Franck) . 
 
 50 
 
7 86 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The dilatation of the os is assisted by the amniotic and allantoic 
 fluids. Each contraction of the uterus is accompanied by a 
 pain; the pains last from fifteen to ninety seconds, and the 
 interval between them is from two to four minutes. The con- 
 tractions of the uterus, from horns to fundus and thence to the 
 os, occur under the influence of a centre in the lumbar portion 
 of the cord; they are not under the control of the will, and occur 
 even' though the animal be unconscious, or the spinal cord 
 divided in the lower cervical region (dog). In multiple births 
 the passage of one foetus is greatly assisted by the pressure of 
 the one immediately behind. 
 
 The mare is remarkable for the rapidity with which delivery 
 is effected; ruminants, on the other hand, are often very slow 
 
 Fig. 269. — -The Revolution completed, Membranes ruptured, and Foal 
 in the Normal Position for Delivery (Franck). 
 
 a, The amnion; b, the allantois. 
 
 and in labour for hours. Parturition in the mare is accompanied 
 by a- complete separation of the chorion from the uterine wall; 
 this is the explanation of why any difficulty in delivery invariably 
 sacrifices the life of the foal. In ruminants, on the contrary, 
 the circulation between the mother and fcetus is to the last kept 
 up by the gradual separation of the cotyledons, so that, though 
 the process may be delayed several hours, the animal is generally 
 born alive. The cause of the first respiration of the foetus is 
 dealt with at p. 142. 
 
 Separation of the Membranes. — After the birth of the animal 
 the fcetal membranes have to be cast off. In the mare this occurs 
 very early, owing to the shortness of the villi; in cattle, delay is 
 common. Retention of the membranes leads to their decom- 
 
GENERATION AND DEVELOPMENT 787 
 
 position inutero, resulting in septic poisoning. The membranes 
 are expelled by a contraction of the walls of the uterus, which, 
 while driving out what is now a foreign body, also closes the 
 vessels and controls haemorrhage. The membranes are subse- 
 quently eaten by the mother in all animals excepting the horse 
 (see p. 796) . 
 
 Involution of the Uterus. — The most remarkable change occurs 
 in the uterus in regaining its normal or almcst normal size. 
 The shrinking of this large sac, which reaches as far forward as 
 the diaphragm, rapidly occurs. The immensely thickened walls 
 are daily reduced; in the case of the mare, if the uterus be 
 examined a few days after parturition, it is difficult to believe 
 that it ever contained a foal, and at the ninth day the animal 
 experiences oestrus. It is obvious from what has been said at 
 p. 765 that' the nature of the connection between foetus and 
 mother must influence the length of time involved in involution ; 
 for example, the destruction of the mucous membrane of the 
 human womb renders its interior a veritable wound. Tt has 
 been supposed that involution is carried out under the influence 
 of a hormone secreted by the corpus luteum. Contraction of 
 the walls of the organ is assisted by suckling. 
 
Section 2. 
 The Secretion of Milk. 
 
 Mammary Glands.— In all animals the mammary glands have a 
 common structural arrangement; there is the pinkish-grey gland 
 substance separated by septa of nbro-elastic tissue derived from 
 the capsule of the gland itself, and linking up the gland tissue into 
 lobes, and these last into lobules. The lobules are built up of 
 secreting alveoli, each alveolus possessing a tubule; these by uniting 
 furnish a duct for each of the numerous lobes, the ducts opening 
 into a sinus. From the sinus, or milk cistern, one or more tubes, 
 according to the species of animal, penetrate the teat. There are 
 as many glands as teats. The skin of the gland is firm, thin, 
 and hairless, and between the right and left glands is a septum 
 of fascia, which not only divides the glands, but supports them in 
 their inverted position. 
 
 In the mare the skin of the mammary gland has a greasy feel 
 owing to the number of sebaceous glands in that structure; the 
 milk cistern is small, the teats are short, and each is penetrated by 
 two or three lactiferous ducts. The mammary glands, even in a 
 state of activity, are small, and this insures that locomotion is not 
 interfered with. 
 
 In the cow there are four glands arranged as pairs on the right 
 and left; these four glands are usually described as quarters. The 
 pair of glands on the right are divided from those on the left by 
 a suspensory septum. There is no communication between the right 
 and left glands, but the two quarters on the same side of the body 
 are described as communicating, though Sisson states that the 
 injection of coloured fluids into the teats demonstrates that the 
 cavities drained by them do not communicate. The milk cistern 
 in the cow is large and capable of holding from a half to one pint of 
 fluid; where it communicates with the teat it possesses a sphincter 
 muscle over which a little control can be exercised. The teat is 
 penetrated by a single canal which possesses a sphincter at the outlet; 
 the sphincter of the teat is not under voluntary control. The teats 
 are about 3 inches in length. 
 
 The milk ducts which drain the lobes and open into the cistern 
 are each provided with a sphincter which is under voluntary control, 
 so that a cow may withhold her milk at pleasure; if her calf has 
 been brought up with her she may refuse to yield any milk in its 
 absence (see in this connection p. 610). No animal, however, 
 has the power to discharge the milk from the gland, and when, 
 owing to accumulation, the tension becomes extreme, pain is pro- 
 duced. The glands in the cow are of variable size; in deep milkers 
 they are enormous. The total volume of the four glands in the cow 
 is said to be about 6,700 c.c, of which 3,000 represent spaces capable 
 of holding milk. In the goat the udder is relatively larger than 
 in the cow. 
 
 In the sheep there are a pair of glands; in the pig ten or twelve 
 arranged in two rows, each teat having two ducts; in the dog the 
 
 788 
 
GENERATION AND DEVELOPMENT 
 
 789 
 
 glands are ten in number, the teats being penetrated by several 
 openings. 
 
 It has been supposed that teats additional to the number normal 
 to each animal indicate fertility. 
 
 The Secretion of Milk.— The real work of the mammary 
 gland takes place in the cells found in the follicles of the lobules, 
 but it is not yet settled in what way these cells contribute to 
 the secretion. It has been suggested that they are broken down 
 in the alveolus and their contents elaborated into milk. The 
 resemblance of the mammary to sebaceous glands affords this 
 view some support. A second view is that the cells are not 
 destroyed, but excrete their contents; and the third and most 
 likely theory is that there is a combination of the two processes. 
 In Fig. 270 are seen the loaded and discharged glands of the 
 dog. In the active condition the cells are large and columnar, 
 with one or more nuclei; fat cells are apparent at the free borders 
 of the cell. In the discharged gland the lumen is large, the 
 
 Loaded. Discharged. 
 
 Fig. 270. — Mammary Gland of Dog during Lactation, 
 Heidenhain (Waller). 
 
 cells flattened, each containing a single nucleus, while in the 
 lumen of the glands are corpuscular elements. It would appear 
 that the cells do undergo partial disintegration followed by rapid 
 reconstruction, that the fat is extruded as globules, and that the 
 detached portion of the cell furnishes the protein, sugar, and 
 salts. The cellular elements of the lobule elaborate all the 
 constituents of milk from the circulating blood and lymph. 
 This appears to be quite clear, for neither casein nor milk sugar 
 occur in an}? other tissue of the body, and the ash of milk differs 
 from the ash composition of the blood-lymph, so that the salts 
 are not merely filtered off. 
 
 The development of the mammary glands at puberty is brought 
 about by a hormone secreted by the ovary; if these be removed 
 in a non-pregnant animal the glands fail to develop. The en- 
 largement of the glands as the period of parturition approaches 
 is a different question. The experiments of Lane-Claypon and 
 Starling have shown that this development is effected by a 
 
790 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 hormone contained in the body of the foetus, which, after absorp- 
 tion by the maternal blood, stimulates the growth of the mam- 
 mary gland. This substance does not produce a secretion of 
 milk; on the other hand, it inhibits it; it is not until the womb 
 is empty that, in consequence of no further foetal hormones 
 being produced, the secretion of milk begins. The experimental 
 evidence is sufficiently positive, but there are yet to be explained 
 certain phenomena connected with the production of milk, in 
 which the foetal hormone can play no part in organising the 
 mammary gland for secretion. For example, it is beyond all 
 doubt that a mule may yield milk and rear a foal. Obviously 
 the foal is a stranger. By what process was the mammary 
 gland of the mule rendered fit for lactation ? Milk may occa- 
 sionally be expressed from the breasts of children and of fillies 
 shortly after birth; this is less difficult of explanation, for the 
 hormone which has been preparing the glands of the mother 
 has originated in the fcetus. Mr. H. Gray informs the writer 
 that he has frequently observed a bitch may come into milk 
 about nine weeks after oestrus, not only though non-pregnant, 
 but without coitus having occurred. She may prepare her 
 bed and even suckle the puppies of another should she get the 
 opportunity. The same observer has seen ewe lambs in the 
 autumn come into milk when fed on young clover. Marshall* 
 states he knew a mare which had never had a foal which could 
 be made to yield milk at any time for years. Heape, in con- 
 sequence of this class of case, holds the view that the source of 
 the stimulus which excites the activity of the mammary glands 
 must be looked for in the ovary. The rudimentary teats of 
 the goat and sheep, and even of a bull, have been known to 
 produce milk.f A dog (with retention of the testicles in the 
 inguinal canal) has been known to secrete milk, and to be followed 
 about as if in oestrus (Gray). Some evidence has been brought 
 forward to show that mammary growth and milk secretion are 
 under the control of the nervous system, but the severance of 
 all nervous connection prevents neither the development of the 
 gland nor the subsequent secretion of milk. The gland has 
 been transplanted in the guinea-pig to the neighbourhood of 
 the ear, and functioned at the next parturition. Nevertheless, 
 in the absence of a nervous connection it is difficult to explain 
 the uterine contractions which occur in a recently emptied 
 uterus during suckling. 
 
 The stimulus to milk secretion is suckling; no artificial method 
 can completely take its place. While sucking is occurring, milk 
 is being secreted under its stimulus (see also p. 796). 
 
 * ' Physiology of Reproduction.' 
 
 f ' The Breeding of Animals,' 1'. B. Mumford; also Marshall, he. cit. 
 
GENERATION AND DEVELOPMENT 
 
 791 
 
 Composition of Milk. — The milk of the herbivora has an 
 alkaline reaction, that of carnivora is acid. Fresh cow's milk 
 may be amphoteric — that is to say, give both an acid and an 
 alkaline reaction to test paper, owing to the presence of acid 
 and alkaline salts. In the cow the specific gravity varies from 
 1-028 to 1-034; m the mare, 1-029 to I-0 39- It is increased by 
 caseinogen and dissolved substances, and decreased by the presence 
 of fat. A high percentage of fat (cream) in milk accordingly 
 lowers the specific gravity of the fluid. The yellowish- white colour 
 of milk is due to carotin and xanthophyl obtained from green 
 food. The fat in milk is in the form of an emulsion ; an emulsion 
 being a subdivision of oil in a hydrated colloid. The proteins 
 are in colloidal solution, the sugar and salts in true solution.* 
 
 The following table given by Heineman is compiled from 
 various sources: 
 
 Man 
 
 Water. Casein. 
 
 Albumin. 
 
 Total 
 Protein. 
 
 Fat. sugar. Ash SoM ^ 
 
 87-58 o-8o 
 
 I-2I 
 
 2-01 
 
 3-74 
 
 6-37 1 0-30 12-42 
 
 Cow 
 
 87-27 2-88 
 
 0-51 
 
 3-39 3-68 
 
 4-94 ^ 0-72 I 12-73 
 
 Cow colos- 
 
 
 
 
 trum 
 
 75-07 i 4-!9 
 
 12-99 
 
 17-18 
 
 3-97 1 2-28 ; 1-53 
 
 24-93 
 
 Ass 
 
 90-12 | 0-79 
 
 1-06 
 
 1-85 
 
 i-37 
 
 6-19 
 
 0-47 
 
 9-88 
 
 Sheep 
 
 83-57 j 4-17 
 
 0-98 
 
 5-15 
 
 6-i8 
 
 4-17 
 
 o-93 
 
 16-43 
 
 Goat 
 
 86-88 2-87 
 
 0-89 
 
 3-76 , 4-07 
 
 4-64 
 
 0-85 
 
 13-12 
 
 Mare 
 
 90-58 : 1-30 
 
 o-75 
 
 2-05 
 
 1-14 
 
 5-87 
 
 0-36 9-42 
 
 Sow 
 
 83-94 — 
 
 — 
 
 7-23 
 
 4-55 
 
 3-23 
 
 1-05 
 
 16-06 
 
 Bitch - 
 
 75-44 — 
 
 ■ 
 
 11-17 
 
 9-57 
 
 3-09 
 
 o-73- 
 
 24-56 
 
 It will be observed that the milk of the sheep is rich in casein, 
 but that of the dog contains the highest percentage of total 
 protein, and likewise of fat, the sheep and pig coming next. 
 The largest sugar content occurs in man, the next largest in the 
 ass, which is closely followed by the horse. The highest amount 
 of ash occurs in the pig, the lowest, next to man, being found 
 in the horse. 
 
 The Proteins in milk are caseinogen, lactalbumin, and lacto- 
 globulin. The latter contains phosphorus, and is probably a 
 mixture of proteins. The proteins in milk are not precipitated 
 by boiling; on the other hand, colostrum is precipitated; this is 
 due to the presence of globulin. 
 
 Caseinogen is a phospho-protein, and the most important 
 protein of milk; it occurs, in a colloidal condition in combination 
 with calcium, and is probably a mixture of several compounds, 
 for it is not a clearly definite chemical substance (Heineman). 
 Under the influence of rennin (p. 196) it is coagulated, forming 
 
 * ' Milk,' P. G. Heineman, Ph.D., 1919. 
 
752 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 a clot and whey, seen in curdled milk. The process is not fully 
 understood, but the presence of calcium is necessary. The 
 protein next in importance to casein is lactalbumin; it is allied 
 to serum albumin, and is found to contain only a trace of phos- 
 phorus. Lactoglobulin contains phosphorus, and is probably 
 identical with serum globulin. The origin of the lactalbumin 
 and lactoglobulin is from similar bodies in the body fluids, but 
 caseinogen has to be built up from the blood, and the manner 
 in which it is constructed is at present unknown. The non- 
 protein nitrogen in the milk of the cow is represented by amino- 
 nitrogen and urea, both of which are increased on a high 
 protein diet; there are also creatinine and creatine, but, accord- 
 ing to Heineman, uric acid is absent. Hypoxanthine, xanthine, 
 adenine, and guanine are present; hippuric acid is absent. The 
 protein content of milk is liable to considerable variation, but 
 is not so variable as that of fat. 
 
 The Carbohydrate in milk is lactose or milk sugar; it is peculiar 
 to the milk of all animals, and is not found elsewhere in the 
 body, nor has it been found in plants. The table of analyses 
 shows its proportion in the milk of different animals. Lactose 
 readily undergoes fermentation, resulting in the production of 
 lactic acid and the curdling of milk. The milk of the mare, 
 cow, and camel, in the presence of suitable ferments, undergoes 
 alcoholic fermentation, resulting in the production of koumiss 
 and kephir. The source of lactose in milk is not decided upon; 
 it is probably derived from the dextrose contained in the blood 
 and food. Lactose is the least variable constituent in milk 
 (see next paragraph). 
 
 The Fat in milk exists in the form of globules in suspension. 
 It forms a true emulsion, each globule being separated by a 
 layer of milk plasma. On standing, the globules rise to the 
 surface of the fluid and form cream. This process can be much 
 accelerated by centrifugalising the mirk. By the process of 
 churning the emulsion is destroyed and the fat obtained as butter. 
 Butter consists of 68 per cent, palmatin and stearin, 30 per cent, 
 of olein, and 2 per cent, specific butter fats (tributyrin and 
 tricaproin). The source of the fat in milk is the fats and carbo- 
 hydrates contained in the food. The question of its origin from 
 the protein of the food has been greatly debated, and the point 
 is not yet settled. The elaborate experiments of Jordan and 
 J enter suggest four sources of fat— the fat in the food, the body 
 fat, protein, and carbohydrate. The most variable constituent 
 of milk is the fat content; there may be 50 per cent, difference 
 between the maximum and minimum; when the fat percentage 
 is high the lactose is decreased. The differences observable in 
 the composition of the milk solids are mainly dependent on the 
 
GENERATION AND DEVELOPMENT 793 
 
 proportion of lactose to fat, though the percentage of these, 
 taken together, is practically constant (Armsby). The first milk 
 drawn is always poorer in fat than that obtained later. This 
 is probably due to the secretion produced as the result of milking. 
 
 The Salts of milk are principally calcium phosphate, together 
 with salts of sodium and potassium. The salts are derived 
 from the blood and lymph, but do not exist in the proportions 
 found in those fluids. Bunge has shown that the salts in milk 
 are found in proportions closely agreeing with the requirements 
 of the young animal. He compared the ash of a puppy with 
 the ash of the milk of its mother, and the correspondence was 
 remarkably close. On the other hand, the ash' of the body of the 
 calf, as determined by Lawes and Gilbert, does not agree with 
 the mineral substances found in the milk of the cow, the calcium 
 and phosphoric acid being twice those found in the milk. The 
 iron also is very greatly- deficient, but it has long been recognised 
 that cow's milk is extremely poor in this metal (p. 780). 
 On reference to the table of analyses striking differences will be 
 observed in the ash content of the milk of various animals. 
 The poorest in salts is that of the horse, the richest the pig's, 
 followed by the milk of the sheep, dog, and cow. The rate 
 at which the young grow can hardly be accepted as an explana- 
 tion; it is also difficult to understand why the calf requires 
 twice the amount of saline matter necessary to the foal. 
 
 The source of the salts in milk is the salts contained in the 
 food. Nevertheless, a study of the ash metabolism of the cow 
 shows very clearly that there are times when she is parting with 
 more salts in the milk than she receives with her food. Even 
 liberal feeding to the extent of a gain in body weight may not 
 arrest this- loss, and it is clear that the excess of salts in the 
 milk over the ash content of the food can only be derived by 
 the animal drawing upon the solid tissues of its own body. 
 That the overdraft is made up in the later stages of lactation is 
 probable, but Forbes, whose name is so closely identified with 
 this work, considers that leguminous roughage or even bone 
 meal should be given to prevent the loss. It was found experi- 
 mentally that increasing the mineral content of the food was 
 not sufficient; the repairing material is evidently required in 
 organic combination. Cows fed on bone flour retain appreciable 
 quantities of calcium and phosphoric acid, but wholly insufficient 
 to maintain equilibrium or produce storage.* Forbes regards 
 
 * All recent observations connected with the mineral metabolism of the 
 milch cow have been made in Experimental Stations in the U.S.A. It is 
 well to note that experience in South Africa indicates that the phosphoric 
 acid of bone flour is absorbed in sufficient quantities to control or prevent 
 the onset of a disease of cattle which is induced by the animal's attempt 
 to remedy » mineral deficiency. During the dry months of the year the 
 
794 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the depletion of the mineral reserves of the body as one of the 
 factors in the natural shrinkage of milk production (Armsby). 
 Cows, however, have been known to remain in milk for some 
 years, as, for example, after the removal of the ovaries. 
 
 The drain on the system caused by the secretion of milk is con- 
 siderable. Lawes and Gilbert published a table showing the 
 weekly loss of nutrients as estimated by the content from various 
 yields of milk. These animals were liberally fed, but the protein 
 content of the ration, which the observers stated affected both 
 the yield and richness of the milk, was not carried to the point 
 of producing. an increase in body weight. 
 
 Body Losses Weekly. 
 
 Daily Yield of 
 Milk. 
 
 Protein. 
 
 Fat. 
 
 Carbo- 
 hydrate. 
 
 Ash. 
 
 1 Total Solid 
 ' Matter. 
 
 4 quarts 
 8 ,, 
 
 12 
 
 16 ,, 
 
 20 
 
 Pound*. 
 
 2-64 
 
 5-28 
 
 7-92 
 
 10-56 
 
 13-20 
 
 Pounds. 
 
 2-53 
 5-06 
 
 7-59 
 
 10-12 
 12-65 
 
 Pounds. 
 3-30 
 
 6-66 
 
 9-99 
 
 I3-32 
 
 16-65 
 
 Pounds. 
 
 o-54 
 1-08 
 1-62 
 
 2-l6 
 
 2-70 
 
 Pounds. 
 9-04 
 
 18-08 
 27-12 
 36-16 
 45-20 
 
 If an average secretion of milk be taken at 6 quarts (6-8 litres) 
 per diem for forty weeks in the year, there is a loss of total 
 solids equal to 542 pounds in weight, including 32 pounds of 
 mineral substances; this is small compared with the loss in the 
 deep milking strains of the present day, which produce 2,000 
 gallons, and even more, of milk in forty-four weeks, and yield half 
 a ton of solid matter every year. These figures convey some 
 idea of the metabolism occurring in the mammary glands and 
 of their extraordinary powers of construction. 
 
 Colostrum. — This is the first milk secreted. It is a yellowish- 
 white, sticky fluid, remarkable for the amount of protein it 
 contains, which coagulates on boiling. Its analysis is shown 
 in the table (p. 791). Not only is the protein content high, but 
 also that of the ash; it is, however, poor in sugar. When 
 examined microscopically, colostrum is found to be full of 
 corpuscles, known as colostrum corpuscles, large granular bodies 
 containing droplets of fat; they possess amoeboid movements. 
 
 grazing is insufficient to provide the needful phosphoric acid, and in con- 
 sequence the cattle become bone-eaters. Putrid bones may be infected 
 w ; th a bacterium which elaborates a poison, or ptomaine, from the decom- 
 posing substance; this toxine produces a very fatal disease. If phos- 
 phorus be given in the form of wheat bran, as bone meal, or pure phosphoric 
 acid, the craving for bones ceases. If the phosphorus be withdrawn the 
 ' craving ' returns (Sir A. Theiler, in the Journal of Agriculture, South 
 Africa, June, 1920). 
 
GENERATION AND DEVELOPMENT 5-95 
 
 These cells are probably leucocytes; they are present immedi- 
 ately before and at the close of lactation; they may also be 
 found during secretion if the glands be insufficiently ' stripped.' 
 Normally the cells disappear from five to eight days after par- 
 turition, but even at this period the milk is not normal, for it 
 is still acid, and it is not possible until the tenth day for the 
 production of casein to occur. By the fourteenth day the milk 
 may be regarded as of normal composition. 
 
 The function of colostrum is to act as a purgative to the 
 young animal. 
 
 The Yield of Milk among cows is governed by various factors, 
 of which, apart from food supply, the most important is the 
 breed of animal. Milk production is essentially hereditary. 
 Holsteins and Frisians hold the palm. On the authority of 
 Alvord,* Shorthorns, Jerseys, Ayrshires, and Guernseys, are 
 placed next in order of average milk production. Not only is 
 quantity hereditary, but also the fat content. But there is no 
 correlation between yield of milk and fat production; the Jersey 
 is credited with the highest average percentage of fat. The 
 influence of food on production is a commonplace. 
 
 Lawes and Gilbert regarded grass, and then swedes, as the 
 best milk-producing substances in conjunction with the ordinary 
 diet. In this protein is the most important constituent, for the 
 food must contain not only the amount of protein contained 
 in the milk, but also that required for the maintenance of the 
 body (p. 393). Protein is believed to exercise a specific secretory 
 effect over the mammary gland; further, it appears undoubted 
 that the cow is able to utilise a larger percentage of the food 
 protein for milk production than the ox can for beef production. 
 At p. 399 is a brief statement of the daily calories required by 
 a milch-cow under definite conditions of production. The 
 amount of assimilable protein required in this case is at least 
 1-5 pounds per diem. Contrary to what might be expected, 
 an increase of fat in the diet of cows above the standard of 
 0-5 pound of assimilable fat per 1,000 pounds of body weight 
 does not lead to an increase in the fat of milk, though in sheep 
 there is some evidence that it does assist. The physiological 
 rule should, however, be that, short of increasing the body weight 
 of a fully-grown animal, the rations cannot be too liberal. An 
 increase in weight diminishes and eventually stops secretion. 
 Underfeeding causes the percentage of fat in milk to rise, and 
 that of protein and ash to fall (Ekles and Palmer). 
 
 Galactagogues are agents which increase the production of 
 milk. Pituitrin causes temporarily an increased flow, but it 
 is not a galactagogue; it merely produces an increased flow by 
 * Heincman, loc. cit. 
 
796 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 causing contraction of the walls of the milk ducts. On the other 
 hand, extracts of placenta cause an increase in production. 
 In women the extract produces an increase both in the protein 
 and lactose; further, it is believed that a growth-promoting 
 factor is also present which is passed on to the infant in the 
 milk. All animals, the horse excepted, eat the placenta, and in 
 cows it not infrequently produces impaction of the oesophagus. 
 It is an instinctive habit which obtained no explanation until 
 the galactagogue effects of placenta were discovered. Sub- 
 cutaneous injection of the animal's own milk acts as a galacta- 
 gogue; this is true of the woman and the cow, though in the 
 latter the limited observations which have been made suggest 
 that the increased secretion is only temporary. No drug can 
 be relied upon to produce an increase of milk or butter. Castor 
 oil decreases the butter fat (Forbes). 
 
 The natural galactagogue is sucking; this causes an increased 
 secretion, and is the chief means by which the activity of the 
 glands is maintained for months. ' Stripping ' is the next best 
 method; when imperfectly performed, the glands soon cease to 
 secrete. Pressure on the gland also plays a part; the violent 
 thrusts given by the lamb and other animals are a secretory 
 excitant. 
 
 Secretion soon ceases if the glands be not emptied, and atropine, 
 or its preparations, also causes it to stop. 
 
 The average duration of lactation in the cow is about forty 
 weeks in the year; as previously noted, if ovariotomy be per- 
 formed when in full milk, the secretion may be maintained for 
 two or three years, some say even longer. Cows have been 
 known to remain in milk for five years after the last visit to the 
 bull (H. Gray). Gestation may hasten the close of lactation, 
 but it does not influence the composition of the milk until towards 
 the end of lactation, when an increase in protein and fat occurs, 
 and frequently a decrease in lactose (Palmer and Eckles). As 
 a rule, the greatest flow of milk occurs within two or three weeks 
 after parturition, and then it gradually becomes reduced as 
 the next period of gestation approaches, but there are marked 
 individual differences. 
 
Section 3.* 
 Heredity. 
 
 As no living thing arises spontaneously, but is built up from a 
 pair of other living things (with certain exceptions in the simpler 
 forms of life), it is probable that so long as man has been able to 
 reason he has regarded the offspring of. a given union as consisting 
 of a mixture of its parents. As knowledge became extended, more 
 especially since the origin of species has been traced almost step 
 by step from the lowest to the highest forms of life, and all life 
 shown to be the outcome of a common primitive form, the facts 
 underlying these extraordinary changes have been the subject of 
 inquiry, in order to determine the natural laws by which they are 
 governed. 
 
 The terms ' evolution ' and ' origin of species ' have become so 
 closely associated with the name of Charles Darwin that it is apt 
 to be forgotten that the idea of a common descent for all living 
 things had been prominent in the minds of naturalists long before 
 Darwin's day. 
 
 The French scientist, Lamarck, and Darwin's grandfather, 
 Erasmus Darwin, though they did not originate the idea of a common 
 origin, formulated the theory that evolution was due to the trans- 
 missibility of modifications acquired during the lifetime of the 
 individual. This parental experience might be positive or negative. 
 In the former case the result took the form of increase in size and 
 function of the parts or organs concerned. On the other hand, 
 Lamarck maintained that the continued lack of use of the same 
 organ sensibly weakened it, leading to deterioration and ultimate 
 disappearance. 
 
 In order to account for the stability in the filial generations of 
 parental reaction to environment and parental experience, Lamarck 
 held that Nature preserved everything that she had caused the 
 individual to acquire or to lose by the influence of the circumstances 
 to which the race had been for a long time exposed, and conse- 
 quently she preserved everything acquired by the predominant 
 use of certain organs (or lost by their continued disuse). She did 
 this by the generation of new individuals, which were produced with 
 the newly acquired organs. This occurred, provided that the 
 acquifed changes were common to the two sexes, or to the indi- 
 viduals that produced the hew forms. 
 
 The Lamarckian position is still very strongly held, especially 
 in France and America, but as pointed out by Professor E. Ray 
 Lankester, Lamarck's laws are contradictory. In the first place, 
 he inferred that old characters had not become fixed and congenital 
 after a multitude of successive generations had developed them in 
 response to environment, but gave place to new characters when 
 new conditions operated on the individual. Why, therefore, should 
 we conclude that new characters are likely to become fixed after 
 a relatively shorter time of responsive existence ? 
 
 * This section has been revised by Mr. J. B. Robertson, M.R.C.V.S. 
 The author takes this opportunity of expressing his indebtedness to Mr. 
 Robertson for the revision and for his many communications and guidance 
 on the subject of heredity. 
 
 797 
 
7 o8 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Charles Darwin's views on the Lamarckian theories varied some- 
 what from time to time. In his ' Origin of Species ' he held that 
 heredity and adaptation enter into close reciprocal action. In the 
 struggle for existence, only those individuals survived which were 
 naturally selected by their environment, and this selection, Darwin 
 contended, took place through the medium of impalpable differences 
 and through the course of a long period of time. In other words, 
 owing to the removal of the least fit, selection took place through 
 the most fit — i.e., most fit for the conditions in which the organism 
 existed. This theory of the survival of the most fit was the central 
 difference between Darwin's conception of evolution and the views 
 of Lamarck, for both believed that somatic acquirements were 
 the fountain-head of corresponding modifications in the germ plasm, 
 and also that Nature did not make ' jumps.' But extended investi- 
 gation has shown that ' jumps ' are occasionally made. Indeed, it 
 is difficult to account for the multitude of specific forms which now 
 inhabit the earth, or for pre-existing forms which have inhabited 
 it and become extinct, if we accept the theory that evolution is 
 brought about solely through the medium of small modifications. 
 
 Weismann, in his work on ' The Continuity of the Germ Plasm.' 
 advanced the opinion that only those characters could be trans- 
 mitted to subsequent generations that were contained in rudi- 
 mentary form in embryonic tissue. Variation in effect commenced 
 in the germ plasm, and not, as postulated by Lamarck and Darwin, 
 in the soma. Weismann consequently very strenuously opposed 
 the doctrine that parental somatic experience was ever transmitted. 
 The germ plasm, he pointed out, is continued from generation to 
 generation, but is subject to inborn transformation, and the environ- 
 ment through its finished product, the soma, selects the fittest 
 or most suitable plasm, the modification so selected being thus 
 transmitted to the germ plasm of the next generation. A clear 
 conception of the two theories is of paramount importance to the 
 student of heredity. We shall see presently that the consensus 
 of present opinion is against the transmission of purely somatic 
 acquirements. Only those characters which are innate, and there- 
 fore the outcome of germinal determinants, are transmissible. 
 
 The mutation theory of de Vries was also in sharp contrast to 
 the fundamental principle of Darwinism, which laid down the central 
 idea that changes were extremely slow and gradual. According to 
 de Vries, no new species can be established without mutation. 
 ' When a mutation has occurred a new species is already in existence, 
 and will remain in existence, unless all the progeny of the mutation 
 are destroyed. . .' The phrase ' survival of the fittest,' as 
 describing a process of evolution, ought, de Vries contended, to be 
 replaced by survival of the fittest species. . . Natural selec- 
 tion may explain the survival of the fittest, but it cannot explain 
 the arrival of the fittest.' De Vries, in his first essay, was a remark- 
 able pioneer in the conception of unit characters which are so inti- 
 mately associated with the name of Mendel, and in his second he 
 very clearly pointed out the marked distinction between fluctuating 
 variations and true genetic variations, or mutations. Each muta- 
 tion, he contended, is the centre of a group of fluctuating variations 
 and unless there be very rigorous selection, fluctuations do not 
 lead to a permanent change in the mean of the species, and even 
 then, if the selection be slackened, there is a regression to the old 
 
GENERATION AND DEVELOPMENT 799 
 
 It has been asserted that the mutation theory of de Vries has 
 been amply verified in plants, but that it cannot be applied with 
 certainty to the animal kingdom. The development of the British 
 thoroughbred horse during at least the last two hundred years is 
 a clear proof of the fallacy of the latter contention. The truly 
 phenomenal strides made by him in physique, speed and racing 
 stamina could not possibly have been effected through the operation 
 of fluctuating variations on the twenty odd filial generations which 
 separate him from his ancestors the Arab, Barb and native English 
 racing Galloway. From time to time mutants have arisen which 
 have served as the parents of the race, and very appreciably raised 
 the mean capacity m the all-important characters which make for 
 success on the turf. 
 
 What has been said of the impregnation of the ovum (p 762) 
 is sufficient evidence that both parents contribute in an equal degree 
 to the construction of the offspring, and there is not much difficulty 
 in conceiving that not only the parents concerned but their 
 ancestors, are represented, though in ever-diminishing degrees. 
 An animal or plant is therefore a mixture of its ancestors, and the 
 question whether in this complex the individual characteristics 
 remain distinct, or whether they are blended, is one which during 
 the last few years has received the closest attention. If they are 
 blended, modifications in various directions may in consequence 
 arise; if they are not blended, variation must be ascribed to the 
 introduction of a new unit character or characters by one of the 
 parents, or alternatively to a character which had been carried in 
 a state of latency becoming patent. A blending of certain characters 
 in the soma is not denied by the Mendelists, but they hold that, as 
 there is no blend of character determinants in the germ plasm, 
 the somatic blend is not transmitted as such to the filial generation! 
 Galtonism. — That an individual is a mixture of its parents and 
 their progenitors is but a bare statement of the law of heredity 
 with which the name of Galton will ever be associated. As the 
 result of his careful inquiries and elaborate statistical data on 
 stature and other qualities in man, and on coat colour in basset 
 hounds, Galton formulated his law of ancestral inheritance, which 
 he stated as follows : ' The two parents between them contribute 
 on the average one-half of each inherited faculty, each of them con- 
 tributing one-quarter of it. The four grandparents contribute 
 among them one -quarter, or each of them one-sixteenth, and so 
 on, the sum of the series i + i + £ + ,V+ . . being equal to 1, as 
 it should be. The bias towards particular diseases, or tendencies 
 to such diseases, conveyed by any particular ancestor in any given 
 pedigree tends to be eliminated by a law which deals only with 
 average contributions, and the various prepotencies of sex in 
 respect to different qualities, such as the familiar sex-limited diseases 
 of pseudo-hypertrophic muscular paralysis, haemophilia, and colour- 
 blindness, are also presumably eliminated.' The essential feature 
 to be borne in mind in connection with Galton's law is that it 
 applies to masses rather than to the individual. It is an average 
 result, the immediate parents in all cases being, on the whole, more 
 largely represented than any of the other progenitors. For the 
 improvement of a breed it is essential, according to Galtonism, that 
 the laws of artificial selection should prevail. These, acting over 
 a sufficient length of time, would blend in desirable proportions the 
 special qualities required in the mass, though they might be absent 
 
800 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 in the individual. In further explanation it is necessary to add 
 that the whole of the parental and ancestral contributions trans- 
 mitted to the offspring must be represented in the germ plasm of 
 the parents. The parental share in the offspring's inherited faculties 
 refers to the outward, visible, demonstrable characters present in 
 the parents, and not to their latent characteristics. 
 
 Although, as already stated, the law of ancestral inheritance is 
 more applicable to the family group than to the individual, never- 
 theless it is of very considerable assistance in working out the 
 probable filial capacity resulting from a given mating, provided 
 always that the ancestral capacity in the trait or quality concerned 
 is accurately known. And ancestral capacity here applies not only 
 to somatic capacity, but also to reproductive capacity. 
 
 Mendelism. — The latest view of heredity is that put forward by 
 Mendel many years ago, though it remained unrecognised until 
 1901. It is difficult to define it in precise terms until examined, 
 but it aims at applying physiological laws of inheritance to the 
 individual rather than to the mass. It does not at present, and 
 may never be able to, deal with all the characteristics of individuals. 
 In the case of the more important faculties in man and animals the 
 application of Mendelian principles is as yet somewhat limited. 
 Mendelian inheritance is known to exist in the matter of coat colour 
 and length of fur in rabbits, and of the fleece in sheep. It applies 
 to the coat of various breeds of dogs — e.g., fox hounds, basset hounds, 
 and terriers. It also applies to eye colour. In the case of cattle 
 and sheep it applies to coat colour and horns, and in dairy stock, 
 it is said, to the quantity and quality of milk.* In poultry, to 
 plumage, combs, feathered or clean legs, and extra toes, as opposed 
 to the normal condition. In the horse it applies to coat colour, and, 
 as Robertson f has shown, to 'staying' power, to the number of 
 lumbar vertebrae, to the shape of the nasal bones, and to the shape 
 of the ears in the race-horse, and in Shetland ponies to hock cal- 
 losities. The principles apply much further in the case of plants, 
 dealing with such features as height, characters of flowering-head, 
 leaves, and stem, and have already been turned to useful economic 
 account in husbandry by the production of new and improved species. 
 The characters spoken of above are described in Mendelian phrase- 
 ology as Unit Characters. 
 
 The union of the ovum and sperm cell and their subsequent 
 division, described at p. 761, enables the essential facts of Mendelism 
 to be visualised. In Mendelian terminology either generative cell 
 is spoken of as a Gamete, and the organism resulting from the union 
 of two gametes is called a Zygote. We have seen that each gamete 
 contains half the number of chromosomes found in the body cells, 
 so that when the zygote is formed it is a double structure consisting 
 of an equal contribution of chromosomes from each parental gamete. 
 The resulting plant or animal retains in its cells throughout life the 
 double structure imparted by the two complete sets of chromo- 
 somes, one derived from the father, the other from the mother. As 
 was stated on p. 755, the inheritance of characters is believed to be 
 connected with the chromosomes. 
 
 Each chromosome contains within it the hereditary factors 
 arranged, it is suggested, end to end like beads on a string. Each 
 
 * Professor J. Wilson, M.A., Proceedings of the Royal Dublin* 7 Societv 
 1910-n. - /J 
 
 f_See footnote, p. 438. 
 
GENERATION AND DEVELOPMENT Sox 
 
 actor corresponds to a characteristic in the animal from which it 
 has been derived — eye and coat colour, height, body-build, temper 
 disposition, sex, longevity, instinct, milking capacity, early maturity' 
 horn production, constitutional vigour, speed, stamina, fertility and 
 the scores of features which distinguish animals of the same class 
 from each other. All the factors are not necessarily transmitted to 
 the offspring, but where they are alike in the parents the 
 chances of their transmission are enormously increased, and some 
 ■ factors are reproduced with certainty. This is the principle of in- 
 breeding. The offspring is a complex of the factors derived from 
 both parents, the whole being rearranged in the chromosomes 
 of the new animal. The number of new arrangements possible in 
 the paired chromosomes of the horse amount to nearly sixty-nine 
 millions, so that some notion may be formed of the capacity which 
 exists for variation. Fortunately, from an animal-breeding point 
 of view, the factors carried by the parental chromosomes may be 
 linked together in groups, though the linkage is elastic, and con- 
 sequently tend to be inherited in groups. 
 
 We have seen that the cells in the living organism are divided 
 into two groups — body or somatic, and reproductive or germ cells 
 (P- 751)- The absence of blending referred to on p. 799 refers solely 
 to the generative cells. The soma, or body, is the envelope con- 
 taining the generative cells, and between one generation and another 
 there is no continuity in the cells of the soma. The link between 
 generations lies in the germ cells. It is generally conceded that 
 environmental conditions may modify the somatic cells of the indi- 
 vidual, but that they exert no appreciable influence on the genera- 
 tive cells. Modification of the somatic cells not being heritable, it 
 is to the influence of natural selection transmitted through the 
 generative cells that variations must be attributed. 
 
 When a pair of zygotes breeds true for any given unit character, 
 their gametes must in respect of that character be identical, and the 
 resulting offspring of their union carries in its generative cells the 
 same unit character. When two z/gotes are mated which breed 
 true for opposite unit characters, such as tallness and shortness in 
 the common pea, the gametes produced by each, for the particular 
 unit character of height, are not identical, but opposite. The 
 resulting offspring in the first generation are not, however, in the 
 matter of height, a mixture of the two zygotes; they take after 
 one parent or the other, not after both, and whichever characteristic 
 they follow is the dominant one ; that which the body cells have not 
 followed is the recessive. 
 
 But though the body, or somatic, cells of the offspring have 
 followed the characteristic of either one or the other parent, their 
 generative cells are a mixture of the unit characters of both. In 
 spite of being mixed, there is no blending; the cells producing tall 
 do not fuse with the cells producing short plants, and subsequently 
 result in those of medium size. The determinant or factor which 
 produces tallness and the determinant or factor producing short- 
 ness, though both present in the generative cells of the zygote, are 
 as clearly and sharply separated as if only one factor were present. 
 When subsequently this zygote produces a generation, it will be 
 found that half its gametes carry the factor for tallness and half 
 carry that for shortness. This unblending or separation of the cells 
 is described as Segregation. A single gamete must, in this way, be 
 pure for either one or the other of two opposite characteristics; in 
 
 5i 
 
802 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the example selected it must be pure for tallness or pure for short- 
 ness, inasmuch as we have seen that no blend of the two is possible. 
 
 The whole Mendelian structure pivots on the segregation of 
 characters as outlined above, and the purity of the cells forming the 
 gamete. 
 
 Mendel's work was done on the common pea, and the essentia 
 facts are known in this plant with the greatest precision. 
 
 Selecting shortness and tallness from among several pairs of 
 contrasting characters which the pea furnishes, Mendel found, on 
 crossing a pure tall with a pure dwarf, that the offspring, or first 
 generation, were all tall. Ihis settled that in the pea tallness is 
 dominant and shortness recessive. If the first generation be self- 
 fertilised, a second generation is obtained. These are no longer all 
 tall; there are some short, and the proportion of tall to short plants 
 is as three to one. The dwarfs of this generation, if bred among 
 themselves, never produce anything but dwarfs, and so on in every 
 succeeding generation. The dwarfs, we have seen, are recessive, 
 and in Mendelian phraseology recessives breed true. The three 
 tails of the second generation, if bred among themselves, behave as 
 follows : One will produce all tall plants, and continue to do so to 
 infinity; the remaining two tails behave differently. They do not 
 breed true, but each produces in a next generation tails and dwarfs 
 in the proportion of three tails to one dwarf. Of these, one tall and 
 one dwarf breed true, while two tails are impure. These facts are 
 shown in the following table : 
 
 Tall x Dwarf (Parents = P) 
 
 All tall (First filial generation = Fi) 
 
 r ~ i ~r "i 
 
 Tall, Tall, Tall, Dwarf (=F2), 
 
 pure for impure impure pure for 
 
 ever ever 
 
 when bred when bred 
 
 inter se inter se 
 
 Reviewing the above classical example, it will be seen that the 
 first generation, though all tall, contained in the generative, as 
 apart from the body cells, the factors both for tallness and shortness. 
 There was nothing in the appearance of these tall plants to indicate 
 that they differed from their tall parent, or that they carried a 
 recessive character segregated, but it became evident in the second 
 generation that three different plants existed — viz., a true tall, 
 a true short, and an impure tall. In the third generation it was 
 shown that the pure tall and pure short bred true, and that the 
 impure tall bred pure tails, impure tails, and dwarfs, in the propor- 
 tion of three tails or dominants to one short or recessive. 
 
 The above example illustrates the simplest form of Mendelian 
 inheritance, and Mendel showed that each of the seven pairs of 
 contrasting characters found in the common pea behaved in the 
 same way— viz., for each of the distinguishing characteristics the 
 dominant was to the recessive as three to one in the second filial 
 generation, F2. 
 
 Examples of the Mendelian law in the breeding of animals are less 
 numerous than in the case of plants. If, however, pure polled 
 
GENERATION AND DEVELOPMENT 803 
 
 cattle are crossed with pure horned cattle, the resulting generations 
 behave precisely as in the contrasting characteristics of the sweet pea 
 
 The first generation will all be horned. If the first generation 
 be interbred the horned and polled factors separate out in the second 
 generation ; one-quarter will be pure polled, one-quarter will be pure 
 horned, and half will be hybrids. There is nothing in the appear- 
 ance of the hybrids to distinguish them from horned cattle but if 
 bred to other hybrid cattle they will produce one-quarter pure 
 polled, one-quarter pure horned, and one-half hybrids. A pure 
 horned or a pure polled breed may thus become established. Diffi- 
 culty is encountered in determining the hybrids, as these carry 
 horns; the hybrids can only be ascertained by the production of 
 further generations. 
 
 Dominance has been illustrated by the above recorded experiments 
 in which it has been seen that the resulting zygote is a pure domi- 
 nant when both gametes are pure for the particular character and 
 impure when only one gamete imparts the dominant variety. 
 There is no difference in appearance between the pure and impure 
 dominant above mentioned, but the test of breeding from them at 
 once settles the question. In some cases a zygote differs from either 
 parent by being intermediate in size or by physiological properties, 
 but the offspring of two such zygotes follow the ordinary Mendelian 
 law — viz., they reproduce the parental and the intermediate charac- 
 teristics in the second generation. The parental characteristics in 
 the next generation breed true ; the intermediates behave as in the 
 first generation — in fact, in accordance with the classical illustra- 
 tion of the peas. In such cases dominance is absent, and it is usual 
 to state the fact by saying that dominance is not a necessary feature 
 of Mendelian inheritance. 
 
 The following are examples of dominance in animals : 
 
 Bay, brown, and black dominant to chestnut in horses. 
 
 Grey dominant to black in rabbits and mice. 
 
 Black dominant to white plumage in fowls (in certain breeds). 
 
 Polled dominant to horns in cattle. 
 
 Short coat dominant to long coat in rabbits, sheep, and dogs. 
 
 Feathered legs dominant to clean legs in poultry. 
 
 Extra toe dominant to normal condition m poultry. 
 
 Six lumbar vertebra? dominant to five lumbar in the horse. 
 
 Convex nasal bones dominant to concave in the horse. 
 
 Melanism dominant to albinism in all animals. 
 
 The inhibitory factor to which greyness in horses is due is dominant 
 to its absence. 
 
 No explanation can at present be offered why dominance is 
 present in one case, absent in another, and irregular in a third. 
 In the following example of irregular dominance in sheep it is 
 supposed that sex is the determining cause : Dorset horned and 
 Suffolk hornless sheep were crossed, and the first generation resulted 
 in the rams being horned and the ewes hornless. Bred among 
 themselves, the several generations produced rams of which only 
 three-fourths were horned, and ewes of which one-quarter bore 
 horns. In this case horns or their absence depended upon sex, 
 horns being dominant in the ram and recessive in the ewe. This 
 was proved by mating a hornless ram with the hornless ewes of the 
 first generation, the resulting progeny showing that half the rams 
 were hornless and all the ewes. 
 
8o 4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Only an outline of Mendelism has been sketched above, and 
 nothing but its simpler side explained. When applied to many 
 cases of inheritance, Mendelism is not found to behave with the 
 simplicity above indicated. For instance, it is no longer considered 
 necessary to assume the existence of two factors to represent con- 
 trasting characters. Tallness or shortness in the pea is not due 
 to a factor for tallness and another for shortness, but to one factor 
 in two possible conditions — i.e., present or absent. If present, the 
 pea is tall; if absent, it is short. Two or more distinct factors in 
 the gamete may also act on each other, and so influence the resulting 
 zygote. This is known as the interaction of factors, and is a question 
 of supreme importance to the Mendelist. Factors may also repel 
 or attract one another; this is the theory designated the repulsion 
 and coupling of factors. These are mentioned in order to illustrate 
 some Of the difficulties and complexities of the doctrine; the con- 
 sideration of these is beyond the scope of this work, and requires 
 expert treatment. 
 
 Evolution. — It is natural that Mendelism should be searched for 
 an explanation of the evolution of the domestic from the original 
 wild species. It is suggested by those to whose opinion great 
 weight is attached* that the addition of factors, the subtraction, or 
 the interpolation of factors will explain the little which is clearly 
 known of evolution. For instance, there appears to be no doubt 
 that the present sweet-pea in its many varieties arose from the wild 
 sweet-pea introduced into this country a little more than two 
 hundred years ago. j In this case the numerous present-day varieties 
 have arisen, as is supposed, by the subtraction of factors. All the 
 necessary elements for the production of the many civilised varieties 
 are believed to have existed in the wild plant, but one by one those 
 inimical to their production have been eliminated. The Mendelist 
 does not reject the influence of variation, but he attaches to it a far 
 stricter interpretation; he does not believe that variation need 
 necessarily be small in amount, nor require ages to become adapted, 
 for it can be shown that marked variations, known as ' sports,' may 
 appear in a single generation. He further recognises two forms of 
 variation : the one which may appear quite suddenly, and is trans- 
 missible by the gametes, since it is due to the existence of ' factors ' ; 
 and the other which is acquired in consequence of the conditions of 
 life under which the organism is living. The first has been called 
 mutation, and has already been referred to; the second is somatic 
 fluctuation, and, not being represented in the gametes, is consequently 
 not transmissible. The process of evolution depends upon muta- 
 tions, and natural selection determines whether their continued 
 production shall be maintained or rejected. It is stated by Punnett J 
 that a rare ' sport ' with 5 per cent, selection in its favour will 
 replace the normal in a few hundred generations. This being so, 
 evolution generally demands a far shorter time for its production than 
 has generally been supposed, and no better evidence of this can be 
 brought forward than the improvement, based on artificial selection, 
 of the breeds of domesticated animals within recent generations. § 
 
 * ' Mendelism,' R. C. Punnett, M.A., third edition 1911. 
 
 t Op. cit. * p. cit. 
 
 § In the matter of the race-horse, certain exact data are available for s 
 considerable period, and the bearing of Mendelian inheritance on this 
 animal has formed the subject of special study. See ' The Principles of 
 Heredity applied to the Race-Horse,' by J. B. Robertson, M.R.C.V S 1910 
 
GENERATION AND DEVELOPMENT 805 
 
 Mendelism is still in its infancy, and its application to the breeding 
 of the larger stock, excluding poultry, has been determined only to 
 the extent recorded at p. 803. There are difficulties in obtaining 
 in animals the results readily effected in plants. One of these is 
 that the sexes have to be provided by two distinct physiological 
 units which may differ widely from each other, whereas in those 
 plants which are produced by self-fertilisation the constitution of 
 the male and female elements is identical. 
 
 The laws of inheritance of such unit characters as coat and eye 
 colour are known with considerable precision. The whole trend of 
 the work on heredity from the time of Darwin onwards has shown 
 that it has a physiological basis, and when regard is had to this fact, 
 even though the breeder is, as has hitherto been the case, working 
 in the dark, results of the highest economic importance have been 
 obtained. It is not too much to hope that the great precision 
 given to heredity by Mendelism will in time replace the policy of 
 ' hit or miss ' by something approaching exactitude. Already it 
 has explained, through the simple term ' recessive,' the previously 
 incomprehensible fact that a character may ' skip a generation ' ; 
 it also offers an explanation of reversion* and atavism.^ 
 
 Telegony is the supposed influence of a male by whom a female 
 has previously conceived on her subsequent offspring by another 
 male. For generations this supposed influence was the dread of 
 breeders. It possessed no physiological explanation, and was 
 finally conclusively shown by Cossar Ewart to be without a shadow 
 of foundation (see also p. 783). 
 
 Heredity in Disease. — Of the influence of heredity in disease there 
 is no possibility for doubt. Arrested development of the fingers 
 in man, night-blindness, colour-blindness, haemophilia, and a few 
 other conditions, have been shown from constructed pedigrees to 
 follow, or closely approximate to, the laws of Mendelian inheritance. 
 So far as the hereditary diseases of animals are concerned, their 
 Mendelian examination has barely begun, and in the very nature 
 of things must take a long time to accomplish. Robertson has 
 shown, from his inquiry into ' Roaring and Ruptured Bloodvessels 
 in Race-Horses, 'J the possibilities awaiting patient investigations. 
 On the question of the inheritance of disease considerable caution is 
 necessarv. There are few veterinary practitioners with experience 
 of stock-breeding who are not impressed by the hereditary nature 
 of such diseases as bone-spavin, ring-bone, side-bone, ' shivering,' 
 springhalt, cataract, and navicular disease. 
 
 It may be that our ideas concerning the actual nature of the 
 transmitted characters which lead to some, if not all, of the above 
 conditions will have to be modified. Chemical composition, or even 
 molecular arrangement, may ultimately turn out to be the deter- 
 mining factor. No biologist would, for instance, assert that bone- 
 spavin was represented by a unit character, but he would accept 
 the view that some particular condition of hock articular cartilage 
 led to bone-spavin. He would also admit that this specific carti- 
 
 * A return to some type of ancestral character, as when a black and 
 a white rabbit produce a grey, which is the colour of the wild form. 
 Chestnut is a reversionary character in the horse (Robertson). 
 
 t ' Throwing back,' or atavism, is the appearance of ancestral racial 
 characteristics, as, for example, the extra digit sometimes found in the 
 horse. 
 
 I Of. cit. 
 
8o6 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 laginous condition might well be represented by a determining unit 
 character, and that this character passed from parent to offspring. 
 In these circumstances it is conceivable that bone-spavin due to a 
 defect or modification in the germ plasm would be heritable. On the 
 other hand, if arising from sprain or trauma, its heredity would be 
 absent, for somatic acquirements are not transmissible (Robertson). 
 Robertson's investigations have shown that roaring, side-bone, 
 ' shivering,' and a tendency to ruptured bloodvessel, follow the laws 
 of Mendelian inheritance. 
 
Section 4. 
 
 Breeding. 
 
 By J. B. Robertson, M.R.C.V.S. 
 
 These islands have for many centuries been famous for their flocks 
 and herds but little progress was made in the improvement of 
 ■III ^ the middle of the eighteenth century. The explanation 
 is that the open field and land-in-common system of farming which 
 prevailed until the close of the seventeenth century gave no'oppor- 
 tumty to create improved sub- varieties ; further, the methods of 
 farming were backward, little or no provision being made for winter 
 keep, so that the stock emerged from the stress of winter in a state 
 of semi-starvation. 
 
 When the production of turnips, clover, and artificial grass was 
 rendered possible by the work of J. Tull and of Viscount Townshend, 
 the middle of the eighteenth century showed a vast improvement in 
 the winter provision of food for stock, concurrently with an all-the- 
 year-round supply of fresh meat for the industrial population in 
 place of the salted food they had previously obtained. 
 
 Then came R. Bakewell (1725-95), a farmer and grazier of extra- 
 ordinarily advanced views, both on agriculture and breeding. He 
 determined to create improved breeds of sheep and cattle, and at the 
 same time to increase the output of meat. He succeeded, not by 
 any haphazard methods, but by the rigid use of experimental 
 inquiry; his Leicester sheep were the best in the world, and were in 
 demand even as far off as America. His long-horn herd of cattle 
 became equally famous. BakewelPs methods were the careful 
 selection of original stock, and the obliteration of all progeny which 
 did not come up to his standard. He established a museum in 
 which werp kept the skeletons and soft parts of his various experi- 
 mental animals, so that a comparison could readily be made to test 
 his progress. He also adopted the principle of in-breeding. In this 
 respect Bakewell was not original. Even as far back as the time 
 of the Stuarts in-breeding had been employed for the racing Gallo- 
 ways of the northern counties of England, also for hounds, and 
 doubtless for fighting game-birds. Bakewell applied the same 
 methods to obtain improved sheep, cattle, and horses for agricul- 
 tural purposes. In respect of the latter, he had the object-lesson 
 afforded by Leedes. of Yorkshire, who, forty years before Bakewell 
 was born, had produced by in-breeding two horses, Cream Cheeks and 
 Fox, whose names are repeated many hundreds of times in the pedigree 
 of every thoroughbred horse in the world. The difference, however, 
 between the methods of Bakewell and of those who preceded him in 
 in-breeding is that, whereas in the case of horses, hounds, and game- 
 fowl their performance had been the test, Bakewell worked on the 
 method of purely artificial selection, guided by his extraordinary 
 judgment, so that his progress was relatively rapid. One of his 
 rules was this, that the male parent was the more important of the 
 two in racial improvement. He selected, for early maturity, thick 
 flesh, light offal, and fair milking qualities in his cattle, early maturity 
 and wool in the case of sheep. It mustbe remembered that Bake- 
 
 807 
 
8o8 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 well could have done nothing but for his improved system of agri- 
 culture; his farm became the Mecca of agricultural students, both at 
 home and abroad. His pupils carried his practice far and wide ; two 
 of the earliest and most distinguished were the brothers Colling, 
 both excellent judges of cattle, who subsequently founded the 
 modern Shorthorn breed. Among their contemporaries were T. 
 Bates (1776-1849) and T. Booth, who, drawing on the Colling herds 
 for stock revolutionised stock-breeding, particularly in Ireland and 
 South America, while the Dairy Shorthorns of Bates furnished 
 the female ' tap-roots ' of many of the best strains of the present 
 day. Certain stocks, such as the Channel Island cattle and the 
 Kerries of the south-west of Ireland, were of purer origin than the 
 modern Shorthorn, and in consequence did not furnish so much 
 scope for the breeder's art. 
 
 The principle of in-breeding applied to the improvement of cattle 
 and sheep has since the days of Bakewell been applied to horses 
 other than thoroughbreds, but the resultant improvement is 
 based on show-ring standards and has not proved an unmixed blessing. 
 Certain of the ' fancier ' points relating to the Shire, Clydesdale, 
 Suffolk Punch, Cleveland Bay, and Hackney are far from being 
 correlated with ability to excel in heavy and light draught work. 
 The long sloping pasterns, large spreading feet, and inturned hocks of 
 the Clydesdale are considered the acme of perfection by his admirers, 
 yet these characteristics are viewed as the worst possible defects in 
 the Shire or Suffolk Punch. In like manner the coarse integument 
 and long hair on the limbs of the Shire have been carried to such a 
 point as to predispose him to pathological conditions such as coro- 
 nitis, lymphangitis, and ' grease.' 
 
 All the horse stocks of the world are referable to three species of 
 Equidae — viz., E. robustus, E. przewalskii, and E. agilis. The Shire 
 horse of the present day presents all the characteristics of E. robustus. 
 and, being pure, he brfeeds true to type, the biggest, coarsest, heaviest, 
 and most hairy-legged variants being unfortunately selected as 
 breeding units. E. przewalskii and E. robustus are combined in the 
 Suffolk Punch, Clydesdale, and Percheron. E. robustus and E. 
 agihs are found in the Cleveland Bay and Hackney. The modern 
 representatives of E. agilis are the Arab, Barb, British Thorough- 
 bred, and a few Celtic ponies. It hybridises with other species, but 
 permanent sub-types can only be evolved with the greatest dfhiculty. 
 Failure to recognise this is the explanation of the many mis-shapen 
 animals included in the category of light horses. Individuals of the 
 second filial generation, when bred inter se, show marked segregation, 
 but not on specific lines, so that in the third and fourth parental 
 generations variations of an undesirable character are frequent. It 
 is also well established that the thoroughbred stallion is considerably 
 less fertile with common mares than with mares of his own species. 
 The ignorance exhibited by the agriculturists of England and Scot- 
 land in putting any degenerate specimens of E. robustus to the 
 thoroughbred horse shows the condition in which the breeding of 
 half-bred horses is at the present day. The farmers in the south of 
 Ireland have made fewer mistakes, for the reason that the mares at 
 their command have in the distant past sprung from a stock saturated 
 with the blood of the thoroughbred, directly or indirectly. To mate 
 these mares with the Clydesdale and Shire, as has been done in 
 recent years, shows the ignorance which exists of the physiology of 
 breeding, 
 
GENERATION AND DEVELOPMENT 809 
 
 Knowledge of the physiology of breeding has of late years been 
 considerably augmented by the Mendelian theory, and by cytolo- 
 gical evidence. Our conception of the pure-bred thing is no longer 
 vague, but clear-cut and definite. 
 
 Formerly it was believed that persistent saturation of several 
 generations was necessary for the fixation of a character, and that 
 this had to take place before a male or female could become ' pre- 
 potent ' as a parent in a particular attribute. We now know that 
 this was erroneous. All that is necessary is that a chromosome in 
 the nucleus of the spermatozoon and a chromosome in the nucleus 
 of the ovum shall bear chromatin granules which are alike in respect 
 of the particular character desired. Hence ' prepotency,' or the 
 power to stamp parental character on the soma of the offspring, is 
 not correlated to length of ancestry, but is governed by clearly 
 definite rules, now styled dominance (see p. 803). 
 
 The thoroughbred's ancestry has been known for upwards of 200 
 years. He is the product in all cases of an exceedingly small number 
 of individual ancestors, and is the most intensely interbred domes- 
 ticated animal in the world. The parents of the breed are selected 
 by the race -course test, which picks out the plus variants in speed, 
 stamina, and ability to stand the severity of training. It is generally 
 accepted that the distribution of the paternal and maternal chromo- 
 somes to the gametes of the filial generation is entirely a matter of 
 chance. On the basis of eighteen chromosomes in the mature repro- 
 ductive cell, the number of combinations possible in the gametes are 
 262,144, an d in the zygotes 68,719,476,786. These figures give some 
 idea of the capacity which exists for variation. The chance of pro- 
 ducing desired combinations are increased by in-breeding with 
 animals possessing the desired correlations. 
 
 The extent to which in-breeding may be carried with safety 
 depends on the degree in which the race generally has been interbred. 
 When the parents of the first generation are in no way related, in- 
 breeding may be carried to extreme lengths in the succeeding genera- 
 tions without harmful consequences. As already pointed out, the 
 thoroughbred has been intensely interbred, no new blood having 
 been introduced for practically 200 years. In place of having the 
 full complement of 16,384 individual ancestors in the fourteenth 
 parental generation, no modern thoroughbred has more than fifty, 
 and many have fewer than that number. In the tenth and twelfth 
 generation the number of male ancestors is reduced to three, and of 
 these three Eclipse enormously predominates. In consequence, it is 
 not possible to find thoroughbred horses which are not related in 
 blood. 
 
 In the racehorse and greyhound the methods by which the parental 
 generations are selected — namely, their performance — tends auto- 
 matically to keep a sharp check on too close in-breeding, for the 
 degenerates have no chance of becoming racial parents. 
 
 Similarly, dairy stock is selected by the capacity to produce deep 
 milkers. Cattle and sheep are judged by the early maturity they 
 attain. Elimination is carried out by the judgment of the breeder. 
 Bakewell, Bates, the Collings, and others did not hesitate to sacri- 
 fice any animal not coming up to their standard, and to this may 
 be attributed the fact that the introduction of new blood was a 
 matter of rarity. Bates used no new blood for the first thirteen 
 years; during the next seventeen years he introduced it three times, 
 always on account of a reduction in fertility, which is the most 
 
Sio A MANUAL OF VETERINARY PHYSIOLOGY 
 
 obvious and the most frequent indication of deterioration. Price, 
 a breeder, stated in 1841 that his cattle had been closely in-bred for 
 eighty years without fresh blood. As surely as good qualities arc 
 transmitted by in-breeding, so are heritable diseased conditions in- 
 tensified (see p. 805). 
 
 The characteristics transmitted to the offspring by each parent 
 have been a subject of fruitful speculation among men of equally 
 wide experience. The liability to variation where the parents are 
 not of fixed type and the prepotency shown by either the male or 
 female in stamping its likeness on the progeny are two factors which 
 must always influence the tranmsission of characteristics. The 
 broad rule more or less generally accepted by breeders is that the 
 male parent influences the external structures, vigour, courage, and 
 organs of locomotion, while the female transmits the internal organ- 
 isation, temperament, and organs of nutrition. Physiologically it 
 seems impossible to accept this, though if we regard the mule as a 
 test case the rule appears to apply. In this animal we have the 
 offspring of two widely contrasting parents, with the result that the 
 male, the ass, is strongly represented in the external structure and 
 organs of locomotion. When the male parent is the horse, the 
 resulting ' jennet ' partakes of the appearance of the horse so closely 
 that at first sight it might pass as a pony. 
 
 The careless manner in which the public discuss the practice of 
 breeding shows the remarkable ignorance which exists of this most 
 complex of all physiological phenomena. There is no branch of 
 industry which requires so long an apprenticeship, and this even is of 
 no avail without the ' breeder's eye,' a gift which few men possess. 
 
CHAPTER XIX 
 
 GROWTH, DECAY, AND DEATH 
 
 Growth. — This subject has already received some consideration 
 in dealing with nutrition (pp. 368, 376) ; it was referred to in 
 connection with vitamines (p. 390), and at pp. 305, 306, and 307 
 isolated references had been made to the influences on growth 
 exercised by certain endocrine glands. 
 
 We have seen that the growth impulse is most pronounced in 
 intra-uterine life, and is diminished from birth to maturity, at 
 first rapidly, then slowly. During foetal life the production of 
 new tissues is the characteristic feature of growth. Broadly 
 speaking, in extra-uterine life no new tissues are produced by the 
 herbivora; the existing tissues are developed by cell multiplica- 
 tion, but especially by an increase in cell contents and intra- 
 cellular material. Growth cannot be explained as resulting 
 from an increase in the size of the body cells, for their limits are 
 fixed. The energy of intra-uterine metabolism in the herbivora 
 is devoted to the production of protein and inorganic tissues to 
 enable these animals to move freely at birth without undergoing 
 the period of helplessness so prominent in man and the car- 
 nivora. To this end the bones and muscles are rendered capable 
 of playing their part as organs of locomotion from the moment 
 of birth. The store of fat, so well seen in the subcutaneous 
 deposits in the human infant, is entirely absent in new-born 
 herbivora. 
 
 With the carnivora the intra-uterine development is incom- 
 plete, and new tissues continue to be produced after birth; for 
 example, the eyes of the puppy are not completed for vision 
 until the fourteenth day after birth, and it is not until the thirty- 
 fourth that a dog avoids obstacles. Similarly, the locomotor 
 apparatus is incomplete at birth, and further development has to 
 take place before standing and walking are possible. 
 
 The importance of protein in the food of the young growing 
 animal has already been dealt with, also the remarkably com- 
 plete manner in which it is utilised by the herbivora. It is this 
 which leads to the deposition of protein in the body, and the 
 facility with which the protein accumulates has been taken 
 
812 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 advantage of in the production of early beef and mutton maturity, 
 which has long been recognised as the basis of economical meat 
 production. As the animal grows older the tendency to store up 
 protein is diminished, nor can it be stimulated by increasing the 
 amount in the diet. 
 
 The increase in weight of very young animals is largely water; 
 as much as 80 per cent, of the gain may be so represented, but 
 as early maturity approaches the water gained by the tissues 
 becomes less and less, and finally drops to 30 or 40 per cent, of 
 the gain. The storage of dry matter in the very young is, as we 
 have seen, largely protein, while as early maturity approaches 
 the increase in weight is largely fat with some protein. The 
 storage of salts is very high, especially in the young while still 
 suckling, and gradually diminishes, though not so rapidly as does 
 that of the organic substances, (see p. 385, where a table is given 
 showing the retention of salts in the body) . 
 
 We have seen that the young of the herbivora very rapidly 
 shake off the helpless condition in which they first find them- 
 selves in this world. This is largely due to the fact that they 
 are born with a nervous and locomotor system in a high state of 
 development ; in the course of an hour or less they learn to stand 
 and walk, and in a day or two can skip and run. The young 
 animal, moreover, is born in full possession of its senses, such as 
 sight, touch, hearing, smell, taste, and with an amount of in- 
 telligence which nearly, if not quite, equals that of its parents. 
 It has practically nothing to learn but obedience to man. The 
 legs of the foal are remarkably long, some of the bones being 
 nearly their full length (see table, p. 813), though, of course, not 
 their full weight; such joints as the knee and hock have very 
 little to grow. We can understand the reason of this develop- 
 ment of the limb from what has been said above, while the length 
 of leg in the foal is undoubtedly for the purpose of enabling the 
 animal to reach the mammary gland. The limb, however, is 
 only partially developed; from the knee and hock to the ground 
 it is nearly the same length as in the adult; from the knee to 
 the elbow and the hock to the stifle it is decidedly below 
 the adult, whilst from the elbow to the withers, and the stifle to 
 the croup, the body has a considerable amount to grow. It has 
 been said, and the statement appears to be true, that the future 
 height of the foal may be ascertained by measuring the fore-limb 
 from the fetlock to the elbow and multiplying it by two. 
 
 The hind-quarters of the foal are in a more advanced state 
 of development than the fore: the shoulders are very oblique, 
 the chest contracted and shrunken-looking, and neither shoulder 
 nor chest contains much muscle. The oblique position of the 
 scapula is due to the weight of the body on the limbs, the weakness 
 
GROWTH, DECAY, AND DEATH 
 
 Si3 
 
 of the muscles at this part allowing the angle formed by the 
 scapula and humerus to be considerably closed, and the shoulder- 
 joint to bulge. The head of the foal is prominent over the brain 
 and depressed over the nasal bones. The hair is fully developed 
 but woolly, that of the mane being scanty and of the tail curly, 
 while the colour of the body-hair is light of its kind. A similar 
 deficiency of pigment is observed in the iris. 
 
 Table showing the 
 
 Length 01 
 
 the Bones 
 
 of the Limbs of the 
 
 
 Foal 
 
 AND 
 
 Adult Horse. 
 
 
 
 Adult Horse. 
 
 Foal of Six 
 Weeks. 
 
 Difference. 
 
 Scapula 
 
 _ 
 
 _ 
 
 15 in. 
 
 8£in. 
 
 6} in. 
 
 Humerus 
 
 - 
 
 - 
 
 12 in. 
 
 8 in. 
 
 4' in. 
 
 Radius and ulna 
 
 . 
 
 - 
 
 18 in. 
 
 12 in. 
 
 6 in. 
 
 Knee-joint 
 
 - 
 
 - 
 
 3£x3jin. 
 
 3 x 3 in- 
 
 Jin. 
 
 Metacarpal - 
 
 - 
 
 - 
 
 9i "I- 
 
 8f in. 
 
 I in. 
 
 Suffraginis 
 
 - 
 
 - 
 
 3* in. 
 
 3 in- 
 
 | in. 
 
 Femur 
 
 - 
 
 - 
 
 17 in. 
 
 10J in. 
 
 6£ in. 
 
 Tibia - 
 
 
 - 
 
 13^ in. 
 
 o|in. 
 
 4 in. 
 
 Calcis - 
 
 
 - 
 
 6 in. 
 
 5 in- 
 
 1 in. 
 
 Metatarsal 
 
 
 
 n in. 
 
 10 in. 
 
 1 in. 
 
 Suffraginis 
 
 
 
 3iin. 
 
 3 in. 
 
 \ in. 
 
 Ihe rate at which the foal increases in weight, and other 
 circumstances connected with its nutrition, were made the 
 subject of inquiry by Boussingault.* He found that the mean 
 weight at birth was 51 kilogrammes (112 pounds), that during 
 the first three months the daily increase in weight was 1 kilo- 
 gramme (2-2 pounds), from three up to six months the increase 
 was o-6 kilogramme (1-3 pounds), and from six months up to 
 three years of age the increase was at the rate of 0-32 kilogramme 
 (07 pound) per diem. The influence of feeding on development 
 is most remarkable. Not only does the body increase m size 
 and weight, but the animal presents the appearance of the adult 
 so that a thoroughbred at two years old is ' furnished and 
 looks as old as an ordinary horse at four years old Caves, 
 according to Torcy,* have a mean weight at birth of 35 kilo- 
 grammes ( 77 pounds), the daily increase during the first two 
 years being 07 kilogramme (i- 5 pounds). With sheep the daily 
 increase in weight is more rapid. A lamb will m ten days gam 
 5 o per cent, on its original weight, will double its ; weight 
 at the end of the first month, and treble it at the end 
 of the second. Swine present, however, the most rapid 
 increase in weight, for, according to the authorities quoted, a 
 * Quoted by Colin. 
 
8i 4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 pig will increase 20 per cent, in its weight per diem during the 
 first week, and up to the end of the first year will add o-2 kilo- 
 gramme (0-44 pound) daily to its body weight. 
 
 The relative rate of growth of each part is not the same. 
 The eyes, ears, brain, kidneys, and liver. grow less rapidly than 
 the other parts, owing to their relatively large size at birth. 
 The greatest increase is in the skeleton and muscles, and to the 
 rate of this increase we have just alluded; the least increase is 
 in the eyes and the ears and the limbs below the knee and hock. 
 Few observations have been made on the rate of growth. 
 Percival* many years ago drew up a table, which he considered 
 very imperfect, of the rates at which some horses of his regi- 
 ment grew, from which he showed that the increase in height 
 between two years and three years was on an average 1 inch, 
 between three years and four years J inch, and between four 
 years and five years J inch. Some of the horses did not grow. 
 
 Of 35 two-year-olds, 2 did not grow during the year. 
 Of 144 three-year-olds, 17 did not grow during the year. 
 Of 48 four-year-olds, 7 did not grow during the year. 
 Of ri five-year-olds, 2 did not grow during the year. 
 
 There can be no doubt that many horses grow much more 
 than f inch between three and five years, and many grow up to 
 their sixth year. 
 
 The writer measured the daily growth of a foal for three months, 
 the observations being made at the withers and croup. The 
 animal during this period grew, on an average, 1 inch every 
 eight days. The most rapid growth occurred shortly after birth 
 and was 1 inch in three days. At the withers and croup the 
 rate of growth was practically equal. 
 
 During the time the calf and foal are receiving their mother's 
 milk the urine is acid, for the reason that the animal is prac- 
 tically carnivorous. Once a vegetable diet is taken, the urine 
 becomes alkaline and, it is probable, decreases in quantity. The 
 activity of certain glands, such as the thymus, becomes con- 
 siderably reduced as the animal grows, and the gland finally 
 disappears at the adult period. One characteristic of the young 
 animal is the necessity for sleep and rest. It is during these 
 that the tissues make the immense strides noticeable during the 
 first few weeks of life. 
 
 Dentition commences immediately at birth, if it has not 
 already commenced in utero. The tables on p. 815 show the 
 period at which changes take place in the teeth from birth to 
 adult age: 
 
 The periods of eruption and change of the molar teeth are 
 liable to considerable variation. 
 
 * ' Lectures -on Form and Action.' 
 
GROWTH, DECAY, AND DEATH 
 Horse. 
 
 815] 
 
 Eruption. 
 
 Change. 
 
 In;isors : 
 
 
 
 Central 
 Lateral 
 Corner 
 
 At birth or shortly after. 
 1 to 2 months. 
 7 to 8 months. 
 
 i\ years. 
 3I years. 
 4I years. 
 
 First 
 Second 
 Third 
 Fourth 
 Fifth 
 Sixth 
 Canines 
 
 1 At birth. 
 
 About 1 year. 
 About 2J years. 
 About 3! to 4 years. 
 About 4J years. 
 
 2 \ years. 
 ■ 3 years. 
 About 3 \ years. 
 
 
 Ox.* 
 
 
 
 Eruption. 
 
 Change. 
 
 Incisors : 
 Central 
 Middle 
 Lateral 
 Corner 
 
 Molars : 
 First 
 Second 
 Third 
 Fourth 
 Fifth 
 Sixth 
 
 (.At or soon after 
 I birth. 
 
 [At birth. 
 
 6 months. 
 
 About 12 months. 
 
 21 months. 
 
 (iA years. 
 J 2^ to 2 T 6 2 years. t 
 I 2/5 to 3 years. f 
 U}4 to 3^ y ears. t 
 
 f About 2j\ years. 
 -j About 2^- years. 
 (.About 2-j% years. 
 
 Sheep. 
 
 
 Eruption. 
 
 Change. 
 
 Incisors : 
 
 
 
 Central 
 
 [At- birth or soon 
 
 ("About I year. 
 
 Middle 
 
 J About 2 years. 
 
 Lateral 
 
 I after. 
 
 1 Soon after 2 years.f 
 
 Corner 
 
 ^About 3 years, t 
 
 Molars : 
 
 
 
 First 
 Second 
 
 [At birth or soon 
 | after. 
 
 fSoon after 18 
 -] months. 
 
 Third 
 
 [About 2 years. 
 
 Fourth 
 
 3 months. 
 
 
 Fifth 
 
 9 months. 
 
 
 Sixth 
 
 18 months. 
 
 
 * The age of the ox, sheep, and pig is tabulated from the data given 
 by Professor Brown in his ' Dentition as. Indicative of the Age of Animals ' ; 
 the observations were made by Professor Simonds. 
 
 f There is considerable variation in the date of development of these teeth. 
 
816 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Pig. 
 
 
 Eruption. 
 
 Change. 
 
 Incisors : 
 
 
 
 Central 
 
 i month. 
 
 12 months. 
 
 Lateral 
 
 2 months. 
 
 1 8 months. 
 
 Corner 
 
 At birth. 
 
 8 months. 
 
 Molars : 
 
 
 
 First 
 
 } 
 
 1 
 
 Second 
 
 [i month. 
 
 } About 15 months. 
 
 Third 
 
 J 
 
 J 
 
 Fourth 
 
 5 months. 
 
 
 Fifth 
 
 10 to 12 months. 
 
 
 Sixth 
 
 1 8 months. 
 
 
 Premolars - 
 
 5 months. 
 
 
 Tusks 
 
 At birth. 
 
 9 months. 
 
 In all these tables the periods given are those of eruption 
 only. The teeth are not fully developed for some time later, 
 which varies from four to six months in the horse to a month 
 in the pig and ruminant. B. Clark, nearly ioo years ago, gave 
 a table showing the rate of growth of the temporary and per- 
 manent incisors of the horse from the time of eruption until the 
 teeth met. It is remarkable that this question does not appear 
 to have since attracted attention. He gave two months as the 
 period of growth of the first and second temporary pair, and 
 three months for the third pair. In the permanent incisors he 
 states the period of growth of the first pair is one year, and of 
 the second and third pairs each six months. 
 
 The completion of dentition usually marks the age of maturity. 
 The uncastrated animal presents very distinctive features as 
 compared with the female — viz., greater bulk, a heavy crest and 
 neck, and a harsher voice; the castrated horse more closely 
 resembles the mare. No such difference in the date of maturity 
 as is observable in the human family exists between the male 
 and female of the horse tribe. The mare arrives at maturity at 
 the same time as the horse, and the castrated animal is not 
 deficient in stamina, strength, or capacity for work. Moreover, 
 castration in the horse does not lead to a deposition of fat in the 
 body (see p. 379). 
 
 Decay.— It is doubtful to what age a horse would live if not 
 subjected to domestication, but we may safely say that at 
 seventeen years old, which probably represents thirteen years' 
 work, the powers of life in the majority of them are on the wane, 
 though at this period some may be found in full possession of life 
 and vigour. These are probably cases of a survival of the fittest, 
 and cannot be taken as a general guide. As a broad rule it may 
 
GROWTH, DECAY, AND DEATH ^7 
 
 be stated that an old horse is liable to be killed by a hard day's 
 work, and in this sense he is certainly old at seventeen; what he is 
 especially deficient of at this period of life is recuperative power: ) 
 
 It has been pointed out (pp. 62 and 100) that the horse, unlike 
 man, does not fail in his heart and arteries in consequence of in- 
 creasing age. It is not, therefore, a matter for surprise that up to 
 a late period of life he does not seriously lose his capacity for work. 
 In this respect he offers a great contrast to man. At forty years 
 of age there are few men capable of undergoing fast muscular 
 work on their own limbs, for not only are the muscles. slower in 
 responding, but the effort required is greater. A child may 
 indeed, execute with ease muscular movements which would 
 produce a punishing effect on a middle-aged man. If it be 
 accepted that a horse of fifteen is comparable to a man fifty 
 years of age, then it is certain no man of fifty can perform the 
 relative amount of work which a horse can at the corresponding 
 age. It may, perhaps, yet be shown that the vertical position of 
 man exercises a strain on the heart and vessels which is absent 
 in the horizontally placed animal. Or there may be some actual 
 defect in the skeletal muscles of man, the outcome of age, which 
 is unknown in the horse. Trotting work soon wears a man out; 
 the Japanese or Natal coolie, who pulls a light cart containing 
 one or two people, only lasts three years. He then has to look 
 for lighter employment. A horse performing relatively equiva- 
 lent work would last much longer. 
 
 Doubtless the work performed by horses is the chief Cause 
 of their decay, for their legs always wear out before their bodies. 
 But apart from this, changes in their teeth, such as the wearing 
 away of the molars, appear to prevent many of them from reach- 
 ing a ripe old age. Instances are on record of horses attaining 
 the age of thirty-five, forty-five, fifty, and one animal is known 
 to have lived to sixty-three years of age.* Blainef appears to 
 
 * Bracy Clark, in his ' Podophthora,' quoting from the Liverpool 
 Advertiser, said there was a cart-horse on the canel near Warrington 
 sixty-three years of age. Clark knew of a hunter fifty-two years of age 
 that had never been out of the hands of the man who bred him. In a 
 morning paper of May 17, 1911, it was stated that a farmer in the Lake 
 district owned a horse forty-three years of age which still occasionally 
 worked . 
 
 The age at death of some famous racehorses is given in the following table : 
 
 Eclipse 
 
 26 years. 
 
 Bend Or 
 
 26 years 
 
 Parrot 
 
 36 „ 
 
 Queen's Birthday 
 
 26 „ 
 
 Pocahontas 
 
 33 ., 
 
 Marcion 
 
 23 ,, 
 
 Touchstone 
 
 - 3° .. 
 
 Ladas - 
 
 23 ,. 
 
 Hermit 
 
 29 ,, 
 
 Cherry Tree 
 
 26 „ 
 
 Melton 
 
 29 ,. 
 
 Collar 
 
 - 19 „ 
 
 St. Simon 
 
 27 >• 
 
 William the Third 
 
 19 ,. 
 
 King Tom 
 
 - 27 „ 
 
 
 
 •f ' Outlines of the Veterinary Art.' 
 
 52 
 
8i8 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 have gone very carefully into the question of old age in equmes, 
 and he drew the following comparison, which is doubtless very 
 close to the truth : ' The first five years of a horse may be con- 
 sidered as equivalent to the first twenty years of a man. Thus, 
 a horse of five years may be comparatively considered as old as a 
 man of twenty; a horse of ten years as a man of forty; a horse 
 of fifteen as a man of fifty; a horse of twenty as a man of sixty; 
 of twenty-five as a man of seventy; of thirty as a man of eighty; 
 and of thirty-five as a man of ninety.' 
 
 The indications of advanced age in horses are well marked, 
 apart from those connected with the length, shape, and colour 
 of the teeth: the edge of the lower jaw becomes sharp, the 
 supra-orbital fossa deep, the eyelids wrinkled; white hairs may 
 appear on the face and elsewhere, while black horses may turn 
 white, though this is unusual. A hollow back is very greatly 
 intensified with age ; the joints of the limbs show the effects of 
 work, and the gait has lost its elasticity. 
 
 In the case of animals intended for food the question of decay 
 does hot arise. In the carnivora Mr. H. Gray informs the writer 
 that in the dog at about eight years old, tumours, carcinoma, car- 
 diac murmurs, chronic orchitis, sarcoma of the testicle, enlarge- 
 ment of the prostate, catarrhal endometritis, chronic mammitis, 
 and cirrhosis of the,diver with ascites are liable to appear. In 
 the cat he states that mammary cancer, especially of the pectoral 
 glands, is common at about fourteen to sixteen years of age. 
 The frequency of malignant tumours is remarkable ; they are not 
 evident in the horse, if we except grey animals, where melanotic 
 growths are a frequent accompaniment of old age. 
 
 The duration of life in the various domesticated animals is 
 given by Crisp* as follows : 
 
 Horse 
 Ass 
 Ox 
 Goat 
 
 25 to 35 years. 
 30 to 40 years. 
 15 to 20 years. 
 15 years. 
 
 Sheep 
 
 Pig 
 
 Dog 
 
 15 years. 
 
 12 to 16 years. 
 
 14 years. 
 
 It is believed these observations were made under the favour- 
 able conditions of animals in captivity (Zoological Gardens), 
 well fed and looked after, and the figures are in consequence high. 
 
 Selecting thoroughbred stock as the type of horse placed under 
 the most favourable conditions, we find the mean age at death 
 of both stallions and mares to be a little under twenty years. 
 
 Mr. H. Gray informs the writer that the larger the breed of dog 
 the shorter its life. Great Danes are old at eight to nine years, 
 spaniels, collies, etc., at ten to twelve years, terriers at twelve to 
 fourteen years. The smaller breeds live sometimes to sixteen 
 
 * Op. cit., p. 757. 
 
GROWTH, DECAY, AND DEATH 
 
 iig 
 
 and eighteen years; dachshunds may live to eighteen or nine- 
 teen years, and Maltese to twenty to twenty-two years. In 
 the dog as with man, a long life is hereditary. The life of the 
 cat is longer than that of the dog. 
 
 We have already seen that in the herbivora and carnivora 
 there is no ' change in life ' as effecting either male or female, 
 and that both are virile in old age. 
 
 Death. — Death from senile decay in the horse is a matter 
 of rare occurrence. It is seldom that an animal is taken such 
 care of that the tissues are worn out by age, or that he is allowed 
 to live until the breath of life passes gradually from the body- 
 
 
 "fc* 
 
 ;.«■«■•■; 
 
 ■■—■ ^.xtr.^wife 
 
 Fig. 271. — Convulsive Limb Movements at the Moment of Brain 
 
 Destruction. 
 Note the tail is affected as well as the limbs. The bandages were put on 
 to assist the plate. 
 
 Sentiment plays no part in horse management. A useless mouth 
 is one to be got rid of. In consequence, the majority of horses 
 meet either with a violent death or one the result of disease. 
 Under good hygienic conditions the general annual death-rate 
 may be taken at 1-5 per cent. To this, however, must be added 
 a fluctuating loss due to destructions for injury, which may be 
 stated at about 1 per cent., depending on the occupation. 
 
 Natural death is described as commencing either at the heart, 
 lungs, brain, or blood. Probably the few cases of natural death 
 which occur may be attributed to failure of the heart's action; 
 but from what is known of the physiology of the heart, respira- 
 tion, and blood, it is very difficult to separate these in discussing 
 
S20 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 the causes of death, as one is so largely dependent on another. 
 The cessation of the heart's action may be looked upon as the 
 termination of life. 
 
 We cannot enter upon the cause of death resulting from disease, 
 except to notice the interesting fact that horses seldom die 
 quietly. A large majority of them die in powerful convul- 
 sions, fighting or struggling to the last, lying on the side, and 
 galloping themselves to death. This is especially marked in 
 acute abdominal trouble, or other painful conditions. The 
 struggles at the end should not be mistaken for pain: the animal 
 is quite unconscious. The violent convulsions which occur at the 
 
 Fig. 
 
 -Brain destroyed by a Charge of Shot. 
 
 The head has slightly dropped; the muscles of the quarters are preparing to 
 contract, as may be seen by their outline; the tail is also turned to one side, 
 and the heel of one fore-limb hasleft the ground. There is nothing, however, 
 to indicate the fact that the horse is dead. 
 
 last moment are not present in death from acute chest diseases; 
 such cases stand persistently to the last, and either drop dead or 
 die very shortly after falling. 
 
 In violent death by destruction of the brain in horses, re- 
 markable muscular contractions of the limbs occur. These 
 cannot be seen with the unaided eye, as they are so rapid, but 
 are readily revealed by the camera (Fig. 271). In spite of their 
 rapidity, a distinct interval occurs between brain destruction 
 and muscular contractions. In Fig. 272 the brain was destroyed 
 by a charge of large shot, yet the horse is still standing, the 
 
GROWTH, DECAY, AND DEATH 821 
 
 * 
 
 impulses producing convulsive limb movements not yet having 
 had time to pass out. At the moment of violent death the 
 bladder and rectum are emptied, the penis protruded, the horse 
 sweats on the inside of the thighs, the pupils dilate widely, and 
 occasionally, when all seems at an end, the ftanniculus is called 
 into play, and the animal may shake the skin with remarkable 
 vigour, as if to dislodge a fly. The heart may continue to beat 
 for a minute or two, but the respirations cease. As life is being 
 extinguished the ligamentum nucha exercises its elastic recoil in 
 the absence of muscular resistance, and the head is drawn back 
 momentarily with slight jerks until the muzzle projects.* 
 
 Soon after death rigor mortis appears (see p. 451), and within 
 a short time tympany of the abdomen is apparent in the her- 
 bivora, reaching such a degree in a few hours, especially during 
 warm weather, that post-mortem ruptures of the diaphragm 
 and other viscera are exceedingly common. The explanation 
 of the tympany is the considerable amount of gas generated by 
 the fermentative decomposition of vegetable food. In death- 
 stiffening the flexor muscles of both fore and hind limbs contract 
 in excess of the extensors, so that the heels of the feet are slightly 
 drawn up ; the ears are also drawn back, and in the course of an 
 hour or two the pupils are closely contracted. 
 
 * At p. 654 reference has been made to the elastic nature of the liga- 
 mentum nuchae. By an oversight its functions were not referred to in 
 dealing with the subject of posture, p. 435. The chief function of this 
 elastic ligament is to assist the muscles in keeping the head extended, as, 
 for example, during the prolonged act of grazing. It also assists in sup- 
 porting the head, but this is a subordinate function as the funicular portion 
 may be completely divided without causing the head to droop. 
 
CHAPTER XX 
 THE CHEMICAL BASIS OF THE BODY* 
 
 By Professor T. H. Milroy, M.D. 
 
 Our knowledge of the chemical composition of the animal organism 
 is almost entirely based upon the analysis of the various tissues and 
 fluids, not as they exist during life, but after their removal from the 
 living structure. A great variety of elements is met with, usually 
 in combination, although in a few cases — for example, the oxygen 
 (partly) and the nitrogen dissolved in the blood — they are met with 
 in the free state. The most important elements are carbon, hydro- 
 gen, nitrogen, oxygen, sulphur, phosphorus, chlorine, iodine, traces 
 of fluorine, sodium, potassium, calcium, magnesium, and iron. 
 These elements are met with either in organic or in inorganic com- 
 pounds. 
 
 The chemical constituents of the body may be considered under 
 the following headings: I. Gases; II. Water; III. Inorganic Salts; 
 IV. Organic Compounds. 
 
 I. The Gases found are oxygen, nitrogen, hydrogen, carbon 
 dioxide, sulphuretted hydrogen, and marsh gas. Oxygen is the most 
 widely distributed of the elements, forming one-quarter by weight 
 of the atmosphere, and eight-ninths by weight of water. By means 
 of its compounds it forms one-half by weight of the earth's crust. 
 It is practically the only element which enters the animal or vege- 
 table body in a free state, and in plants it does so only to a limited 
 extent, for these obtain the bulk of their oxygen through the decom- 
 position of carbon dioxide and water. In the animal body it exists 
 free and combined in some of the body fluids, such as blood; others, 
 such as lymph, contain only traces, and none can be obtained from 
 the most bulky tissue of the body — i.e., muscle. Of the two great 
 cavities of the body — the chest and abdomen — one is remarkable for 
 containing the oxygen absorbing and distributing apparatus, the 
 other contains the digestive canal, which carries out its work in the 
 entire absence of oxvgen. Nitrogen exists largely in a free state, 
 since it forms no less than four-fifths of the atmosphere, while it 
 has but little affinity for other elements. In the form of ammonia, 
 nitrous and nitric acids, it enters the plant through its roots; as 
 protein it enters the animal, leaving it as urea, etc., which by decom- 
 position readily yields ammonia. The animal cannot utilise free 
 nitrogen any more than the plant can, though leguminous plants 
 
 * It is not intended in this chapter to do more than to elucidate and 
 supplement some of the chemical statements scattered throughout the 
 prevjjus chapters. 
 
 832 
 
THE CHEMICAL BASIS OF THE BODY 823 
 
 utilise atmospheric nitrogen by symbiotic co-operation with nitrify- 
 ing bacteria. There are nitrifying and denitrifying bacteria The 
 former oxidise ammonia and nitrites into nitrates, and are capable 
 of assimilating the free nitrogen of the atmosphere which thev fix 
 and supply to the plant The latter set nitrogen free by reducing 
 nitrates to nitrites, and decomposing nitrites into nitrogen From 
 the nitrifying bacteria in plants protein can be built up out of 
 inorganic salts in the absence of chlorophyll. Carbon is present 
 in the atmosphere in small amounts united to oxygen— i e in the 
 form of carbon dioxide. It is only in this form that it can be taken 
 up by plants which in their special laboratory split off the oxygen 
 molecule and store up the carbon, returning the oxygen to the air 
 and thus supply to the atmosphere that element of which animals 
 are continually depriving it. Under the influence of the ultra- 
 violet rays in light, the green leaves of plants are capable of manu- 
 facturing sugars and starches from carbon dioxide and water 
 Carbon enters the animal system with the carbon of the food and 
 leaves it either as carbon dioxide or in compounds, such as urea • as 
 carbon dioxide it is again taken up by the plant. There is no solid 
 or fluid tissue of the body free from carbon dioxide. It is the most 
 widely distributed gas in the body. 
 
 II. Water forms 60 per cent, of the whole body, the bulk of which 
 is taken in with the food, only a small quantity being produced in 
 the organism. The water supplied to the system furnishes no 
 potential energy, but it is essential in order that the sources of the 
 potential energy may be enabled to transform their potential energy 
 into the kinetic form. The amount of water met with in various 
 tissues ranges from the mere trace existing in such structures as 
 the enamel and dentine of the fully developed tooth to the very 
 high percentage met with in such fluids as the sweat and saliva (over 
 99 per cent, in certain cases). As the physiological significance of 
 many dissolved constituents depends upon the degree of their disso- 
 ciation, which increases with dilution and diminishes with increasing 
 concentration, any interference with the normal intake gives rise to 
 the most serious disturbances (see p. 392). 
 
 III. Inorganic Salts. — The most important members of this class 
 are the chlorides, phosphates, carbonates, and sulphates of sodium, 
 potassium, calcium, and magnesium (see p. 382 et seq.). 
 
 1 . The chlorides are the most widely distributed salts in the body, 
 appearing as valuable constituents practically in all the tissues 
 and fluids. They contribute largely to the total saline concen» 
 tration in the various fluids, and thus must play an important 
 part in the regulation of fluid exchange within the tissues. Certain 
 proteins — for example, globulins — require these salts for their solution. 
 As the sole source is from preformed salt in the food, and as the 
 urine and sweat carry away daily a fairly large amount, their re- 
 placement necessitates an adequate supply in the dietary. As, the 
 source of the hydrochloric acid of the gastric juice, the chloride is 
 necessary for certain ferment actions. The chloride present in 
 largest quantity in the body is the sodium salt, and it is in the 
 tissue fluids rather than in the intracellular substance that the con- 
 centration is highest. 
 
 2. The phosphates may be derived from various sources. In the 
 first place, they may be taken as inorganic salts in such foods as oats, 
 mainly as the magnesium salts. By collecting in the bowels, these 
 salts produce the ammonio-magnesium phosphate calculi so common 
 in horses. The foods richest in phosphoric acid are oil cake and 
 
8-2 4 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 bran, while hay and straw are poorest in this substance. Phos- 
 phoric acid is principally excreted by herbivora with the faeces, only 
 small quantities passing away with the urine. 
 
 A portion of the phosphates excreted is derived from organically 
 bound phosphorus in foods or tissue elements, as, for example, the 
 lecithins, nucleo-proteins, and phospho-proteins. 
 
 The phosphates of the alkalies form important constituents of the 
 body fluids and also of the cell protoplasm, while those of calcium 
 and magnesium contribute largely to the ash of bone. 
 
 3. The carbonates, and especially the bicarbonates, of the alkalies 
 are extremely important constituents of the body fluids. The 
 bicarbonates play an important role in determining the reaction of 
 the body fluids, as has been described under the Reaction of the 
 Blood. They constitute the end products of the oxidation of the 
 salts of such organic acids as citric, malic, and tartaric acids. They 
 are met with in the form of calcium carbonate in the urine of her- 
 bivora, being kept more or less completely in solution by carbonic 
 acid. Calcium carbonate also occurs along with the phosphate in 
 the ash of bones. 
 
 4. Sulphates occur in small quantity in the body fluids, being partly 
 derived from the sulphur present in organic combination in the food 
 proteins (exogenous), and partly from the disintegration of the tissue 
 proteins (endogenous). 
 
 The forms in which the sulphur occurs in the urine and in the 
 bile are referred to in the sections dealing with those fluids. 
 
 The important functions of the various salts are discussed under 
 metabolism (p. 382 etseq.), but these may here be shortly summarised. 
 Upon the saline concentration of the body fluids the osmotic pressure 
 most largely depends, and thus a certain "isotonic" value of 
 these fluids is necessary in order to maintain the integrity of the 
 cell elements in contact with the fluids. Thus any solution which 
 shows a different molecular concentration from the normal blood- 
 plasma would disturb the osmotic equilibrium of the medium 
 were it introduced into the circulation. Hamburger has largely 
 made use of the red cells for determining the isotonic values of 
 various saline solutions. If the salt solution be above the normal 
 v^lue (hypertonic), shrinkage of the red cells, or crenation, occurs; 
 while if it be below the normal (hypotonic), distension and finally 
 rupture of the red cells occur, with resultant haemolysis. The alka- 
 line carbonates play an important part at least as storehouses of the 
 important end product of metabolism, carbonic acid. Very slight 
 disturbances in the amounts of other salts may give rise to very 
 distinct alterations in the excitability of tissues. Thus, removal 
 of the small quantity of calcium from the fluids used for heart per- 
 fusion will give rise to cessation of the beat, while a slight excess 
 will produce an increased tonus of the cardiac muscle. Potassium 
 salts act as the antagonists of the calcium ones. 
 
 IV. Organic Compounds are most conveniently subdivided into . 
 (1) nitrogenous, and (2) non-nitrogenous, under the former being 
 included the proteins and their cleavage products, and under the 
 latter the fats and carbohydrates. The proteins constitute the 
 most important constituents of the body. As food-stuffs and con- 
 stituents of the tissues they are irreplaceable, and the large nitrogen 
 output from the body daily shows how active a part they play in 
 bodily metabolism. All are characterised by certain very dis- 
 tinctive physical and chemical properties. They belong to the 
 important group of f.olloids referred to as emulsoid colloids. An 
 
THE CHEMICAL BASIS OF THE BODY 825 
 
 emulsoid colloidal system shows a high viscosity and a great ten- 
 dency to foam formation. It does not show any difference in elec- 
 trical charge between the internal and the external phase. High 
 electrolyte concentration is necessary to bring about a coagulation, 
 which is in most cases reversible. In each of the above respects the 
 emulsoid differs from the suspensoid system. 
 
 The emulsoid system is closely allied to a true solution, and hence 
 protein solutions may be studied in the same way as true solutions.* 
 Such solutions show an infinitely slow rate of diffusion through 
 dialysing membranes, owing to the large size of the protein molecule. 
 In virtue of their colloidal character they constitute the important 
 viscous element of the blood, and by means of their indifrusibility 
 they regulate and control fluid exchange, thus often counteracting 
 filtration, which would otherwise occur under a hydrostatic pressure. 
 Chemically they give a number of reactions which can only be 
 satisfactorily dealt with in practical textbooks. These mainly 
 depend upon the existence in the protein molecule of certain groups 
 with characteristic reactions. The protein molecule is a very large 
 one, as is shown by the extremely small osmotic pressure exerted by 
 fairly strong solutions. Determinations of the molecular weight 
 are extremely difficult, and have given rise to very divergent results , 
 partly due to variations in the size of molecular aggregates and 
 partly to admixture with salts. Hiifner and Gausser's determina- 
 tions of crystallised haemoglobin most carefully prepared gave 
 molecular weights of 15,000 to 18,000, while Sorensen's determina- 
 tions of crystallised egg-albumin gave the molecular weight of the 
 anhydrous protein as 34,000. 
 
 The percentage composition of proteins varies, as is shown in the 
 following table: 
 
 Per Cent. 
 
 Carbon - 5 I- 5 to 54"5 
 
 Hydrogen 6-9 ,, 7-3 
 
 Oxygen 20-9 ,, 23-5 
 
 Nitrogen i5 -2 .. I 7 , ° 
 
 Sulphur 0-3 ,, 2-0 
 
 The proteins may be classified as follows: 
 
 1. Protamines. 
 
 2. Histones. 
 
 3. Albumins and globulins. 
 
 4. Glutelins and gliadins (vegetable proteins). 
 
 5. Conjugated proteins (nucleo-proteins, gluco-proteins, chromo- 
 proteins) . 
 
 6. Phospho-proteins. 
 
 7. Sclero-proteins. 
 
 8. Derived proteins (meta-proteins, proteoses, peptones, poly- 
 peptides). , , . 
 
 The protamines are of strongly basic character, and are found in 
 combination with nucleic acid in the form of nucleo-proteins. They 
 may be obtained from ripe fish spermatozoa, arginme, a diamino- 
 acid, forming a characteristic constituent. 
 
 The histones are also strongly basic. They have a high nitrogen 
 percentage (up to 20 per cent.), and evidently are intermediate 
 between protamines and the albumin class. Their yield of arginme 
 is smaller. 
 
 * Sorensen's ' Studies on Proteins,' Comptes Rendus des Tmvaux d% 
 Laboratoire de Carlsberg, vol. xii., 1917. 
 
826 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 The albumins and globulins are heat-coagulable proteins, which 
 contain sulphur, but little or no phosphorus. The albumins are 
 soluble in water, while the globulins are only soluble in dilute salt 
 solutions. The most important examples of this class are the 
 serum-albumin and serum-globulin of the blood, but there are also 
 many vegetable globulins, such as edestin, a crystallisable protein 
 obtainable from hemp-seed. 
 
 The glutelins and gliadins are vegetable proteins present in many 
 kinds of seeds. The former are insoluble in alcohol and neutral 
 aqueous solutions, the latter (present in the seeds of cereals — e.g., 
 gliadin of wheat and rye, hordein of barley, zein of maize) are 
 soluble in alcohol, but not in water. The products of their hydro- 
 lysis are different; thus gliadin and hordein yield more glutamic 
 acid and less arginine than the glutelins. Zein yields more leucine 
 and less glutamic acid than gliadin and hordein. 
 
 The conjugated proteins are interesting bodies which possess in 
 addition to the simple protein another substance, known as the 
 prosthetic group. The nucleo-proteins vary in complexity, the 
 simplest being those which occur in fish spermatozoa already referred 
 to. They are also present in large quantity in such organs as the 
 thymus and pancreas. Under the action of ferments such as trypsin 
 they break down into nucleic acid and the usual products of pro- 
 teolysis. 
 
 The nucleic acid can be broken down by acids into a number of 
 products — carbohydrates, phosphoric acid, pyrimidine, and purine 
 bases. The structure of the nucleic acids is a complex one, the 
 simplest forms consisting of one molecule each of acid, base, and 
 sugar, a combination found in guanylic and inosinic acids (Levene 
 and Jacobs).. 
 
 Chromo-proteins : The most important representative of this 
 group is haemoglobin, the properties of which have been already 
 described. 
 
 The most important gluco-proteins belong to the class of mucins 
 and mucoids. An important derivative of these bodies is an amino- 
 glucose, glucosamine. 
 
 The phospho-proteins, like the nucleo-proteins, contain phosphorus, 
 but differ from the latter in yielding on hydrolysis no purine nor 
 pyrimidine bases. Caseinogen of milk and vitellin of egg-yolk are 
 the most important representatives. 
 
 Sclero-proteins, found in skeletal and connective tissues, are 
 represented by gelatin derived from skin and cartilage, elastin from 
 elastic fibrous tissue, keratin from horn, hair, and nails, and many 
 others. 
 
 Derived proteins are obtained by the partial hydrolysis of pro- 
 teins. Examples of these are the meta-proteins (acid and alkali), 
 and later products of hydrolysis such as proteoses, peptones, and 
 polypeptides. The polypeptides are the most interesting repre- 
 sentatives of this class, because their structure is better understood 
 than that of the others. They are made up of amino-acids which 
 are joined up with one another in amide form, the carboxyl group 
 of one amino-acid being linked to the amino-group of another. Two 
 three, four, or more amino-acids may be so joined together, consti- 
 tuting the bodies known as di-, tri-, tetra-, and poly-peptides. A 
 simple example of a di-peptide is glycyl-glycine, two molecules of 
 glycine (amino-acetic acid) being linked together 
 
 H 2 N.CH 2 .CO.NH,CH 3 .COOH. 
 
THE CHEMICAL BASIS OF THE BODY 827 
 
 ■ ^ r j d !"1l of t B e Hydrolysis of Proteins.— When proteins are sub- 
 jected to the action of acids, alkalies, or ferments under suitable tem- 
 perature conditions, they undergo hydrolytic decomposition, and 
 furnish a great variety of cleavage products, many of which are 
 met with in the animal organism. 
 
 These products may be classified as follows: 
 Monobasic mono-amino-acids : 
 
 Glycine (amino-acetic acid) — 
 
 (NH 2 ).CH 2 .COOH. 
 Alanine (a-amino-propionic acid) — ■ 
 
 CH 3 .CH.(NH 2 ).COOH. 
 Valine (a-amino-isovaleric acid) — 
 
 CH*/ CH - CH (NH a ) .COOH. 
 
 Leucine (a-amino-isobutyl-acetic acid) — 
 
 Ch 3 / CHCH 2-CH(NH 2 ),COOH. 
 
 Dibasic mono-amino-acids : 
 Aspartic acid (a-amino-succinic acid) — ■ 
 CH(NH 2 ).COOH 
 
 CH 2 .COOH. 
 Glutamic acid (a-amino-glutaric acid) — 
 CH(NH a ).COOH 
 
 CHg.CHjj.COOH. 
 
 Hydroxy- and thio-amino-acids : 
 
 Serine (a-amino-/3-hydroxy-propionic acid) — 
 CH 2 .OH.CH.NH 2 COOH. 
 Cysteine (a-amino-/3-thiolactic acid) — 
 
 CH 2 .SH.CH(NH 2 ).COOH. 
 Cystine (dithio-amino-propionic acid) — 
 
 S.CH g CH(NH is ).COOH 
 
 S.CH 2 .CH(NH 2 ).COOH. 
 Diamino-acids : 
 
 Ornithine (aS-diamino-valeric acid) — 
 
 (NH 2 ) .CH 2 .CH 2 .CH 2 .CH (NH 2 ) .COOH. 
 Lysine (ae-diamino-caproic acid) — 
 
 (NH 2 ).CH 2 .(CH 2 )3.CH.NH 2 .COOH. 
 Arginine (a-amino-S-guanido-valeric acid) — 
 NH 
 
 (NH 3 ).C.NH.CH 2 .CH 3 .CH 2 .CH.NH 3 .COOH. 
 
828 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Heterocyclic amino-acids : 
 
 Proline (a-pyrrolidine-carboxylic acid)- 
 H,C CH„ 
 
 I 
 H,C 
 
 A 
 
 H.COOH 
 
 NH 
 
 Histidine (a-amino-/3-iminazole-propionic acid) . 
 Tryptophane (indole-a-amino-propionic acid) . 
 
 Aromatic amino-acids : 
 
 Phenylalanine (a-amino-/3-phenyl-propionic a_cid)._ 
 Tyrosine (a-amino-p-hydroxy-phenyl-propionic acid). 
 
 From the action of anaerobic or putrefactive bacteria the- amino- 
 acids are broken down into various amines, generally with a loss of 
 carbonic acid. Thus leucine furnishes isoamylamine, ornithine 
 tetramethylene diamine, lysine pentamethylene diamine, trypto- 
 phane indole ethylamine and histidine, iminazple ethylamine. 
 Aerobic bacteria oxidise the amino-acids to fatty acids with one 
 atom less with the loss of NH 3 and C0 2 . 
 
 It is important to know the nature and amount of the amino- 
 acids obtainable by hydrolysis from various proteins. 
 
 
 Hemo- 
 globin. 
 
 Caseino- 
 
 
 
 
 
 
 gen 
 (Cow). 
 
 Keratin. 
 
 Gliadin. 
 
 Edestin. 
 
 Salmine. 
 
 Glycine 
 
 
 
 °-3 
 
 0-02 
 
 3-8 
 
 _ 
 
 Alanine 
 
 4-19 
 
 0-9 
 
 1-2 
 
 2-0 
 
 3-6 
 
 
 
 Valine 
 
 
 1-0 
 
 5-7 
 
 0-2 
 
 + 
 
 4-3 
 
 Leucine and 
 
 
 
 
 
 
 
 isoleucine 
 
 30-0 
 
 10-5 
 
 18-3 
 
 5-6i 
 
 20-9 
 
 — 
 
 Aspartic acid 
 
 4-43 
 
 1-2 
 
 2-5 
 
 0-58 
 
 4-25 
 
 — - 
 
 Glutamic acid - 
 
 1-73 
 
 II'O 
 
 3-0 
 
 57-53 
 
 6-3 
 
 — 
 
 Serine 
 
 0-56 
 
 0-23 
 
 0-7 
 
 0-13 
 
 o-33 
 
 7-8 
 
 Cystine 
 
 0-31 
 
 + 
 
 7-0 
 
 o-45 
 
 0-23 
 
 
 Lysine 
 
 4-28 
 
 6-o 
 
 + 
 
 — • 
 
 1-0 
 
 — 
 
 Arginine 
 
 5'4 2 
 
 4 -8 
 
 2-3 
 
 3-16 
 
 n-7 
 
 87-4 
 
 Proline 
 
 2-34 
 
 3-1 
 
 3-6 
 
 7-06 
 
 4-1 
 
 II-O 
 
 Histidine 
 
 10-76 
 
 2-6 
 
 
 o-6i 
 
 i-i 
 
 — 
 
 Tryptophane 
 
 + 
 
 i-5 
 
 — 
 
 + 
 
 + 
 
 — ■ 
 
 Tyrosine - 
 
 1-33 
 
 4-5 
 
 4-6 
 
 1-2 
 
 2-II 
 
 
 The various proteins met with in the animal organism are thus 
 best characterised by the amounts of the various amino-acid de- 
 rivatives obtainable from them. These constitute the building- 
 stones of the proteins, and metabolism is largely concerned in the 
 separation and selection of these acids, some for complete disin- 
 tegration and others for synthesis. Although, as already stated, 
 the tests for proteins can only be satisfactorily dealt with in practical 
 textbooks, it is advisable to refer briefly to the most characteristig 
 reactions. 
 
THE CHEMICAL BASIS OF THE BODY 829 
 
 Tests for Proteins.— There are certain chemical tests which apply 
 to the entire group of protein bodies, and others which are distinctive 
 of members of the group. Their reactions are based on colour tests 
 precipitation, or coagulation. 
 
 Colour Tests— Xantho-Proteic Reaction.— Solutions of protein 
 heated with strong nitric acid turn yellow, and on the addition 
 of ammonia or caustic soda change to orange. This reaction is 
 considered to be due to the presence in the molecule of some group 
 of the aromatic series. r 
 
 Milton's Reaction.— Millon's reagent is a mixture of mercurous 
 and mercuric nitrates in the presence of nitric acid. When boiled 
 with a protein solution, the mixture turns red, and the same colour 
 is imparted to a precipitate should it occur. This reaction is 
 supposed to be due to the presence of tyrosine in the molecular 
 grouping, and is therefore not given when this is absent. 
 
 Piotrowski's Reaction.— To the solution of protein an excess 
 of strong solution of caustic soda is added, and one or two drops 
 
 Fxg. 273. — Albumin Crystals from Horse-Serum (Gurber). 
 
 of a I per cent, solution of copper sulphate. A violet colour results 
 in the presence of native protein, and a rose-red colour in the presence 
 of proteoses and peptone. The essential feature is that the colour 
 must be a purple, either red or blue. When protein in any form is 
 absent, the solution remains blue. The term biuret is applied to this 
 test, as a similar reaction is given with biuret, which is formed 
 by heating urea. 
 
 Adamkiewicz' s Reaction. — A mixture of one volume of con- 
 centrated sulphuric acid and two volumes of glacial acetic acid, 
 when added to a solution of protein, produces a reddish-violet colour 
 and slight fluorescence. The reaction is due to the presence of 
 tryptophane in the molecule. 
 
 The precipitation of proteins is brought about on the addition of 
 mineral acids, such as nitric; salts of the heavy metals — for instance, 
 acetate of lead or mercuric chloride — and by boiling; also by excess 
 of alcohol; neutral salts of the alkalies — i.e., sodium chloride and 
 ammonium sulphate — to the point of saturation, and in many other 
 
830 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 ways. Halliburton draws attention to the necessity for distin- 
 guishing between coagulation and precipitation. For instance, 
 a protein, on heating or on adding nitric acid, is coagulated; the 
 precipitate is insoluble, but precipitates obtained by the addition 
 of such substances as ammonium sulphate are soluble, and can 
 readily be sent back into solution. To these the term ' precipitate ' 
 especially applies. The circumstances under which such precipitates 
 are obtained have been mentioned under the head of -Albumins 
 and Globulins. 
 
 As already stated, several proteins *nay be obtained in crystalline 
 form — for example, egg-albumin, the serum-albumin of the horse 
 (Fig. 273), haemoglobin, and many plant proteins. In all careful 
 analyses of the proteins it is necessary to obtain the material in as 
 pure a form as possible, hence the great advantage of crystallised 
 proteins. 
 
 The Non-Nitrogenous Constituents. 
 
 1. The fats and oils are glycerol esters (glycerides) of the fatty 
 acids. The fatty acids may differ in number and in character; thus 
 there may be di- or tri-glycerides with the same or different fatty 
 acid radicles. They may be extracted from the tissues by means of 
 ether and other solvents of a like nature. They constitute the most 
 important ether-soluble constituents of the tissues, but there are 
 also included in such extracts other bodies of a somewhat fatty 
 nature which differ from the fats and oils in containing nitrogen, 
 phosphorus, or a carbohydrate group. The term lipoid has been 
 used, not always with the same significance, to include bodies of this 
 nature. There are also found in the animal and vegetable kingdom 
 esters of higher alcohols — the waxes — such as spermaceti and bees- 
 wax. 
 
 Cholesterol, a secondary alcohol of still uncertain constitution, is 
 very widely distributed, and forms an important constituent of 
 certain gall-stones. It is evidently a cyclic compound with a double 
 bond. It can replace glycerol and form esters with certain fatty 
 'acids. Iso-cholesterol is present along with cholesterol in the wool- 
 fat of sheep, and also in sebum. 
 
 The various substances which may be found in an ethereal or 
 alcoholic extract of the tissues may be classified as follows: 
 
 1. Neutral fats (including waxes), oils, fatty acids. 
 
 2. Cholesterol and allied bodies and their compounds. 
 
 3. Phosphatides, such as lecithin, containing fatty acids, nitrogen, 
 and phosphorus. 
 
 4. Cerebrosides, containing fatty acids, nitrogen, no phosphorus, 
 but a carbohydrate group. 
 
 It is evident, therefore, that the fats are closely associated with 
 bodies containing nitrogen, and hence the classification into nitro- 
 genous and non-nitrogenous, including under the latter fatty-like 
 bodies, is not a correct one. 
 
 1. Fats. — The fats found in body-fat, milk, and the marrow of 
 bones, are compounds formed by the union of fatty acids with 
 glycerol. These fats are palmitin, stearin, and olein. 
 
 Fatty acids are formed by the oxidation of alcohols, the group 
 being a large one. Some of the earlier members of the series, such 
 as acetic, propionic, valeric, caproic, have been referred to in speak- 
 ing of the amino-acids, for these form glycine, alanine, valine, and 
 leucine, in the manner already described. The acids responsible 
 for the body fats— palmitic and stearic acids — are the sixteenth 
 
THE CHEMICAL BASIS OF THE BODY 831 
 
 and eighteenth in the acetic series, while oleic is the eighteenth in 
 the acrylic series. A certain proportion of the fats in milk, and hence 
 in butter, is formed from acids lower down in the acetic series, 
 such as caproic, caprylic, and capric acids. 
 
 Fat is insoluble in water, and only slightly so in alcohol, but 
 freely soluble in ether, chloroform, and benzene. When pure, it 
 is neutral in reaction, tasteless and colourless, and by the action of 
 caustic alkalies or superheated steam may be decomposed into its 
 respective fatty acid and glycerol. When this splitting is brought 
 about by an alkali, the base, sodium or potassium, at once unites 
 with the free fatty acid and forms a salt (soap). This decomposi- 
 tion and saponification takes place to a greater or less extent in 
 the intestine under the influence of the pancreatic juice and bile. 
 
 The solid fat of the body is composed principally of stearin, such 
 as is found in the ox and sheep. The more liquid fat, such as is 
 found in the horse and carnivora, contains palmitin, but in all 
 cases a mixture of the three fats is obtained. Fat as it exists in 
 the cells of the living body is, of course, in a liquid condition. Since 
 the melting-point of palmitin is 45° C, and that of stearin 55° to 
 6o° C, it is evident that the fluidity of living fat is due to the olein 
 it contains, the melting-point of which is - 5 C. The amount of 
 fat in the body must depend upon the feeding of the animal, and will 
 obviously vary within extreme limits. In individual tissues marrow 
 has the largest amount; nerve, brain, milk, muscle, liver, bone, 
 bile, and blood, have proportions which decrease in the order given. 
 The change which the fats undergo in the alimentary canal has been 
 discussed in the chapter on the pancreas (p. 264), while the origin 
 of fat in the body, and its function is dealt with under nutrition 
 
 (P- 375)- 
 
 2. Cholesterol (C 27 H 45 OH) is found in bile and the tissues generally, 
 especially in nervous tissue and bone-marrow, and also very com- 
 monly in pus and various pathological effusions. It is present in 
 small quantity in blood-serum in combination with fatty acids as 
 esters, and also in lanolin or sheep-wool ' fat.' It crystallises from 
 hot alcohol in thin rhombic plates with notched angles. It is readily 
 soluble in hot alcohol, and also in acetone, chloroform, ether, and 
 benzene. In chloroform solution it is lawo-rotatory. The chloro- 
 form solution gives certain characteristic reactions, one of the most 
 commonly employed being Salkowski's test. On adding an equal 
 quantity of strong sulphuric acid to the chloroform solution, the 
 upper (CHCI3) layer reddens and the lower (H 2 S0 4 ) one appears 
 yellow with a green fluorescence. 
 
 3. Phosphatides occur in various forms with different ratios of 
 N : P, from 1 : 0-5 to 1 : 2. 
 
 Lecithin is the most common phosphatide. It is classified by 
 
 Maclean as a mono-amino-phosphatide (N : P = 1 : 1) . It is met with 
 
 in practically all tissues along with fat, and its occurrence has been 
 
 already frequently referred to. Its constitution may be represented as: 
 
 Fatty acid radicle. 
 
 Glycerol —Fatty acid radicle. 
 
 ^Phosphoric acid radicle — choline. 
 
 Choline is closely related to muscarine obtainable from certain 
 fungi. Its constitution is : 
 
 /C 2 H 4 OH 
 N= (CH 3 ) 3 
 \OH 
 
832 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 or trimethyl-/3-hydroxy-ethyl-ammonium hydroxide. Lecithin dis- 
 solves very readily in alcohol, and differs in this respect from kephahn, 
 which belongs to the same group of phosphatides. Kephahn does 
 not contain choline, but another base, oxy-ethylamine. 
 
 4. Cerebrosides, as their name signifies, are present in bram tissue 
 but are also met with in other parts. On hydrolysis they yield 
 galactose. 
 
 Phrenosin and kerasin are two members of this class.* 
 
 Carbohydrates.— This important class is of the greatest interest 
 to the physiologist, inasmuch as the bulk of material consumed 
 as food, especially in the herbivora, consists of carbohydrate matter. 
 It is an extensive group of bodies consisting of such substances as 
 starch and its derivatives, the various forms of sugar, and cellulose. 
 Though so much carbohydrate material enters the body, but little 
 can be found in the tissues. An animal starch (glycogen) is found 
 in the liver and other organs, minute amounts of sugar are found in 
 the blood, and a sugar exists in milk; but very much less carbo- 
 hydrate is recoverable from the body than enters it as food, for the 
 reason that the bulk of it becomes converted into fat, or is rapidly 
 oxidised to carbonic acid and water as a source of heat and energy 
 to the body. 
 
 The carbohydrates may be divided into the — 
 
 Starch group, or poly saccharoses. 
 Cane-sugar group, or disaccharoses. 
 Dextrose group, or monosaccharoses. 
 
 Polysaccharoses — Starch. — The formula for starch is unknown; 
 it is considered to be (C 6 H 10 O 5 )w, where n is not less than 5 or 6, and 
 is probably very much larger. The molecular weight is thus unknown. 
 
 Starch exists in plants in the form of grains, the shape of which 
 depends upon the group from which they are derived; thus potato, 
 bean, wheat, and other starch grains, have each a distinctive shape. 
 The grain is composed of two parts, an envelope known as cellulose 
 (amylopectin) and an interior called granulose (amylose). The granu- 
 lose is the true starch; the cellulose is not, however, identical with 
 the ordinary cellulose of plants. Starch is insoluble in cold water, 
 but when boiled the grains burst, and a viscid, opaque, pasty mass 
 results, which is not, however, a true solution of starch. A solution 
 of starch can be obtained from this mass by careful and limited 
 digestion with an enzyme, such, for instance, as human saliva, or by 
 the action of dilute acid ; when this takes place the material becomes 
 watery, perfectly transparent, and filters readily, while previously 
 this was impossible. To this limpid fluid the name ' soluble starch ' 
 has been given. The characteristic test for starch is the blue colour 
 produced on the addition of iodine. Starch has no reducing action 
 on Fehling's solution. 
 
 Dextrin. — When starch paste is acted upon by dilute mineral 
 acid, or the enzymes found in the saliva and pancreatic juice, soluble 
 starch is first formed as above described; but if the process be 
 allowed to continue, further changes rapidly occur, leading to the 
 production of dextrin and finally of sugar. There are probably 
 several dextrins, though two are generally more particularly described 
 —viz., erythro-dextrin and achroo-dextrin. These are distinguished 
 
 * ' Lecithin and Allied Substances,' by H. Maclean. Longmans, Green 
 and Co., 1918. 
 
THE CHEMICAL BASIS OF THE BODY 
 
 S33 
 
 from starch and from each other by their colour reactions with 
 iodine, erythro-dextrin giving a reddish colour, while achroo- 
 dextrin gives no colour. Much the same change which can thus be 
 brought about by acting upon starch out of the body takes place 
 more rapidly within the body. The sugar formed from the action 
 of amylase on amylose is maltose, while hydrolysis of starch by 
 acids furnishes glucose. The conversion of starch into dextrin and 
 finally into sugar under the influence of certain enzymes performs 
 a most important physiological function ; neither starch nor dextrin 
 is capable of being absorbed as such, whereas the sugar which results 
 from this conversion is readily assimilable. 
 
 Glycogen closely resembles starch. It is found in several of the 
 tissues of the body, and its origin and use have been previously 
 discussed (see p. 252). It may be obtained as an amorphous white 
 powder, readily soluble in water, and gives with iodine a port-wine 
 colour instead of blue. By the action of acids or enzymes it is 
 readily converted into dextrin, and finally into sugar. The sugar 
 resulting from the action of acid is dextrose, whereas that produced 
 by the enzyme is maltose ; in the liver the sugar produced is dextrose 
 and not maltose, and the method by which this conversion is obtained 
 has been previously dealt with. 
 
 Cellulose, though not found in the animal body, is of great interest 
 to the physiologist from its intimate relation to the feeding of the 
 herbivora. The food substance in plants is locked up in a cellulose 
 envelope, and until this envelope is broken down the material within 
 cannot be acted upon by the digestive juices. This breaking down 
 is accomplished by laceration during the process of mastication, but 
 also by a subsequent digestion of the covering, by which means it is 
 removed and the food substance exposed. The digestion of cellulose 
 is a physiological puzzle, for the reason that no vertebrate is known 
 to secrete a cellulose-dissolving enzyme. In certain invertebrates 
 a true cellulose enzyme (cytase) is met with, and is known to be 
 secreted in the intestinal canal. Inasmuch as the herbivora are 
 capable of dealing with cellulose, the question of its solution is of 
 the greatest interest. Bunge has shown that sheep can digest 
 from 30 to 40 per cent, of the cellulose of sawdust and paper when 
 mixed with hay. There is every reason to think that all the herbiv- 
 ora deal quite as thoroughly with the cellulose naturally found in 
 their food. The question has been before us in dealing with diges- 
 tion (p. 199), in which it was shown that in the case of oats the grain 
 provided its own cellulose-dissolving ferment. It is hardly likely 
 that the sawdust or paper in Bunge's experiment provided their 
 own enzyme, so that it is probable the hay furnished it. Cellulose 
 however, may be digested by the action of putrefactive organisms, 
 either outside or inside the body. In both cases it is attended by 
 the formation of acetic and butyric acids, and the evolution of 
 marsh gas, carbon dioxide, and other substances. We have previously 
 studied the facilities which exist within the body for the necessarily 
 slow maceration of cellulose, which is the essential prelude to its 
 solution (see pp. 224, 227 and 373). 
 
 Disaccharoses (C] 2 H 22 O n ). — Sucrose, or cane-sugar, is not found 
 as part of the animal body, but exists largely in plants, and forms 
 a well-known supply of carhohydrate to the system. Cane-sugar does 
 not give some of the characteristic sugar reactions ; among others, 
 it has no reducing action upon salts of copper, but by boiling with 
 dilute mineral acids it is converted into equal parts of dextrose and 
 
 53 
 
834 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 laevulose, and the same change may be effected by enzymes in tne 
 stomach and small intestines. This conversion of cane-sugar is 
 recognised by the changed action of the solution on polarised "■§"*, 
 the rotation of the plane of polarisation being now to the left 
 instead of to the right, as it was previously to the conversion; 
 that is to say, it is inverted, hence the name invert sugar. If cane- 
 sugar be injected into the circulation, it passes out of the system 
 unaltered. Before this sugar can be assimilated, it must be con- 
 verted into dextrose (see pp. 218 and 296). 
 
 Maltose is formed by the action of malt extract (diastase) on starch 
 paste, also by the action of saliva and pancreatic juice upon starch 
 paste and glycogen. In its reactions it corresponds closely to 
 dextrose, but it has a one-third less reducing action upon Fehling's 
 solution, and, unlike it, does not reduce Barfoed's reagent.* Its 
 specific activity in rotating the plane of polarised light is considerably 
 greater than that of dextrose, being about +140 , as against +52 
 for dextrose. Maltose yields an osazone when heated with phenyl- 
 hydrazine hydrochloride. When heated the crystals (phenyl-malto- 
 sazone) melt at 206 C, and this, together with the shape of the 
 crystals and their specific solubility in 75 parts of boiling water, 
 renders the identification of maltose easy. Maltose, like cane-sugar, 
 is non-assimilable, for, if injected into the circulation, it is excreted 
 unchanged. Before absorption it has to be converted into dextrose, 
 and this is effected by a ferment, maltase (pp. 218, 264 and 296). 
 
 Lactose, or milk-sugar, is found solely in milk. It reduces Fehling's 
 solution, and has the same rotatory power as dextrose, but it does 
 not reduce Barfoed's reagent, nor does it undergo direct alcoholic 
 fermentation with yeast. If boiled with dilute mineral acids, it 
 is converted into equal parts of dextrose and galactose. Lactose 
 readily undergoes lactic fermentation, as, for instance, in souring 
 milk. The cause of this is a micro-organism ; but there are reasons 
 for believing that an enzyme may also bring it about. In spite of 
 the fact that isolated lactose is unable to ferment in the presence 
 of yeast, yet an alcoholic fermentation is capable of occurring in 
 milk, such, for instance, as in the koumiss from mare's milk and 
 kephir from cow's milk. It is probable that the changes which 
 bring this about are very complex, and due to several organisms. 
 
 Lactose, like sucrose and maltose, is non-assimilable as such, 
 and it is probable that it is changed into dextrose before absorption, 
 not necessarily as the result of the action of any digestive secretion, 
 but during its passage through the intestinal wall. Like maltose, 
 lactose yields an osazone, phenyl-lactosazone, which crystallises 
 in characteristic rounded clumps of yellow crystals. These crystals 
 melt at 200 C, and are soluble in 80 to 90 parts of boiling water. 
 
 Monosaccharoses, or Simple Sugars. — A simple sugar may be either 
 an aldehyde or ketone linked to one or more alcohol groups. Such 
 sugars may contain from two to nine carbon atoms, and so are 
 termed bioses, trioses, tetroses, pentoses, hexoses, etc. Of these, the 
 most important are the pentoses (C 5 H 10 O 5 ) and the hexoses 
 (C 6 H I2 6 ), the former being widely distributed in plants, the latter 
 occurring in three important forms in the animal organism — glucose, 
 galactose, and fructose. Glucose and galactose are aldo-hexoses, 
 fructose a keto-hexose. The di- and poly-saccharoses furnish on 
 hydrolysis the monosaccharoses, or simple sugars. Of these, the 
 most important one is — 
 
 * A solution of cupric acetate to which acetic acid is added. 
 
THE CHEMICAL BASIS OF THE BODY 835 
 
 Dextrose, Glucose, or Grape-Sugar. — This is probably the form to 
 which all sugars must be reduced in the alimentary canal, whether 
 before or during absorption, in order that they may be assimilable 
 by the tissues. In its ordinary reactions dextrose resembles maltose, 
 but may be easily distinguished from it by the following differences' 
 in behaviour. Its specific rotatory power is only + 52 It reduces 
 Barfoed's reagent (see Maltose). The osazone it forms, phenyl, 
 glucosazone, crystallises in fine yellow needles; these melt at 205° C, 
 and, unlike the corresponding compound of maltose, are almost 
 insoluble in water. Dextrose is capable of undergoing three fer- 
 mentations — i.e., alcoholic, lactic, and butyric; the last two are 
 probably always present in the intestinal canals of animals, especi- 
 ally after a carbohydrate diet. Dextrose is found in the blood and 
 many organs of the body. It is also the form in which sugar escapes 
 by the kidneys in diabetes. 
 
 Laevulose (Fructose) . — This occurs in fruits and honey mixed with 
 glucose. It may also be prepared by acting upon cane-sugar with 
 sulphuric acid, by which means the cane-sugar is converted into 
 equal parts of dextrose and laevulose. 
 
 Pentoses occur in nature either free or in the form of condensation 
 products, gum-like bodies, or celluloses (C 5 H 8 4 )w, the latter on 
 hydrolysis furnishing the pentoses (C 5 H, O 5 ) . Arabinose is derived 
 from the pentosan (araban) of gum arabic. They are obtained also 
 from the decomposition of certain nucleo-proteins, and may occur 
 in the urine. They reduce metallic oxides in alkaline solution, furnish 
 osazones, and are dextro-rotatory or inactive. They are not fer- 
 mented by yeast. 
 
 Tests for Sugar — 1. Trommer's. — An excess of caustic potash and 
 a small amount of dilute solution of copper sulphate are added to 
 the fluid and the whole heated. The copper is reduced to suboxide 
 by the sugar, and a red precipitate falls. Fehling's solution, which 
 is used as a quantitative test for sugar, consists of hydrated cupric 
 oxide in caustic soda, and the double tartrate of sodium and potas- 
 sium. The principle of this test is the same — viz., the reducing 
 action of the sugar, which robs the cupric compound of its oxygen. 
 
 2. Moore's. — A solution of sugar boiled with caustic potash turns 
 brown. 
 
 3. Bottcher's. — Bismuth oxide and excess of caustic potash are 
 added to the fluid containing sugar and heated. The solution be- 
 comes grey and then black, from the deposition of metallic bismuth. 
 
 4. Picric Acid Test. — Boil the solution of sugar with a little picric 
 acid and caustic soda in small quantities; a browd-red opaque 
 coloration is obtained. 
 
 5. Fermentation Test. — The fluid containing a piece of yeast is 
 placed in a tube and inverted over mercury. If sugar is present it 
 undergoes fermentation, and carbonic acid is given off, which collects 
 in the tube. 
 
 The osazone tests have already been described under the re- 
 spective sugars. They are very important to chemists for the 
 discrimination of the various sugars, as well as for their identifica- 
 tion. 
 
 The inositols do not belong to the carbohydrates. They consti- 
 tute a small group of natural compounds which are hydro-aromatic 
 compounds (hexa-oxy-hexa-hydro-benzene) . Some are optically 
 active, others inactive. The form which is found in heart muscle is 
 optically inactive. It may be separated in large colourless crystals 
 
8 3 6 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 which, on solution have a sweetish taste, hence the old name applied 
 to this body, ' muscle sugar.' It does not reduce Fehling's solution. 
 Many compounds of this group are met with in plants, quercitol 
 (in acorns), i-inositol (in peas and beans, and in combination with 
 phosphoric acid in the husk of various cereals) . 
 
 The Ferments. 
 
 The conversion of one substance into another by the process of 
 fermentation must have acted as a stimulus in the search for the 
 philosopher's stone. To the ordinary mind, if sugar could be 
 converted into alcohol, why not lead into gold ? The extraordinary 
 phenomenon of a body being wholly changed into another, and bear- 
 ing not the faintest resemblance to its progenitor, is not only one of 
 the astonishing facts recorded by chemistry, but one of the wonders 
 of science. The name ' fermentation ' was applied to the oldest 
 example of this phenomenon, and indicated a restlessness or agitation 
 in the fluid, the outcome of the generation of gas. When, later on, 
 this was shown to be due to the activity of- micro-organisms, these 
 latter were naturally described as ferments. This term has remained, 
 though fermentation is now known not to be exclusively produced 
 by living organisms, but that non-living material may also excite 
 it. To these non-living ferments the distinguishing name of enzymes 
 is applied. 
 
 The action of enzymes may be regarded as a catalytic process, in 
 which the velocity of a reaction is increased from the presence of a 
 catalyst. Like most catalytic processes, enzyme reactions are usually 
 reversible — that is to say, whether one starts with the mother sub- 
 stance on which the enzyme acts or with the products of the reaction 
 the same position of equilibrium is finally reached. Inorganic cata- 
 lysts have not the very specific character of organic enzymes. The 
 enzyme acts upon a special substance (substrate), and on it alone. 
 The conditions under which the reaction takes place — for example, 
 temperature and reaction of medium — influence the velocity of the 
 reaction. 
 
 The larger number of enzymatic processes are hydrolytic, as, for 
 example, the diastases, maltase, invertase, pepsin, trypsin, erepsin, 
 arginase, guanase, adenase, lipase, and urease; but there are also 
 oxidising (oxidases) and reducing enzymes (reductases) which un- 
 doubtedly play an important part in tissue metabolism (see p. 403). 
 
 The reversibility of enzyme actions is a most important property, 
 enabling the enzyme under one set of conditions to produce a de- 
 composition of the mother substance and under bther conditions to 
 synthetise the products of decomposition. Thus, lipase may not 
 only break down olein into oleic acid and glycerol, but may also 
 synthetise these bodies to olein. Pepsin, trypsin, and erepsin may 
 also in all probability act reversibly. Throughout the text the 
 various important enzymes have been referred to and the nature 
 of their reactions discussed. 
 
INDEX TO AUTHORITIES 
 
 Adams, 690 
 Adrian, 465 
 Arloing, 164, 165, 321, 
 
 533 
 Armsby, viii, 341, 
 
 343. 345. 358, 373, 
 383, 384, 385, 396, 
 398, 418, 420, 421, 
 
 44°. 793. 794 
 Assheton, R., 767, 770 
 Atwater, 362 
 Avebury, 547 
 
 Bainbridge, 445 
 Bakewell, 807] 
 Banting, 367 
 Barrett, 597 
 Barrier, 725 
 Bates, T., 808 
 Bayliss, 2, 4, 93, 133, 
 I 37. 237, 260, 300, 
 445. 456, 493 
 Beccari, 343 
 Begg, H., 737, 758 
 Bell, W. B., 305 
 Berlin, 534 
 Bernard, C, 86, 89, 
 
 168, 253, 268, 407 
 Bidder, 51 
 Bischoff, 354 
 Blaine, 817 
 Blenkinsop, 408 
 Bolton, J. S., 536 
 Booth, T., 808 
 Borelli, 667 
 Bouley, 324 
 , Boussingault, 813 
 Bowman, 330 
 Brodie, T. G., 332 
 Brown, G. T., 199, 815 
 Brown, J., 737 
 Bunge, 25, 248, 257, 
 780, 793, 833 
 
 Chapin, Catherine, 783 
 Chauveau, 34, 37, 38, 
 
 41.43.46, 54. 8 4. 133. 
 155, 162, 163, 165, 
 212, 324, 443, 510, 
 55 6 - 557, 66 °,7° 8 , 737 
 
 Chick, Harriette, 391 
 Clark, Bracy, 725, 729, 
 
 731, 816, 817 
 Clark, James, 721 
 Colin, ix, 3, 37, 61, 
 142, 146, 154, 162, 
 167, 169, 170, 184, 
 185, 200, 201, 203, 
 206, 207, 212, 213, 
 215, 218, 220, 224, 
 232, 233, 251, 259, 
 266, 284, 286, 288, 
 289, 290, 291, 292, 
 3i8, 393, 405, 445, 
 512, 519, 526, 527, 
 534, 564, 610, 619, 
 
 635 
 Crisp, E., 757, 818 
 Culley, 697 
 Cushny, 332, 387 
 Cuvier, 610 
 
 Darwin, 416, 797 
 Desaguliers, 694, 697 
 De Vries, 798 
 Dewar, 393 
 Dexler, 500, 553 
 Dupas, 784 
 
 Durham, Florence M., 
 314 
 
 Eckles, 796 
 
 Edkins, 197 
 
 Ehrlich, 17 
 
 Ellenberger, 7, 14, 107, 
 172, 180, 187, 189, 
 190, 191, 199, 203, 
 204, 205, 217, 219, 
 222, 223, 247, 324, 
 
 349, 35°. 785 
 Elliot, 563 
 Ewart, T. Cossar, 745, 
 
 756, 767, 775, 805 
 
 Faas, 351 
 Fischer, E., 339 
 Fish, P., 663 
 Flack, M., 33 
 Flechsig, 521, 536 
 Fleming, 707, 783 
 Fletcher, 177, 445 
 
 837 
 
 Flourens, 203, 516, 525, 
 
 529, 564 
 Forbes, 793, 796 
 Franck, 705, 785 
 
 Gader, 725 
 
 Galton, 621, 681, 799 
 
 Galvani, 442 
 
 Gamgee, 238, 653, 667, 
 
 669 
 Gaskell, 52, 60, 558 
 Gilbert, viii, 358, 366, 
 
 375. 379, 385, 396, 
 
 793, 794, 795 
 Goltz, 527 
 Goodall, 749 
 Gordon, W. J., 605, 697 
 Goubaux, 725 
 Gowers, 7 
 
 Grandeau, 3ig, 398, 399 
 Gray, H., 568, 570, 578, 
 
 585, 593, 737, 79°, 
 
 796, 818 
 Green, Edridge, 605 
 Green, H. H., 380 
 Grenside, 663 
 Guenon, 312 
 
 Haldane, 26, 122, 126, 
 
 127, 131,134,140,445 
 
 Halliburton, 540, 609, 
 
 830 
 Hamilton, M. G., 758 
 Hammarsten, 20, 248 
 Harris, W., 574, 579 
 Hart, Berry D., 782 
 Hartridge, 606 
 Harvey, F.T., 737, 757, 
 
 774 
 Haydon, 171 
 Head, A. S., 349, 747 
 Head, H., 511, 614 
 Heape, W., 741, 747, 
 
 781, 782, 790 
 Heidenhain, 175, 282, 
 
 331 
 Helmholtz, 588, 627 
 Henderson, T., 577 
 Henle, 35, 328 
 Henson, 744 
 
838 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Hering, 86 
 
 Herring, 305 
 
 Hess, H. F., 391 
 
 Hickes, R. J., 737 
 
 Hill, A. V., 445 
 
 Hill, L., 73, 74, 97, 99, 
 
 133 
 His, 33 
 
 Hobday, 405, 406, 408 
 Hofmeister, 190, 199, 
 
 205, 252 
 Hopkins, 388, 445 
 Hoppe-Seyler, 3, 259 
 Hume, E. M., 391 
 Hutcheon, 391 
 Hutchinson, J., 316 
 
 Iwanow, 740, 741, 742, 
 759, 761 
 
 Jackson, F. G., 415 
 Jacobs, 826 
 Jenter, 792 
 Johnson, G. L., 574, 
 
 581, 597. 6°5 
 Jolly, 748 
 Jordan, 792 
 
 Kaufmann, 133 
 
 Keith A., 33, 35, 36, 
 
 106, 307 
 Kellner, 691 
 Kendall, 303 
 Kent, 33 
 King, J. L., 500, 507, 
 
 531 
 Krall, 548 
 Kfihne, 195, 266 
 
 Lamarck, 797 
 Lane-Claypon, Jane, 789 
 Lang, 597 
 Langley, 87, 176, 177, 
 
 192, 304, 455, 55 8, 
 
 560 
 Lankester, E. Ray, 797 
 Laulanie, 145, 148 
 Lawes, J. B., viii, 
 
 358, 366, 375, 379, 
 
 385, 396, 793. 794. 
 
 795 
 Lawrence, J., 690, 697 
 
 698 
 Lea, Sheridan, 266 
 Le'Clerc, 398, 399 
 Le Conte , J . , 602 
 Leedes, 807 
 Leeney, 745, 746, 750 
 Lehmann, 143, 145 
 Levene, 826 
 Lewis, T., 50, 63, 74, 
 
 116, 740, 741 
 
 Liebig, 341, 366 
 Lillie, F., 783 
 Linton, 556 
 Loeb, 279, 763 
 Longet, 527, 557 
 Lortet, 85 
 Lucas, K., 465 
 Ludwig, 84, 277, 331 
 Lungwitz, 720, 729 
 
 McCollum, E. V., 391 
 McKendrick, 613 
 Mackenzie, J., 63 
 Maclean, 831, 832 
 Majendie, 219, 529 
 Malassez, 7 
 Marey, 41, 43 
 Marshall, 741, 746, 748, 
 
 749, 767, 768, 790 
 Martell, 618 
 Matthiessen, 574, 579, 
 
 594 
 Mellanby, 269, 390 
 Mendel, 798, 800 
 Metschnikoff, 18, 228 
 Mettam, 704, 705, 733 
 Millar, M., 711 
 Milroy, T. H., ix, 822 
 Mitchell, 566 
 Moeller, 353, 556, 707, 
 
 708, 725 
 Moor, 747 
 Morgan, 546 
 Morin, 691 
 Mott, F.W., 586 
 Mottram, 605 
 Miiller, G., 14 
 Mumford, F. B., 377, 
 
 740,790 
 Munk, 48, 146, 347, 348, 
 
 351 
 Muybridge, 665, 667, 
 681 
 
 Nasse, 19 
 
 Nicholas, 568, 578, 585, 
 593 
 
 Oliver, 304 
 Orr,J.B., 33 8 
 
 Palmer, 796 
 Parnell, 693 
 Paton, Noel, 390 
 Pavlov, 167, 193, 197, 
 
 198, 217, 262, 265 
 Pavy, 268 
 Peach, J. L., 603 
 Peddie, J., 737 
 Percivall, 27, 243, 653, 
 
 667, 814 
 
 Pezard, 746 
 Pfliiger, 368, 462 
 Pliny, 621 
 Priestley, 131, 140 
 Punnett, N. C, 804 
 
 Rankine, 691 
 Redtenbacher, 691 
 Remak, 51 
 Ringer, 52 
 Rivers, 614 
 Robertson, J. B., 438, 
 
 566, 797, 800, 804, 
 
 805, 807 
 Roger, 240 
 Rogers, F. T., 512 
 Romanes, 546 
 Rubner, 395 
 
 Sahlstedt, 350 
 Salkowski, 248, 347, 
 
 35°. 354 
 Salmon, 542 
 Sato, 741 
 Seymour, 171 
 Sharpey-Schafer, 304, 
 
 306 
 Sheppard, W., 663 
 Sherrington, 152, 434, 
 
 435. 436. 489, 49°, 
 
 493. 5l8. 528, 553, 
 
 616, 629, 633 
 Siedamgrotzky, 408, 416 
 Simonds, 815 
 Simpson, Sutherland, 
 
 3°3. 531 
 Sisson, 553, 556, 623, 
 
 635, 657, 738, 742, 
 
 788 
 Skelton, 391 
 Smith, Lorrain, 26 
 Smith, Meade, 172 
 Smith, Sydney, 546 
 Sowton, 493 
 Spooner, W. C, 704, 
 
 707, 729, 732 
 Starling, 47, 49, 237, 
 
 260, 262, 281, 290, 
 
 300, 789 
 Stewart, G. N., 45, 76, 
 
 86 
 Stewart, J., 692, 696 
 Storch, C, 732 
 Srmner, F. B., 760, 783 
 Susfdorf, 26 
 Sutherland, 784 
 
 Taplin, 690 
 Tappeiner, 199 
 Tereg, 349, 351, 353 
 Tessier, 784 
 
INDEX TO AUTHORITIES 
 
 839 
 
 Theiler, 793 
 Thompson, 338 
 Torcy, 813 
 Toussaint, 212 
 Townshend, 807 
 Trowbridge, 378 
 Tubby, 290 
 Tull, J., 807 
 
 Underbill, F. P., 338 
 Unger, 391 
 
 Valentin, F., 607 
 Verworn, 465 
 
 Voit, 252, 354, 368, 369 
 Volkmann, 84 
 Von Bezold, 51 
 Von Oaten, 548 
 
 Wallenberg, 553 
 Waller, 428, 466 
 Ward, W., 690 
 Watson, G. A., 536, 538 
 Watt, 690 
 Weber, 438 
 Weiss, 284 
 Weissmann, 798 
 Willis, 408 
 
 Wilson, J., 800 
 Wilson, P., 737, 758 
 Wodsedalek, 753, 782 
 Wolff, E.., 350, 691 
 Wooldridge, 405, 408 
 Wriglitson, 628 
 
 Xenophon, 736 
 
 Youatt, 690, 697 
 Young, T., 589 
 
 Zuntz, 143, 145, 440, 
 441, 44^ 
 
INDEX OF SUBJECTS 
 
 Abdominal f oetation, 744 
 
 muscles, 112, 113, 138, 531 
 Abducens nerve, 550, 553, 573 
 Abduction, 631 
 Aberration chromatic, 592 
 
 spherical, 592 
 Abomasum, 202, 205, 216 
 Absorption, 220, 272, 289 
 
 bands, 13 
 
 by bloodvessels, 290, 297 
 
 by chyle vessels, 297 
 
 by lymph vessels, 290, 292 
 
 by the skin, 290 
 
 from cellular tissue, 290 
 
 from conjunctiva, 290 
 
 from peritoneum, 290 
 
 from pleura, 290 
 
 from respiratory passages, 289 
 
 from stomach, 207, 291, 298 
 
 from vagina, 290 
 
 intestinal, 220, 223, 226, 229, 
 286, 291, 292, 299 
 
 mechanism of, 276, 277, 292 
 
 of carbohydrates, 254, 296 
 
 of fat, 252, 264, 292, 375 
 
 of food, 295 
 
 of oxygen, 12, 125, 438 
 
 of proteins, 292, 297, 369 
 
 of salts, 299, 383, 386 
 
 of water, 289, 298 
 
 paths of , 291 
 
 rectal, 229, 292 
 
 selective power of cells, 299, 386 
 Accelerator nerves, 54, 59 
 Accessory chromosome, 753, 781 
 
 organs of generation, 738, 759 
 Accommodation (eye), 581, 587, 
 
 588, 589 
 Acetone, 270 
 Achroo-dextrin, 171, 832 
 Acid, acetic, 190, 202, 236, 239, 
 373- 387. 830 
 
 aceto-acetic, 270 
 
 albumin, 195 
 
 aspartic, 256, 263, 337, 827 
 
 benzoic, 249, 333, 335, 340 
 
 butyric, 190, 202, 239, 373, 
 387 
 
 Acid, caproic, 830, 831 
 caprylic, 236, 831 
 carbolic, 290 
 carbonic, 358, 824. See Carbon 
 
 dioxide 
 cholalic, 240, 249 
 citric, 824 
 formic, 239 
 
 glutamic, 256, 826, 827 
 glutaminic, 263 
 glycuronic, 345, 354 
 guanylic, 826 
 hippuric, 314, 333, 34°. 35L 
 
 353. 354- 358, 392. 792- See 
 
 Urine 
 hydrochloric, 190, 206, 260 
 hydrocyanic, 291 
 inosinic, 826 
 lactic, 136, 140, 146, 190, 196, 
 
 199, 202, 206, 239, 339, 373, 
 
 387, 4°3. 444. 445. 45°. 54 2 . 
 
 773. 792 
 malic, 239, 387, 824 
 nucleic, 370, 740, 826 
 oleic, 831 
 oxalic, 346, 387 
 oxybutyric, 270 
 palmitic, 830 
 phenylsulphuric, 344 
 phosphate of soda, 339, 348, 354 
 phosphoric, 239,346, 347, 348, 
 
 35i. 354. 369. 793. 824 
 propionic, 236, 830 
 sarcolactic, 140, 146, 336, 444, 
 
 445, 45 .45i. 54 2 
 stearic, 830 
 succinic, 239 
 sulphocyanic, 344, 354 
 sulphuric, 257, 343, 384 
 tartaric, 387, 824 
 uric, 339. See Urine 
 valeric, 830 
 Acidosis, 134, 140, 270, 342, 343, 
 
 348, 387 
 Acids, amino-. See Amino-acids 
 biliary, 249 
 
 fatty, 228, 252, 264, 323, 375, 
 37°. 830 
 
 840 
 
INDEX OF SUBJECTS 
 
 Acids of stomach, 189, 190, 194, 
 
 195, zo6 
 neutralisation of, in the body, 
 
 342, 348, 382, 384, 385, 387 
 Acromegaly, 306 
 Adamkiewicz's reaction, 829 
 Addison's disease, 304 
 Adduction, 631 
 Adenase, 836 
 Adenine, 339, 792 
 Adenoid tissue, 274, 288 
 Adrenal glands, 62, 94, 96, 304, 563, 
 
 740 
 Adrenalin, 62, 94, 96, 304, 563 
 Afferent nerves, 423, 452, 524, 558, 
 
 564. 
 Age, duration life in animals, 818 
 
 horse compared with man, 818 
 
 indicated by teeth, 815 
 
 influence on work, 817, 818 
 
 of famous horses, 817 
 
 of puberty, 756 
 
 old, 314, 818 
 
 virility, 757, 819 
 Agglutination, 10 
 Agglutinin, 10 
 Air, alveolar, 122 
 
 amount of, required, 142, 143, 
 
 145 
 
 complemental, 142 
 
 composition of, 119 
 
 reserve, 142 
 
 residual, 142 
 
 respiratory changes in, 119 
 
 tidal, 142 
 
 vitiated, 147 
 Alanine, 256, 827 
 Albinism, 316, 578, 711 
 Albino horses, 315, 316, 576 
 Albumin acid, 195 
 
 crystals, 826 
 Albuminoids, 371, 826 
 Albumins, 450, 791, 825, 826 
 Alkalosis, 134 
 Allantoic fluid, 774 
 Allantoine, 340, 353, 369, 774 
 Allantois, 766, 767, 77:, 773, 774 
 Alpha iodine, 303 
 Alveolar air, 122 
 Alveoli, 122 
 
 Amble, notation of, 674 
 American trotting, 671, 673, 698 
 Amids, 368 
 
 Amino-acids, 228, 257, 263, 368, 
 369, 370, 376, 826, 828, 830. See 
 also Proteins, chemistry of 
 Amino -glucose, 826 
 Ammonia, 336, 339, 822 
 Ammonium carbamate, 336 
 
 carbonate, 336 
 
 lactate, 336 
 
 Ammonium in muscle, 336 
 
 murine, 335, 342, 343, 349 
 magnesium phosphate, 239 
 
 Amnion, 770, 771 
 
 Amniotic plates, 771 
 
 Amoeba, 16, 471, 754 
 
 Amoeboid movement, 16, 17, 275, 
 
 754. 794 
 Amphoteric reaction, 791 
 Ampullae of vas deferens, 742 
 Amylase, 296 
 Amyloly tic action , 171 
 Amylopectin, 832 
 Amylopsin, 261, 264 
 Amy lose, 832 
 Anabolism, 60, 360 
 Ancestral inheritance, law of, 799 
 Anelectrotonus, 460 
 Angle of repose (roads), 694 
 
 visual, 595 
 Animal heat, 402. See Heat 
 Ancestrum, 747 
 Ano-spinal central, 505 
 Anoxaemia, 126, 134 
 Antagonistic muscles, 112,489, 633, 
 
 634, 638. See Muscle antagonism 
 Antibody, 10, 23, 208 
 Anti-concussion mechanism. See 
 
 Concussion 
 Antidromic impulses, 456 
 Antineuritic factor, 388 
 Antipepsin, 208 
 Antiperistalsis, 232 
 Antirachitic factor, 390 
 Antirennin, 196 
 Antiscorbutic factor, 388, 391 
 Antithrombin, 23, 24 
 Antitoxin, 10 
 
 Antlers, growth of, 307, 745 
 Antrum pylori, 181 
 Aorta, 65, 67 
 Aortic notch, 79 
 Apex beat, 42 
 Apnoea, 135 
 Apoplexy of lungs, 156 
 Aqueo s humour, 574, 577, 580, 585 
 Arabinose, 341, 835 
 Arborisation, 469. See Synapse, 
 
 dendrites 
 Arc, reflex. See Nervous system 
 Areas, motor. See Nervous system 
 
 sensory. See Nervous system 
 Arginase, 337, 836 
 Arginine, 263, 337, 338, 825, 826, 
 
 827 
 Argon, 119, 124 
 Arm nucleus, 501, 508 
 Aromatic bodies, 228, 344 
 Arteries, 65, 100, 817 
 Arterioles, 70, 71 
 Artificial insemination, 741, 756, 760 
 
842 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Aryepiglottic folds, 151 
 Aspartic acid, 256, 263, 337, 827 
 Asphyxia, 92, 114, 132, 135, 142, 
 
 55i, 557 
 Ass, chestnut of, 324 
 
 duration of pregnancy, 784 
 of life, average, 818 
 
 maturity of, 756 
 
 milk of, 791 
 
 stupidity of, 543 
 
 sweating of, 318 
 Association areas, 535 
 
 fibres, 522, 525 
 
 tracts, 480, 501 
 Astigmatism, 574, 597 
 Astragalus, screw action of, 660 
 Astrosphere, 753 
 Atavism, 805 
 Ataxia, cerebellar, 517 
 Atropine (ciliary muscle), 579, 581, 
 
 589 
 (heart), 59 
 
 (milk secretion), 796 
 (pancreatic secretion), 261 
 (salivary secretion), 175 
 Attitude, 636, 648, 649, 661. See 
 
 Posture 
 Attracti n sphere, 744, 753 
 Atwater-Rosa calorimeter, 362 
 Audition, theories of, 627, 628 
 Auditory centre, 534 
 
 sensations, 626 
 Auricular fibrillation, 50, 63 
 Auriculo-ventricular bundle, 33, 34, 
 
 51 
 groove, 40, 41 
 node, 33, 51 
 
 valves, 31, 36, 38, 40, 41, 45 
 Autonomic system of nerves, 558. 
 
 See Sympathetic system 
 Autoxidation, 403 
 Axial stream, 77 
 Axis cylinder, 456, 457, 470, 471, 
 
 47 2 
 Axon, 467, 469, 470, 472, 478, 497, 
 
 5°4. 513. 520, 559 
 Azoturia, 9, 14, 463 
 
 Back and draught work, 694 
 
 and weight carrying power, 
 
 693. 697, 698 
 breaking of, 446 
 
 Bacterial action, intestinal, 227, 228, 
 
 373. 374 
 in rumen, 202, 373, 374 
 Bacteriolysins, 18 
 Barfoed's reagent, 834, 835 
 Bars (foot), 712 
 
 use of the, 726 
 Basal ganglia, 510, 534 
 Basket cells, 513 
 
 Bellini, duct of, 329 
 
 Bellowing, 155 
 
 Bell's experiment, 554, 617 
 
 Benzoic acid, 249, 333, 335, 340 
 
 Beri-beri, 388 
 
 Betz cells, 520, 538 
 
 Bezoar stones, 239 
 
 Bicuspid valves, 36, 38 
 
 Bile, 246 
 
 acids, origin of, 249 
 
 Pettenkofer's test, 249 
 
 amount of, 251 
 
 composition, 247 
 
 concretions, 248, 271 
 
 fat absorption, effect on, 252, 
 264 
 
 Gmelin's test, 248 
 
 pigments, 247, 248 
 
 salts, 249, 604 
 
 secretion of, 250 
 
 use of, 251 
 
 See also Liver 
 Biliary calculi, 248, 271 
 Bilirubin, 16, 247, 248, 354 
 Biliverdin, 247, 248 
 Binocular vision, 573, 584, 586, 597 
 Bipolar cells, 468 
 Birth, growth after, 812 
 Biuret test, 829 
 Black horses, 315, 605 
 Bladder, calculi, 356 
 
 capacity, increase of, 437 
 
 gall, 251 
 
 muscle of, 355, 437 
 
 nerves of, 355, 492, 563 
 
 urinary, 354, 773 
 Blastodermic vesicle, 763 
 Bleating, 155 
 Blind spot, 583 
 Blood, 1 
 
 agglutination, 10 
 
 amboceptor, 10 
 
 antibodies in, 10, 23 
 
 antithrombin, 23, 24 
 
 arterial, 25 
 
 bacteriolysins in, 18 
 
 bilirubin, 16, 248 
 
 buffer salts of, 2 
 
 bufiy coat in, 3, 18 
 
 carbon dioxide in, 3, 25, 92, 
 120-124, 127, 130, 132, 
 
 774 
 
 monoxide, 12, 134 
 carbonic oxide haemoglobin, 12 
 cells, 8 
 
 amoeboid movement, 16, 
 
 J 7. 2 75. 754, 794 
 
 chromoprotein in, 826 
 
 circulation of, 30, 77. See 
 
 Circulation 
 duration, 86 
 
INDEX OF SUBJECTS 
 
 843 
 
 Blood, coagulation of, 18, 22, 24 
 cause of, 20 
 circumstances affecting 
 
 2 4 
 
 intravascular, 23 
 
 retarded, 24 
 
 time of, 19 
 colour of, 6, 11 
 composition of, 3, 28 
 corpuscles, 6, 8. See Red and 
 
 white 
 crystals, 14, 15 
 defibrination of , 19 
 diapedesis, 77 
 distribution of, 27 
 eosinophiles, 17 
 erythroblasts, 8 
 extractives, 25 
 fat in, 25, 831 
 fibrin ferment, 20, 21 
 
 in 30 
 fibrinogen in, 5, 20, 275 
 fibrino-globulin in, 6 
 fluid lost by, 619 
 functions of, 1 
 gases of. See Respiration 
 globin, 11, 15 
 haematin, 11, 14, 248 
 haematoidin, 15, 248 
 haematoporphyrin, 15 
 haemin, 15 
 haemochromogen, 15 
 haemoglobin, 1, 7, 8, 11, 14, 
 125, 247, 248, 780, 826, 
 828 
 
 compounds of, 12, 126 
 
 derivatives of, 12 
 haemolysis, 9, 824 
 haemorrhage, 29, 76, 137 
 histon, 23 
 
 hydrobilirubin, 15, 16 
 hydrogen-ion concentration, 2 
 immune body, 10 
 in disease, 28 
 ' laky,' 8, 9 
 
 leucocytes, 16. See Leucocytes 
 leuconuclein, 2 5 
 liquor sanguinis, 3 
 lymphocytes, 17 
 mast cells, 17 
 mechanics of circulation, 67 
 methaemoglobin, 14, 29 
 nitric oxide haemoglobin, 12 
 nitrogen, 124, 128 
 nucleo-protein, 6, 17, 23 
 opsonins in, 18 
 oxygen in, 8, 12, 25, 123-127, 
 
 133, 774. 822 
 oxyhemoglobin, 8, 12, 126, 
 
 127. 134 
 paraglobulin, 4, 275 
 
 Blood, peptone and proteose absent 
 from, 298 
 phagocytosis in, 18 
 physical character of, 1 
 physiological salt solution, 8, 
 
 28,52,137,445,824 
 plasma, 3, 4, 5 
 platelets, 11 
 pooling of, 97, 98 
 precipitins in, 10 
 pressure, 67. See Blood- 
 pressure 
 proteins of, 4 
 pro-thrombin, 22 
 quantity, 26 
 reaction, 1, 824 
 red corpuscles, 6, 248 
 
 destruction, 8, 9 
 number, 7 
 seat of formation, 8 
 size of, 6 
 structure, 6 
 reduced haemoglobin, 7, 12, 13 
 regeneration, 27 
 respiratory changes in, 119 
 salts of, 25, 126 
 serum, 5, 18, 28 
 
 albumin, 4, 21, 275, 826, 
 
 830 
 globulin, 4, 268, 792, 826 
 smell, 3 
 
 specific gravity, 3 
 stromuhr, 84 
 
 supply, chemical stimulus in, 96 
 taste, 3 
 
 temperature, 26, 83, 407 
 thrombin, 21 
 thrombogen, 22 
 thrombokinase, 22 
 velocity, 78, 83, 85 
 venous, 25 
 
 and arterial, differences, 25 
 viscosity, 3, 825 
 white corpuscles, 16. See also 
 Leucocytes 
 Blood-letting, 29, 76 
 Blood-pressure, 67, 71 
 centre for, 510 
 chemical stimulation of, 62, 
 
 96, 304, 305 
 depressor fibres, 89 
 effect of dividing medulla, 88, 
 
 510 
 of gravity, 73, 75, 97, 98 
 
 of transfusion, 76 
 
 fall of, 71, 75, 76 
 
 haemorrhage and, 76 
 
 in capillaries, 74 
 
 - influence of adrenals on, 96, 
 
 3°4, 3°5 
 of heart-beat on, 71, 74 
 
8 4 4 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Blood-pressure, influence or mus- 
 cular support on, 97 
 of pituitary body on, 96, 
 
 3°5 
 of sympathetic on, 86, 87, 
 
 562, 563 
 of work, 72 
 
 in pulmonary system, 75 
 
 in various parts of system, 74 
 
 in veins, 75 
 
 manometer, 68, 72, 73, 75 
 
 mean, 69 
 
 arterial, 72 
 
 not related to size of animal, 72 
 
 peripheral resistance in, 67-71 
 
 pressor fibres , 89 
 
 pressure velocity, 69 
 
 regulated by nervous system, 86 
 
 respiratory influence, 74, 115 
 
 surgical shock, 92, 95, 136 
 
 variations in, 72, 74 
 Bloodvessels, 65 
 
 and nervous system, 86-94, 49 2 . 
 5 IQ . 563. See also Nerves, 
 vasomotor 
 
 arterial tone, 89 
 
 arteries, 65, 100, 817 
 
 capillaries, 66 
 
 chemical stimulation, 96, 304 
 
 contractility, 65 
 
 course of blood through, 77 
 
 dilatation of, 443 
 
 disease of, 100, 444 
 
 elasticity of, 65 el seq. 
 
 inflammation in, 77 
 
 mean pressure in. See Blood- 
 pressure 
 
 mechanics of circulation, 67 
 
 muscular support to, 97 
 
 peripheral resistance . See 
 Blood-pressure 
 
 plethysmograph, 80, 82, 90 
 
 pulse, 67, 70, 77, 78, 100 
 rate 81, 82, 83, 449 
 tracings, 79, 80 
 wave, 78 
 
 recoil wave, 81 
 
 ruptured, 448, 806 
 
 sphygmogram, 79 
 
 tension, 80, 81 
 
 tone of , 88 
 
 vasomotor centre, 88, 92, 510 
 subcentre, 91 
 
 veins, 66 
 
 venous system, 56, 98 
 
 valves in, 67, 710, 733 
 
 volume pulse, 80 
 
 See also Circulation 
 Body, chemical basis of, 822 
 
 composition, 357 
 
 expenditure, 358, 371 
 
 Body, growth, 811 
 
 income, 358, 371 
 
 maintenance, 393 
 
 temperature, 405, 419 
 
 See also Nutrition 
 Bottcher's test, 835 
 Bowel, diseases of, 232, 233, 243, 
 
 245. 564 
 Bowman, capsule of, 327 
 Brain, circulation in, 98, 99 
 coverings of, 538 
 proportion to body weight, 519 
 See also Nervous system, Cere- 
 brum, Cerebellum, Medulla, 
 etc. 
 Bran, effect of large quantity, 239, 
 
 384, 4 3 i 
 
 Braying, 155 
 
 ' Breaking service,' 775 
 
 Breeding, viii, 807. See also Here- 
 dity 
 dominance, 803, 809 
 fixation of character, 809 
 in-breeding, 807, 808, 809, 810 
 inheritance of disease, 805, 810 
 Mendelism, 809 
 mutants, 808 
 
 parental transmission of char- 
 acteristics, 810 
 prepotency, 809, 810 
 selection, artificial, 807 
 by performance, 809 
 
 Broken back, 446 
 wind, 156, 401 
 
 Bronchi, 107, no, 289 
 
 Bronchitis, 156 
 
 ' Brushing,' 632, 652 
 
 Buck-jumping, 689 
 
 Buffer salts of blood, 2 
 
 Buffy coat, 3,18 
 
 Bulb, 507. See also Medulla 
 
 Bulbo-urethral glands, 738, 742, 759 
 
 Bundle of Monakow, 515 
 
 Burdach, bulbar nucleus of, 501 
 column of, 501, 503, 508 
 
 Burmese pony, 697 
 
 Butter, 792 
 
 Butyric acid, 190, 202, 239, 373, 
 387 
 
 Caecum, 222, 226, 232, 233 
 Calcium, 383, 384,385,386,451,824 
 
 and blood-clotting, 22 
 
 and heart, 52, 824 
 
 and milk-clotting, 196, 792 
 
 and nutrition, 383 
 
 and thyroid, 302 
 
 in milk, 770, 791 
 
 in saliva, 168, 169 
 
 in urine, 345, 346, 347 
 
 phosphate, 391, 793 
 
INDEX OF SUBJECTS 
 
 845 
 
 Calculi, biliary, 248, 271 
 
 intestinal, 239, 383, 384, 401, 
 565, 823 
 
 vesical, 356 
 Calf, dummy, 610 
 
 mean weight at birth, 813 
 
 urine of, 353 
 Calorie, definition, 374, 395, 419 
 Calories, daily production, 420 
 
 in food, 400 
 
 for maintenance, 398 
 
 for work, 399 
 Calorimeter, 395, 420 
 
 Atwater-Rosa, 362 
 
 Pennsylvania, 363 
 
 respiration, 360 
 
 Regnault-Reisel, 361 
 Camel, duration of pregnancy, 784 
 
 rutting of, 750 
 
 trot, 674 
 
 walk, 670 
 Camphor, effect on urine, 354 
 Canals, semicircular, 515, 517, 549, 
 
 572, 616, 625, 628, 630 
 Cane sugar, 218, 296, 833, 835. See 
 
 Sugar 
 Canker (foot), 736 
 Canter, 674, 698 
 
 analysis of, 675 
 
 notation of, 675 
 
 ■speed of, 698 
 
 stride of, 677, 698 
 Capillaries, 66 
 
 blood-pressure in, 74 
 Capons, 379, 746, 781 
 'Capped elbow,' 649 
 Capped hock, 649 
 Caproic acid, 830, 831 
 Caprylic acid, 236, 831 
 Capsule of Bowman, 327 
 
 of Tenon, 573 
 Carbamide, 335. See Urea 
 Carbohydrates, 357, 364, 373, 792, 
 832 
 
 absorption of, 296 
 
 chemistry of, 832 
 
 composition of, 357, 372 
 
 depression of digestibility, 374 
 
 digestion of, 372 
 
 energy from, 374, 375, 395. 444 
 
 excretion of, 373 
 
 fat from, 375 
 
 ferments. See Ferments 
 
 function of, 366, 374. See 
 Sugar, glycogen 
 
 metabolism of, 372, 375, 780 
 
 oxidation of, 120, 373, 375, 404 
 
 starch conversion. See Sugar 
 
 source of, 366, 372, 375, 396 
 
 sugars, variety of. See Sugar 
 and Nutrition 
 
 Carbolic acid, absorption of, 290 
 Carbon, 823 
 
 dioxide, 358, 373, 392, 822, 823, 
 824 
 in blood, 3, 25, 92, 120-124, 
 
 127, 128, 130, 132 
 in cell, 754 
 in expired air, 129 et seq., 
 
 145 
 in lymph, 275 
 in muscle, 443, 451, 822 
 in saliva, 169 
 of foetus, 774 
 equilibrium, 371 
 excretion of, 358 
 monoxide-poisoning, 12, 134 
 pressure, alveolar, 130 
 Carbonates, 824 
 
 in urine, 345, 348, 824 
 in saliva, 169 
 Carbonic acid. See Carbon dioxide 
 Carboxyl group, 826 
 Cardia, stomach, 180, 182, 183, 209 
 Cardiac cycle, 39 
 
 impulse, 32, 41, 43, 47 
 muscle, properties of, 53, 425, 
 
 43° 
 
 output, 48, 447 
 
 sound, 43 
 
 sounds, 42. See also Heart 
 Cardiograph, 47 
 Cardio-inhibitory centre, 58 
 Cardiometer, 47 
 Carnine, 451 
 Carotin, 375, 791 
 Cartilages, laryngeal, 151, 153 
 
 lateral, 701, 704, 724, 726, 
 727 
 
 ossified, 724, 728, 734 
 Cartilago nictitans, 569 
 Casein, 196, 789, 791, 795 
 Caseinogen, 791, 792, 826, 828 
 Castration, effects of, 154, 155, 305, 
 
 3°7. 379. 74 2 . 745. 74&, 8l6 
 Cat, decapitated, 528 
 
 duration of pregnancy, 784 
 
 effect of castration, 746 
 
 hearing, 621 
 
 old age in, 818 
 
 reflexes in, 488, 630 
 
 starvation of, 393 
 
 stomach digestion in, 215 
 
 sweating of, 318, 320, 321 
 
 temperature, 406 
 
 vision of, 569, 578, 579, 581, 
 586, 597, 601, 605 
 Catacrotic limb and waves, 79 
 Catalase, 404 
 Catalysis, 836 
 Catalyst, 403, 836 
 Cataract, horse, 590, 594, 606, 805 
 
846 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Catarrh, 157 
 ' Cat hairs,' 309 
 Catoptric test, 590 
 Cattle, act of rising, 650 
 
 temperature of, 405 
 Cell division, 753, 755, 761 
 
 structure of, 751-754 
 Cells, basket, 513 
 
 Betz, 520, 538 
 
 bipolar, 468 
 
 body, 466, 478, 751 
 
 germ, 743, 751, 801 
 
 goblet, 287 
 
 Golgi's. See Nerve cells 
 
 Leydig's, 740, 751 
 
 mammary gland, 789 
 
 mast, 17 
 
 Purkinje's, 513 
 
 Sertoli's, 739, 740 
 
 somatic, 743, 751, 801 
 
 specific resistance of, 208 
 
 work of, how effected, 754 
 
 See also Nerves 
 Cellular tissue, absorption from, 290 
 Cellulose, 372, 833 
 
 contributes body heat, 418 
 
 digestion of, 199, 223, 224, 227, 
 228,230, 373,374, 833 
 
 fermentation of, igg, 202, 227, 
 
 373, 374. 387. 418, 833 
 in large intestine, 223, 227 
 in stomach, 199 
 Centre, cilio-spinal, 505 
 diabetic, 268, 510 
 of gravity, 649 
 
 vasomotor. See Medulla ob- 
 longata 
 Centres, medulla, 510. See also 
 Medulla 
 spinal 90, 505. See also Spinal 
 cord 
 Centrosome, 744, 753, 755, 761, 762, 
 
 763 
 Cerebellar ataxia, 517 
 Cerebellum, 513 
 
 a reflex arc, 514, 518 
 
 connecting paths in, 515 
 
 equilibrium and, 517, 518, 629 
 
 functions of, 516 
 
 ganglion of proprioceptive 
 system, 491 
 
 may act independently, 516 
 
 muscle sense and, 517, 616 
 
 peduncles, 515 
 
 receptor system and, 517 
 
 reflex tonus and, 517 
 
 removal of, 516 
 
 rubro-spinal tract, 515 
 
 structure of, 513 
 Cerebral anaemia, 74 
 
 centres. See Cerebrum 
 
 Cerebral circulation, 98 
 fluid, 538, 540 
 peduncles, 509, 510, 515, 522, 
 
 5 2 7 
 Cerebrosides, 458, 459, 830, 832 
 
 Cerebrum, 519 
 
 association areas, 535 
 
 fibres, 522, 525 
 Betz cell area, 520, 538 
 calcarine fissure, 586 
 circulation, 98 
 commissural fibres, 522, 525 
 connection with skeletal 
 
 muscles, 522 
 convolutions, 519 
 corpus callosum, 525, 527 
 cortex of man and animals 
 compared, 536 
 
 structure of, 520 
 crucial sulcus, 529 
 development of fibres, 536 
 fissure of Rolando, 520, 524, 
 
 525. 5 2 9. 535 
 functions of, 525, 536, 543 
 Gennari, line of, 586 
 great afferent path, 524 
 
 efferent path, 522 
 motor areas, 528, 529, 534 
 
 inherbivora, 531 
 
 path, 522 
 movements, 99 
 muscle sense and, 616 
 prefrontal area, 538 
 projection fibres, 522 
 psychical function, 543 
 pyramidal cells, 520, 522, 586 
 removal of, in dog, 527 
 
 in horse, 526 
 sensory areas, 528, 534 
 audition, 534 
 olfaction, 535, 609 
 taste, 535, 613 
 vision, 534, 537, 586 
 silent areas, 535 
 visuo-psychic area, 538, 587 
 
 -sensory area, 534, 537, 586 
 weight of, proportional to body., 
 
 519 
 white substance, function, 521 
 See Mid-brain 
 Cerumen, 323, 622 
 Check ligaments, 635, 640, 6C2, 663. 
 
 See also Ligaments 
 Chemical basis of the body, 822 
 energy, 395 
 engine, 439 
 
 excitants, 197, 199, 218, 237, 
 250, 260, 300, 795. See 
 Hormones 
 stimulus to circulation, 62, 96, 
 3°4. 3°5 
 
INDEX OF SUBJECTS 
 
 847 
 
 Cherry gum, 341 
 Chestnut horses, 315 
 ' Chestnuts,' 324 
 
 reflex, 488 
 Chest wall, 101. See also Respiration 
 Chimpanzee, brain of, 530 
 Chloral, effect on urine, 354 
 Chlorides in body, 823 
 
 in food, 383 
 
 in urine, 335, 347 
 Chloroform, effect on inhibition, 490, 
 493 
 
 intratracheal injection of, 289 
 Chlorophyll, 239, 288, 319, 323 
 Cholalic acid, 240, 249 
 Cholesterin, 7, 250, 275, 323, 450 
 Cholesterol, 627, 830, 831 
 Choline, 459, 831 
 Cholohaematin, 248 
 Chorda tympani. 67, 93, 160, 173, 
 
 455. 55o, 562, 613 
 Chordae tendinese, 36, 38, 40 
 Chorion, 767, 773, 774, 786 
 Choroid, 575, 584, 588 
 Chromatic aberration, 592 
 Chromatin, 753, 809 
 Chromatolysis, 466 
 Chromogen, 345 
 Chromophile substance, 466 
 Chromo-protein, 825, 826 
 Chromosomes, 739, 740, 744, 753, 
 755. 761, 762, 781, 800, 801, 
 809 
 
 accessory, 753, 781 
 Chyle, 286, 297 
 
 intestinal villi, 286, 292, 293 
 I molecular basis in, 286 
 1 movements of, 287, 288 
 "} nature of, 286, 288 
 "] See also Absorption, Thoracic 
 Duct, and Lacteal 
 Chyme, 218, 219, 220 
 Ciliary body, 576 
 
 ganglion, 554, 562, 580, 581 
 
 muscle, 581, 588, 589 
 
 nerves, 562, 580 
 
 processes, 576, 577, 5S4, 585, 
 588 
 Cilio-spinal centre, 505 
 Circle of Willis. 98 
 Circulation, aids to the, 95 
 
 arterial tension, 81 
 tone, 89 
 
 blood-pressure, 67 
 
 brain, 98 
 
 capillary, 74 
 
 chemical aids to, 62, 96, 304, 
 
 3°5 
 compensations, 73 
 'condition' and, 147, 447. See 
 
 Condition 
 
 Circulation, constrictor nerves, 87, 
 
 94. 563 
 
 coronary, 47, 49 
 
 course of the, 31 
 
 dilator nerves, 94, 562 
 
 diseases of, 100 
 
 duration of, 86 
 
 fcetal, 768, 777 
 
 foot, 710, 732 
 
 gravity, effect of, 73 
 
 hollow viscera, 100 
 
 inflammation and, 77 
 
 living 'tissues, 77 
 
 mechanics, 67 
 
 muxular aids to, 97 
 
 nerve centres connected with, 
 88, 91, 510 
 
 nervous system, influence of, 
 86, 87, 510, 562, 563 
 
 peculiarities in, 98 
 
 penis, 99, 757 
 
 placental, 765, 766, 768 
 
 pulmonary, 30, 75, 85 
 
 pulse, 67, 70, 77, 100 
 
 pulse-wave, 78 
 
 respiration, effect of, 115 
 
 systemic, 31 
 
 transfusion, effect of, 76 
 
 velocity of, 78, 83-85 
 
 venous, 75 
 
 See also Pulse, Blood-pressure, 
 Bloodvessels, Heart 
 Circumvallate papillae, 612 
 Clarke's column, 497, 501, 503, 525 
 Cleavage products, 827. See Pro- 
 teins 
 Clipping, 309, 316, 416 
 Coagulation, blood, 18, 20, 22, 24. 
 
 See also Blood 
 Cocaine, effect on eye, 579 
 
 on peristalsis, 232, 233 
 Cochlea, 625, 627 
 Cochlear canal, 626 
 
 nerve, 515, 550, 627, 628 
 Cock, castration of, 379, 746, 781 
 Cold, influence of, 413, 414 
 Colic, horse, 242, 564 
 Collaterals (nerves), 469, 505, 523 
 Colliculi '(corpora quadrigemina), 
 
 491, 510, 582 
 Colloids, 277, 333, 543, 752, 791, 
 824 
 
 emulsoid, 79r, 825 
 Colon, 224, 225, 231, 233 
 Colostrum, 791, 794 
 Colour tests, 829 
 
 vision, 605 
 Colours of horses, 314, 605 
 Columnae carneae, 35 
 Common path, 480-481 
 Complement, 10 
 
A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Complemental air, 142 
 Composition of blood, 3, 28 
 
 of body, 357 
 Compression of navicular bone, 703 
 
 of wall (foot), 731 
 Concentrated foods, 237 
 Concussion, 663 
 
 prevention of, in foot, 701, 702, 
 721, 726,728, 733 
 in limbs, 650, 654, 656, 
 662, 663, 700, 701 
 'Condition,' 147, 316, 379, 447, 448, 
 
 449, 684, 694, 698, 699 
 Conduction of heat, 410, 413, 414, 
 
 455 
 in nerves, 455, 459, 470, 479 
 Cones and rods, 583, 584, 599, 604 
 Conjugated proteins, 826 
 Conjunctiva, absorption from, 290 
 Connecting stalk, 765, 766 
 Contractility, 425, 462 
 Co-operative antagonism, 638 
 Co-ordinate movement, 448, 476, 
 481, 482, 489, 516, 518, 617, 623, 
 
 633 
 Cord, spinal. See Spinal cord 
 Corium, foot, 705 
 Corn, foot, 735 
 Cornea, 568," 570, 571, 572, 574, 
 
 57 8 , 579. 585. 586, 590 
 Cornua, spinal cord, 496 
 Coronary body (foot), 706, 708, 710, 
 
 713 
 
 circulation, 47, 49 
 Coronet, fistula of, 734 
 
 suppuration around, 735 
 
 See also Foot 
 Corpora nigra, 581 
 
 quadrigemina (colliculi), 491, 
 510. 582 
 Corpus Arantii, 38 
 
 callosum, 525, 527 
 
 cavernosum, 99, 757 
 
 luteum, 307, 743, 787 
 
 nigrum, 581 
 
 striatum, 409, 512 
 Corpuscles in colostrum, 794 
 
 in milk glands, 789 
 
 red. See Blood * 
 
 white. See Leucocytes 
 Corresponding points (retina), 598 
 Corti, organ of, 626, 627 
 . Cortico-pontine fibres, 509 
 Cotyledons, 766, 767, 768, 769, 770, 
 
 774, 786 
 Coughing, 155 
 Cow, dummy calf, 610 
 
 duration of pregnancy, 784 
 
 faeces, 238, 239, 240, 241 
 
 milk of, 791. See Milk 
 
 oestrus, 748 
 
 Cow-hocks, horse, 657 
 
 -kicking horse, 632, 657, 686 
 Cowper's glands, 738, 742, 759 
 Cranial autonomic system, 562. 
 See also Sympathetic system 
 nerves, 549 
 
 modern classification of, 
 
 549 
 first pair, 549, 550, 608 
 second pair, 549, 550, 575, 
 
 581, 582, 586, 599, 603 
 third pair, 517, 518, 549, 
 
 55°, 552, 553. 562. 573. 
 580, 581 
 fourth pair, 518, 549, 550, 
 
 553. 573 
 fifth pair, 138, 160, 165, 
 
 173. 364. 549, 55°. 
 551. 553, 580, 608, 
 613 
 section of superior 
 maxillary division, 
 
 554, 617 
 sixth pair, 549, 550, 553, 
 
 573 
 
 seventh pair, 67, 93, 137, 
 142, 160, 173, 549, 550, 
 562, 613 
 
 eighth pair, 515, 517, 549, 
 55°, 628 
 
 ninth pair, 165, 173, 549, 
 55°, 555, 562, 613, 618 
 
 tenth pair, 54, 60, 131, 
 137, 138, 139, 152, 165, 
 197, 204, 215, 216, 217, 
 234, 268, 269, 364, 549, 
 55°, 555, 556, 562. See 
 Pneumogastric nerve, 
 and Vagus 
 
 eleventh pair, 156, 455, 
 
 549, 55°, 551, 562 
 twelfth pair, 160, 166, 549, 
 55°, 552, 613 
 
 Crassamentum, 18 
 
 Creatine, 338, 421, 450, 451, 792 
 
 Creatinine, 338, 353, 421, 792 
 
 Crescents of Gianuzzi, 167 
 
 Critical temperature, 417, 418 
 
 Crura cerebri, 509. See Cerebral 
 peduncles 
 
 Cryptorchidism, 739, 790 
 
 Crystalloids, 277 
 
 Curare, absorption of, 289 
 
 and heat production, 408 
 and nerve end-plates, 423 
 
 Curdling of milk, 196, 792 
 
 Current of action, 460 
 of injury, 459 
 
 Currents, electrotonic, 460 
 
 Cutaneous sensation, 614 
 
 Cysteine, 827 
 
INDEX OF SUBJECTS 
 
 849 
 
 Cystine, 249, 344, 345, 384, 827, 828 
 
 Cytase, 833 
 
 Cytoplasm, 752, 753, 755, 761, 762, 
 
 763 
 Cytose, 760 
 
 Dandruff, 323. See Skin 
 
 Dartmoor ponies, 697 
 
 Darwinian theory, 797 
 
 De Vries' theory, 798 
 
 Death, 811, 819 
 
 Deafness, 551 
 
 Decay, 811, 816 
 
 Decerebrate rigidity, 435, 436, 491 
 
 unfatiguable, 436 
 Decidua, 765, 766, 768 
 Decussation, optic nerves, 582, 587, 
 
 600 
 Deep sensibility, 616 
 Deer, antlers, 307, 745 
 Defalcation, act of, 241 
 Defensive mechanisms, 18 
 Defibrinated blood, 19 
 Deficiency diseases, 388 
 
 in oxygen, 133 
 Degeneration of nerves, 470, 472 
 Deglutition, 163, 165 
 Deiters, nucleus of, 500, 509, 517 
 Delirium cordis, 50 
 Demilunes, 167 
 Dendrites, 468, 469, 470, 478, 513, 
 
 520, 559 
 Denitrifying bacteria, 823 
 Dentition, 814 
 Depressor fibres, nerve, 89, 94 
 
 nerve, 60, 90, 269 
 Derived proteins, 826 
 Descemet, membrane of, 574, 586 
 Descending tracts (cord) , 499 
 Determinant factor, 801 
 Determination of sex, 781 
 Deutoplasm, 744 
 Development of ovum, 761. See 
 
 Generation 
 Dextrin, 176, 218, 832 
 Dextrose, 25, 52, 172, 218, 254. 255, 
 
 296. 33*. 35°. 372, 444. 74L 792, 
 835. See Glycogen and Sugar 
 Diabetes, 257, 267, 270 
 
 pancreatic, 267, 270 
 
 phloridzin, 255 
 
 temporary, 268 
 Diabetic centre, 268, 510 
 
 puncture, 268 
 Dialysis, 277, 278 279 
 Diamino acids, 263, 827 
 Diapedesis, 77 
 Diaphragm, innervation of, 141 
 
 movements of, 104, 105, no 
 
 position of, 104, 105 
 
 rupture of, 157, 446, 821 
 
 Diaphragm, spasm of, 155, 156. 
 
 See Respiration 
 Diarihcea, 245 
 Diastase, 169, 836 
 Diastole, 39 
 Dicrotic waves, 79, 81 
 Diet and bran. See Bran 
 
 and chemical excitants, 197, 
 
 199, 218, 237, 250, 260, 300 
 and respiratory quotient, 120 
 boiled food, disadvantage of, 
 
 199 
 effect on faxes, 238, 240 
 
 on urine, 342, 348, 350, 
 392 
 errors of, 186, 198, 265 
 maintenance and production 
 
 rations, 393, 394, 398, 417 
 sudden changes in, 186, 198, 
 265. See Nutrition 
 Dieting of horses, 296, 297, 298, 393 
 Diffusion, 129, 279 
 Digestion, 158 
 
 abomasum, 202, 205, 216 
 antiperistalsis, 232 
 bacterial, 202, 227, 228, 373 
 caecum, 222, 226, 232, 233 
 carbohydrate. See Carbo- 
 hydrate 
 cellulose, 199, 202, 223, 224, 
 
 227, 228, 373, 387, 833 
 chemical excitants, 197, 199, 
 
 202, 218, 237, 250, 260, 300 
 chyme, character 01, 219 
 colon of horse, 224, 225, 231, 
 
 233 
 of ox, 230 
 defalcation, 241 
 
 deglutition, 163, 165, 289 (foot- 
 note), 510 
 digestive ferments, X99, 202, 
 218,833. See Ferments 
 processes, regulation of, 
 
 237 
 tract, sensation in, 237, 
 
 564 
 diseases affecting apparatus, 
 
 242 
 drinking, 160, 396 
 
 period of fluid in transit, 
 216, 220 
 faeces, 238. See Fasces 
 fat. See Fat 
 gastric juice, 190, 193, 205 
 
 secretion of, 1 go. See 
 Stomach 
 hay, 183, 295, 297, 298 
 ileo-caecal valve, 222 
 ileum, function of, 220 
 intestinal absorption. See 
 Absorption 
 
 54 
 
850 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Digestion, intestinal, in dog, 230 
 in horse, 219 
 in pig, 230 
 in ruminant, 230 
 movements, 231 
 nervous mechanism of, 
 234, 562, 564 
 intestines, gases in, 228, 236, 
 
 237, 358, 821 
 large intestine, muscula; bands, 
 222, 231 
 intestines, 221, 227, 228, 
 299 
 mastication, 161, 510, 554 
 in horse, 162 
 in ox, 163 
 meconium, 241, 773, 780 
 oats, 185, 199, 297, 298 
 oesophageal groove, 200, 210 
 omasum, 203. See Omasum 
 prehension of food, 158 
 rectum, 228, 229, 241, 292 
 reticulum, 202. See Reticulum 
 rumen, arrangement of food in, 
 201 
 function of, 201. See 
 Rumen 
 rumination, 210. See Rumina- 
 tion 
 saliva, 166. See Saliva 
 secretin, 218, 250, 260, 300, 
 
 3°7 
 secretion of gastric juice, 190, 
 
 197 
 starch conversion, 172, 198, 
 
 264, 833 
 stomach, absorption from, 207, 
 291 
 acids, 189, 190, 192, 193, 
 
 J 94. 195. z ° 2 . 206 
 arrangement of food in 
 (horse), 186 
 of food in (ox), 201 
 digestion in dog, 206 
 
 in horse, 178. See 
 
 Stomach 
 in pig, 205 
 in ruminant, 200 
 gases of, 208 
 
 gastric juice, 190, 193, 205. 
 
 See Stomach 
 
 nervous system 
 
 and secretion 
 
 of, 197 
 
 (horse), arrangement of 
 
 food in, 186 
 movements of food in, 2 1 5 
 mucin in, 181, 189, 192 
 nervous mechanism of, 
 197, 204, 212, 215, 216, 
 433. 557 
 
 Digestion, stomach (ox), arrange- 
 ment of food in, 201 
 pepsin, 192, 195 
 periods of, 199 
 reaction of contents, 183, 
 
 189 
 rennin, 194, 196. 2 °5 
 rupture of, 182, 209, 210, 
 
 242, 245 
 self-digestion of, 208 
 vomiting, 209, 210 
 X-ray observations, 215, 
 233 
 Dilator nerves, 86, 87, 88, 92, 94, 
 
 562, 757 
 Dioestrum, 747 
 Diphasic variation, 463 
 Disaccharides, 296 
 Disaccharoses, 832, 833 
 Discus proligerus, 743 
 Disease, heredity in, 805, 810 
 Dislocation of patella, 648 
 Dissociation, 278 
 
 of gases, 126, 130 
 Distribution of weight on limbs, 
 
 648 
 Diuretics, 333 
 Dog, absorption, carbolic acid, 290 
 
 bile, 247 
 
 blood, 5, 14, 19, 26 
 
 brain, motor areas, 529, 534 
 
 demented, 527 
 
 duration of pregnancy, 784 
 
 ejaculation, 742, 759 
 
 expression, of feeling, 621 
 
 gallop of, 681 
 
 extensor thrust, 481, 482, 
 
 483 
 
 intestinal digestion in, 230 
 
 life, average duration of, 818 
 
 lymph, 276 
 
 milk of, 791 
 
 moral sense of, 543 
 
 cestrous cycle, 748 
 
 old age in, 818 
 
 puberty of, 756 
 
 reasoning in, 543 
 
 scratch reflex, 481, 483, 485, 
 
 spinal, 482, 491, 617, 
 
 stomach digestion in, 206 
 
 sweating of, 318, 320 
 
 temperature of, 406 
 
 urine of, 353 
 
 vision of, 579, 581, 586, 597, 
 601, 605, 811 
 Dominance, 803 
 Dominant characters, 781, 801, 802, 
 
 803, 809 
 Donkey. See Ass 
 Draught, angle of repose, 694 
 
 back muscles in, 694 
 
INDEX OF SUBJECTS 
 
 851 
 
 Draught, body-weight, influence 
 of, 694, 695, 698 
 climbing hills, 440, 692, 694 
 effort in, 694 
 
 force of traction, 691, 693 
 height and weight, 694 
 horizontal, 694 
 horse-power, 690 
 influence of gradient, 440, 693, 
 
 694. 695 
 
 maximum effort possible, 695 
 
 physiology of, 693 
 
 single and pairs, 696 
 
 spirit and determination, 694, 
 
 695, 699 
 strength of horses, 694 
 tables of work, 692 
 theory of, 694 
 
 weights drawn, 692, 694, 695, 
 696 
 Drinking, 160, 216, 220, 396 
 Dromograph, Chauveau's, 83 
 Duct of Bellini, 329 
 Ductless glands, 300. See Endocrine 
 
 glands 
 Ductus arteriosus, 779 
 venpsus, 777, 779 
 Duodenum, syphon trap of, 181 
 Dura mater, 538 
 Duration of pregnancy, 784 
 Dynamometer, 691 
 Dyspnoea, 114, 134 
 
 Ear, 620 
 
 annual cartilage, 621 
 auditory centre, 534 
 
 nerve, 515, 517, 550, 627, 
 628 
 
 sensations, 626 
 bridge of bones, 624, 625, 627 
 cerumen, 323, 622 
 cochlea, 625, 627 
 conchial cartilage, 621, 624 
 Eustachian tube, 624 
 external, 621 
 
 hair on, 622 
 
 muscles of, 623 
 fenestra ovalis, 625, 627 
 
 rotunda, 625, 627 
 guttural pouches, 160, 624 
 hair cells, 626, 627, 629 
 hearing, centre for, 534 
 
 theories of, 627, 628 
 internal, 515, 517, 549, 617, 
 
 625 
 labyrinth, 436, 451, 491, 492, 
 
 625, 627, 629, 630 
 lymph, endo- and peri-, 625, 
 
 627,629 
 macula acoustica, 629 
 membrana basilaris, 626, 627 
 
 Ear, membrana reticularis, 626 
 tectoria, 626 
 membrane of Reissner, 626, 627 
 middle, 624 
 movements of, 621 
 nature of sound, 620 
 noise, 620 
 
 organ of Corti, 626, 627 
 otoliths, 629 
 position receptors, 628 
 rotation of, 622, 623 
 scutiform cartilage, 623 
 semicircular canals, 515, 517, 
 549, 572, 616, 625, 628, 630 
 structure of, 624 
 tympanum (membrane of), 
 
 624 
 vestibular nerve, 515, 517, 628, 
 
 629 . 
 vestibule, 625, 628 
 wax, 323, 622 
 Edestin, 334, 826, 828 
 Efferent nerves, 423, 452, 453, 558 
 Egg of hen, 770 
 
 Einthoven, galvanometer of, 463 
 Ejaculation, 742, 759 
 Elasticity, arteries, 65, 70, 78 
 
 foot, 712, 721, 722, 724, 726, 727 
 muscle, 434, 635 
 tendon and ligament, 635, 653, 
 654, 662, 821 
 Elastin, 826 
 Elbow, ' capped,' 649 
 
 -joint, 638, 651, 652 
 Electric phenomena in muscle, 441, 
 459. 462, 463 
 in nerve, 459, 463 
 Electro-cardiogram, 464 
 Electrolytes, 278 
 Electrolytic action, 279, 754 
 Electrotonus, 460. See a ho Nervous 
 
 system and Nerves 
 Elephant, duration of pregnancy, 
 784 
 reasoning in, 543 , 
 rutting of, 750 
 testicles of, 739 
 Embryo, death of, 775 
 heart, 50, 777, 779 
 membranes of, 770 
 nutrition of, 767, 769, 770, 772, 
 
 775. 779 
 unity of type, 754 
 Emmetropia, 596, 597 
 Empty weight, 358 
 Emulsification, 252, 264, 292, 791 
 Emulsoid colloids, 791, 825 
 End-plates in muscle, 423, 446, 458, 
 
 523 
 Endocardiac pressure, 44 
 Endocardium, 36 
 
8 5 2 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Endocrine glands, 300 
 
 adrenals, 62, 94, 96, 304, 
 
 3°5. 3°6, 563. 74° 
 corpus luteum, 307, 743, 
 
 787 
 function of, 300, 306, 307 
 hormones, 300. See Hor- 
 mones 
 internal secretions, 300. 
 
 See Hormones 
 iodothyrin, 303 
 parathyroid, 302. See 
 
 Thyroid 
 pineal, 306 
 
 pituitary, 62, 96, 305, 795 
 spleen, 300. See Spleen 
 thymus, 303, 814, 826 
 thyroid and parathyroid, 
 302. See Thyroid gland 
 See also Mammary glands, 
 Ovaries, Testicles 
 Endolymph, 625, 627, 629 
 Endurance tests, 698 
 Energy, 366, 371, 373. See Heat 
 chemical, 395 
 gross, 396 
 metabolised, 396 
 net, 396 
 theoretical, 395 
 yielded by food. See Food 
 Enteritis, 243 
 Enterokinase, 218, 262 
 Enzymes, 22, 133, 194, 198, 199, 
 202, 218, 261, 262, 264, 340, 
 
 37 2 . 373. 384, 403. 4°4. 765, 
 833. 836 
 
 intracellular, 369 
 
 See also Ferments and Hor- 
 mones 
 Eosinophiles, 17 
 Epiblast, 763 
 
 tissues from, 764 
 Epicritic fibres, 6x4 . 
 Epidermis, 308 
 Epididymis, 738, 740, 745, 751, 759, 
 
 760 
 Epiglottis, 151 
 
 action of, 164 
 
 See Larynx 
 Epilepsy, 534 
 Equilibrium of body, 517, 518, 541, 
 
 629 
 Erection, act of, 757 
 
 of hairs, 317, 562 
 Erepsin, 218, 263, 264, 297, 836 
 Ergograph, 439 
 Ergots, 324, 700 
 Erythroblasts, 8 
 Erythrodextrin, 171, 832 
 Eserine, effect on eye, 579 
 Ether, absorption of, 289 
 
 Ether, absorptionof, per rectum, 229 
 Ethyl sulphide, 344 
 Eustachian tube, 624 
 
 valve, 777 
 Evaporation, 410, 411 
 Evolution, 797 
 Excretion, definition of, 325 
 
 salts, 239, 319, 342, 359, 382, 
 386, 387, 793, 8 2 3- See 
 Urine 
 Exmoor ponies, 697 
 Expansion (foot), 712. See Foot 
 Expenditure of body, 358 
 Expiration, 112 
 muscles of, 113 
 See Respiration 
 Expired air, composition of, 119 
 Extensibility, 434 
 Extension, 631 
 
 Extensor thrust, 481, 482, 483 
 Exteroceptive field, 479. See 
 
 Nervous System 
 Eye, 566, 568 
 
 aberration, chromatic, 592 
 
 spherical, 592 
 accommodation, 581, 587-589 
 ciliary muscle, 588 
 focussing, 588, 589 
 muscles of eyeball, 589 
 anterior chamber, 577, 578, 585 
 aqueous humour, 574, 577, 580, 
 
 585 
 astigmatism, 574, 597 
 atropine, effect of , 579, 581, 589 
 bat, 604 
 binocular vision, 573, 584, 586, 
 
 597 
 bird, 574, 600, 606 
 blind spot, 583 
 capsule of Tenon, 573 
 cardinal points of, 593 
 cat. See Cat, vision of 
 cataract, 590, 594, 606, 805 
 catoptric test, 590 
 choroid, 575, 584, 588 
 ciliary body, 576 
 
 ganglion, 554, 562, 580, 
 58 c 
 
 muscle, 581, 588, 589 
 
 nerves, 562, 580 
 
 processes, 576, 577, 584, 
 585, 588 
 cilio-spinal centre, 505 
 cocaine, effect on, 579 
 colour vision, 605 
 cornea, 568, 570, 571, 572, 574, 
 
 578, 579, 585. 586, 590 
 corpora nigra, 581 
 corpus quadrigeminum ant., 
 
 510, 582 
 defects of,^68, 596, 597 
 
INDEX OF SUBJECTS 
 
 853 
 
 Eye, diseases of, 606 
 
 dog. See Dog, vision of 
 
 duck, 598 
 
 electrical changes in, 603 
 
 emmetropic, 596, 597 
 
 errors of refraction, 596, 606 
 
 eserine, effect on, 579 
 
 field of vision, 574, 577, 579, 
 
 583, 606 
 focussing, 588, 589 
 formation of retinal image, 594 
 fovea centralis, 583, 599, 604 
 fundus, 576, 578 
 Gennari, line of, 586 
 glaucoma, 585 
 Harderian gland, 570 
 hare, 598, 600, 606 
 hen, 604 
 
 humours, 585, 586 
 hyaloid membrane, 577, 584, 
 
 586 
 hypermetropia, 596, 597 
 injuries, 606 
 
 intra-ocular pressure, 589 
 iris, 510, 562, 578, 580, 585, 
 
 813 
 innervation of, 562, 580 
 jumping, visual judgment, 61 7, 
 
 682 
 lachrymal apparatus, 570 
 lens, 577, 578, 584, 588, 590 
 lenses, passage of lightthrough, 
 
 590 
 ligament, suspensory, 584, 585, 
 
 586, 588 
 ligamentum pectinatum, 
 
 585 
 light, nature of, 566 
 lymph in, 585 
 measurements of, 575 
 membrana nictitans, 569 
 membrane of Descemet, 
 
 586 
 monocular vision, 587, 597, 601 
 morphine, effect of, 579 
 muscles. See Eye, ocular 
 
 muscles, and Eyeball, 
 
 muscles of 
 mydriasis, 579 
 myopia, 596, 597 
 myosis, 579 
 night vision, 584 
 nodal point, 593, 595 
 occipito-thalamic fibres, 586 
 ocular muscles and equilibrium, 
 5i8 
 control of, 630 
 innervation of, . 550, 
 
 552, 573 
 movements of, 518, 
 568, 570, 571, 572 
 
 579, 
 
 574. 
 
 Eye, ocular muscles and pupil, 571 
 ophthalmia, specific, 606, 607 
 
 sympathetic, 607 
 ophthalmoscope, 590, 595 
 optic axis, 576, 584, 593, 605 
 chiasma, 582, 587, 600 
 nerve, 549, 550, 566, 572, 
 575. 581, 584, 586, 
 587, 599, 603 
 decussation of, 582, 
 
 587, 600 
 direct fibres, 582, 600, 
 603 
 papilla, 583 
 thalamus, 504, 511, 524, 
 
 525, 553, 582 
 optogram, 604 
 orbit, 570 
 owl, 604 
 
 panoramic vision , 583, 602 
 papilla, 583 
 periscopic vision, 602 
 photo-chemical theory, 603 
 physiological optics, 590 
 pigeon, 604 
 
 position of, 597, 598, 600 
 posterior chamber, 577 
 pupil, 492, 510, 571, 578, 630, 
 
 821 
 puppy, 811 
 reduced, 593 
 
 refraction, 567, 574, 585, 586, 
 594, 602 
 
 errors of, 596, 606 
 retina, 580, 581, 584, 586, 588, 
 604 
 
 corresponding points, 598 
 
 picture, 594, 595 
 
 rods and cones, 583, 584, 
 
 599, 604 
 rhodopsin, 584, 604 
 schematic, 592 
 
 sclerotic, 568, 571, 572, 575, 582 
 shying, 606 
 squint, 573, 600 
 stereoscopic vision, 602 
 suspensory ligament, 584, 585, 
 
 586, 588 
 tapetum lucidum, 575, 576, 
 
 583, 584, 605 
 nigrum, 576 
 tears, 568, 570 
 vision, binocular, 573, 584, 586, 
 
 597 
 central, 583 
 
 field of, 606. See Vision 
 monocular, 587, 597, 601 
 panoramic, 583, 602 
 stereoscopic, 602 
 theory of, 584 
 visual angle, 595 
 
§54 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Eye, visual axis, 571 , 572 
 
 centre, 534, 537, 586 
 cortex, 586 
 purple, 584, 604 
 visuo-psychic area, 538, 587 
 visuo-sensory area, 534, 537, 
 
 586 1 
 
 vitreous humour, 577, 584, 586 
 zonule of Zinn, 577, 584, 588 
 Eyeball, 566, 567 
 
 action, conjugate, 573, 574 
 co-ordinate, 573 
 opposite, 573, 574 
 length of, 575 
 movements of, 518, 570, 571, 
 
 572, 573 630 
 muscles of, 510, 518, 550, 552, 
 553. 57°. 571. 57 2 . 573. 
 589, 630 
 oblique, 571, 572, 573 
 recti, 571, 572, 573 
 retractor, 572, 573 
 tonus of, 553 
 Eyelashes, 569 
 Eyelids, 568, 818 
 glands, 568 
 movements, 568 
 muscles of, 568 
 secretion, 323, 568 
 
 Face markings, 316 
 
 Facial nerve, 67, 93, 137, 142, 160, 
 
 i/3. 549. 55°. 562, 613 
 
 sinuses, 118, 119, 624 
 Factors (breeding), 801 
 
 interaction of, 804. See 
 Heredity 
 Faeces, 238 
 
 amount of, 240 
 
 composition of, 238 
 
 dog, 238, 383 
 
 horse, 240 
 
 inorganic matter in, 239 
 
 meconium, 241, 773, 780 
 
 odour of, 241 
 
 ox, 238, 239, 240, 241 
 
 pig, 238, 241 
 
 pigment of, 239 
 
 sheep, 238, 240 
 Fallopian tube, 739, 744, 745, 756, 
 
 761, 767 
 False nostril, 117 
 Fascia, use of, 424 
 Fat, absorption of, 292, 375 
 
 blood in, 25, 831 
 
 chyle in, 288, 294 
 
 colour of, 375 
 
 composition of, 357, 830 
 
 depots, 375, 378, 379 
 
 differences in animals, 375, 831 
 
 digestion in large intestines, 227 
 
 Fat, digestion in small intestines, 
 252, 264, 292, 375, 396. 
 
 831 V 
 in stomach, 199 
 
 embolism, 265, 293 
 
 emulsification, 252, 264, 292 
 
 energy from, 375, 395 
 
 ferments, 261, 264, 294, 375. 
 
 See Ferments and Lipase 
 
 foetus, in, 811 
 
 function of, 376 
 
 glycogen-producing, 256 
 
 lean and fat condition, amount 
 
 of, 358 
 metabolism of, 375, 
 milk, in, 264, 375, 830. See 
 
 Milk 
 nerve, in, 458 
 oxidation of, 375, 402, 403, 
 
 404, 416 
 protein economiser, 367, 371 
 reconstruction of, 265, 294, 375, 
 
 830 
 removal from tissues, 376, 830 
 reserves, 375, 378, 379 
 saponification, 264, 292, 831 
 skin in, 322 
 source of, 256, 367, 375, 379. 
 
 832 
 splitting, 252, 264, 375, 831 
 storage of, 265, 375, 379, 812 
 synthesis of, 265, 294, 375, 830 
 tissues, various, 831 
 transport of, 376, 379 
 utilisation of stored, 376 
 Fatigue, 429, 436, 445, 446, 447, 
 640, 662 
 fever, 408 
 
 inhibition, effect on, 490 
 muscle, 429, 446 
 
 when absent, 436 
 nerve, 465, 467 
 Fattening animals for food, 29, 296, 
 37°. 37 8 . 379, 380, 393. 394. 399. 
 745 
 Fatty acids. See Acids, fatty 
 ' Feather,' horse, 311, 312, 808 
 Fehling'stest, 172, 832, 834, 835 
 Fenestra ovalis, 625, 627 
 
 rotunda, 625, 627 
 Fermentation test, 835 
 Ferments, 836 
 blood, 21 
 cellulose, 199, 227, 373, 374, 
 
 418, 833 
 diastatic, 169, 172, 254, 256, 
 
 261, 264, 296, 832-835 
 intestinal, 199, 218, 227, 255, 
 
 262, 264, 296, 297, 301, 373, 
 374, 418, 833-836 
 
 intracellular, 369, 404, 836 
 
INDEX OF SUBJECTS 
 
 855 
 
 Ferments, kidney, 307 
 kinases, 22, 262 
 lipolytic, 261, 264, 294, 375 
 nuclease, 340 
 
 oxydases, 133, 340, 404, 836 
 pancreatic, 261, 264, 294, 296, 
 
 2 97, 375 
 pepsin, 195, 297, 836 
 peroxydases, 404 
 proteolytic, 261, 262, 297, 836 
 rennin, 194, 196, 205, 791 
 reversible reaction, 265, 836 
 salivary, 169, 172 
 stomach, 195, 196, 297, 836 
 skin, 314 
 
 velocity of reaction, 836 
 Fertilisation of ovum, 279, 307, 
 
 753, 755, 7 61 - See Ovum 
 Fetlock-joint, 640, 653, 654, 656 
 Fever, 400, 421 
 Fibrillar contraction, 50, 434 
 Fibrils, muscle, 33, 422 
 Fibrin, 20 
 
 ferment, 20, 21 
 Fibrinogen, 5, 20, 275 
 Fibrino-globulin, 6 
 Fifth cranial nerve, 138, 160, 165, 
 173, 364. 549, 55°. 
 551, 553, 580, 608, 
 613 
 section of superior 
 maxillary branch, 
 554, 617 
 Fight or flight, 305, 306, 511 
 Filiform papillae, 612 
 Fillet (pons), 508, 525 
 Filtration, 277 
 Fissure of Rolando, 520, 524, 525, 
 
 529, 535 
 Fistula of the coronet, 734 
 Flechsig, tract of, 501 
 Fleece, potash in, 324 
 Flesh-formers, 366 
 Flexion, 482, 631 
 Fluctuations (Mendel), 804 
 Fluids, swallowing, 160, 216, 220 
 Flying Childers, speed of, 698 
 Foal, characteristics of, 812 
 
 future height of, 812 
 
 iris, 578, 813 
 
 mean weight of, 813 
 
 rate of growth, 814 
 Foetal circulation, 768, 777 
 
 heart, 50, 777, 779 
 
 liver, 777, 780 
 
 lung, 113, 779 
 
 membranes, 770, 786 
 P' nutrition. See Embryo 
 
 respiration, 113, 142, 771, 774 
 Fcetation, abdominal, 744 
 Foliate papillae, 612 
 
 Food, absorption of, 295 
 
 accessory factors, 388. See 
 
 Vitamines 
 amount of, required, 370, 393, 
 
 447, 
 energy, 366, 371,373,374, 395 
 
 loss of, 395, 396 ■ 
 inorganic, 823 
 See also Nutrition and Diet 
 Foot, 700 
 
 anti-concussion mechanism, 
 
 701, 702, 721, 726, 728, 733 
 bars, 712 
 
 use of, 726 
 blood-supply, 710, 732 
 bones of, 701 
 canker, 736 
 
 cartilages, 701, 704, 724, 726, 
 727 
 
 ossified, 724, 728, 734 
 
 use of, 724, 727 
 cleft, 705, 717 
 compression of wall, 731 
 contraction of, 726, 735 
 corium of, 705 
 cor n, 735 
 coronary body, 706, 708, 710, 
 
 713 
 diseases of, 734 
 elasticity, provision for, 712, 
 
 721, 722, 724, 726, 727 
 evaporation from, 711, 720, 721 
 evulsion of, 735 
 expansion, 712, 717, 724, 726, 
 727, 728, 729, 730, 731, 
 732, 733 
 area of, 729 
 measurement of, 729 
 external and internal, 700 
 functions of, 701 
 fat in plantar cushion, 704 
 frog, 716 
 
 atrophy of, 726 
 -stay, 717 
 
 use of, 726. See Footpad 
 gelatinous degeneration of, 735 
 ground, makes contact with, 
 
 673, 722, 726, 728 
 growth of, 713, 722 
 hoof, 710 
 
 action of alkalies on, 718, 
 
 721 
 colour of, 315, 711 
 concussion, 701, 721 
 differences between fore 
 
 and hind, 712 • 
 hardness of, 736 
 white line, 715 
 horn cells, 717 
 
 chemistry of, 721 
 moisture, 713, 716,720,721 
 
856 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Foot, horn, old and young, 722 
 secretion of, 705, 719 
 soft, 716 
 
 structure of, 717-720 
 tubular and intertubular, 
 719 
 -joint, 656, 702 
 keratin, 717, 721, 826, 828 
 laminae, horn, 708, 709, 713, 
 714, 716, 719, 723, 
 724 
 origin of, 708 
 
 of secondary, 708, 
 709, 714, 716, 
 
 7 2 3 
 sensitive, 706, 707, 708, 
 714, 716, 723, 724, 
 
 733 
 secondary, 707, 708, 
 
 7°9, 714, 7 2 3 
 area of, 725 
 number of, 723 
 structure of, 706, 
 
 7°7 
 laminal tissue, 706, 723 
 laminitis affecting, 364, 661, 
 
 7 2 5 
 navicular bone and bursa, 656, 
 701, 702 
 disease, 648, 664, 703, 805 
 nerve-supply of, 733 
 neurectomy, 364, 471, 473 
 -pad, 705, 712, 716, 717, 721, 
 
 726 
 papillae. 705, 710, 713, 714, 715, 
 
 716, 719 
 peak 717 
 
 pedal bone, descent of, 725, 731 
 periople, 710, 711 
 physiological shoeing, 734 
 plantar cushion, 704, 710, 716, 
 
 717, 726, 733 
 reflex, 485 
 rings on, 736 
 sand crack, 735 
 seedy toe, 736 
 shoeing of, 734 
 side bone, 724, 728 
 sloughing of, 364, 471, 473 
 sole, horn, 714 
 
 use of, 726, 732 
 vascular, 710 
 
 papillae on, 714 
 sweat glands in, 705 
 thrush, 736 
 
 touch organ, 517, 717, 733 
 toughness, 722 
 vascular, 706 
 
 mechanism, 732, 733 
 sole, 710, 714 
 wall, 706 
 
 Foot, venous plexus, 732 
 wall, 710, 722 
 
 compression of, 731 
 -secreting body, 710, 713 
 thickness of, 713 
 weight, how carried, 712, 714, 
 
 723. 725. 726 
 
 white line, 715 
 
 yielding body, 712, 716, 731 
 Footprints, horse, 681 
 Foot-tons (work), 438, 691 
 Foot-tracks, 683 
 Foramen ovale, 777, 779 
 Foreign bodies in heart, 61, 64, 203 
 Formic acid, 239 
 Fourth cranial nerve, 518, 549, 550, 
 
 553, 573 
 Fovea centralis, 583, 599, 604 
 Free-martin, 782 
 Frog, decerebrated, 476 
 
 (foot), 716 
 
 rheoscopic, 442 
 
 stay (foot), 717 
 Frontal sulcus, 529 
 Fructose, 834. See Laevulose 
 Fundus glands, 192 
 Fungiform papillae, 612 
 Furfurol, 250 
 
 Galactagogue, 306, 795 
 Galactose, 255, 296, 832, 834 
 Gall-bladder, 250. See Liver 
 Gallop, analysis of, 678 
 
 conventional, 681 
 
 dog, 681 
 
 extensor thrust in, 482, 483 
 
 rotatory, 677, 681 
 
 speed of, 698 
 
 stride in, 679, 698 
 
 transverse, 677 
 Galton's law, 799 
 Galvanometer of Einthoven, 463 
 Gamete, 800, 803, 809 
 Ganglia, abdominal, 234 
 
 basal, 510, 534 
 
 cardiac, 50, 51 
 
 cervical, 269 
 
 ciliary, 554, 562, 580, 581 
 
 cranial, 491, 550, 553 
 
 function of, 470, 471, 474, 559, 
 563 
 
 Gasserian, 553, 580 
 
 lateral. See Vertebral 
 
 prevertebral, 560, 561, 563 
 
 Remak's, 51 
 
 sacral, 563 
 
 spinal, 470, 493, 494, 502 
 
 stellate, 56 
 
 sympathetic, 558, 559, 560 
 function of, 559, 563 
 
 terminal, 560, 563 
 
INDEX OF SUBJECTS 
 
 857 
 
 Ganglia, thoracic, 269 
 
 vertebral, 559, 560, 561, 562, 
 563 
 Ganglion cells, 458, 468, 470 558 
 
 559 
 Gaseous exchange in lungs, 129 
 Gases, absorption of, 130 
 
 diffusion, 129 
 
 dissociation, 126, 130 
 
 in atmosphere, 119 
 
 in blood, 123, 131, 135, 140, 
 
 774. 780 
 tension of, 128 
 
 in body, 132, 822 
 
 in expired air, 119, 121 
 
 in intestines, 228, 236, 237, 358, 
 821 
 
 in lymph, 275 
 
 in muscle, 451 
 
 in saliva, 169 
 
 in swim-bladder, 131 
 
 in tissues, 131, 132 
 
 of rumen, 202 
 
 of stomach, 208, 821 
 
 partial pressure, 130 
 
 secretion of, 130 
 Gaskell's myogenic path, 52 
 Gasserian ganglion, 553, 580 . 
 Gastric juice, 193, See Stomach 
 Gastrin, 197, 307 
 Gastrocnemius muscle (frog), 426 
 Gelatin, 826 
 
 in diet, 368, 371 
 Generation, 737 
 
 abdominal fcetation, 744 
 
 accessory organs of generation, 
 738, 759 
 
 allantois, 766, 767, 771, 773, 
 
 774 
 amnion, 770, 771 
 ancestrum, 747 
 artificial insemination, 741, 
 
 756 760 
 astrosphere, 753 
 attraction sphere, 744, 753 
 ' breaking service,' 775 
 breeding, 807. See Breeding 
 bulbo-urethral glands, 738, 742, 
 
 759 
 cell division, 753, 755 
 features of, 751-754 
 nucleus, 752, 753, 754, 
 761, 762 
 cells, body, 751 
 
 germ, 743, 751, 801 
 Leydig's, 740, 751 
 Sertoli's, 739, 740 
 somatic, 743, 751, 801 
 centrosome, 744, 753, 755, 761, 
 
 762, 763 
 chorion, 767, 773, 774, 786 
 
 Generation, chromatin, 753, 809 
 chromosomes, 739, 740, 744, 
 
 753. 755. 761, 762, 781, 
 800, 801, 809 
 accessory, 753, 781 
 corpus luteum, 307, 743, 787 
 Cowper's glands, 738, 742, 759 
 cryptorchidism, 739, 790 
 cytoplasm, 752, 753, 755, 761, 
 
 762, 763 
 decidua, 765, 766, 768 
 determination of sex, 781 
 dicestrum, 747 
 ejaculation, 742, 759 
 emasculation, effect of , 154, 155, 
 
 305, 3°7, 379, 74 2 . 745. 746, 
 816 
 embryo, death, of, 775 
 
 nutrition of, 767, 769, 770, 
 
 772, 775, 779 
 unity of type, 754 
 epiblast, 763, 764 
 epididymis, 738, 740, 745, 751, 
 
 759, 760 
 erection, 757 
 Fallopian tubes, 739, 744, 745, 
 
 756, 761, 767 
 fcetal circulation, 768, 777 
 membranes, 770 
 
 separation of, 786 
 Graafian follicle, 743, 744, 755 
 heredity, 797. See Heredity 
 hypoblast, 763, 764 
 implantation of ovum. See 
 
 Ovum 
 liquor amnii, 773 
 mammary gland, 307, 788 
 maturation of ovum, 754 
 
 of spermatozoon, 739, 740, 
 
 74 2 . 751 
 maturity, 307, 756, 813, 816 
 meconium, 241, 773, 780 
 mesoblast, 764 
 metcestrum, 747 
 milk, secretion of, 788. See 
 
 Milk 
 multiple births, 756, 782 
 neural arch, 764 
 notochord, 764 
 cestrus, 747. See OEstrus 
 
 cause of, 307, 749 
 organs of, 737 
 
 accessory, 738, 759 
 ovaries, 738, 742. See Ovaries 
 ovulation, 755 
 ovum, fertilisation of , 307, 761. 
 
 See Ovum 
 parturition, 784 
 placenta, 765. See Placenta 
 pregnancy, 783 
 pronucleus, 761 
 
858 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Generation, pro-oestrum, cause of, 
 
 749 
 signs of, 750 
 prostate gland, 738, 742, 751, 
 
 759 
 puberty, 307, 756 
 rutting, 307, 750, 751 
 segmentation nucleus, 761 . 
 
 See Segmentation 
 seminal vesicles, 738, 741, 751, 
 
 759 
 sexual characteristics, develop- 
 ment of, 307, 740, 745, 
 746, 756, 816 
 cycle, 747-751 
 intercourse, 758 
 inversion, 746 
 season, 747-751 
 spermatic fluid, 739, 741, 759, 
 
 760 
 spermatozoon. See Sperma- 
 tozoa 
 superfecundation, 783 
 superfcetation, 756, 783 
 testicles, descent of, 739, 751. 
 
 See Testicles 
 trophoblast, 765, 766, 767, 768, 
 
 769, 770, 775 
 umbilical cord, 769, 774, 776 
 uterine milk, 768, 769 
 uterus, 738. See Uterus 
 
 change during sexual 
 cycle, 750 
 vas deferens, 742, 759, 760 
 yolk sac, 765, 767, 769, 776 
 spherules, 744 
 Genito-spinal centre, 505 
 Gennari, line of, 586 
 Germ-plasm theory, 798 
 Germinal area, 764 
 spot, 744 
 vesicle, 744, 767 
 Gestation, periods of, 784 
 Gland, Harderian, 570 
 hibernal, 542 
 lachrymal, 570 
 parathyroid, 302. See Thyroid 
 
 glands 
 thyroid, 302,385. See Thyroid 
 glands 
 Glands, Brunner, 218 
 
 Cowper's, 738, 742, 759 
 endocrine. See Endocrine 
 
 glands 
 fundus, 192 
 gastric, 192 
 
 lymphatic, 273, 286, 292 
 mammary, 788. See Milk 
 of eyelid, 568, 570 
 pyloric, 192 
 salivary, 167, 613 
 
 Glaucoma, 585 
 
 Gliadin, 368, 825, 826, 828 
 
 Globin, 11, 15 
 
 Globulins, 450, 823, 825, 826 
 
 Glomerulus, 326 
 
 Glosso-pharyngeal nerve, 165, 173, 
 
 549. 55°. 555. 5 62 , 6l 3. Dl8 
 Glottis, 118. See Larynx 
 Gluco-protein, 825 
 Glucosamine, 826 
 Glucose, 403, 834, 835. See 
 
 Dextrose, Glycogen, and Sugar 
 Glutamic acid, 256, 826, 827 
 Glutaminic acid, 263 
 Glutelins, 825, 826 
 Glycsemic centre, 269 
 Glycerides, 830 
 Glycerol, 264 
 
 Glycine, 249, 256, 337, 826, 827 
 Glycocholate soda, 249 
 Glycocholic acid, 249 
 Glycogen, characteristics of, 252 
 
 disappearance of, 253 
 
 ferment of, 256, 372 
 
 formation from fat, 255, 256, 
 
 372 
 from protein, 255, 367 
 from sugar, 255, 372 
 in amnion, 772 
 in body, 267, 373 
 in liver, 252, 254, 256, 372 
 
 amount of, 253, 254 
 in muscle, 254, 255, 268, 269, 
 37 2 , 373. 374. 444. 45°. 
 451 
 amount of, 254, 255, 444 
 in placenta, 254, 780 
 muscular work and, 253, 444 
 oxidation as dextrose, 255, 256, 
 267, 269, 270, 372, 373, 374. 
 
 833 
 pancreas, removal of, 267 
 sources of, 255, 307 
 starvation, effect of, 253, 254, 
 
 256, 444 
 storing up of, 253, 254 
 sugar yielded by, 254, 373, 833 
 supply, how regulated, 253, 
 254, 256, 267 
 Bernard's view, 268 
 Pavy's view,. 268 
 See Carbohydrates, Liver, 
 and Sugar 
 Glycolysis, 255, 269 
 Glycosuria, 255, 257, 269, 270, 2,96 
 
 phloridzin, 255 
 Glycuronic acid, 345, 354 
 Gmelin's test, 248 
 Goat, average duration of life, 818 
 maturity, 756 
 milk of, 791 
 
INDEX OF SUBJECTS 
 
 859 
 
 Goblet cells, 287 
 
 Golgi cells, 468, 513, 520 
 
 tendon organs of, 423, 458, 489 
 Goll, bulbar nucleus of, 501 
 
 column of, 501, 503, 508 
 Gowers, tract of, 501, 515 
 Graafian follicle, 743, 744 , 755 
 (j-ranulose, 171 
 Grape sugar, 835. See Sugar and 
 
 Glycogen 
 Grey horses, 315 
 Growling, 155 
 Growth, 376, 378, 722, 811 
 
 amino-acids necessary, 368, 376 
 
 fat in, 378 
 
 intra-uterine, 781, 811 
 
 protein in, 376, 377, 811 
 
 rate of, 814 
 
 underfeeding, 378 
 
 vitamines, 376 
 Guanase, 836 
 Guanine, 339, 792 
 Gustatory pore, 612 
 Guttural pouches, 160, 624 
 
 Haematin, 11, 14, 248 
 Haematoidin, 15, 248 
 Haematoporphyrin, 15 
 Haemin, 15 
 Haemochromogen, 15 
 Haemoglobin, 1, 7, 8, n, 12, 14, 
 124-126, 247, 248, 780, 826, 828 
 Haemoglobinuria, 29, 400 
 Haemolymph glands, 285 
 Haemolysis, 9, 824 
 Haemorrhage, 27, 29, 76, 137 
 Hair, 309. See Skin 
 Hairballs, 201, 216, 240 
 Harderian gland, 570 
 Hay, absorption of, 295, 297, 298 
 
 digestion of, 183 
 Hearing, 620. See Ear 
 
 centre for, 534 
 
 theories of, 627, 628 
 Heart, 30 
 
 accelerator nerves, 54, 59 
 
 adrenals, influence of, 62, 304 
 
 anabolic nerves, 563 
 
 anabolism of, 60 
 
 apex, 32, 34, 35, 37, 41, 42 
 
 arrangement of, 30 
 
 atropine and, 59 
 
 augmentor fibres of, 54 
 
 auricle, fibres of, 35 
 
 auricular fibrillation, 50, 63 
 
 auriculo-ventricular bundle, 33, 
 
 34. 51 
 
 groove, 40, 41 
 
 node, 33, 51 
 automaticity, 52 
 beat, cause of, 50 
 
 Heart, block, 51, 54 
 bone, 36 
 calcium, 52, 824 
 capacity of, 47, 447 
 cardiac sound, 43 
 cardiograph, 47 
 cardio-inhibitory centre, 58, 
 
 centre. See Medulla 
 changes in shape on contract- 
 ing, 40 
 chemical stimulus, 62, 304, 305 
 chorda? tendineae, 36, 38, 40 
 circulation through, 31 
 columnae carneae, 35 
 contraction of auricles, 40, 41, 
 
 53 
 
 of ventricles, 40, 41 
 coronary circulation, 47, 49 
 corpus Arantii, 38 
 cycle of movements, 39, 40, 41 
 depressor nerve, 60 
 diastole, 39 
 embryonic 50 
 endocardium, 36 
 feeding of, 52 
 fibres, 35, 425 
 
 fibrillar contraction, 50, 434 
 fibrous rings in, 33, 36 
 fcetal, 50, 777, 779 
 foreign bodies in, 61, 64, 203 
 fulcrum, 36 
 function of, 30 
 
 of sympathetic, 54, 59, 562 
 ganglia, 50, 51 
 impulse of, 32, 41, 43, 47 
 inhibition, 58, 60 
 inhibitory fibres of, 54, 562 
 inner stimulus, 52 
 insensibility to pain, 61, 564 
 intermittent action, 63 
 intersystolic period, 41 
 irregular action of, 63 
 katabolic nerves, 563 
 katabolism of, 60 
 latent period, 57 
 Mackenzie's views, 63 
 moderator bands, 34, 37 
 movements of, 39, 51 
 muscarine and, 59 
 muscle, 33, 34, 39, 53, 425, 430, 
 447 (footnote) 
 
 glycogen in, 269 
 
 physiological properties of, 
 53. 425. 438 (footnote) 
 
 stimulation of, 53 
 muscular wall of, 34 
 musculi papillares, 34, 35, 36, 
 
 38, 4°. 41 
 myogenic theory, 51 
 nervous mechanism of, 54, 510 
 
A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Heart, neurogenic theory, 51 
 nodal tissue in, 33 
 output of, 48, 447 
 pathological, 62, 430, 817 
 pause, 39, 41, 53 
 pericardium, 32, 36, 74, 116, 273 
 
 use of, 42 
 pilocarpine and, 59 
 pituitary body, influence on, 
 
 62, 305 
 plexus of nerves, 56 
 poisons and, 59 
 position of, 32 
 postsphygmic period, 45 
 presphygmic period, 45 
 pressure within, 40, 43, 44, 45, 
 
 46, 47 
 pulse. See under Pulse 
 rate of, 41 
 
 refractory period, 53 
 rotation of, 40, 41 
 rupture of, 63, 447 
 salts, influence on, 52 
 secondary augmentation, 58 
 septum, interauricular, 34, 51 
 
 interventricular, 34 
 sounds of, 40, 41, 42 
 standard movements of, 45 
 Stannius's experiment, 52 
 structure of, 30 
 sympathetic nerves, 54, 59, 562 
 systole, 39 
 systolic plateau, 44 
 temperatures of right and 
 
 left, 405 
 tension sounds, 39 
 tetanus, 53 
 tonus of, 425, 824 
 vagus nerves, 54, 56, 555 
 valves, 31, 36 
 
 aortic, semilunar, or sig- 
 moid, 31, 38, 39, 41, 45 
 
 auriculo-ventricular, 31, 
 36, 38,40,41,45 
 
 corpus Arantii, 38 
 
 disease of, 63 
 
 method of closing, 36, 37, 
 38 
 
 mitral or bicuspid, 31, 36, 
 38, 45 
 
 position of, 31 
 
 pulmonary, 39, 41, 44 
 
 tricuspid, 31, 36 
 
 use of, 31 
 venous aspiration, 40 
 ventricle, fibres of, 34 
 ventricular fibrillation, 50 
 
 relaxation, 41 
 vortex currents, 39 
 
 fibres, 34 
 walls of, 34 
 
 Heart, work of the, 48, 447 
 Heat, amount produced, 419 
 animal, 402 
 
 animals which regulate from 
 birth, 414 
 which fail to regulate from 
 birth, 414 
 autoxidation, 403 
 blood, 405 
 
 body temperature, 404 
 brain, 405 
 calories, their value, 374, 395, 
 
 419 c 
 
 calorimeter, 362, 395, 420. See 
 
 Calories 
 centres, 409, 512 
 clipping, effect on, 316, 416 
 conduction, 410, 413, 414 
 critical temperature, 417, 418 
 energy, cause of, 402 
 
 conservation of, 403 
 
 forms of, 403 
 
 transformation of, 403 
 evaporation, 410, 411 
 fatigue fever, 408, 421 
 hibernating animals, 405, 416, 
 
 542 
 homoiothermal animals, 404 
 increase in working horses, 407 
 influence of coat colour on, 412 
 
 of high temperatures on, 
 
 413 
 of low temperatures on, 
 
 413.414. 418 
 in hibernation, 416, 542 
 in large and small animals, 411, 
 
 419,420 
 loss, 409, 411, 412, 414 
 
 due to varnishing, 324 
 muscle tonus and, 407, 438 
 muscular contraction and, 407, 
 
 438, 439, 445 
 nerves, 408 
 
 normal temperature, 405 
 oxidations, 402, 403, 404, 413 
 pathological considerations, 42 1 
 poikilothermal animals, 404 
 post-mortem rise, 421 
 produced during digestion, 407, 
 408, 418 
 during feeding, 408 
 during glandular activity, 
 
 407 
 during muscular contrac- 
 tion, 407, 438, 439, 445 
 during rest, 407, 408, 417 
 production, 407, 419, 445 
 
 chloroform narcosis and, 
 
 409 
 clipping and, 416 
 curare and, 408 
 
INDEX OF SUBJECTS 
 
 861 
 
 Heat production, nervous system 
 and, 408, 512 
 puncture, 409 
 radiation of, 410, 413 
 regulation, 410, 512 
 
 of body temperature 
 
 (chemical), 410 
 of body temperature 
 (physical), 410 
 rigor, 430, 444, 451 
 shivering, 324, 396, 409, 411, 
 
 417 
 spots and cold spots, 615 
 standing, effect on, 407, 417 
 surface and deep, "405 
 thermotaxis, 410 
 units, 394, 395, 419 
 variations in body temperature, 
 
 406 
 See Temperature 
 Helicotrema, 626, 627 
 Henle, loop of, 328 
 Hensen's cells, 626 
 Heredity, 797. See also Breeding 
 atavism, 805 
 blending, 799, 801 
 chromosomes, 755, 762, 800, 
 
 801, 809 
 common origin, 797 
 determinant, 801, 805 
 dominant characters, 781, 801, 
 802, 803, 809 
 irregularity in, 803 
 evolution, 797 
 factors, 801 
 
 coupling of, 804 
 grouping of, 801 
 interaction of, 804 
 repulsion of, 804 
 gamete, 800, 803, 809 
 germ cells, 751, 801 
 in-breeding, 801-803 
 in disease, 805, 810 
 jumps, 798 
 
 Alendelian inheritance in polled 
 and horned cattle, 803 
 in sweet-pea, 802 
 mutants, 799, 808 
 mutation, 798 
 natural selection, 798 
 prepotency, 782, 799, 809, 810 
 segregation, 801, 808 
 somatic cells, 751, 801 
 fluctuations, 804 
 ' sports,' 804 
 staying power, 438 (footnote), 
 
 800 
 telegony, 783, 805 
 theories, Darwinian, 797, 798 
 de Vries, 798 
 Galtonian, 799 
 
 Heredity theories, germ plasm, 798, 
 800, 806 
 Lamarckian, 797 
 law of ancestral inheri- 
 tance, 799 
 Mendelian, 798, 800 
 mutation, 798, 804 
 Weismannian, 798 
 ' throwing back,' 805 
 unit characters, 800, 801, 804 
 zygote, 800, 803, 809 
 Hermaphrodites, 781, 783 
 Hetero-cyclic amino-acids, 828 
 Hexone bases, 218 
 Hibernal gland, 542 
 
 sleep, 542 
 Hibernating animals, 121, 405, 416 
 Hibernation, 121, 405, 416, 542 
 
 respiratory quotient, 121 
 Hiccough, 155, 156 
 Hip-joint, 641. 656 
 Hippomanes, 774 
 Hippuric acid, 314, 333, 340, 351. 
 
 353, 358, 392. 792- See Urine 
 Hirudin, 24 
 
 Histidine, 263, 337, 828 
 Histon, 23 
 Histones, 825 
 
 Hock-joint, 644, 646, 648, 658, 664 
 Homoiothermal animals, 404 
 Hoof, 710. See Foot 
 ^ growth of, 722 
 wall of, 710 
 Hordein, 826 
 
 Hormones, 197, 198, 219, 237, 250, 
 260, 270, 300, 740, 743, 744, 751, 
 783, 787, 789, 790, 795 
 Horn, chemistry of, 721 
 
 laminae, 708, 709, 713, 714, 
 716, 719, 7 2 3. 7 2 4 
 origin of, 708 
 moisture in, 713, 716, 720, 721 
 structure of, 717-720 
 See Foot. 
 Horns, growth of, 307, 803 
 Horse, act of rising, 649 
 affection, 544 
 albino, 315, 316, 576 
 arterial degeneration, 100, 817 
 astigmatism of, 574, 597 
 azoturia, 9, 14, 463 
 bile, 247 
 
 blood, 2, 13, 19. 26 
 boiled food for, 199 
 bolting, 545 
 broken wind, 156, 401 
 ' brushing,' 632, 652 
 buck-jumping, 689 
 caecum of, 222, 226, 232, 233 
 capacity for work, 817 
 capped elbow, 649 
 
862 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Horse, capped hock, 649 
 
 cataract in, 590, 594, 606, 805 
 
 catarrh, 157 
 
 ' cat ' hairs, 309 
 
 centre of gravity, 649 
 
 colic, 242 
 
 colon of, 224, 231, 233. See 
 
 Digestion 
 colour of, 314 
 conservatism of, 545 
 ' cow-hock,' 657 
 ' cow-kicking,' 632, 657, 686 
 dandruff from, 323 
 death-rate, 819 
 dentition, 814 
 dieting of, 297. See Diet 
 digestive diseases of, 242 
 draught, 692, 694, 695, 696 
 elbow-joint. See Joints 
 enteritis in, 243 
 eye disease in, 606 
 eyelashes of, 569 
 faeces of, 240 
 ' feather,' 311, 312, 808 
 food, absorption of, 295 
 foot, 700. See Foot 
 diseases of, 734 
 -prints, 681 
 gall-bladder absent, 251 
 gregarious instincts of, 545 
 growth of, 814 
 hair, 309 
 hay, digestion of, 183, 295, 297, 
 
 298 
 hip-joint. See Joints 
 identification of , 3 1 1 
 intestinal digestion in, 219 
 'jibbing,' 544 
 jump of, 682 
 kicking, 632, 657, 686 
 lameness, 661 
 ' left-handed,' 316 
 life, average duration of, 818 
 lungs, diseases of, 156 
 lying down, 649 
 lymph, 276 
 markings of, 316 
 maturity, 756, 816 
 moral sense in, 543 
 muscles of, 631 
 myopia of, 596 
 
 natural standing attitude, 648 
 neigh of, 154, 155, 745 
 oats, digestion of, 185, 199, 297, 
 
 298 
 oldagein, 314, 818 
 paces of, 666 
 panic in, 545 
 ' pointing,' 648 
 polyuria, 271 
 -power, 438, 690 
 
 Horse, puberty of, 756 
 rate of growth, 814 
 rearing of, 686 
 respires by nasal passages, 
 
 117 
 ' roaring,' 152, 156, 455, 557 
 ruptured stomach, 182, 209, 
 
 210, 242, 245 
 ' sham watered,' 618 
 shivering, 324, 396, 409, 411, 
 
 565 
 
 shoulder-joint, 637, 650 
 
 ' shunting,' 617 
 
 shying, 606 
 
 sleep, standing. See Sleep 
 
 speed of, 698. See Speed 
 
 sprains of, 662 
 
 staleness, 447 
 
 standing attitude . See Posture 
 
 starvation of , 393, 618 
 
 stomach of, 178. See Stomach 
 digestion in, 199 
 
 strangulation, intestines, 243, 
 244, 564 
 
 strength of, 695 
 
 sweating of, 318, 319 
 
 teeth, sharp, 159 
 
 temperature of, 405 
 
 tumour, brain, 534 
 
 urine of, 343, 346, 349, 352 
 
 vomiting of, 209, 510 
 
 walk of, 666, 667 
 
 wall-eyed, 316, 578 
 
 watering of, 187, 618 
 
 ' weaver,' 632 
 
 weight he should carry, 697 
 he should draw, 695 
 
 'willing,' 543 
 
 work of, 440, 690 
 Horses, famous, age at death, 817 
 Hunger, 618 
 
 Hyaloid membrane, 577, 584, 586 
 Hydrobilirubin, 15, 16 
 Hydrochloric acid, 190, 206, 260 
 Hydrocyanic acid, absorption of, 
 
 291 
 Hydrogen, 121, 373 
 
 excretion of, 358 
 Hydrogen-ion concentration, 2 
 Hydrolysis, 263, 264, 373, 393, 826, 
 
 827, 833 
 Hydrolytic action, 827 
 Hydroxy-amino-acids, 827 
 Hyperglyceemia, 257 
 Hypermetropia, 596, 597 
 Hyperpncea, 134 
 Hypertonic solutions, 280, 824 
 Hypoblast, 763, 764 
 Hypogastric plexus, 235 
 Hypoglossal nerve, 160, 166, 549, 
 550, 552, 613 
 
INDEX OF SUBJECTS 
 
 863 
 
 Hypotonic solutions, 280, 824 
 Hypoxanthine, 329, 450, 451, 792 
 
 Ileum, function of, 220 
 
 Iliac arteries, thrombosis of, 100 
 
 444 
 Impaction, 245, 796 
 Impregnation, 761. See Ovum 
 In-breeding. See Breeding 
 Income of body, 358 
 Inco-ordinate movements, 476, 516, 
 
 630 
 Incus, 624 
 
 Indeciduate animals, 768 
 Indican, 344, 354 
 Indol, 220, 228, 239, 264, 344 
 Indoxyl, 344 
 Inflammation, 77 
 Infundibulin, 300, 305 
 Inhibition, 60, 483, 488, 489, 490, 
 
 .493 
 
 intraspinal, 483 
 nature of, 60 
 Innervation, antagonistic, 434, 435, 
 633 
 identical, 490, 634 
 reciprocal, 152, 436, 481, 489, 
 5°5, 633 
 Inorganic substances in body, 382, 
 
 823 
 Inositol, 835 
 
 Insemination, artificial, 741 
 Insensible perspiration, 318, 411 
 Inspiration, 107, hi, 118, 138 
 first cause of, 142 
 muse es of, 113 
 See Respiration 
 Inspiratory centre, 137, 138, 139, 
 
 448, 510 
 Instinct and reason, 546 
 Intelligence, 546 
 
 Internal ear, 515, 517, 549, 617, 625 
 respiration, 131 
 secretions, 256, 269, 300. See 
 
 Hormones 
 sensations, 614 
 Interoceptive field, 454, 479 
 Intersystolic period, 41 
 Intestinal absorption, 220, 291, 299 
 calculus, horse, 239, 383, 384, 
 
 401, 565, 823 
 canal, nervous mechanism of, 
 
 234. 562, 564 
 digestion, 218. See Digestion 
 horse, 219 
 other animals, 230 
 ruminants, 229 
 Intestines, gases of, 227, 228, 236, 
 237, 358, 821 
 large, 221, 222, 228, 231, 299 
 digestive changes in, 227 
 
 Intestines, movements of, 231 
 
 sensibility of, 237, 564 
 
 small, 219 
 
 pendular movements of, 
 232 
 
 See Digestion 
 Intracardiac pressures, 43, 46 
 Intracellular enzymes, 369 
 Intra-ocular pressure, 589 
 Intrapulmonic pressure, 114 
 Intrathoracic pressure, 114 
 Intussusception, 234, 244 
 Invert sugar, 218, 296, 834 
 Invertase, 218, 296, 836 
 Inverting ferments, 218 
 Involuntary muscle. See Muscle, 
 
 pale 
 Iodine, 303, 384, 386 
 Iodothyrin, 303 
 Ionic action, 279 
 Ions, hydrogen, 1, 445 
 
 hydroxyl, 1 
 
 theory of, 278 
 Iris, 510, 562, 578, 580, 585, 813 
 Iron, 15, 247, 248, 384, 386, 404, 
 
 780, 793 
 Irradiation, 478 
 Irritability, 425 
 
 muscle, 425, 443 
 
 nerve, 459 
 Islands of Langerhans, 270 
 Isotonic solutions, 280, 824 
 
 Jacobson, organ of, 608 
 Jaundice, 271 
 Joints, 631, 650 
 
 compression of, 663 
 
 elastic, 653, 654, 656, 662, 702 
 
 elbow, 638, 651, 652 
 
 fetlock, 640, 653 
 
 hip, 641, 656 
 
 hock, 644, 646, 648, 658, 664 
 
 knee, 638, 653 
 
 lameness, 661, 663 
 
 -locking, 641, 655, 656, 663 
 
 movements, 631 et seq. 
 
 pastern, 640, 655 
 
 pedal, 656, 702 
 
 reciprocating action, 648 
 
 rotation, 632 
 
 sesamoid bones, 634, 654, 656 
 
 shoulder, 637, 650 
 
 sprains of, 663 
 
 stifle, 641, 642, 648, 657 
 
 yielding, 653, 654, 656, 662, 702 
 Jump of horse, 682 
 Jumping, judgment in, 617, 682 
 
 vision in, 589, 602 
 
 Katabolic nerves, 60, 563 
 Katabolism, 360, 373 
 
864 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Katabolism, products of, 368-387 
 
 Kathelectrotonus, 460 
 
 Kephalin, 832 
 
 Kephir, 792, 834 
 
 Kerasin, 832 
 
 Keratin, 717, 721, 773, 826, 828 
 
 Ketonic acid, 370 
 
 Kicking horse, 686 
 
 Kidney insensibility, 564 
 
 structure of, 326 
 
 vascular mechanism of, 326, 
 
 33° 
 See Urine 
 Kilogramme calorie, xv, 374, 419, 
 
 438 
 Kilogrammeter, xv, 691 
 Kinsesthetic sense, 616 
 Kinases, 22, 262 
 Knee-jerk, 488 
 
 -joint, 638, 653 
 Korean pony, 697 
 Koumiss, 792, 834 
 Krause's end-bulbs, 458 
 Kresol, 344 
 Kresyl, 344 
 
 Labyrinth, 436, 451, 491, 492, 625, 
 
 627 629, 630 
 Lachrymal gland, 570 
 Lactalbumin, 791, 792 
 Lactase, 218, 296 
 Lacteals, absorption of, 291, 292, 
 298 
 structure of, 286 
 See Absorption, Chyle, and 
 Thoracic duct 
 Lactic acid, 136, 140, 146, 190, 196, 
 199, 202, 206, 239, 339, 373, 387, 
 
 4°3. 444. 445. 45°. 54^, 773. 
 
 792 
 Lactoglobulin, 791, 792 
 Lactose, 218, 255, 296, 789, 791, 
 
 792 794, 796, 834 
 Laevulose, 218, 255, 296, 780, 791, 
 
 835 
 
 Lamarckian doctrine, 797 
 
 Lameness, causes of, 661, 734. 
 chief seat of, 664 
 expensive in energy, 448 
 pain not necessarily a cause, 
 
 may be reflex, 528 
 specific, 663 
 thrombosis, 100, 444 
 
 Lamina spiralis, 626 
 
 Laminal tissue. See Foot 
 
 Laminitis, 364, 661, 725 
 
 Langerhans, islands of, 270 
 
 Lanolin, 323, 831 
 
 Lapping, 160 
 
 Large intestines, 221. See Diges- 
 tion and Intestines 
 
 Laryngeal nerves, inferior. See 
 Larynx and Roaring 
 superior, 138, 140, 152, 
 
 165, 556 
 Laryngitis, 157 
 Larynx, 149 
 
 cartilages of, 151, 153 
 epiglottis, 151 
 
 during swallowing, 151, 
 164 
 glottis, 118, 148, 149, 151. 153 
 
 closure of, 151 
 muscles of, 149, 531 
 
 paralysis of, 152, 557 
 phonatory, 150 
 respiratory, 150 
 nervous mechanism of, 152,510, 
 
 53i. 555 
 and Lathy rus saHvus, 
 
 557 
 and roaring, .152, 156, 
 
 455, 556 
 phonation, 153, 510 
 use of, 149 
 ventricles, 153 
 vocal cords, 149, 153 
 See Vocal cords 
 Lateral cartilages (foot), 701, 704, 
 
 724, 726, 727, 728, 734 
 Lathyrus sativus, 557 
 Lecithin, 7, 250, 275, 375, 384, 458, 
 
 543,824,830, 831 
 Leech extract in coagulation, 24 
 ' Left-handed ' horse, 316 
 Leg nucleus, 501, 508 
 Lemniscus, 508, 525 
 Lens, 577, 578, 584, 586, 588, 590 
 suspensory ligament of, 5S4, 
 585, 586, 588 
 Lenses, passage of light through, 
 
 59° 
 Leucine, 239, 257, 263, 337, 826, 
 
 827 
 Leucocytes, 16, 168, 275, 293, 754, 
 
 794 
 Leucocytosis, 29, 768 
 Leuconuclein, 23 
 Leucopsenia, 29 
 Levers, 636 
 
 Leydig, cells of, 740, 751 
 Lieberkiihn's follicles, 218 
 Life, average duration of, 818 
 Ligaments, check, 635, 640, 662, 663 
 elasticity of, 635, 653, 654, 662, 
 
 821 
 sprain of, 446, 655, 662, 663 
 suspensory, 640, 653, 654, 655, 
 669 
 effect of dividing, 655 
 (lens), 584, 585, 586, 588 
 function of, 653, 654, 669 
 
INDEX OF SUBJECTS 
 
 Ligaments, tendo-, 635, 641, 658, 660 
 
 uses of, 446, 634 
 Ligamentum nuchae, 654, 821 
 function of, 821 
 
 pectinatum, 579, 585 
 
 teres, 656 
 Light, 566 
 Lignin, 373 
 Limb markings, 316 
 
 velocities, 666 
 Limbs, functions of, in relation to 
 
 lameness, 661 
 Lingual nerve, 160, 166 549 550 
 
 552. 613 
 .Lipase, 194, 261, 264, 265, 294, 375, 
 
 Lipoids, 375, 384, 458, 607, 740, 830 
 Lips, 158 
 Liquor amnii, 773 
 
 sanguinis, 3 
 Liver, 246, 564 
 
 bile, acids, 249 
 
 amount of, 251 
 characters of, 246 
 cholohaematin in, 248 
 composition of, 247 
 furfurol in, 250 
 gall-stones, 248 
 pigments, 247, 248 
 
 and stercobilin, 249 
 salts, 249 
 
 in various animals, 
 
 249 
 tests for, 249 
 secretion of, 250 
 
 influence of secretin 
 on, 250 
 test for, 249 
 use of, 251 
 bilirubin and biliverdin, 247, 
 
 248 
 blood-supply, 246 
 cholalic acid, 249 
 cholesterin in, 250 
 diseases of, 271 
 fat depot, 258, 379 
 ferment, 256 
 foetal, 777, 780 
 gall-bladder, function of, 250 
 
 .animals without, 251 
 glycine in, 249 
 glycocholate soda, 249 
 glycogen. See Glycogen 
 lecithin, internal secretion of, 
 
 250, 3°°. 3°7 
 protein poison neutraliser, 257 
 summary of uses, 257 
 taurin in, 249 
 taurocholatesoda, 249 
 urea formed in, 257, 307, 337 
 See also Bile 
 
 865 
 
 Living test-tube experiment, 23 
 Llama, rumination in, 212 
 Loads, drawn, 692, 694, 695 696 
 
 697 
 Locomotion, 665 
 
 amble, 673 
 
 buck-jumping, 689 
 
 canter, 674, 698 
 
 and gallop compared, 681 
 
 concussion, strain of, 662 
 
 extensor thrust, 481, 482, 483 
 
 fascia, use of, 424 
 
 foot, contact with ground, 672, 
 
 673 
 -prints, 681 
 gallop, 491, 677 
 pace, 698 
 
 stride, 698. See Gallop 
 jumping, 589, 602, 617, 682 
 kicking, 686 
 leading leg, 675, 677 
 limb combinations in, 490, 665 
 666 
 movements, 491, 631 
 velocities, 666, 669 
 muscles. See Locomotor 
 
 system 
 pacing, 674 
 
 propulsion, strain of, 662 
 racking, 674 
 rearing 686 
 reflex act, 476, 477, 481, 482, 
 
 491 
 speed, 698. See Speed 
 step and stride, 665, 677 
 stumbling, 670, 673 
 trot, 671 
 
 endurance, 698 
 pace, 671, 672, 698 
 stride, 672, 698 
 walk, 666, 669. See Trot 
 pace, 669, 698 
 stride, 670, 671, 698 
 Locomotor system, 631 
 abduction, 631 
 adduction, 631 
 attitude. See Muscle, 
 posture 
 factors essential to, 
 
 636 
 lying down, 649 
 rising, 649 
 
 standing, 417, 435, 
 491, 636, 648, 
 
 654 
 stress of, 395,661, 
 
 7°3 
 ' brushing,' 632, 652 
 concussion reduced, 650, 
 654, 656, 662, 663, 700, 
 
 701 
 
 55 
 
866 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Locomotor system, developed at 
 birth, 811, 812 
 draught. See Draught 
 endurance trials, 698 
 equilibrium, 517, 518, 541 
 extension. See Muscular 
 
 contraction 
 gravity, centre of, 649 
 impact, strain of, 662, 663 
 joint-locking, 641, 656, 663 
 reciprocating action, 
 648 
 joints. See Joints 
 lameness and function, 
 
 661 . See Lameness 
 levers, 636 
 
 ligaments . See Ligaments 
 ligamentum nuchae, 654 
 
 function of, 821 
 limb combinations, 490, 
 
 665, 666 
 limbs, fore, suffer most, 
 662 
 longer on ground than 
 
 off, 669 
 move faster than 
 
 horse, 666 
 movements, pendular, 
 491.631 
 muscles, 631. See Muscles 
 and Muscular con- 
 traction 
 antagonism of, 434, 
 
 435, 448, 482, 488, 
 489, 505, 633, 634, 
 638 
 
 breaking strain, 635, 
 
 662 
 co-operation of, 634, 
 
 635.637.638 
 elasticity of, 434, 635 
 flexion and extension, 
 
 436, 482, 491, 492, 
 632 
 
 function of, 636 
 
 may contract from 
 either end, 634 
 
 producing attitude, 
 435. 436, 492, 636 
 
 provision against 
 
 fatigue, 436, 640 
 
 rupture of, 446, 635, 
 662 
 
 strain, effect on ten- 
 don, 446, 640, 655 
 
 uniting limb to trunk, 
 
 637 
 pace which kills, 699 
 pastern, fracture of, 663 
 propulsion, strain of, 662 
 ' speedy cutting,' 652 
 
 Locomotor system, sprains, cause 
 of, 446, 655, 662 
 stamina and body height, 
 
 699 
 tendo-ligaments. See Liga- 
 ments 
 tendons, 635 
 
 elasticity, of, 635, 654, 
 
 662, 821 
 extensor, 638, 641 , 662 
 not sprained, 662 
 flexor, 640, 641 
 
 division of, 655 
 sprain of, 446, 
 640, 655, 662, 
 663 
 See Tendons 
 weight, back muscles must 
 be conditioned, 684, 
 694, 698 
 carried by horses, 697, 
 698 
 by men, 697 
 by ponies, 697 
 carrying, and muscles 
 of back, 693, 697, 
 698 
 effective, a horse can 
 
 carry, 698 
 of vehicles, 696 
 on the limbs, 648 
 stamina weakens with 
 increase in body 
 height, 699 
 work, amount to be ex- 
 pected from 
 horses, 691, 692 
 in the past, 690, 
 
 697 
 back muscles, con- 
 dition of, for, 684, 
 
 694, 698 
 comparison between 
 
 man and horse, 695, 
 
 697, 817 
 ' condition,' 147, 447, 
 
 448, 449, 698, 699 
 draught, theory of, 
 
 694. See Draught 
 effort to start load, 
 
 694. See Draught 
 fatigue, 446. See 
 
 Muscle, Fatigue 
 foot-tons of, 438, 691 
 calculation of, 
 692 
 force required to move 
 
 animal's body, 441 , 
 
 692, 694 
 force required to move 
 
 given load, 691, 692 
 
INDEX OF SUBJECTS 
 
 867 
 
 Locomotor system, work, height 
 
 and weight, 694, 699 
 
 horse-power, 438, 690 
 
 influence of age on 
 
 817, 818 
 load and body weight, 
 
 694. 695, 698 
 maximum muscular 
 
 effort, 695 
 single and pairs, 696 
 spirit and determina- 
 tion, 694, 695, 699 
 ' staleness,' 447 
 staying power, 438 
 (footnote), 697, 800 
 strength of horses, 694 
 training, 447 
 uphill, 440, 692, 694 
 wear and tear, 441, 
 661, 690, 694, 817 
 Lungs, 107 
 
 diseases of, 156 
 gaseous exchange in, 129 
 ventilation of, 122, 143. See 
 Respiration 
 Lutein, 743 
 
 Luxus consumption, 371 
 Lying down, 649 
 Lymph, 272 
 
 capillary, 272 
 
 circulation, muscle and tendon, 
 
 283 
 enters vascular system, 283 
 formation of, 276 
 
 dialysis, 277, 278 
 diffusion, 277, 278 
 filtration, 277 
 in resting limb, 282 
 osmosis, 277, 278, 279, 754 
 osmotic pressure, 277, 279, 
 
 280, 824 
 physical theory, 277 
 secretory theory, 282 
 haemolymph glands, 285 
 in eye, 585 
 
 movement of, 283, 284 
 nature of, 272, 275 
 oedema, 285 
 oxygen in, 822 
 plasma, 275 
 
 post-mortem flow of, 281 
 quantity of, 275 
 secretory theory, 282 
 sinus, 274, 288 
 source of milk, 789 
 spaces, 272 
 theories of, 277 
 thoracic duct, 273, 283, 284 
 velocity of, 284 
 Lymphagogues, 282 
 Lymphangitis, 400 
 
 Lymphatic glands, 273, 
 vessels, 273 
 
 function of, 272 
 Lymphatics, perivascular, 272 
 Lymphocytes, 17 
 Lysine, 263, 337, 368, 376, 827 
 
 Macula acustica, 629 
 Magnesium, 823, 824 
 
 in muscle, 451 
 
 in nutrition, 383 
 
 in urine, 345, 347 
 Maintenance ration, 394, 398, 417, 
 
 418 
 Male pronucleus, 761 
 Malic acid, 239, 387, 824 
 Malleus, 624 
 
 Malpighian tufts, 326, 331 
 Maltase, 218, 264, 296, 834, 836 
 Maltose, 171, 218, 254, 255, 264, 
 
 296, 833, 834. Sec Sugar 
 Mammary glands. See Milk 
 
 secretion. See Milk 
 Manganese, 404 
 Manometer, 68, 72, 73, 75 
 Man and horse, underfeeding of, 393 
 Mare, breaking service, 775 
 
 duration of pregnancy, 784 
 
 milk of, 791 
 
 oestrous cycle, 748 
 
 twin births, 756, 782 
 
 urine of, 350 
 Marsh gas, 121, 202, 208, 227, 237, 
 
 358, 373 
 Masseter muscle, 98, 162, 407, 445, 
 
 54i 
 Mast cells, 17 
 Mastication, 161, 510, 554 
 Maternal nutrition (foetus), 779 
 Maturity, 307, 756, 813, 816 
 ' Mealy ' bays, 314 
 Mean arterial pressure, 72 
 
 pressure, 69 
 Mechanical daily work, 691 
 Meconium, 241, 773, 780 
 Medulla oblongata, 507 
 
 centres in, cardiac, 58, 59, 60, 
 62, 510 
 diabetic, 268, 510 
 mastication, 510 
 respiratory, 137, 138, 139, 
 
 448, 510 
 salivary, 510 
 swallowing, 165, 510 
 sweat, 321, 510 
 vasomotor, 87, 88, 92 
 vomiting, 209, 510 
 function of, 510 
 paths in, 508 
 structure of, 507 
 Medullary sheath, 456,471, 521, 536 
 
A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Melanin, 314 
 
 Membrana basilaris, 626, 627 
 granulosa, 743 
 nictitans, 569 
 tectoria, 626 
 
 tympani (tympanum), 624, 
 627 
 Membrane of Reissner, 626, 627 
 Mendelism, 800, 809 
 Menstruation, 749, 750, 765 
 Mercurial air pump, 124 
 Mesentery, 219 
 Mesoblast, 764 
 
 Metabolism, 303, 360. See Nutrition 
 foetal nutrition, 779 
 light, influence on, 364 
 nervous system, influence on, 
 
 360 
 storage of tissue, 379 
 tables, 380-381 
 Metaprotein, 826 
 Methaemoglobin, 14, 29 
 Methane, 373. See Marsh Gas 
 Metcestrum, 747 
 Micrococcus urem, 335 
 Micturition, 354 
 
 position during, 355 
 Mid-brain, 509, 586 
 
 cerebral peduncles, 509, 510, 
 
 515, 522, 527 
 corpora quadrigemina (colli- 
 culi), 491, 510, 582 
 division of, 491 
 function of, 510 
 Middle ear, 624 
 Milk, 788 
 
 average secretion, 794 
 body loss due to, 793, 794 
 clotting, 196, 792 
 colostrum, 791, 794 
 composition of various animals, 
 
 791 
 fats in, 264, 375, 769, 791, 792, 
 
 794, 796, 830 
 fermentation of, 196, 792, 834 
 galactagogues, 306, 795 
 mammary glands, 788 
 
 changes in cells of, 
 
 789 
 f oetus, hormone, effect 
 
 on, 307, 790 
 ovary, effect on, 307, 
 
 789, 790 
 protein, effect on, 795 
 proteins of, 791 , 794, 796 
 salts of, 789, 791, 793, 794 
 secretion, 789 
 
 atropine, effect of, 796 
 average, 794 
 duration of, 794, 796 
 non-pregnant animals, 790 
 
 Milk secretion, ovariotomy, effect 
 on, 307, 746, 794, 796 
 stimulus to, 790, 795, 796 
 theories of, 789 
 sugar, 218, 296, 789, 791, 792, 
 794, 796, 834. See Lactose 
 uterine, 768, 769 
 yield, 795 
 
 under voluntary control, 
 610, 788, 790 
 Millon's reaction, 829 
 Mind, influence on digestion, 217 
 Mitosis, 753 
 
 Mitral valves, 31, 36, 38, 45 
 Moderator bands, 34, 37 
 Molars, 159, 815 
 Monakow, bundle of, 515 
 Mon-amino acids, 263, 827. See 
 
 Amino acids 
 Monkey, dicestrous cycles, 749 
 Monocular vision, 587, 597, 601 
 Moncestrous animals, 747 
 Monosaccharides, 296 
 Monosaccharoses, 832, 834 
 Moore's test, 835 
 Morphia, absorption of, 289 
 
 effect on eye, 579 
 Moss fibres, 514 
 Mosso's ergograph, 439 
 Motor area, cerebrum, 528, 529, 534 
 areas, herbivora, 531 
 oculi nerves, 517, 518, 549, 550, 
 
 552, 553. 562, 573, 580, 581 
 paralysis, 528 
 Motorial sense, 616. See also 
 
 Muscle sense 
 Mouth, digestion in, 158 
 
 movements of, 162 
 Movements of the intestines, 231 
 Mucigen, 177 
 Mucin, 168, 181, 189, 192, 220, 268, 
 
 287, 350, 586, 826 
 Mule, chestnut of, 324 
 lactating, 790 
 obstinacy of, 543 
 sweating of, 318 
 Multiple birth, 756, 782 
 Murmur, respiratory, 147 
 Muscarine, 59, 831 
 Muscle, 422, 631 
 
 absolute power of, 438 
 ammonium salts in, 336 
 antagonism of; 112, 434, 435, 
 448, 482, 488, 489, 505, 
 
 633, 634. 638 
 partial, 634 
 atrophy, 463, 473 
 attachment, 424 
 bladder, 355, 437 
 blood-supply of, 47, 49, 443 . 
 body weight and, 422 
 
INDEX OF SUBJECTS 
 
 869 
 
 Muscle, breaking strain of, 635, 662 
 carnine, 450 
 changes in active and resting, 
 
 443 
 chemical composition of, .358, 
 
 449 
 engine, 439 
 colour, 423 
 'condition,' 147, 447, 448,698, 
 
 699 
 contractility, 425, 462 
 contraction, cause of, 425. See 
 Muscular contraction 
 and antagonistic innerva- 
 tion, 434, 435, 633 
 and identical innervation, 
 
 49°, 634 
 and reciprocal innervation, 
 112, 152, 423, 436, 448, 
 477, 481, 489, 505, 633, 
 648 
 and reflex inhibition, 436, 
 488, 490 
 co-operation of, 634, 635, 637, 
 
 638 
 creatine in, 338, 450, 451 
 creatinine in, 338 
 current of action, 442 
 
 rest, 441 
 currents, 441 
 
 diphasic variation, 442 
 negative variation, 441 
 of injury, 441 
 curve, 428 
 
 diaphragm, rupture of, 446 
 diphasic variation, 442 
 education, 448, 698 
 elastic tension, 662 
 elasticity of, 434, 635 
 electric phenomena, 441, 459. 
 
 462, 463 
 energy liberated by, 162, 440, 
 
 817 
 extensibility, 434 
 extractives, 450 
 fascia, use of, 424 
 fatigue, 429, 436, 438, 441, 445. 
 446, 447, 602, 640, 655, 
 662, 694 
 effect on tendons, 446, 655, 
 
 662 
 when absent, 436 
 fibres, 422 
 
 fibrillar contraction, 50, 434 
 fibrils, 422 
 
 fibrous strengthening of, 639 
 function of, 422, 636 
 gases of, 443, 45 1 . 822 
 glycogen. See Glycogen 
 heart, 33, 425, 430. See also 
 Heart 
 
 Muscle, heat in, 407, 417, 438, 445 
 rigor, 430, 444, 451 
 hypoxanthine, 450, 451 
 injury currents, 441 
 insects, 432 
 
 involuntary. See Muscle, pale 
 irritability, 425, 443 
 iso-electric, 459 
 lactic acid in, 336, 444, 445, 
 
 451. See Muscle, Sarco-lac- 
 
 tic acid 
 law of contraction, 462 
 lengthening reaction of, 436, 
 
 437. 492 
 levers, 636 
 
 locomotion a reflex act, 476, 
 477,491. See also Locomotor 
 system 
 magnesium salts in, 451 
 mastication, 161, 510, 554 
 myosin, 450 
 myosinogen, 450 
 nerve-muscle preparation, 426, 
 
 442 
 nerves, afferent, 423 
 
 degeneration, test for, 463 
 efferent, 423 
 
 inhibitory, unknown, 489 
 motor, 423 
 
 supply, 423, 425, 446, 489, 
 616 
 antagonistic inner- 
 vation, 434, 435, 
 
 633 
 end-plates in, 423, 
 
 446, 458, 523 
 identical innervation, 
 
 490, 634 
 inhibition of innerva- 
 tion, 488, 489, 490 
 reciprocal innerva- 
 tion, 112, 152, 423, 
 436, 447. 448, 481, 
 489, 505, 633, 648 
 reflexadjustments,436 
 inhibition, 423, 
 477, 488, 490 
 sensory, 423, 446, 489, 
 
 524, 616 
 spinal centres, 477, 
 
 491 
 spindles, 423, 616 
 neurones, 446. See Nerves 
 oxygen, 443, 822 
 
 storage of, 443 
 pale, 422, 424, 433, 451. 453. 
 
 58i 
 contraction of, 433, 451 
 nerves of, 425, 433, 5 6z 
 structure of, 424 
 plasma, 450 
 
870 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Muscle pose, 435 
 
 post-inhibitory rebound, 490 
 posture, 97, 407, 435, 436, 491, 
 492, 517. 541. 633, 636, 
 641, 648, 654, 659, 661 
 reflex character of, 435, 
 491 
 potassium salts in, 451 
 quality of, 438 
 reciprocal innervation. See 
 
 Muscle, Nerves 
 reflex excitation, 490 
 
 inhibition, 423, 477, 482, 
 488, 490 
 relaxation, 429, 433 
 repose, action during, 633, 636, 
 
 649 
 respiration in, 133 
 respiratory, 113 
 rigor mortis, 444, 451, 630, 821 
 Hitter's tetanus, 463 
 rupture, 446, 635, 636, 662 
 salts of, 451 
 sarco-lactic acid, 140, 146, 444, 
 
 445. 45°. 54 2 
 sarcolemma, 422 
 sarcomere, 422 
 sarcoplasm, 422, 436 
 sarcostyle, 33, 422, 436 
 sarcous elements, 422 
 secondary contraction, 442 
 sense, 423, 446, 491, 504, 517, 
 528, 554, 616, 617, 629 
 centre; 517 
 shortening reaction, 436, 437, 
 
 492 
 smooth, contraction of, 433, 
 
 451. See Pale 
 sodium salts in, 451 
 standing, act of, 435, 436, 491, 
 
 492, 510, 636, 648, 654, 661, 
 
 703. See Posture 
 staying power, 438 (footnote) 
 striped, 422 
 structure of, 422, 424 
 sugar, 254, 267, 444. See Sugar 
 summation, 430, 433 
 synapse, 446 
 taurine, 450 
 
 tendinous strengthening of , 424 
 tetanus, 53, 430, 433 
 thrombosis, effect of, 100, 444 
 tone, 58, 89, 448, 492, 517, 541, 
 
 635. 636 
 tonus, 407, 434, 435, 437, 438, 
 451. 489, 553. 630, 633, 
 636, 821, 824 
 latitude of pose, 436 
 lengthening and shortening 
 
 reactions, 437 
 unfatiguable, 436 
 
 Muscle, training, 447 
 
 unpractised movements, 448 
 
 urea, 336, 444, 450 
 
 uric acid, 450 
 
 • veratrine curve, 431 
 
 voluntary, 422 
 
 water in, 391 
 
 wave, 433 
 
 work, 438, 440, 448, 690 
 effect of age on, 817 
 
 xanthine, 450 
 Muscular contraction, 425, 433, 436 
 all or none, 53, 437 
 at death, 820 
 blood-supply during, 443 
 beneficial effects of, 429 
 carbohydrates, source of, 
 
 374. 444 
 cause of, 425, 444, 445 
 changes during, 425 
 contraction period, 429, 
 
 433 
 co-ordination of, 448, 476, 
 
 481, 482, 489, 516, 518, 
 617, 623, 633 
 
 curve, 428 
 
 decerebrated rigidity, 435, 
 436, 491 
 unfatiguable, 436 
 effect, heat on, 430 
 
 induction shocks, 53, 
 
 428 
 weight increase, 429 
 ergograph, 439 
 extensors and flexors, 435, 
 436, 481, 482, 488, 491, 
 492, 632, 633, 634, 641. 
 See also Locomotor sys- 
 tem, Muscles, Flexion, 
 and Extension 
 fatigue, 429, 436, 438, 441, 
 445, 446, 447, 602, 640, 
 655, 662, 694 
 fats, source of, 375, 444 
 fibrillar, 50, 62, 434 
 heart and skeletal com- 
 pared, 53, 422 
 heat production, 407, 438, 
 
 439, 445 
 rigor, 430, 444, 451 
 inco-ordination of, 476, 
 
 516, 630 
 latent period, 433, 442, 
 
 . 477 
 limb muscles in action, 
 
 482, 632, 633 
 maximal stimulus, 429 
 metabolism of, 436 
 myograph, 428 
 
 rate of, 433 
 
 relaxation period, 429, 433 
 
INDEX OF SUBJECTS 
 
 871 
 
 Muscular contraction, staircase trac- 
 ing, 429 
 smooth muscle, 433, 451. 
 
 See Pale 
 time of, 429, 433 
 veratrine-poisoning, 430 
 wave, 433, 434 
 strain, 424, 446, 640 
 system, 422 
 Musculi papillares, 34, 35, 36, 38, 
 
 40, 41 
 Mutation (Mendel), 804 
 
 theory of de Vries, 798 
 Mydriasis, 579 
 Mydriatics, 579 
 Myelin, 471, 521, 536 
 Myogenic theory (heart), 51 
 Myograph, 428 
 Myopia, 596, 597 
 Myosin, 450 
 Myosinogen, 450 
 Myosis, 579 
 Myotics, 579 
 Myxoedema, 302 
 
 Navicular bone, 656, 701, 702 
 
 bursa, 702 
 
 disease, 648, 664, 703, 805 
 Neigh of horse, 154, 155, 745 
 Nerves, afferent, 423, 452, 524, 558, 
 
 564 
 anelectrotonus, 460 
 antidromic, 456 
 arborisation, 469 
 association tracts, 480, 522 
 autonomic, 558 
 axis cylinder, 456, 457, 470, 
 
 47L 472 
 axon. See under Cell 
 cell, 466 
 
 axon, 467, 469, 470, 472, 
 
 47 8 , 497. 5°4. 513. 52°. 
 
 559. 586 
 basket, 513 
 Betz, 520, 538 
 bipolar, 467, 468 
 body, 466, 478, 751 
 cerebellar, 513 
 cerebral, 520, 536 
 collaterals, 469, 505, 523 
 conduction, path of, 470 
 dendrites, 468, 469, 470, 
 
 47 8 , 513,520, 559 
 degeneration of, 470, 471 
 division of , 470, 753, 755, 
 
 761 
 fatigue, 465, 467 
 Golgi, 468, 513, 520 
 multipolar, 467, 468, 559 
 Nissl's granules, 466 
 nucleus, 466, 471, 753, 761 
 
 Nerves, cell, nutritive function, 470, 
 47i 
 paralysis, 176, 561 
 perikaryon, 466, 478 
 polymorphous, 520 
 processes, 457, 468, 469 
 Purkinje, 513 
 pyramidal, 520, 522, 586 
 regeneration of, 470, 471 
 station, 469, 560, 561, 562, 
 
 563 
 
 synapses, 446, 465, 469, 
 
 523, 524. 586 
 chemistry, 458 
 classification of, 452 
 conducting path, 470 
 conduction in, 455 
 neuronic, 479 
 synaptic, 479 
 conductivity, 459 
 constrictor. See Nerves, vaso- 
 motor, and Vasomotor nerves 
 cranial. See Cranial nerves 
 current of action, 460 
 degeneration, 470, 472 
 dendrites. See Nerves, Cells 
 depressor, 60, 89, 90, 94, 269 
 dilator. See Nerves, vaso- 
 motor, and Vaso-dilator 
 nerves 
 diphasic variation, 463 
 disynaptic arc, 485 
 division of, 470 
 
 efferent, 423, 4-52, 453, 522, 558 
 elasticity, 458 
 
 electric phenomena, 459, 463 
 current of injury, 459 
 currents, 460 
 diphasic variation, 463 
 electrotonus, 460, 463 
 negative variation, 
 460, 463 
 electrotonic currents, 460, 463 
 exteroceptive, 454, 479, 517 
 fatigue, 465, 467 
 fibre, chemistry of, 458 
 
 structure of, 456 
 Golgi cells, 468, 513, 520 
 impulse, ' all or nothing,' 465 
 chemical changes, 465 
 direction, 455, 470 
 inhibitory, 54, 60, 425, 489 
 narcotics, effect on, 465 
 nature, 279, 455, 464, 474 
 not generated by, 474 
 passage of, 463, 465 
 paths, 478. See Spinal 
 
 cord 
 velocity, 464 
 innervation, antagonistic, 434, 
 435,633 
 
872 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Nerves, innervation, identical, 490, 
 
 634 
 inhibition of, 488, 489, 490 
 reciprocal, 112, 152, 423, 
 436, 448, 477. 481, 489. 
 505.633.648 
 interoceptive, 454, 479 
 intraspinal excitation, 482 
 
 inhibition, 483 
 irritability, 459 
 kathelectrotonus, 460 
 medullary sheath, 456, 471, 
 
 521, 536 
 medullated, 456 
 moss fibres, 514 
 multipolar cells, 467, 468, 559 
 -muscle preparation, 426 
 
 supply to, 423, 425, 446, 
 489, 616 
 myelination of, 521, 536 
 negative variation, 460, 463^ 
 neurone, 446, 469, 505, 561, 
 580, 586 
 doctrine, 465, 469, 478, 
 542 
 Nissl degeneration, 472 
 non-medullated, 456 
 nutrition of, 473 
 path, neuronic, 475, 479, 523, 
 
 524 
 synaptic, 475, 479 
 pilomotor, 317, 562 
 plexus in heart, 56 
 post-inhibitory rebound, 490 
 pressor, 89. See Nerves, vaso- 
 motor 
 proprioceptive, 454, 479, 488, 
 
 49L5I7 
 reaction of degeneration, 463 
 receptive field, 479 
 receptor system, 454, 479 488, 
 
 491,517,628 
 regeneration, 470 
 secretory, 173-176 
 spinal, 470, 471, 494, 495, 502, 
 
 5°4 
 dog, 482. See Spinal 
 nerves 
 structure of, 456 
 suturing, 455, 472 
 sympathetic, 558 
 synapses, 446, 465, 469, 523, 
 
 524, 586 
 taste, 613 
 tendril fibres, 514 
 terminations, 458 
 trophic, 175, 364, 453, 473 
 vasomotor, 87, 88, 92, 562, 563, 
 
 See Vasomotor centre 
 Wallerian degeneration, 471 
 Nervi erigentes, 93, 757 
 
 Nervous system, 452 
 
 association areas, 535 
 system, 522, 525 
 tracts, 4S0, 501 
 autonomic, 558 
 body equilibrium, 517, 518, 
 
 541, 629 
 bulb, 507. See Medulla 
 cerebellum, 513. See 
 
 Cerebellum 
 cerebrum, 519. See Cere- 
 brum 
 common path, 480-481 
 co-ordinate movements, 
 448, 476, 481, 482, 489, 
 516, 518, 617, 623, 633 
 corpus striatunij 512 
 
 body heat and, 
 
 409, 512 
 frequently dis- 
 eased, 512 
 injury to, 512 
 cranial nerves, 549. See 
 
 Cranial nerves 
 developed at birth, 812 
 electrocardiogram, 464 
 fillet, 508, 525 
 function revealed by 
 
 structure, 536 
 ganglia . See Ganglia , 
 Nerves, Spinal cord , 
 Sympathetic system , 
 Cranial nerves 
 impulse paths, 478. See 
 Spinal cord 
 final common, 
 
 480, 481, 485 
 in nerve arc , 
 
 478, 480, 510 
 internuncial, 480 
 private, 480 
 public, 480 
 struggle for, 481, 
 485, 487, 489 
 impulses, origin of, 474. 
 See also Nerves, impulse 
 inco-ordinate movements, 
 
 476, 516, 630 
 influence on heat produc- 
 tion, 408 
 irradiation, 478, 479 
 medulla oblongata, 507. 
 
 See Medulla 
 mid-brain, 509. See Mid- 
 brain 
 Monakow's bundle, 515 
 motor areas, 528, 529, 531, 
 
 534. See Cerebrum 
 muscle sense, 423, 446, 491, 
 504. 517. 528, 554, 616, 
 617, 629 
 
INDEX OF SUBJECTS 
 
 873 
 
 Nervous system, muscle sense, 
 centre 517 
 tone, 89, 407, 
 434. 481, 489, 
 492, 517. 541. 
 633, 636 
 See Muscle, Tone, and 
 Tonus 
 neurone doctrine, 465, 469, 
 
 478, 542 
 nuclei (medulla), 507 
 pontis, 509 
 sensory, 549 
 nucleus, arm, 501, 508 
 cuneatus, 501, 508 
 gracilis, 508 
 leg, 501, 508 
 of Deiters, 500, 509, 
 
 517 
 red, 500,511, 515 
 optic thalamus, 504, 511, 
 
 524, 525, 553. 582 
 pain. See Sensations 
 pathological, 565 
 pons, 509 
 
 position receptors, 628 
 psychical function, 542 
 receptor system, 454, 479, 
 628 
 exteroceptive, 454, 
 
 479, 517 
 interoceptive, 454, 
 
 479 ' 
 proprioceptive, 454, 
 479, 488, 491, 517, 
 628 
 reflex action, 474 
 
 acts, 474, 477, 491, 
 
 492, 5°5- 547, 581 
 arc, 474, 478 479, 
 480, 487, 493, 
 
 5°5 
 disynaptic, 485 
 chestnut, 488 
 excitation, 490 
 extensor, 481, 482, 483 
 falling, 488, 630 
 foot, 485 
 frog, 476 
 head-'hake, 488 • 
 impulses, 478 
 inhibition, 423 477, 
 482, 488, 489 
 function of, 490 
 intraspinal, 483 
 ischial, 488 
 lameness, 528 
 locomotion, 476, 477, 
 
 491 
 peripheral, 493 
 pinna, 488 
 
 Nervous 
 
 system, reflex action, 
 reversal of, 492 
 scratch, 481, 483, 485 
 standing, 491, 510 
 stepping, 481 
 tendon, 488 
 time occupied, 493 
 tonus, 58 
 refractory period, 465 
 restiform body, 509 
 rubro-spinal tract, 515 
 sensory areas, 528, 534 
 silent areas, 535 
 sleep, 546. See Sleep 
 spinal cord. See Spinal 
 
 cord 
 substantia nigra, 509 
 sympathetic. See Sympa- 
 thetic system 
 tegmentum, 509 
 See Reflex act, Sense, 
 and Sensations 
 Neural arch, 764 
 Neurectomy, 364, 471, 473, 554, 
 
 615, 617, 733 
 Neurilemma, 456 
 Neurogenic theory (heart), 51 
 Neuroglia, 497 
 Neuro-keratin, muscular spindles, 
 
 423, 616 
 Neurone doctrine, 465, 469, 478, 
 
 542 
 Neurones, 446, 469, 505, 561, 580, 
 
 586 
 Nicotine, 76, 168, 232, 233, 561 
 Nissl's granules, 466 
 Nitrifying bacteria, 823 
 Nitrogen, 822 
 
 excretion of , 358, 365, 371 
 in blood, 124, 128 
 Nitrogenous bodies, 824 
 
 equilibrium, 365, 371 
 Nodal points (eye), 593, 595 
 
 tissue in heart, 33 
 Noise, 620 
 
 Non-nitrogenous bodies, 824, 
 food, 372 
 
 function of, 376 
 Normal temperature of animals, 405 
 Nostril, false, 117 
 Nostrils, 116 
 
 Notation of paces of horse, 669 
 Notochord, 764 
 Nuclease, 340 
 Nucleic acid, 370, 740, 826 
 Nuclein, 339 
 Nucleo-albumin, 268 
 
 protein, 6, 17, 23, 339. 3 6 9, 37°, 
 384, 780, 824, 825, 826 
 Nucleus of cell, 466, 471, 752, 753, 
 754, 761, 762 
 
 830 
 
8 7 4 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Nucleus, Deiter's, 500, 509, 517 
 red, 500, 511, 515 
 segmentation, 761 
 sensory, 549 
 
 Nutrient enemata, 229 
 
 Nutrition, 357 
 
 albuminoids, nature of , 371,826 
 amino-acids, 368, 369, 376. See 
 
 Amino acids 
 anabolism, 60, 360 
 ash metabolism, 382, 386 
 balance experiments, 397 
 ' building stones,' 369, 372, 828 
 carbohydrates, 357, 364, 367, 
 372, 373, 792, 832 
 energy-producing, 374, 375, 
 
 395. 444 
 ferments, 832, 833, 834. 
 
 See Ferments 
 metabolism of, 372, 375, 
 780 
 organic acids from, 
 
 373. 387 
 oxidation of , 120, 373, 404 
 respiratory quotient, 120 
 See Carbohydrates and 
 Glycogen 
 carbon equilibrium, 371 
 
 excretion, 358 
 composition of body, 357 
 deficiency diseases, 388, 390, 
 
 391, 793 (footnote) 
 diseases of, 383, 400. Seeabove, 
 
 deficiency diseases 
 embryonic, 767, 769, 770, 772, 
 
 775, 779 
 empty weight, 358 
 energy, chemical, 395 
 
 metabolised, 396 
 
 values, 394, 400 
 enzymes . See Ferments 
 fat, composition of, 357, 830 
 
 depots, 373, 378, 379 
 
 differences in animals, 375, 
 
 831 
 digestion of, 375, 376, 396 
 energy from, 375 
 ferment, 261,264, 294, 375. 
 
 See Ferments 
 metabolism of, 375 
 oxidation of, 375, 404 
 source of, 367, 375, 379, 832 
 utilisation of stored, 376, 
 
 379 
 fattening castration, effect 
 on, 379, 745 
 of animals, 370, 378, 
 379, 380, 394, 399. 
 See Fat 
 food, essential constituents of, 
 364 
 
 Nutrition, food substances, amount 
 
 required, 370, 398 
 
 energy from, 370, 371, 
 
 373, 374, 394, 
 
 395, 396, 4°°. 
 
 448 
 
 loss of, 395, 396, 
 
 440 
 
 function of protein, 
 
 371 
 of carbohydrate 
 
 and fat, 376 
 ferments, 832-836. See 
 
 Ferments 
 oxidation of, 375, 395, 
 402 
 glycogen, 367, 372. See Glyco- 
 gen 
 growth, 376. See Growth 
 heat units, 394, 395, 419 
 hydrogen excretion, 358 
 income and expenditure, 358, 
 
 37i 
 iodine, 303, 384, 386 
 iron, 384, 386 
 katabolism, 59, 360, 373 
 metabolism, 360 
 
 light, effect on, 364 
 nervous system, effect on, 
 
 360 
 of carbohydrates, 372. See 
 
 Carbohydrates 
 of fat, 375 
 of protein. See Nutrition, 
 
 protein 
 vitamines, effect on, 389 
 younggrowinganimals, 811 
 methods of inquiry, 359, 394, 
 
 398 
 nitrogen excretion, 358, 365, 
 
 371 
 income, 358, 371 
 nitrogenous equilibrium, 365, 
 366, 367, 371 
 on protein diet, 366, 
 
 367 
 
 under starvation, 366 
 non-nitrogenous foods, 372, 376 
 spare proteins, 367, 371 
 residue, 367 
 nucleo-protein, 369, 370, 384 
 organic acids, 373, 387 
 oxidations, 395, 402, 404, 443 
 peptid, 369 
 
 phosphorus, 384, 391, 793 
 production and maintenance 
 requirements, 393, 394, 398, 
 417,418 
 protein, amount required, 370, 
 
 376, 447 
 behaviour in body, 369 
 
INDEX OF SUBJECTS 
 
 875 
 
 Nutrition, protein, chemistry of, 
 357. 367. 369, 824, 827, 
 830 
 circulating, 369 
 conversion into living tis- 
 sue, 368 
 into urea, 335, 365, 
 367, 368, 369, 370, 
 economisers 367, 371 
 energy directed by, 371 
 
 not furnished by, 370 
 excess, effect on urine, 382, 
 
 384 
 fat, economiser of , 367, 371 
 formation from, 367, 
 
 375 
 ferments, 261, 263, 369. 
 
 See Ferments 
 flesh formers and, 366 
 glycogen, formation from, 
 
 255, 367 
 heat production, 395 
 luxus consumption, 371 
 metabolism of, 365, 367, 
 
 368, 828 
 non-nitrogenous residue, 
 
 367 
 nucleo-proteins and uric 
 
 acid, 339, 369, 370 
 oxidation of, 395, 404 
 splitting, method of, 369, 
 
 824, 827 
 storage of, 371, 811, 812 
 tissue, 365, 368 
 urea resulting from, 335, 
 
 365. 367. 369, 37° 
 wasteful diets, 367, 370^ 
 
 377 
 work and, 370 
 See Proteins 
 proximate principles of food, 
 
 364 
 puberty, effect of food on, 756 
 salts of body, 357, 359. 3°4. 
 382, 386, 793. 812, 
 823 
 use of, 382, 383, 384, 
 812, 823 
 calcium, 345, 383-386, 824 
 iodine, 303, 384, 386 
 iron. See Iron 
 magnesium, 345, 347, 3 8 3. 
 
 451,823,824 
 phosphorous, 347, 384-386, 
 
 39i.793.823 
 potassium, 345, 347, 382, 
 
 383, 385. 824 
 sodium, 345, 382, 383,385 
 sulphur, 343, 382, 384, 824 
 See Salts 
 starvation, 392, 444 
 
 Nutrition, storage of tissues (fat- 
 tening), 397 
 tables, 380, 381, 385, 393, 395, 
 
 397. 398, 399, 4°° 
 thirst, 392. See Thirst 
 tissue, disordered nutrition of, 
 
 .364 
 vitamines, 364, 368, 376, 377, 
 
 378,388. See Vitamines 
 wasting from disease, 400 
 water essential to digestion, 
 392 
 in tissues, 358, 391, 812 
 lost by tissues, 392 
 Nux vomica, absorption of, 289. 
 See Strychnine 
 
 Oats, digestion of, 185, 199, 297, 298 
 Occipito-thalamic fibres, 586 
 Ocular muscles. See Eye 
 (Edema, 276 
 
 (Esophageal groove, 200, 210 
 (Esophagus, 163 
 
 liquids, passage of, 164 
 
 nervous mechanism, 165, 557 
 
 solids, passage of, 163 
 
 structure, 165 
 CEstrus, 307, 743, 747, 775 
 
 cause of, 307, 749 
 
 duration of, 748 
 
 signs of, 750 
 
 uterine changes during, 750, 
 
 755 
 Oleic acid, 831, 836. See Fat 
 Olein, 792, 831,836 
 Olfactory centre, 535, 608, 609 
 
 nervous mechanism, 608 
 
 organ, 608 
 
 sensations, 610 
 
 stimulus, 609 
 Olivary bodies, 507, 509 
 Omasum, 203, 216 
 
 canal, 203 
 
 contents, character of, 205 
 
 function, 205 
 
 ingesta, movement of, 204, 
 216, 217 
 
 laminae, 204 
 
 function, 204 
 
 nerve-supply, 204 
 
 position, 204 
 Oncometer, Roy's, 326 
 Ophthalmia, specific, 606, 607 
 
 sympathetic, 607 
 Ophthalmoscope, 590, 595 
 Opsonin, 18 
 Optic axis, 576, 584, 593. 605 
 
 chiasma, 582, 587, 600 
 
 nerve, 549. 55°. 5°6, 575, 581, 
 586, 587, 599, 603 
 decussation of, 582, 600 
 
876 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Optic thalamus, 504, 511, 524, 525, 
 
 553. 582 
 Optics, physiological, 590 
 Optogram, 604 
 Organ of Corti, 626, 627 
 
 of Jacobson, 608 
 Ornithine, 337, 827 
 Osazone, 334, 835 
 
 Osmosis, 277, 278, 279, 280, 754. 
 See Absorption and Lymph 
 explanation of, 278, 279 
 osmotic pressure, 277, 279, 280, 
 824 
 and lymph formation, 
 
 280 
 how ascertained, 279 
 hypertonic solutions, 
 
 280, 824 
 hypotonic solutions, 
 
 280, 824 
 isotonic solutions, 280, 
 824 
 Osteomalacia in horses, 391 
 Osteoporosis, 391 
 Ostium abdominale, 744 
 Ostrich, rutting of, 751 
 Otoliths, 629 
 Ovaries, 738, 742 
 
 effect of removal, 745, 746 
 
 on general develop- 
 ment, 746 
 on lactation, 307, 746, 
 
 794. 796 
 on mammary gland, 
 
 307, 746, 789, 790 
 on cestrus, 746 
 on psychic condition, 
 305 
 Ovulation, 755 
 Ovum, 742 
 
 blastodermic vesicle, 763 
 cell division, 755 
 degeneration, 745, 775, 782 
 epiblast, 763, 764 
 fertilisation of, 297, 307, 753, 
 
 755. 761 
 hypoblast, 763, 764 
 implantation, 765, 767, 768, 
 
 775 
 
 maturation, 754 
 
 mesoblast, 764 
 
 number, 756 
 
 polar bodies, 755 
 
 segmentation, 761 
 
 structure, 744 
 Owl, eye of, 604 
 Ox, absorption of food, 296 
 
 bile, 247 
 
 blood, 5, 14, 19, 26 
 
 dentition, 815 
 
 digestion, stomach, 200, 215 
 
 Ox, digestion, intestines, 229 
 
 erection in bull, 737, 757 
 
 faeces, 238, 240, 241 
 
 life, average duration, 818 
 
 lymph, 276 
 
 maturity, 756 
 
 pancreatic fluid, 266 
 
 pneumonia, 157 
 
 puberty, 756 
 
 rumination, 210, 229. See 
 Rumination 
 
 salivary secretion, 167, 169, 
 170, 214 
 
 sweating, 318, 321 
 
 temperature, 405 
 
 "urine, 351 
 
 work, 441 
 Oxalates, in urine, 346, 387 
 Oxalic acid, 346, 387 
 Oxidase ferment, 133, 340 
 Oxidases, 133, 340, 404, 836 
 Oxidations, 132, 133, 340, 373, 395, 
 402, 404, 443 
 
 autoxidation, 403 
 
 catalases, 404 
 
 catalyst, 403, 836 
 
 cellular elements in, 404 
 
 food, 404 
 
 oxidases, 404 
 
 peroxidases, 404 
 
 peroxide, 403 
 Oxygen, 744, 822 
 
 deficiency, 133 
 
 excess of, 135 
 
 in air, 119 
 
 in blood, 8, 12, 25, 123, 127, 
 133, 774, 822 
 
 in body fluids, 132, 822 
 
 in cell, 754,774 
 
 in muscle, 132, 443, 451, 822 
 
 in respiratory exchange, 143, 
 
 145 
 in swim-bladder. 131 
 intramolecular, 133 
 liberation in tissues, 131 
 Oxyhemoglobin, 8, 12, 126, 127, 
 132, 134 
 
 Pace. See Speed 
 Paces of horses, notation of, 668 
 Pacinian corpuscles, 458, 733 
 Pain centre, 616 
 
 ' referred,' 564, 616 
 
 sense, 615 
 Palate, hard, 160 
 
 soft, 163 
 Pale muscle, 422, 424, 433, 451, 453 
 
 581 
 Palmitic acid, 830 
 Palmitin, 792, 831 
 Pancreas, 259, 826 
 
INDEX OF SUBJECTS 
 
 877 
 
 Pancreas, abscess, 271 
 cell changes in, 266 
 cells of, 266 
 function of, 261 
 internal secretion, 256, 269, 
 
 3°°. 3°7 
 removal, 267 
 structure of, 270 
 Pancreatic diabetes, 267, 270 
 fluid, 259 
 
 amino-acids and, 263 
 amount secreted, 265 
 amylopsin, 264 
 
 action of, 264 
 aromatic bodies, 263, 264 
 atropine, effect of, 261 
 composition, 259 
 diet, influence of, 265 
 ferments, 261 
 function of, 261 
 lipase, 261, 264 
 action of, 264 
 reversible action of, 
 265 294, 375,836 
 peptids, 263 
 polypeptids, 263 
 secretin, 260, 300 
 secretion, 260 
 
 supply of secretory 
 
 fibres, 562 
 two sources, 262 
 use of, 261 
 trypsin, 261 
 
 action, 262, 826 
 calcium salts and, 263 
 trypsinogen,262 
 
 enterokinase and, 262 
 use of, 261 
 
 See Glycogen and Sugar 
 Panoramic vision, 602 
 Panting respirations, 141, 146, 411 
 Papilla (optic), 576, 583 
 Papilla? (tongue), 612 
 Paracasein, 196 
 Paraglobulin, 4, 275 
 Paralytic secretion (saliva), 175 
 Parasitic diseases, 245 
 Parathyroid gland, 302 
 Parotid gland, 167, 169. See Saliva 
 Pars trigemini, 553- See Cranial 
 
 nerves, fifth pair 
 Parthenogenesis, 758 
 Partial pressure, gases, 130 
 Parturition, 784 
 centre, 505 
 Pastern-joint, 640, 655 
 Pathetic nerve, 553. See Cranial 
 
 nerves, fourth pair 
 Pavlov's fistula, 197, 217 
 Peak (foot), 717 
 Pedal bone, 701, 706, 710 
 
 Pedal bone, descent of, 725, 731 
 
 joint, 656, 702 
 Pedis, fracture of, 735 
 Pellagra, 388 
 Pendular movements (small gut), 
 
 232 
 Penis, 737, 738, 757 
 
 venous plexuses of, 99, 757 
 Pennsylvania calorimeter, 360, 363 
 Pentoses, 369, 835 
 Pepsin, 192, 195, 297, 836. See 
 
 Stomach 
 Peptid, 196, 263, 369, 825 
 Peptones, 24, 195, 202, 205, 218, 
 
 228, 297, 825, 826 
 Peptonuria, 298 
 Pericarditis, 64 
 Pericardium, 32, 36, 273 
 
 use of, 42, 74, 116 
 Perikaryon, 466, 478 
 Perilymph, 625, 627, 629 
 Periople, 710, 711 
 Peripheral reflex centres, 493 
 
 resistance, 67-71 
 Peristalsis, 165, 232, 492, 559 
 Peritoneum, 273, 283 
 
 absorption from, 290 
 Peroxidases, 404 
 Peroxide, 403 
 Perspiration, 318. See Skin 
 
 insensible, 318, 411 
 Pettenkofer's test, 249 
 Peyer's patches, 287 
 Pffuger's law of contraction, 462 
 Phagocytosis, 18, 765, 768, 770 
 Pharynx, 163, 166, 618 
 Phenol, 228, 239, 264, 344, 384 
 Phenyl-acetic acid, 228 
 
 -alanine, 828 
 
 -oxidase, 404 
 
 -propionic acid, 228 
 
 -sulphuric acid, 344 
 Phenyl sulphate, 344 
 Phloridzin, 255 
 Phonation, 153 
 Phosphates in body, 347, 384, 391, 
 
 793,823,824 
 Phosphatides, 384, 831 
 Phospho-proteins, 384, 824, 825, 
 
 826 
 Phosphoric acid, 823, 826, 830, 831 
 in faeces, 239, 824 
 innutrition, 375, 382, 384- 
 
 386, 391, 793. 823 
 in secretions, 169, 247, 
 
 259, 3 J 9. 791, 793 
 in urine, 346, 347, 824 
 Phosphorus, 347, 384, 391, 791,792, 
 
 793, 822, 823 
 Photo-chemical theory of vision, 
 603 
 
A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Phrenic nerves, 137, 141, 155, 157, 
 
 215 
 Phrenosin, 832 
 Physiological optics, 590 
 
 salt solution, 8, 28, 52, 137, 
 445. 824 
 
 shoeing, 734 
 Physostigmine, absorption of, 289 
 Pia mater, 538 
 Picric acid test, 835 
 Pig, bile, 247 
 
 blood, 14, 19, 26 
 
 dentition, 816 
 
 duration of pregnancy, 784 
 
 faeces, 238, 241 
 
 fattening, 380, 399, 813 
 
 intestinal digestion in, 230 
 
 life, average duration of, 818 
 
 maturity, 756 
 
 puberty, 756 
 
 rate of growth, 813 
 
 starvation, 618 
 
 stomach digestion, 205 
 
 sweating, 318, 321 
 
 temperature, 406 
 
 urine, 353 
 Pigment, bile, 247, 248 
 
 blood, 1, 11 
 
 faeces, 239 
 
 iris, 578, 813 
 
 retina, 583, 604 
 
 urine, 345 
 Pigments, hair, 314, 813 
 Pilocarpine, absorption of , 289 
 
 (heart), 59 
 
 (sweating), 320 
 Pilomotor nerves, 317, 562 
 Pineal body, 306 
 Piotrowski's reaction, 829 
 Pitch of voice, 153, 756 
 Pituitarin, 62, 96, 305, 795 
 Pituitary body, 62, 96, 305 
 Placenta, 765, 767, 770 
 
 biliverdin in, 249 
 
 circulation in, 768, 777 
 
 cotyledonary, 766 
 
 deciduate, 766 
 
 diffuse, 767 
 
 eaten by mother, 787, 796 
 
 filter, 780 
 
 galactagogue, 796 
 
 green border, 768 
 pockets, 768 
 
 non-deciduate, 766 
 
 praevia, 766 
 
 zonary, 766 
 Plantar cushion, 704, 710, 716, 717, 
 7 2 6, 733 
 
 neurectomy, 364, 733 
 Plasma, 3, 4, 5 
 Platelets (blood), n 
 
 Plethysmograph, 80, 82, 90 
 Pleura, 114, 273, 283 
 
 absorption from, 290 
 Pleurisy, horse, 156 
 Pleuro-peritoneal space, 764 
 Pneumogastric nerve, 555 
 heart, 54, 56, 562 
 intestines, 234, 562 
 larynx, 137, 138, 152, 556 
 respiration, 131, 137, 138, 
 139, 152, 364. 555. 556, 
 562 
 stomach, 204, 215, 216, 
 
 217. 234. 557. 562 
 
 section of, 557 
 
 See Respiration, Tenth 
 pair, and Vagus. 
 Pneumonia, horse, no, 156, 430 
 Poikilothermal animal, 404 
 ' Pointing ' horse, 648 
 Polar bodies, 755 
 Polyoestrous animals, 747 
 Polypeptid, 263, 825, 826 
 Polysaccharoses, 832 
 Pons, 509 
 
 Pony, strength of, 697 
 Position receptors, 628 
 Post-ganglionic fibre, 88, 559, 560- 
 
 563 
 -inhibitory rebound, 490 
 -mortem rises of temperature, 
 
 421 
 -sphygmic period, 45 
 Posture, 97, 407, 435, 436, 491, 517, 
 541, 633, 634, 635, 636, 637, 641, 
 648, 654, 661, 821. See Standing 
 Potassium ferrocyanide, absorption 
 of, 289, 290 
 indoxyl sulphate, 344 
 in urine, 345, 347 
 iodide, absorption of, 290 
 kresyl sulphate, 344 
 phenyl, 344 
 
 sulphate, 344 
 salts, in bile, 247 
 in blood, 25, 26 
 on heart's action, 52, 824 
 lymph, 275 
 milk, 793 
 in muscle, 451 
 in nutrition, 382, 383, 385 
 sebum, 324 
 sweat, 319 
 urine, 345, 347 
 skatoxyl sulphate, 34^ 
 See Salts 
 Precipitins, 10 
 
 Preganglionic fibre, 88, 495, 559-56! 
 Pregnancy, duration of, 783 
 Prehension of food, dog, 159, 160 
 horse, 158 
 
INDEX OF SUBJECTS 
 
 879 
 
 Prehension of food, ox, 158 
 
 sheep, 158 
 Prepotency, 782, 799 
 Prepuce, secretion of, 323 
 Presphygmic period, 45 
 Pressor nerve fibres, 89 
 Pressure, blood, 67 
 
 intracardiac, 43, 46 
 intraocular, 589 
 intrapericardial, 74, 116 
 intrapulmonic, 114 
 intrathoracic, 114 
 intraventricular, 40 
 sense, 615, 616 
 Primitive groove, 764 
 Prochorion, 767 
 Procreation, fitness for, 756 
 Production rations, 394, 399 
 Proline, 828 
 
 Pronucleus, male and female, 761 
 Pro-oestrum, 747 
 cause of, 749 
 signs of, 750 
 Propepsin, 195 
 Proprioceptive system, 454, 479, 
 
 488, 491, 517 
 Prorennin, 196 
 Prosecretin, 250, 260 
 Prostate, 738, 742, 751, 759 
 Protagon, 458 
 Protamines, 825 
 
 Proteins, 357, 364, 368, 370, 824. 
 See Nutrition 
 absorption of, 292, 297, 369 
 albumins, 825 
 albuminoids, 371 
 
 as food, 368, 371 
 amount in body, 358 
 
 required, 370, 376,398,447 
 assimilation of, 298, 369 
 bacterial decomposition of , 228, 
 
 264, 828 
 chemistry of, 824-830 
 
 amino-acids, 228, 257, 368, 
 369, 370, 376, 826, 827, 
 828, 830 
 heat evolved by, 395 
 not fully oxidised, 335, 365, 
 
 367.369,395 
 tests for, 829 
 chromoproteins, 826 
 circulating, 369 
 classification of , 825 
 cleavage products, 827 
 colloids, 824 
 conjugated, 825, 826 
 crystallisation of, 830 
 derived, 826 
 disintegration of, 257, 262, 
 
 263, 298, 337, 338, 344. 368, 
 
 369, 828 
 
 Proteins, disintegration, poisonous 
 
 bodies, 257, 344 
 economisers, 366, 367 
 emulsoids, 791, 825 
 energy of, 395 
 endogenous, 824 
 excess of, 382, 384, 400 
 excretion of, 358 
 exogenous, 824 
 fasting, in, 366 
 fattening animals, 370, 378, 
 
 380, 394, 399 
 ferments, 261, 263, 369. See 
 
 Ferments 
 food, 364, 366, 370, 371 
 function of, 371 
 globulins, 825 
 g-lucoproteins, 826 
 history of, in body, 368 
 histones, 825 
 hydrolysis of, 827, 828 
 in blood, 4 
 in cell, 789 
 
 in mammary gland, 795 
 in spermatic fluid, 741 
 in starvation, 366 
 intestinal digestion of , 219, 227, 
 
 298 
 luxus consumption, 371 
 metabolism. - See Nutrition 
 molecule, 357, 825 
 muscle, 450 
 
 clotting of, 450 
 muscular energy, 371 
 nitrogenous equilibrium, 365 
 
 et seq . 
 non-protein nitrogen, 368 
 nucleo-, 339, 369, 370. See Nu- 
 
 cleo-protein, 825 
 of milk, 769, 791, 794. 796 
 oxidation of, 335, 366, 367, 369, 
 
 395 „ , 
 
 phospho-, 791, 826 
 
 poisons, 257, 344 
 polypeptide, 826 
 precipitation of, 829 
 protamines, 825, 826 
 reconstruction of , 298, 369, 836 
 repair tissues, 368, 371 
 resynthesis of, 298, 369, 836 
 sclero-, 825, 826 
 stomach digestion of, 195 
 storingup, 365, 368,371,811,812 
 sulphates conjugated m liver, 
 
 257. 344. 384 
 tests for, 829 
 tissue, 368, 
 urea formation, 335, 365. 369. 
 
 37° , „ 
 vegetable, 825 
 
 Proteoses, 195, 228, 297, 825, 826 
 
880 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Prothrombin, 22 
 Protopathic fibres, 614 
 Protoplasm, 752, 753, 784 
 Psychical function, 543 
 
 secretion, 197 
 Ptyalin, 169, 172 
 Puberty, 307, 756 
 Pulmonary system, blood-pressure 
 
 in, 75 
 
 valve, 39, 41, 44 
 Pulse, 67, 70, 77 
 
 cause of, 67, 77 
 
 character of, 100 
 
 rate, 81, 82, 83, 449 
 
 sphygmogram, 79 
 
 tension, 81 
 
 tracings, 79, 80 
 
 volume, 80 
 
 wave 78 
 Puncture, diabetic, 268 
 Pupil (eye), 492, 571, 578, 630, 821 
 Purin bases, 369, 826 
 
 bodies, 339 
 Purkinje, cells of, 509, 513 
 
 fibres of, 33 
 
 -Sanson images, 590 
 Purpura, 29 
 Purring, 155 
 Pyloric glands, 192 
 Pyramids (medulla), 507, 523 
 Pyrimidine bases, 369, 826 
 Pyrocatechin, 345 
 
 Rabbit, duration of pregnancy, 784 
 Racking, horse, 674 
 Radiation, 410, 413 
 Radiolarian, 471 
 
 Ramus communicans, grey, 559- 
 5.63 
 white, 56, 87, 268, 321 , 495, 
 559-561 
 Ranvier, node of, 456 
 Ration, maintenance, 393, 398,417, 
 479 
 production, 393, 399 
 Reaction of degeneration, 463 
 
 velocity of, 403, 836 
 Reagents. See Tests 
 Rearing, 686 
 Reason in animals, 544 
 
 instinct and, 546 
 Receptaculum chyli, 284, 286 
 Receptors, 454, 479, 491, 517, 628 
 Recessive characteristics, 801, 802 
 Reciprocal innervation, 112, 152, 
 4-3. 436, 448, 477, 481, 489, 505! 
 633. 648 
 Reciprocating action, stifle and 
 
 hock, 648 
 Rectum, absorption from, 229, 292 
 sphincters of, 241 
 
 Recurrent laryngeal nerve, 152, 
 156, 53L 555, 556 
 sensibility, 495 
 Red corpuscles. See Blood 
 
 nucleus (brain), 500, 511, 515 
 Reduced eye, 593 
 Reductases, 836 
 Referred pain, 564, 616 
 Reflex act, time of, 493 
 
 action, 474, 541 . See Nervous 
 
 system 
 acts, 215, 474, 477, 491, 492, 
 
 5°5. 547.58i 
 arc, 474, 478, 479, 480, 487, 
 
 493. 5°5. 514. 51 8 
 centres, peripheral, 493 
 chestnut, 488 
 
 extensor thrust, 481, 482, 483 
 falling, 488, 630 
 foot, 485 
 frog, 476 
 head-shake, 488 
 impulses, 478 
 inhibition, 423, 477, 482, 488, 
 
 489, 490, 493 
 ischial, 488 
 
 locomotion, 476, 477, 491 
 pinna, 488 
 reversal of, 492 
 scratch, 481, 483, 485 
 standing, 491, 510 
 stepping, 481 
 tendon, 488 
 
 tonus, 58. See Muscle tonus 
 Refraction, errors of, 596, 606 
 Reissner, membrane of, 626, 627 
 Remak's ganglion, 51 
 Rennin, 194, 196, 205, 791. See 
 
 Stomach 
 Reserve air, 142 
 Residual air, 142 
 Respiration, 101 
 abdominal, 113 
 
 muscles, 112, 113, 138, 531 
 acid, lactic, in blood, 136, 140, 
 
 146 
 acidosis, 134, 136, 141 
 air, alveolar, 122, 131 
 
 amount of, required, 142 
 at rest, 142 
 at work, 143, 145 
 complemental, 142 
 composition, 119 
 reserve, 142 
 residual, 142 
 
 respiratory changes in, iig 
 sacs (alveoli), 122 
 
 dead space in, 122, 
 123 
 tidal, 142 
 vitiated, 147 
 
INDEX OF SUBJECTS 
 
 Respiration, alkalosis, 134 
 
 allantois, organ of, 771, 774 
 
 alveolar air, 122 
 
 anoxemia, 126, 134 
 
 apncea, 136 
 
 argon, 119, 124 
 
 asphyxia, 92, 114, I32 , 135 
 
 ,142. 551,557 
 blood-pressure, effect on 74 
 
 "5 
 calorimeter, 360 
 carbon dioxide, 119, 120, 122, 
 123, 127. 132, 133! 136! 
 14°. 141, 143, 145 
 monoxide, 134 
 cause of first, 142 
 centre, 137, i 39j I4 o, 448, 510 
 chamber, 360 
 changes in air, 119, 123 
 
 in blood, 119 
 chemical stimulus to, 140 
 chemistry of, 123-136, 143 
 chest cavity, air-tight, in, 112, 
 .» "3. "5 
 increased size, inspiration, 
 
 112 
 pressure within, 114 
 rapid development in 
 
 young animals, 113 
 rib movements, 102, 112, 
 114 
 shape of, 102 
 sternum movements of, 
 
 114 
 wall, horizontal section 
 of, 103 
 movements during re- 
 spiration, 112, 114 
 vertical section of, 
 
 104' 
 width of, 101 
 circulation, effect on, 115 
 'condition' and, 147, 447, 448, 
 
 449 
 costal, 113 
 coughing, 155 
 dead space, 122, 123 
 diaphragm, movements of, 104, 
 105, no 
 blood-pressure and, 
 106 
 position of, 104, 105 
 
 lungs on, 105, 107 
 rupture, 157, 446, 821 
 spasm, 155, 156 
 diffusion of gases, 129 
 dissociation, 126, J 30 
 dyspnoea, 114, 134 
 exchange between lungs and 
 
 tissues, 123-132 
 expiration, 112, 140 
 
 881 
 
 Respiration, expiration, blood - 
 pressure, 115 
 muscles of, 112, 113 
 expired air, composition, 119 
 , I2 3- 143 
 lacial sinuses, 118 
 false nostril, 115 
 foetal, 771, 774 
 
 first inspiration, 142 
 lung, 113, 779 
 gases in blood, 123, 128, 131, 
 
 135. 14°. 774. 780 
 glottis, 118, 148, 149, 151, 1 52, 
 
 heart-beats, ratio to, 115, 449 
 heat, effect of , on, 141, 413 
 hiccough, 155, 156 
 hydrogen in expired air, 121, 
 
 hyperpnoea, 134 
 inspiration, 107, in, 118, 138 
 cause of, 111, 140, 142 
 forced, 112, 114, 117 
 muscles of, 113, 531 
 inspired air, 119 
 internal, 131 
 larynx. See Larynx 
 lungs, 107 
 
 anterior lobes during dis- 
 tension, no 
 apoplexy of, 156 
 areas affected by pneu- 
 monia, no, in 
 broken wind, 156 
 capacity, 142 
 collapse of, no, in, 114 
 fill chest cavity, no 
 filling of, 107 
 method of distension, 107, 
 
 no, in 
 movements, rest and work, 
 
 in 
 nerve supply, 137 
 pressure within, 114 
 recoil of, 112 
 ventilation of , 122, 143 
 marsh gas expired, 121, 358 
 murmurs, 147 
 muscles of, 113, 138, 531 
 muscular work at, 145 
 nasal chamber, 117 
 negative pressure in chest, no, 
 
 113, 114, 115 
 neighing, 155 
 nerves employed in, 137 
 nervous mechanism of, 137, 
 
 531 
 
 centre, 137, 138, 139, 
 140, 448, 510 
 
 cerebral representa- 
 tion, 531 
 
 56 
 
882 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Respiration, nervous mechanism 
 of, chest and abdo- 
 minal wall, 531 
 nature of impulse, 
 
 138 
 nerves employed, 137 
 phrenics, influence on, 
 
 137. 141 
 phrenicus centre, 493 
 reciprocal innerva- 
 tion, 112 
 
 recurrent laryngeal, 
 
 152, 156, 53L 555. 
 
 556 
 seventh pair, influence 
 
 on, 137, 142, 550, 
 
 562 
 superior laryngeal, 
 
 138, 140, 152, 556 
 vagus, influence on, 
 
 131, 137, 139, 152, 
 
 556, 562 
 nitrogen, 119 
 nostrils, 116 
 number of, 115 
 
 effect of work on, 115 
 oxygen, 119, 123, 124, 133, 135, 
 
 143. 145,443 
 deficiency of, 133 
 excess of, 135 
 expired air, 119, 123, 143 
 inhalation in disease, 135 
 in inspired air, 119 
 in tissues. See Respira- 
 tion, chemistry of 
 panting, 141, 146 
 partial pressure gases, 130 
 pause in, 114 
 phonation, 153 
 phrenic nerves, 141 
 pleural sacs, 111, 114, 273 
 pleurisy, 156 
 pneumonia, no, 156 
 ratio to heart-beats, 115, 449 
 ribs in, 102, 104, 114 
 'roaring,' 152, 156, 455, 557 
 seventh pair, nerves, 137, 142, 
 
 55i. 562 
 sternum movement of , 114 
 tissue, 131 
 vagus in, 131, 137, 138, 139, 
 
 r 52, 555. 556, 562 
 voice, 153 
 
 work and, 145, 447, 448, 449 
 Respiratory centre, 137, 138, 139, 
 448, 510 
 exchange, 123-132 
 function, skin, 324 
 movements represented in the 
 
 brain, 531 
 murmur, 147 
 
 Respiration passages, absorption 
 from, 289 
 quotient, 120, 121 
 Restiform body, 509 
 Rete mirabile, 98 
 Reticulum, 202, 203, 213, 216 
 Retina, 580, 581, 584, 586, 588, 604 
 Retinal image, 594, 595 
 Reversible action of lipase, 265, 
 
 294. 375. 836 
 Reversion, 805 
 Rheoscopic frog, 442 
 Rhodopsin, 584, 604 
 Rhythmical movements, intestines, 
 
 232 
 Ribose, 341 
 Ribs, 100-103, 114 
 Rickets, 388, 390 
 Rigidity, decerebrate, 491 
 Rigor, heat, 430, 444, 451 
 mortis, 444, 451, 821 
 
 and labyrinth, 451, 630 
 Ringbone, 664 
 Rising, act of, 649 
 Ritter's tetanus, 463 
 Roads, angle of repose, 694 
 Roans, 315 
 
 Roaring, 152, 156, 455, 557 
 Rods and cones, 583, 584, 599, 604 
 Rolando, fissure of, 520, 524, 525, 
 
 529, 535 
 Rotation (joints), 632 
 Rothamstead experiments viii, 366 
 Roy's oncometer, 326 
 Rumen, 200, 201, 202, 216 
 
 churning movement, 201, 216 
 hair balls in, 201, 216 
 movement of ingesta in, 201, 
 
 216 
 oesophageal groove, 200, 210 
 Ruminants. See Ox and Sheep 
 Rumination, 210, 230 
 
 cud arrives in mouth, 213 
 how effected, 212 
 
 function of oeso- 
 phagus, 212 
 necessary factors, 214 
 nervous mechanism, 215 
 oesophageal groove, 200, 210 
 in rumination, 212 
 movements of, 212 
 oesophagus, structure of, 210 
 period of remastication, 213 
 reflex act, 215 
 weight of cud, 213 
 Rutting, 307, 750, 751 
 
 Saccharose, 832 
 Saccule, 629 
 Saliva, 166 
 
 action on foodstuff, 171 
 
INDEX OF SUBJECTS 
 
 883 
 
 Saliva, amount secreted, 169, 170 
 difference, horse and 
 ox, 169 
 amylolytic action, 171 
 calcium in, 168, 169 
 carbonic acid in, 169 
 classification of glands, 166 
 function of watery and viscid, 
 
 169 
 gases in, 169 
 gland-cells, active and resting, 
 
 176 
 glands, response to stimuli, 
 
 167, 613 
 in ruminants, 200, 205 
 in taste, 613 
 mechanism of secretion, 173 
 
 atropine, effect of, 175 
 nicotine, action of, on sub- 
 maxillary ganglion, 176 
 paralytic secretion, 175 
 physical characters, 168, 169, 
 
 386 
 products, starch digestion, 171- 
 
 173 
 psychical influences on secre- 
 tion, 168 
 ptyalin, 169, 172 
 secretion, 173 
 
 paralytic, 175 
 secretory and trophic fibres, 175 
 serous and mucinous, 167, 169 
 systems in different animals, 
 
 167 
 type depends on food, 167, 
 170 
 Salivary glands, 166 
 
 cell changes in, 1 76 
 Salkowski's test, 831 
 Salmine, 828 
 Salt, common, 823 
 licks, 383 
 
 solution, 8, 28, 52, 137, 445, 
 824 
 Salts, absorption of, 298, 299, 385, 
 386 
 deficiency of, 382, 383, 793 
 distribution of , 382, 823, 824 
 drafts on tissues for, 383, 386, 
 
 387. 793 
 excess of, 382, 383, 384, 385, 
 
 400, 823 
 excretion, 239, 319, 342, 359, 
 
 382, 383-387, 793, 794. 823 
 food, deficient in, 364, 376, 
 382, 383 
 influence of, 383 
 mineral feeding, 383, 384- 
 
 386, 793 
 
 source of, 364, 382, 386, 
 
 387, 793 
 
 Salts, function, 382, .823, 824 
 
 growth and, 376, 378, 383, 
 
 793. 812 
 metabolism of , 382, 793, 823 
 nutrition in, 382, 823 
 organic from inorganic, 383- 
 
 386, 793 
 source of. See Salts, food 
 storage of, 380, Tables ii. and 
 
 v., 385. 386, 812 
 of bile, 247, 249 
 of blood, 25, 26, 126, 824 
 of body, 382, 823, 824 
 
 compared with secretions, 
 
 793 
 
 of fasces, 240 
 
 of gastric juice, 193 
 
 of horn, 721 
 
 of lymph, 275 
 
 of milk, 791, 793, 794 
 
 of muscle, 336, 451 
 
 of pancreatic fluid, 259 
 
 of saliva, 169 
 
 of sweat, 319, 323, 324 
 
 of urine, organic, 335-345 
 inorganic, 345-354 
 ' Salvator,' speed of, 698 
 Sand crack, 735 
 Saponification, 264, 292, 831 
 Sarcolactic acid, 140, 146, 444, 445, 
 
 45°. 54 2 
 Sarcolemma, 33, 422 
 Sarcomere, 422 
 Sarcoplasm, 422, 436 
 Sarcostyles, 33, 422, 436 
 Sawdust, digestion of, 833 
 Scala tympani, 626, 627 
 
 vestibuli, 626, 627 
 Schematic eye, 592 
 Sclero-proteins, 825, 826 
 Sclerotic, 568, 571, 572, 575, 582 
 Scratch reflex, 481, 483, 485 
 Scream of horse, 154 
 Scurvy, 388, 391 
 Scutiform cartilage, 623 
 Sea-sickness, 210 
 Sebaceous glands, 788 
 
 secretion, 322. See Skin 
 Secretagogues, 197 
 Secretin, 218, 250, 260, 300 
 Secretion, antilytic, 175 
 
 antiparalytic, 175 
 
 chemical, 197 
 
 gastric juice, 190, 193, 197,205 
 
 internal, 256, 269, 300, 307 
 
 paralytic, 175, 321, 322 
 
 psychical, 197, 217 
 
 saliva, 173 
 Secretory nerves, 173-176, 492 
 Seedy toe, 736 
 Segmentation, 763 
 
A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Segmentation, cavity, 763 
 cell, 752, 761 
 nucleus, 761 
 Segregation, 801, 808 
 Self-digestion of stomach, 208 
 Semen, 739. See Spermatic fluid 
 Semicircular canals, 515, 517, 549, 
 
 572, 616, 625, 628, 630 
 Semilunar valves, 31, 36, 38, 39, 
 
 41. 45 
 Seminal vesicles, 738, 741, 751, 759 
 Sensations, auditory, 620 
 
 cutaneous, 614 
 
 hunger, 618 
 
 internal, 614 
 
 muscle and motorial, 616 
 
 pain, 615 
 
 pressure, deep, 616 
 light, 615 
 
 temperature, 615 
 
 thirst, 618 
 Sense, body, 504 
 
 of direction, 518, 546 
 
 motorial, 616. Seealsonextentry 
 
 muscle, 423, 446, 491, 504, 517, 
 528, 554, 616, 617, 629 
 Senses, the, 534, 566 
 
 hearing, 534, 620 
 
 sight, 534, 566 
 
 smell, 535, 608 
 
 taste, 535, 612 
 Sensory areas, 534 
 
 nuclei, 549 
 
 paralysis, 528 
 Serine, 827 
 Serous cavities, 273 
 Sertoli, cells of, 739 
 Serum, 5, 18, 828 
 
 albumin, 4, 21, 275, 826, 830 
 
 globulin, 4, 268, 792, 826 
 
 hemolytic, 8, 10 
 Sesamoid, 634, 640, 653, 654, 656 
 Seventh pair of nerves, 67, 93, 137, 
 
 142, 160, 173, 549, 550, 562, 613 
 Sexual characters, 307, 740, 745, 
 746; 756, 816 
 
 intercourse, 758 
 
 independent of nervous 
 system, 749 
 
 inversion, 746 
 
 season, 747-751 
 Sheep, absorption food, 296 
 
 bleating, 155 
 
 blood, 5, 14, 19, 20 
 
 brain, motor area, 531, 532, 533 
 
 castration, effect of, 307, 745, 
 746 
 
 composition of body, 358 
 
 cord, pyramidal tract, 500 
 
 dentition, 815 
 
 dominance in, 803 
 
 Sheep, duration of pregnancy, 784 
 
 faeces of, 238, 241 
 
 fattening, 399, 813 
 
 horns, 307, 746 
 
 lanolin, 323, 324, 831 
 
 life, average duration of, 818 
 
 lymph, 276 
 
 maturity, 756 
 
 metabolism, 420 
 
 milk, 791 
 
 motor area, 531 
 
 oestrus, 749 
 
 old age in, 818 
 
 ovum, implantation of, 767 
 
 puberty, 756 
 
 rate of growth, 813 
 
 starvation, 393, 618 
 
 suint, 324 
 
 sweating, 318 
 
 temperature, 406, 420 
 
 urine, 353 
 
 uterine glands, 468, 469 
 Shetland pony, 697 
 Shield (hair stream), 312 
 Shivering, horses, 324, 396, 409, 411, 
 
 565 
 Shock, surgical, 92, 95, 136 
 Shoeing, injuries by, 735 
 
 physiological, 734 
 Shoulder-joint, 637, 650 
 ' Show,' fattening for, 378 
 ' Shying,' 606 
 Side-bone, 724, 728, 734 
 Sight, 566. See Eye 
 
 eye and head movements, 571, 
 
 57 2 
 Sigmoid valves, 38, 40, 41 
 Silent areas, 535 
 Sinuses, facial, 118 
 Skatol, 220, 229, 239, 264, 344 
 Skatoxyl, 344 
 Skin, 308 
 
 absorption by, 290 
 appendages, chestnutand ergot, 
 
 324, 700 
 dandruff, 323 
 differences in type, 308 
 effect of varnishing, 324, 413 
 hair, albinism, 316, 578, 711 
 cat, 309, 615 
 clipping, 309, 316, 416 
 colour and stamina, 315, 
 
 7H,745 
 
 foal, 813 
 
 horses, 314 
 
 Iimbsandface,3i5,3i6 
 
 protective, 314 
 crest, 311, 312 
 distribution, 309 
 erection of, 317, 562 
 feathering, 311 
 
INDEX OF SUBJECTS 
 
 885 
 
 Skin, hair, permanent and tempor- 
 ary, 309 
 pigment, 314 
 shedding, 310 
 ' shield,' 312 
 streams, 310 
 tufts, 311 
 use of, 309, 317 
 weight of, 309 
 whorls, 311 
 organ of touch, 308 
 respiratory function, 324 
 sebum, in fleece of sheep, 324 
 nature of, 322, 830 
 preputial, 323 
 skin of ears and eyes, 323 
 uses, 322 
 sweat, amount of, 319 
 at death, 322, 821 
 centres, 321 
 
 composition of, 319, 823 
 glands in foot, 705 
 nerves, 321 
 
 nervous mechanism of, 320 
 occurrence in animals, 318 
 pigments in, 319 
 produced by pilocarpine, 
 
 320 
 proteins in, 319 
 repeated and permanent, 
 
 322 
 salts in, 319, 386 
 sensible and insensible, 
 
 318, 411 
 vascular mechanism, 320 
 Sleep, 540, 569, 814 
 hibernal, 542 
 theories of, 542 
 while standing, 541, 635, 636, 
 649 
 Smell, 608 
 
 centre, 535, 609 
 Jacobson, organ of, 608 
 olfactory organ, 608 
 
 nervous mechanism, 550, 
 
 608 
 sensations, 610 
 stimulus, 609 
 Smooth muscle, 422, 424, 433, 451, 
 
 453. 58i 
 salts of, 451. See Muscle, 
 
 pale 
 Sneezing, 155 
 Sniffing, 609 
 Snorting, 117, 154 
 Sodium phosphate, acid, 339, 348, 
 
 354 
 salts, 25, 26, 240, 247, 259, 275, 
 
 319. 344. 345. 347. 382, 383, 
 384, 385, 451, 721, 793, 823. 
 See Salts 
 
 Solar plexus, 216, 234, 555 
 Sole, 710, 726, 732. See Foot 
 Solitary follicles, 287 
 Somatic cells, 743 
 Somatopleure, 764 
 Sound, nature of, 620 
 Sounds, cardiac, 41, 42 
 Sow, duration of pregnancy, 784 
 
 milk of, 791 
 
 restrus of, 749 
 Spasm of diaphragm, 155, 156 
 Spastic paralysis, 524 
 Spavin, 661, 664 
 Specific resistance of tissues, 208 
 Spectroscope, 13 
 Speech, centre for, 535 
 Speed of horse, 698 
 
 canter, 674, 698 
 gallop, 698 . 
 trot, 671, 672, 698 
 walk, 698 
 ' Speedy cutting,' 652 
 Spermatic fluid, 739, 741, 759, 760 
 Spermatids, 739 
 Spermatocyte, 739 
 Spermatogenesis, 740 
 Spermatogonia, 739 
 Spermatozoa, 739, 761 
 
 function of, 755 
 
 maturation of, 739, 740, 742, 
 
 751 
 
 movement of, 740, 761 
 
 number of, 742 
 
 vitality of, 740 760 
 Spherical aberration, 592 
 Sphincters, rectal, 241 
 Sphygmogram, 79 
 Spinal accessory nerve 156, 455, 
 
 549, 55°. 551, 562 
 autonomic system, 562 
 cord, 471,474, 494 
 
 centres, special, in, 505 
 ano-spinal, 505 
 cilio-spinal, 505, 562, 
 
 581 
 erection, 505, 757 
 genito-spinal, 505 
 partur;tion, 505 
 vasomotor, 505 
 vesico- spinal, 355, 
 
 5°5 
 columns of, 495, 497 
 grey matter in, 496, 497 
 structure, 497 
 reflex action, 474, et seq. 
 tracts, ascending, 498, 501 
 function of, 501 
 association, 480 
 fibres, 501 
 Clarke's column, 497, 
 5°i. 5°3. 5 2 5 
 
886 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Spinal accessory tracts, column of 
 Burdach, 501, 
 508 
 of Goll, 501, 503, 
 508 
 cortico-spinal, 499 
 crossed pyramidal, 
 
 498, 499, 524 
 descending, 498, 499 
 direct pyramidal, 498, 
 
 499 
 spino - cerebellar, 
 
 497. 5°i. 5°4 
 dorsal spino-cerebel- 
 
 lar, 497, 501, 504 
 of Flechsig, 501 
 of Gowers, 501, 515 
 hetero-lateral pyra- 
 midal, 499 
 homo-lateral pyra- 
 midal, 499, 500 
 lateral pyramidal, 499 
 
 superior, 501 
 median superior, 501 
 Monakow's, 500 
 prepyramidal, 500 
 proprio-spinal de- 
 scending, 499, 500 
 rubro-spinal, 499, 500, 
 
 515, 524 
 ventral spino-cerebel- 
 
 lar, 501, 504 
 ventro-lateral ascend- 
 ing. 5°i. 515 
 descending, 500 
 vestibulospinal, 499, 
 500 
 white matter, 497 
 
 neuroglia, 497 
 paths in, 498 
 dog, 482, 491, 617 
 nerves, 470, 494, 495, 502, 504 
 dorsal root, method of 
 
 entering cord, 504 
 function of, 495 
 ganglia on, 470, 494, 502 
 
 not reflex centres, 493 
 recurrent sensibility, 495 
 Wallerian degeneration of, 
 
 471 
 white matter, 497 
 segment, 495 
 Splanchnic nerves, 88, 234, 268, 301, 
 
 304. 563 
 
 Splanchnopleure, 764 
 
 Spleen, 300, 301, 564 
 
 ' Sports ' in heredity, 804 
 
 Sprains. See Tendons and Loco- 
 motion 
 
 Squint, 573, 600 
 
 Stag, castration of, 745 
 
 Stag, rutting of, 750 
 ' Staircase ' tracing, 429 
 ' Staleness,' 447 
 Stamina, 694, 695, 699, 816 
 Standing attitude, 435, 491. 492, 
 510, 636, 654, 661, 703. 
 See Posture 
 reflex, 491, 510 
 stress of, 417, 435, 49*. 
 640, 648, 661, 703. See 
 Sleep 
 Stannius's experiment, 52 
 Stapes, 624 
 Star on forehead, 316 
 Starch, 171, 832 
 
 conversion, 172, 198, 264, 832 
 plants of, 171 
 
 sparing effect, 366, 367, 371 
 Starvation, 366, 392, 618 
 Steapsin. See Lipase 
 Stearic acid, 830 
 Stearin, 792, 831 
 Step, 665, 677 
 Stercobilin, 239, 249 
 Stereoscopic vision, 602 
 Stifle and hock, reciprocating action 
 of, 648 
 dislocation (?) of, 648 
 -joint, 641, 642, 648, 657 
 Stokes's fluid, 13 
 
 Stomach, absorption from, 207, 291 
 acids, 189, 190, 192, 193, 194, 
 195, 202, 206 
 variation in different 
 animals, 190 
 arrangement of food in, 186 
 capacity of, 179 
 cellulose in, 199 
 
 digestion in, 199 
 ferment, 199, 202 
 digestion, horse, 178. Seebelow 
 dog, 206 
 periods of, 199 
 pig, 205 
 
 ruminants, 200. See Abo- 
 masum, Omasum, Reti- 
 culum, Rumen 
 fats in, 199 
 gases of, 208 
 gastric juice, 190, 193, 205 
 
 amount secreted, 194 
 characters of, 193 
 chemical secretion, 197 
 enzymes of, 194, 195, 
 196. See Ferments 
 glands for, 190, 192 
 influence of character 
 
 of food on, 198 
 nervous mechanism, 
 
 197 
 non-putrefactive, 195 
 
INDEX OF SUBJECTS 
 
 887 
 
 Stomach, gastric juice, psychical 
 secretion, 197, 217 
 secretagogues, action 
 of, 197 
 general considerations, 178 
 movement of food in, 215 
 nerve supply, 197, 204, 212, 
 
 215, 216, 217, 557 
 nervous mechanism of, in dif- 
 ferent animals, 216, 217 
 nomenclature, 180 
 no peptone in, 207 
 not essential to life, 178, 196 
 of horse, 178 
 
 absorption, none, 207 
 arrangement of food in 
 
 186 
 capacity, 179, 437 
 churning movement, none, 
 
 183, 186, 215 
 fermentation in, 182 
 fundus, 180 
 gastric glands, 190 
 hay digestion, 183 
 
 duration of, 183, 
 
 185 
 influence of watei 
 on, 185 
 mucin in, 181, 189, 192 
 mucous membrane, 179 
 oats, digestion of, 185, 199 
 duration of, 185 
 passage of fluid through, 
 216, 220 
 of food through, 179, 
 216 
 periods of digestion, 199 
 rarely emptied, 179 
 reaction, acid, in, 183 
 rules for watering and 
 
 feeding, 187 
 special physiological fea- 
 tures, 183 
 vomiting rare, 209 
 See Diet and Digestion 
 pepsin, 192, 194 
 
 its action, 195 
 products of digestion in, 195 
 pyloric activity, 215 
 pylorus, 180, 181 
 reaction of contents, 189 
 rennin, 194, 205 
 
 action of, 196 
 rupture of, 182, 209, 210, 242, 
 
 245 
 self -digestion of, 208 
 simple and complex, 178 
 starch digestion, 198 
 
 and boiled foods, 199 
 tonus of, 437 
 villous coat, 1 80 
 
 Stomach, vomiting, 209, 210 
 
 emetics no effect on her- 
 
 bivora, 210 
 why horse cannot, 209 
 why ox cannot, 209 
 X-ray experiments, 215 
 Storage of tissue, 379 
 
 tables, 380-381 
 Strangulation of bowels, horse, 243, 
 
 244.245, 564 
 Stratum periostale, 707 
 
 vasculosum, 707 
 Strength of horses, 695 
 ' Stretching,' 492, 659 
 Stride, 665, 677 
 
 canter, 677, 698 
 gallop, 679, 698 
 trot, 672, 698 
 walk, 670, 671, 698 
 Stroma of ovary, 749 
 Stromuhr, Ludwig, 84 
 Strongylus armatus, 100 
 Strychnine, absorption of, 229, 
 290 
 effect on reflex inhibition, 
 
 49°. 493 
 stomach, horse, 207 
 Stumbling, 670, 673 
 Subarachnoid fluid, 538 
 Subdural fluid, 538 
 Sublingual ganglion, 562 
 gland, 167, 170, 613 
 Submaxillary ganglion, 562 
 
 gland, 67, 167, 168, 170, 173, 
 
 177.613 
 Substantia gelatinosa, 508 
 
 nigra, 509 
 Succinic acid, 239 
 Succus entericus, 218, 221 
 Sucking, 160, 533, 790, 796 
 Sucrose, 833. See Cane sugar 
 Suffraginis, fracture of, 663 
 Sugar, absorption of, 296 
 
 cane into dextrose, etc., 218, 
 227, 255, 296, 833 
 
 carbohydrate, conversion into, 
 
 372 
 katabolism of, 372, 373, 
 
 374 
 chemistry of, 832 
 destruction of, 269. See Sugar 
 
 oxidation 
 dextrine into, 218, 227 
 dextrose. See Sugar, grape 
 diffusibility of, 277 
 excretion of, 332 
 grape, 171, 172, 202, 255, 256, 
 
 833, 834, 835 
 glucose. See Sugar, grape 
 glycogen into, 254, 373, 833 
 glycosuria, 296 
 
A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Sugar in blood, 25, 253, 256, 268, 
 270, 297 
 in chyle, 297 
 
 in liver, 254, 268, 373, 833 
 in lymph, 280, 281 
 in milk. See Lactose 
 in muscle, 254, 268, 269, 373, 
 
 444. 45° 
 in urine, 257, 356 
 invert, 218, 296, 834 
 laevulose, 835. See Laevulose 
 maltose into, 218, 227, 255, 
 883, 884 
 from carbohydrate, 261, 
 264, 296 
 milk, into dextrose, etc., 218, 
 
 227, 296, 834 
 muscular work and, 444 
 osmotic pressure of, 279 
 oxidation of, 255, 269, 270, 272, 
 
 373. 374 
 regulation of supply, 256, 267 
 starch converted into, 171, 172, 
 
 173, 198, 205, 206, 207, 296, 
 
 833 
 
 tests for, 835 
 
 See Pancreas, Glycogen, Dex- 
 trose, Maltose, Cane Sugar, 
 and Carbohydrate 
 Suint, 324 
 
 Sulpho-cyanic acid, 344, 354 
 Sulphur, 824 
 
 in bile, 247 
 
 in body, 358 
 
 in nutrition, 382, 384 
 
 murine, 343, 344, 384 
 
 neutral, 384 
 
 See Salts 
 Sulphuric acid, 257, 343, 384 
 Summation of contractions, 430, 
 
 433 
 Superfecundation, 783 
 Superfcetation, 756, 783 
 Superior laryngeal nerve, 138, 152, 
 
 165. 556 
 Surgical shock, 92, 95, 136 
 Suspensory ligament, 640, 653, 654, 
 669 
 (eye), 584, 585, 586, 588 
 Swallowing, 163, 165, 289 (foot- 
 note), 510. See Digestion, Fluids, 
 Thirst 
 Sweat, 318, 322, 386, 411, 821, 823. 
 See Skin 
 amount of, 319 
 centres, 321 
 glands in foot, 705 
 pilocarpine, action of, 320 
 Sweating, nervous mechanism of, 
 
 320 
 Swim -bladder, fishes, 131 
 
 Swine. See Pig, Sow 
 Sympathetic or Autonomic system, 
 558 
 
 activation of, 563 
 
 afferent fibres in, 558, 564 
 
 anabolism, 563 
 
 cell-stations in, 560, 561, 
 
 5°2. 563 
 effect of nicotine on, 
 
 561 
 cranial nerves and, 559, 
 
 562. 580 
 to ciliary muscle, 562, 580 
 to digestive organs, 234 
 
 562 
 to heart, 54, 59, 562 
 to iris, 562, 580 
 to lungs, 562 
 to salivary glands, 173, 
 
 562 
 to secretory glands, 562 
 efferent fibres in, 558 
 fibres, character of, 559 
 function of, 558, 562 
 ganglia, 558, 559, 560, 564 
 cervical sympathetic, 
 86, 321, 559, 562, 
 
 564 
 ciliary, 562, 580 
 prevertebral or col- 
 lateral, 560, 561, 
 
 563 
 structure of, 558 
 terminal, 560, 563 
 vertebral or lateral, 
 559, 560, 561, 562, 
 563 
 grey rami communicantes 
 in, 559, 560, 561, 562, 
 5°3 
 katabolism, 563 
 neurones in, 561 
 no reflex action in, 493, 
 
 559 
 organisation of, 495, 559- 
 
 562 
 preganglionic fibres. See 
 below, white rami com- 
 municantes 
 post -ganglionic fibres. See 
 above, grey rami com- 
 municantes 
 sacral system, 563 
 sensory phenomena in, 563 
 referred pain, 564, 
 616 
 spinal cord, centres, 505 
 system, 562 
 
 supply to blood- 
 vessels, 86, 87, 
 562, 563 
 
INDEX OF SUBJECTS 
 
 Sympathetic or Autonomic system, 
 
 spinal system, supply to 
 
 pilomotor fibres , 
 
 3I7> 562 
 
 to sweat fibres, 320, 
 
 562 
 
 to vasomotor fibres, 
 86, 562, 563 
 variety of impulses trans- 
 mitted by, 563 
 white rami communicantes, 
 56, 87, 495, 559i ' 
 . 560, 561 
 origin of, 559, 5 6o, 
 561 
 Synapses, 446, 465, 469, 523, 524 
 
 586 
 Syntonin, 195 
 
 Syphon trap, duodenum, 181 
 Systole, 39 
 Systolic plateau, 44 
 
 Tactile cells, 308, 615, 717, 733 
 
 sensations, 517, 528, 553, 614, 
 615, 717, 733 
 Tail, organ of expression, 527, 533, 
 621 
 
 representation in cortex, 533 
 Tapetum lucidum, 575, 576, 583, 
 584. 605 
 
 nigrum, 576 
 Taste, 612 
 
 centre for, 535, 613 
 Taurin, 249, 384, 450 
 Tears, 568, 570 
 Teeth, 158, 159, 814, 817 
 
 age indicated by, 815 
 Tegmentum, 509 
 Telegony, 783, 805 
 Temperature, critical, 417, 418 
 
 low, effects of, 413, 414 
 
 of blood, 26, 83, 407 
 
 of body, 405, 410 
 
 variations in, 406 
 
 normal, of animals, 405 
 
 of stables, etc., 418 
 
 post-mortem rises of, 421 
 
 sense ,615. See Heat 
 Temporo-maxillary articulation, 161 
 Tendo-ligaments, 635, 641, 658, 660 
 Tendon organs of Golgi, ^23, 458,489 
 
 reflexes, 488 
 Tendons, 424, 635 
 
 effect of continuous strain on, 
 446, 640, 662 
 
 extensor, do not sprain, 662 
 
 flexor, division of, 655 
 
 low breaking strain, 635, 662 
 
 not elastic, 635, 654, 662 
 
 physiological causes of sprain, 
 446, 640, 655, 662, 663 
 
 Tendons, ruptures before muscle, 
 446, 635, 662 
 sprain at definite points, 662 
 See Locomotor system, Mus- 
 cular system, and Ligaments 
 Tension, pulse, 80, 81 
 Tenth pair of nerves, 54, 131, 137, 
 J 38, 139, 152, 165, 197, 204, 215, 
 216, 217, 234, 268, 269, 364, 549, 
 55°. 555-557- 562. See Pneumo- 
 gastric 
 Testicles, 737, 738, 739, 740 
 descent of, 739, 751, 790 
 removal, effect of, 305, 307, 
 
 379, 745 
 spermatic fluid, 739, 741, 759, 
 
 760 
 spermatozoa, maturation, 739, 
 
 740, 742, 751 
 X rays, effect of, 751 
 Tests, Adamkiewicz's, 829 
 Barfoed's, 834, 835 
 Bottcher's, 835 
 colour, 829 
 endurance, 698 
 Fehling's, 835 
 fermentation, 835 
 Gmelin's, 248 
 • Millon's, 829 
 Moore's, 835 
 Pettenkofer's, 249 
 picric acid, 835 
 Piotrowski's, 829 
 proteins, 829 
 Salkowski's, 831 
 sugar, 835 
 Trommer's, 835 
 xantho-proteic, 829 
 Tetanus, 432, 490, 565, 570 
 of heart absent, 53 
 of muscle, 430, 432, 490 
 Tetany, 302 
 Thalami optici, 504, 511, 524, 525, 
 
 553.582 
 Theca folliculi, 743 
 Thermotaxis, 410 
 
 Thigh, tendo-ligaments of, 635, 641 
 Thio-amino acids, 827 
 Thiocyanates, 344 
 Third pair of nerves, 517, 518, 549, 
 550. 552, 553. 562, 573, 580, 581 
 Thirst, 160, 392, 618 
 Thoracic duct, 273, 283, 284 
 
 amount of fluid passing 
 
 through, 275 
 circulation through, 283, 
 
 284 
 contents, 273, 276 
 mode of emptying, 283 
 pressure within, 284 
 See Lacteals 
 
890 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Thrombin, 21 
 Thrombogen, 22 
 Thrombokinase, 22 
 Thrombosis oiiliacs, 100, 444 
 ' Throwing back,' 805 
 Thrush, foot, 736 
 Thymus, 303, 814, 826 
 Thyroid gland, 302, 385 
 
 colloid substance in, 302, 
 
 3°3 
 effect of removal, 302 
 iodine-containing body in, 
 
 3°3 
 parathyroids, 302 
 function of, 303 
 Tidal air, 142 
 
 Tissue respiration, 131. See 
 Respiration 
 storage, 379 
 
 tables, 380, 381. See 
 Nutrition 
 Tone, arterial, 89 
 
 muscle, 89, 407, 434, 481, 489, 
 
 492.517. 541,635.636 
 See Muscle, Tone, and Tonus 
 Tongue, 612 
 
 movements, 159 
 nerves, 160, 550, 552, 613 
 papillae,, 612. See Taste 
 Tonus. See Muscle 
 Touch, sense of, 308, 614 
 Tracts (spinal cord), 498. See 
 
 Spinal cord 
 Training, 147, 447, 449 
 Transfusion, 28, 76 
 
 of fatigue substances, 445 
 Traube-Hering curves, 92 
 Tributyrin, 792 
 Tricaproin, 792 
 Trommer's test, 835 
 Trophic nerves, 175, 364, 453, 473, 
 
 554 
 Trophoblast, 765, 766, 767, 768, 
 
 769, 770, 775 
 Trot, 671 
 
 notation of, 671, 673 
 speed of, 672, 698 
 stride of, 672, 698 
 See Locomotion 
 Trypsin, 261, 262, 263, 297, 301, 
 
 826, 836 
 Trypsinogen, 262, 301 
 Tryptophan, 264, 368, 371, 376, 828, 
 
 829 
 Tubes, flow of fluids in, 67 
 Turbinated bones, 117, 118 
 Turpentine, absorption of, 289 
 Twelfth nerve, 160, 166, 549, 550, 
 
 552,613 
 Twin births, 756, 782 
 Twist of bowel, 233, 243, 244, 564 
 
 Tympanum (membrana tympani), 
 
 624 
 Tympany, 182, 202, 219, 245, 821 
 Tyrosinase, 314, 404 
 Tyrosine, 239, 257, 263, 828 
 
 from protein, 259, 263, 368, 
 
 371, 828, 829 
 in faeces, 239 
 pigment from, 314 
 
 Umbilical cord, 769, 774, 776 
 Unit characters, 800, 801, 804 
 Urachus, 773 , 776 
 Urea, 275, 335, 365-370, 404, 444, 
 
 450, 792. See Urine , 
 Urease, 335 
 Ureter, 354 
 Urethra, 355, 356 
 Uric acid, 337, 339, 353, 450, 792. 
 
 See Urine 
 Uricase, 404 
 Urinary secretions, theories of, 
 
 330-332 
 Urine, 325 
 
 acidity of, 334, 338, 343, 345, 
 
 348, 382, 384 
 alkalinity of, 343, 348, 382, 384 
 allantoine, 340, 353, 369, 774 
 ammonia in, 342 
 
 salts in, 342 
 benzoic acid, 333, 335, 340 
 
 influence of work on, 341 
 calcium in, 345, 346 
 calculus, 347, 356 
 camphor, effect of, 354 
 carbonates in, 345, 348, 824 
 chloral, effect of, 354 
 chlorides in, 335, 347 
 colouring matter, 345 
 comparison with blood, 325 
 composition of, 333 
 
 nitrogenous substances, 
 
 335 
 
 variations in animals, 334 
 creatine origin, 338, 421 
 creatinine origin, 338, 353, 421 
 cystine in, 344, 345, 384 
 discharge of, 354 
 
 nervous mechanism of, 355 
 diuretics, 333 
 during fever, 400 
 foetal, 774 
 
 glycuronic acid, 345, 354 
 hippuric acid, 340, 358, 383 
 
 amount of, 342, 351. 352, 
 353. 354 
 
 formation by synthesis, 
 
 333. 34° 
 substances infood, 340 
 influence of diet on, 342, 
 392 
 
INDEX OF SUBJECTS 
 
 Urine, hippuric acid, influence of 
 work on, 341 
 nature of, 340 
 inorganic substances, 345 
 kidneys are filters, 325 
 
 Bowman's capsules, 327 
 glomeruli and tubules, 
 
 function, 326 
 Malpighian bodies, 326, 
 
 331 
 oncometer, 326 
 physical condition during 
 
 secretion, 332 
 secretion, cortical, 331 
 
 effect of ligature of 
 
 renal vein, 330 
 effected by blood flow, 
 
 33° 
 glomerular, 331, 332 
 protein, pathological, 
 
 325. 332 
 sugar, pathological, 
 
 325, 332 
 theories, 330, 332 
 tubular, 332 
 tubules, seat of syn- 
 thesis, 333 
 secretory nerves absent, 
 
 332 
 structure of, 326 
 uriniferous tubules, 326, 
 
 332 
 vascular arrangement, 326, 
 
 33° 
 magnesium in, 345, 347 
 nitrogen of, 335 
 of calf, 814 
 of dog, 353 
 of foetus, 774 
 
 of horse, 343, 346, 349, 352 
 of ox, 351 
 
 of Pig. 353. 
 of sheep, 353 
 pathological, 356 
 phosphates in, 347, 824 
 phosphoric acid in, 347 
 potassium in, 345, 347 
 reaction of, 348 
 
 alkaline, cause of. See 
 Urine acidity and alka- 
 linity 
 ■ influence of food on, 348 
 secretion, 330-332, 354 
 sodium, 345, 347 
 sugar in, 356 
 sulphates in, 257, 343, 344, 345. 
 
 384 
 
 indican and, 344 
 skin dressing, effect of, 349 
 urea, acidosis, 338, 342, 343 
 
 characters, 335 
 
 Urine, urea, fermentation, 337 
 
 liver seat of production, 
 
 257. 307. 337 
 origin, 335 
 
 fromamino-acids, 337, 
 
 369 
 from ammonia com- 
 pounds, 337 
 from creatinine, 337 
 from protein, 335, 365, 
 
 367. 368, 369, 370 
 from uric acid, 337 
 proportion in blood, 335 
 in muscle, 336 
 murine, 337, 351,353 
 uric acid m, 339,354 
 
 in carnivora, 340 
 in herbivora, 340 
 water, amount of, secreted, 
 348 
 Uriniferous tubules, 326, 332 
 Urobilin, 16, 345 
 Urochrome, 344, 345 
 Uterine milk, 768, 769 
 Uterus, 307, 738 
 
 contraction, a reflex, 492, 749, 
 
 786 
 during intercourse, 758, 760 
 during oestrus, 750 
 during pregnancy, 768 
 involution of, 744, 787 
 torsion of, 785 
 Utricle, 629 
 
 Vagina, absorption by, 290 
 Vago-sympathetic nerve, 54 
 Vagus, function of, 54, 56, 555, 562 
 respiration and, 131, 137, 139, 
 152, 556, 562. See Pneurno- 
 gastric nerve and Tenth pair 
 Valine, 827 
 Vallate papillae, 612 
 Valves of Heart . See Heart 
 Variations in body temperature, 406 
 Vas deferens, 742, 759, 760 
 Vaso-constrictor nerves, 86, 87, 562, 
 
 563 
 -dilator nerves, 86, 87, 92, 95, 
 
 562, 563 
 See Nerves, vasomotor 
 Vasomotor centre, 60, 87, 88, 92, 
 
 5°5. 51° 
 
 nerves, 87. See Nerves 
 
 subcentres, 91 
 Vegetable proteins, 825 
 Veins, 66 
 
 blood-pressure in, 75 
 
 valves in, 67, 710, 733 
 
 See Bloodvessels 
 Velocity of blood, 84, 85 
 
 of reaction, 403, 836 
 
892 
 
 A MANUAL OF VETERINARY PHYSIOLOGY 
 
 Venesection, 27 
 Venous pulse, 67, 78, 80 
 Ventilation of lungs, 122 
 Ventricles of brain, 507, 552, 554 
 
 of heart. See Heart 
 Veratrine curve, muscle, 430 
 Vermis, cerebellum, 509, 513, 515, 
 
 516, 518,625 
 Vesical calculus, 356 
 Vesico-spinal centre, 355, 505 
 Vesicular murmur, 148 
 Vestibular nerve, 515, 517, 628, 629 
 Vestibule, 625, 628 
 Villi, chorionic, 765, 766, 767, 768, 
 
 774. 77° 
 
 intestinal, 286, 292, 293 
 Vision, binocular, 573, 584, 586, 597 
 
 centre for, 534, 537, 586 
 
 colour, 605 
 
 field of, 574, 577, 579, 583, 606 
 
 monocular, 587, 597, 601 
 
 night, 584 
 
 panoramic, 583, 602 
 
 periscopic, 602 
 
 stereoscopic, 602 
 
 theory of, 584. See Eye 
 Visual angle, 595 
 
 axis, 571, 572 
 
 centre, 534 
 
 purple, 584, 604 
 Visuo-psychic area, 538, 587 
 
 -sensory area, 534, 537, 586 
 Vitamines, 364, 368, 376, 388 
 
 antineuritic factor, 388 
 
 antirachitic factor, 390 
 
 antiscorbutic factor, 388, 391 
 
 chemical nature of, 391 
 
 fat soluble A, 388, 390, 391 
 
 function of, 388, 389 
 
 reserve stores of, 388, 390 
 
 water soluble B, 388 
 Vitellin, 826 
 
 Vitreous humour, 577, 584, 586 
 Vocal cords, 149 
 
 innervation of, 154 
 movements of, 151, 153 
 production of voice, 153 
 Voice, pitch of, 153 
 
 quality of, 153 
 
 See Larynx 
 Volume pulse, 80 
 Volvulus, 219 
 Vomiting, 209, 510. See Stomach 
 
 Wallerian degeneration, 471 
 Wall-eyed horses, 316, 578 
 Wall-secreting body (foot), 710, 713 
 Walk, the, 666, 667 
 notation_of, 669 
 
 Walk, speed of, 667, 698 
 stride of, 670, 671, 698 
 
 Water, absorption of, 289, 298 
 
 amount required, 391-392, 619 
 importance of, 392, 619 
 in tissues, 358, 392, 823 
 loss of, 348, 358, 392, 618 
 
 Watering horses, 187 
 
 heat lost by, 396, 409, 411 
 
 Weather warnings by cattle, 317 
 
 Weber's paradox, 438 
 
 Weeping in animals, 570 
 
 Weight horse should carry, 697 
 should draw, 695 
 how carried by foot, 712, 714, 
 
 7 2 3. 7 2 5. 726 
 See Locomotor system 
 Weismann's theory, 798 
 Whartonian jelly, 776 
 Whey, 196 
 Whinny of horse, 154 
 White corpuscles. See Leucocytes 
 
 legs, 315 
 Whorls (hair), 311 
 Work, 438-441, 448 
 amount of, 690 
 
 ' condition ' for, 147, 316. 379, 
 447, 448, 449, 684, 6c 698, 
 699 
 muscular, measurement of , 438, 
 
 690 
 respiration, 'influence on, 145, 
 
 449 
 training, 447 
 
 unpractised movements, 448 
 See Locomotor system and 
 Energy 
 
 Xanthanase, 404 
 Xanthine, 339, 450, 792 
 Xanthophyll, 791 
 Xantho-proteic reaction, 829 
 X-ray experiments, digestion, 215, 
 
 233 
 X-ray experiments, testicles, 751 
 Xylose, 341 
 
 Yawning, 155 
 Yeast extract, 389 
 Yolk sac, 765, 767, 769, 776 
 spherules, 744 
 
 Zebra, bars of foot, 726 
 
 duration of pregnancy, 784 
 Zein, 368, 826 
 Zona pellucida, 744, 767 
 Zonule of Zinn, 577, 584, 588 
 Zygote, 800, 803, 809 
 Zymogen, 177, 192, 195 
 
 HUNTED IN GREAT liKITAlK BY BILLING AND SONS, LTD., GUILDFORD AND ESHER