.. .:■' • -- .■■■'-. *;■■ lillllP'^ ■ ■>' 1*1 of - -^ CORNELL UNIVERSITY. THE 31ostuelI p. %ltxm*v Ubrarg THE GIFT OF ROSWELL P. FLOWER FOR THE USE OF THE N. Y. STATE VETERINARY COLLEGE. 1897 Digitized by Microsoft® Cornell University Library QP 31.S64 1907 A manual of veterinary physiology, 3 1924 001 040 017 Digitized by Microsoft® This book was digitized by Microsoft Corporation in cooperation with Cornell University Libraries, 2007. You may use and print this copy in limited quantity for your personal purposes, but may not distribute or provide access to it (or modified or partial versions of it) for revenue-generating or other commercial purposes. Digitized by Microsoft® Cornell University Library 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/cu31924001040017 Digitized by Microsoft® Digitized by Microsoft® A MANUAL OF VETERINARY PHYSIOLOGY Digitized by Microsoft® Digitized by Microsoft® A MANUAL OF VETERINARY PHYSIOLOGY COLONEL F. SMITH C.B., (1M.G. Army Veterinary Staff fellow of the royal college of veterinary surgeons fellow of the institute of ciemistry '^author of 'a manual of veterinary hv-tIenr, 1 etc. l -i „ < ■;-.;!. v. THIRD EDITION COMPLETELY REVISED AND W PARTS RE-WRITTEN New Yore WILLIAM R. JENKINS CO. PUBLISHERS 851-853 Sixth Avenue 1907 (All Rights Reserved) Digitized by Microsoft® Digitized by Microsoft® TO THE MEMORY OF SIB MICHAEL FOSTEB K.C.B., M.A., M.D., LL.D., D.C.L., F.R.S. THIS ATTEMPT TO DEAL WITH A BHANCH OF PHYSIOLOGY IS DEDICATED IN ACKNOWLEDGMENT OF THE ENCOURAGEMENT AND ASSISTANCE HE GAVE THE AUTHOR IN PROSECUTING THE STUDY OF VETERINARY PHYSIOLOGY Digitized by Microsoft® Digitized by Microsoft® PREFACE TO THE THIRD EDITION Circumstances beyond my control have delayed the re- vision of this manual. During the twelve years which have elapsed since the last edition was published con- siderable additions have been made to our knowledge of physiology. This has necessitated the manual being practically rewritten ; only the chapters on the Senses, Locomotion, and the Foot stand nearly as they were ; the others have been partly or wholly rewritten. This book is essentially a veterinary, and not a com- parative, physiology. It treats of physiology not only from its theoretical aspect, but from the point of view of clinical utility. The requirements of the student and practitioner have consequently not been lost sight of, and every oppor- tunity has been taken in the text to point out the clinical application of physiological facts. To several chapters a special pathological appendix is added, in order to enforce the lesson that pathology is only physiology out of health. In the chapter on the Nervous System the appendix has been omitted, not because the pathological side is wanting in interest, but for the reason that it is at present so defective in exactitude. As in previous editions, the horse has been taken as the type. Though he offers so many physiological peculiarities and differences from other animals, still his physiology among quadrupeds must always be of the first importance, and of the greatest practical interest. By the process of elimination and compression, room has been found for much more material than existed in the previous editions, without adding unduly to the bulk of the book. The digestive system, owing to its extraordinary importance in herbivora, is dealt with very fully. My cordial thanks are due to Professor Sherrington, F.R.S., who has again very kindly read the Nervous vii Digitized by Microsoft® viii PEEFACE TO THE THIRD EDITION System, and contributed some new matter on the ' Scratch Reflex ' and ' Stepping ' in the dog. My friend Dr. Sheridan Lea, F.R.S., has taken a deep interest in the production of this edition. As an old teacher of physiology he was able to advise me of those points which most students find some difficulty in grasping, and he has rendered the text of these portions clearer and more accurate by his careful revision and additions. He has kindly read all the proofs, amplified the chapter on the Muscular System, and brought the final chemical chapter up to date. I am glad of this opportunity of thanking him for his criticism and invaluable assistance. Mr. Goodall, F.R.C.V.S., Christchurch, and Mr. Leeney, M.R.C.V.S., Hove, have both been good enough to supply me with information for the chapter on Generation and Development, based on their special experience. For the information contained in the footnote on p. 608 I am indebted to Mr. Leach, F.R.C.V.S., Newmarket. As a rule no references have been made in the text to published works and papers, excepting where such appeared desirable. The literature of the subject is immense, but I must not omit to mention the help I have obtained from the Manuals and Text-books on Physiology published by Professors Halliburton, Howell (of Baltimore), Starling, Stewart (of Chicago), and from Dr. Leonard Hill's ' Recent Advances in Physiology and Bio-Chemistry.' The number of figures in the text has been considerably increased, some of them being new and original. I am indebted to Professor Stewart for permission to use many of those illustrating his 'Manual of Physiology,' to Pro- fessor Cossar Ewart, F.R.S., for the figures illustrating the early embryos of the horse, and to Messrs. Macmillan for the use of certain figures in Foster's ' Text-book of Physi- ology ' and Huxley's ' Lessons in Elementary Physiology.' My thanks are also due to Messrs. Stahel of Wiirzburg, for permission to publish Fig. 159 from Dr. Michel's paper 'Zur Kenntnis der Giirber'schen Serum-Albumin-Krystalle.' F. S. London, August, 1907. Digitized by Microsoft® PREFACE TO THE SECOND EDITION I greatly regret the considerable delay which has occurred in the production of this edition, but it has been unavoid- able ; the work has been rewritten in order to admit of its scope being enlarged, and this has taken longer than I anticipated. In the revision of the sheets I have received great assistance from Professors M'Kendrick, Halliburton, Hay- craft, Sherrington, and Dr. Waller. Professor Sherrington revised the whole of the Nervous System and supplied Figure 58. Professor Mettam, of the Eoyal Veterinary College, Edinburgh, kindly wrote the chapter dealing with the Development of the Ovum, while to Professor Macqueen, of the London Veterinary College, I am indebted for many useful suggestions and valuable criticism. To all these gentlemen I offer my cordial thanks ; their corrections, suggestions, and criticisms have been of the greatest help, and cannot fail to enhance the value of the book. As in the first edition, I have avoided dealing with histology, excepting where such was necessary to the clear understanding of the subject under consideration. After due deliberation, I determined not to introduce for the present the metrical system of weights and measures. The number of illustrations has been doubled, and for electrotypes of blocks I am indebted to Professor Foster, of Cambridge ; Professor M'Kendrick, of Glasgow ; Professor Hamilton, of Aberdeen ; and Dr. Waller, of London. The rewriting of this edition has unfortunately necessi- tated an increase in the size of the book. Woolwich, September, 1895. Digitized by Microsoft® Digitized by Microsoft® PREFACE TO THE FIRST EDITION My object throughout this manual haB been to condense the information as much as possible, for which purpose I have omitted all special reference to the physiology of the dog, and have not touched upon the histology of the tissues, or methods of physiological inquiry. The reasons for these omissions are obvious : special canine physiology is of subordinate interest to the pro- fession, and our information about this animal is so com- plete, that when required no difficulty is experienced in obtaining it from human text-books. The histology of the tissues is already before the profession, and methods of physiological inquiry are only needed for laboratory work, for which purpose this book is not intended. In the description of the physiology of the various organs and tissues the horse is necessarily taken as the type, but the ox, sheep, and pig are dealt with wherever their special physiology requires it. • It was my original intention to publish nothing until I had gone over the field of equine physiology, but I found after several years of work, that the information I had collected was a mere drop in the ocean, for inquiries of this kind are necessarily slow, and as there appeared no reason- able prospect of covering within the space of one life the ground I had mapped out, I was advised that only good could result from placing on record what little we know of veterinary physiology. I have, therefore, ventured, I know well how imperfectly, to state the broad facts of the science, so as to render them of use to the student and practitioner. The work does not pretend to be anything more than a stepping-stone to the study of physiology ; for those requiring more detailed information, reference must be made to the various text- books of human and comparative physiology which are available. Digitized by Microsoft® xii PEEFACE TO THE FIEST EDITION Incomplete as the work is, it would have been still more so but for the assistance I have received from my friend Dr. Sheridan Lea, F.E.S., of Caius College, Cambridge, who, at great personal inconvenience, has kindly read and revised nearly all the sheets as they passed through the press. In saying this, and expressing to him my very great indebtedness, I in no way wish to shift the respon- sibility for error or inaccuracy which may exist, but I feel that whatever merit the book possesses is entirely due to him. I have to thank Professor Michael Foster, F.E.S., for the loan of many of the woodcuts which illustrate this manual, and elsewhere I have acknowledged how much I owe to his encouragement. To my friend and colleague, Assistant-Professor Butler, A.V.D., my best thanks are due for assistance in revising the proofs, and in the preparation of the index ; to Mr. W. Hunting, F.B.C.V.S., for suggestions on the chapter dealing with Locomotion ; and to Professor M'Fadyean for the loan of two woodcuts illustrating the chapter on the Foot. To facilitate the study of locomotion, I have had the plates so arranged as to face as nearly as possible the letterpress describing the movements. I have laid under contribution Colin's invaluable ' Traite" de Physiologic compared des Animaux ' ; Ellenberger's 'Physiologie der Haussaugethiere ' ; Foster's, M'Kendrick's, and Landois and Stirling's Text-books of Physiology; Gamgee's translation of ' Hermann's Physiology ' ; the same author's ' Physiological Chemistry of the Animal Body '; Halliburton's ' Text-book of Chemical Physiology and Pathology ' ; Bunge's ' Physiological and Pathological Chemistry ' ; Meade Smith's ' Physiology of the Domestic Animals,' and others mentioned in the text. With reference to Dr. Meade Smith's work, I regret to find that on page 105 I have inadvertently given the title as ' A Text- book of Comparative Physiology.' I have endeavoured to acknowledge all sources of in- formation, though it is possible that in drawing from such a wide area I may have omitted in places to do so. Army Veterinary School, Aldershot, August, 1892. Digitized by Microsoft® CONTENTS CHAPTER PAGE I. THE BLOOD ------ 1 II. THE HEART- - - - - - 28 III. THE BLOODVESSELS- - - - -55 IV. RESPIRATION - - - - - 84 V. DIGESTION - - - - - - 131 VI. THE LIVER AND PANCREAS - - - 218 VII. ABSORPTION ..... 242 VIII. DUCTLESS GLANDS AND INTERNAL SECRETIONS - 264 IX. THE SKIN ------ 271 X. THE URINE- ..... 285 XI. NUTRITION - - - - - - 314 XII. ANIMAL HEAT - - - - - 336 XIII. THE MUSCULAR SYSTEM ... - 352 XIV. THE NERVOUS SYSTEM - 382 XV. THE SENSES ..... 454 XVI. THE LOCOMOTOR APPARATUS - - - 505 XVII. THE FOOT ------ 537 XVIII. GENERATION AND DEVELOPMENT - - - 577 XIX. GROWTH, DECAY, AND DEATH - - - 623 XX. THE CHEMICAL CONSTITUENTS OF THE BODY - 632 INDEX 667 xm Digitized by Microsoft® v aiirenneit. uenugi wi- Water bolls ,_„* e/o e — 2/2° r Inches Centimetres 14- 10 ^z 1 ^z: — 5" /\ Digitized by Microsoft® 32° = ■ jo" — A COMPARISON OF SOME BRITISH AND METRIC UNITS. Degrees Fahrenheit = § C.° + 32. Degrees Centigrade = § (F.°-32). Length 25-4 millimetres 304-8 1 inch: 1 foot= 1 yard 1 mile = 1609 - 3 metres 5 miles = 2 - 54 centimetres. = 30-48 = 91-44 = 1-609 kilometres. = 8 kilometres (nearly). Weight 1 metre = 1,000 millimetres = 39'37 inches. 1 centimetre = yj^ metre = - 39 inch. 1 kilometre — 1,000 metres = 0-62 mile. ^8 kilometres - - = 5 miles (nearly). 1 grain =0'064 gramme. 1 ounce (avoir.) = 2835 grammes = 457'5 grains. 1 pound „ =453 - 60 „ = J kilogramme (approx.). 1 cwt. „ - - - = 50"8 kilogrammes. 1 ton „ - - =1,016 „ 1 kilogramme = 1,000 grammes 1 gramme 1 milligramme =xr>m gramme = 22 pounds (avoir.) . = 15-432 grains. = 0'0154 grain. '1 fluid ounce = 28-4 cubic centimetres. lpint =568-0 ,, „ 1 gallon = 4 - 54 litres. lpeck = 9-08 „ 1 bushel = 36 32 „ ,, ., 1 cubic inch = 16-38 cubic centimetres. Capacity** foot = 2 8-31 litres. 1 litre = 1,000 cubic centimetres = 1 cubic decimetre = l - 76 pint (imperial). 1 cubic centimetre = 0'061 cubic inch. ,1 cubic metre = 1,000 cubic decimetres = 35-3 cubic feet. Work 1 1 1 foot-ton =309-12 1 kilogramme-metre = 7*25 foot-pounds. 1 unit of heat (British) = heat necessary to raise 1 pound of water through 1° F. 1 calorie (Metric) = heat necessary to raise 1 gramme of water through 1°C. Mechanical equivalent of heat-unit = 772 foot-pounds. „ ,, calorie =424 gramme-metres. „ ,, kilo-calorie =424 kilogramme-metres. Digitized by Microsoft® CORRIGENDA Page 19, top line, for. ' action ' read ' difference.' 59, line 7 from bottom, for 'all' read 'during.' 94, line 17 from bottom, for 'occupy' read 'occupying.' 140, line 19 from top, for 'Appendix ' read ' Chapter XX. 171, footnote, ' H. J. Brown ' should be ' H. T. Brown.' 185, line 10 from bottom, the semicolon to be a comma. 225, line 9 from top, after ' such ' insert ' power.' 337, bottom line, /or ' come ' read ' comes.' 429, line 5 from bottom, for ' dilatation ' read ' dilating. ' 439, line 2 from bottom, for ' cochlea ' read ' cochlear.' 472, line 8 from bottom, for ' revolves ' read ' rotates. ' 584, line 18 from top, for ' kreatine ' read 'creatine.' 636, line 15 from top, delete comma after ' albumin.' 636, line 22 from top, delete hyphen after ' blood.' 637, line 10 from top, for ' albumose ' read ' albumoses.' 637, line 14 from top, for ' alchol ' read ' alcohol.' Digitized by Microsoft® A MANUAL OF VETEBINAKY 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 pro- ducts of their activity. To enable all this to be carried out the blood is constantly in circulation, is rapidly renew ed, is instantaneously puxi^ed in the lungs and, by means of certain channels, is placed directly in communication with the nourishing fluid absorbed from the intestines, 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 haemoglobin 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 difference in the amount of oxygen with which the haemoglobin is combined. The reaction of blood is alkaline; as the process of coagulation occurs this alkalinity diminishes. The alkaline reaction is due to the phosphate and bicarbonate of soda found in the fluid; the decreasing alkalinity observed on 1 Digitized by Microsoft® 2 A MANUAL OF VETERINAEY PHYSIOLOGY standing is probably due to the formation of an acid. The alkalinity of the blood is reduced by muscular work, owing to the production of an acid by the muscles. The o^pjir of blood is believed to be due to a volatile body of the fatty acid series. The 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, though it is said that the o dour o f butyric acid can always be obtained from the blood of the latter by heating it with sulphuric acid. 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, 1060 ; sheep, 1050-1058 ; dog, 1050 (Colin). According to Hoppe-Seyler the specific gravity of the liquor sanguinis of the horse is 1027 to 1028, and the specific gravity of the cells 1105. 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 horses' blood when drawn from the body, producing during the process of clotting the so-called ' buffy coat.' The composition of the blood is almost absolutely uniform so far as the presence of various substances is concerned ; the amount of these substances, however, varies in animals of different classes. The source from which the blood is taken also affects its composition/; the blood from an artery does not represent exactly that found in a vein. Blood consists of : 1. A fluid part, Liquor sanguinis or Plasma, containing in solution proteids, extractives, mineral matter and gases, the latter in a state of loose chemical combination. 2. Corpuscles. a. Red corpuscles. /3. White corpuscles. 7. Platelets. The liquor Sanguinis, or Plasma, forms about 66 per cent, of the total blood ; it is an albuminous fluid containing a small and variable amount of a yellow colouring matter of Digitized by Microsoft® TliJU KLiVVV 3 a fatty nature. It holds in solution th ree proteid s — viz., fibrinogen, serum globuhn (par aglobufin) ," and seram albumin. Eecent researches have shown~that what has always been regarded as serum globulin consists in reality of two proteids, to which distinctive names have been given. It is a simple matter to separate these proteids from plasma, as they are differently acted upon by neutral saltg. For example, fibrinogen is precipitated by balf saturation with common salt, serum globulin is precipitated by saturation with magnesium sulphate, serum albumin may be wholly precipitated by ammonium sulphate. During the life of the blood the liquor sanguinis is termed the plasma, but after it has been shed from the body and coagulation has occurred, the liquid residue is called 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, we may say that serum is plasma minus the fibrin-forming elements. Perhaps the nearest approach to pure plasma is the fluid found in the peri- cardium and abdominal cavity. The fluid effused into the pleural cavity during pleurisy is plasma to start with, but if the fibrin in it becomes thrown down (forming the so-called false membranes), the remain- ing fluid is serum which is no longer capable of clotting. The Proteids of Serum are serum globulin, serum albu- min, and a ferment produced as the result of coagulation. As fibrinogen is used up in the process of coagulation it is not found in serum, but a proteid known as fibrino-globulin appears, though in small quantities. Fibrino-globulin is produced from fibrinogen during the process of fibrin formation. In the following table a comparison is made between the proteids of the plasma and serum : Proteids of the Plasma. Proteids of the Serum. Fibrinogen. Serum globulin. Serum globulin. Serum albumin. Serum albumin. Fibrin ferment (nucleo- proteid). Fibrino-globulin. 1—2 Digitized by Microsoft® 4 A MANUAL OP VETERINARY PHYSIOLOGY It has been shown that the proportion in which serum globulin and serum albumin exist in the blood varies in different animals. In the horse and _ox the globulins are in exgess of the alburn-ins ; in man and the rabbit this is reversed. Analyses show that the amount of total proteids is rather more uniform than is that of the different albumins of which they are composed, as may be seen from the following table, which represents the weight of proteid (grammes in 100 c.c.) of blood plasma of different animals : Total Serum Para- Proteids. Albumin. globulin. Fibrinogen. Dog - - 6-03 3-17 2-26 0-60 Sheep - - 7'29 3-83 3-00 0-46 Horse - - 8'04 2-80 4-79 0-45 Pig - - 8'05 4-42 2-98 0-65 Fibrinogen is the precursor of fibrin, a substance of which we shall have more to say when dealing with coagu- lation ; it is found in blood plasma, but not in the serum, since it is converted into fibrin during the process of clotting ; it also" exists in the fluids exuded into the cavity of the chest, pericardium, etc. Corpuscles. — Blood examined under the microscope is found to consist of an enormous number of bodies termed corpuscles floating in the liquor sanguinis. These cor puscle s are of two kinds, rgd 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 be biconcave jdiscs, circular in shape, and possess- ing no nucleus (Plate I.) ; they are soft, flexible, elastic bodies, capable of having their shape""readily alterecTby pr§saure, which enables them to pass along the~finest capillaries. The colour of a single corpuscle is yellow, but when heaped together, they are red, and thus "give the colour to the blood. In all mammals excepting the Camel tribe the red cells are circular ; in all verieht&tes below mammals they are bi- conyex, oval, and nucleated. The corpuscles vary in size Digitized by Microsoft® THE BLOOD 5 in different animals, as may be seen in the diagram (Fig. 1). 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 tendency to lie in long rows, with their flat surfaces in close contact, resembling the appearance of a pile of pennies. This condition is not marked in horses' blood. A red blood-cell is composed of a s pongy stro ma, holding in its meshes the red colouring matter. The stroma or S*" "N: — ' ELeflllcmf ■O09i-mm / s^Z-~^^r\- ~ M an •0077 / /v^^^vvV '- — -Cat ■0 055 1 fl/fP~~\\YY\-\- Sheep ■0 050 [ [ [( ( ■ ) M-I-J-H- -, - -Goat , ■0041 \ \ \ \Vv'_ ■*-/-}-]-}— -j - - - - Musk-deer ■ 0025 VvC^v// ^-^ Fig. 1. — Diagram showing Eelative Size of Red Corpuscles of Various Animals (Stewart). framework of the corpuscle consists chiefly of nucle o- albumm, and also contains lecithin, cholesterin, and salts ; the r^ed colouring matter consists of an al buminou s. c rysEal - line substance, ha emog lobin, which forms no less than 90 to 94 per cent, of the total solid matter of the dried corpuscle. The number of corpuscles in the blood is determined approximately either by the method of Gowers or Malagsez. The principle on which these methods are based is the same — a known qua ntity of blo od is diluted with a known bulk of artificia l seru nTand thoroughly mixed ; of this a small drop is placed in a counting-chamber, which is ruled into squares, and examined under the microscope. The blood cells occu- pying 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 7,212,500, and in the ox 5,073,000. Taking the amount of blood in the horse as 66 lbs. (50 pints or 29 litres), Digitized by Microsoft® 6 A MANUAL OF VETEEINARY PHYSIOLOGY this gives 204,113,750,000,000 as the approximate number of red cells in the body (Bllenberger).* It is evident that a loss of water from the blood means a larger relative pro- portion 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 pregnancy and copious draughts of water. But apart from these conditions, it is undoubted that an actual increase or decrease in the number of red cells may occur, this 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 conse- quence it shrinks and becomes wrinkled. If the plasma be diluted the red cells swell. A *9 per cent, solution of sodium chloride causes the corpuscles neither to shrink nor swell ; this strength is known as ' physiological salt 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 something enormous, being equal for the horse to a square having a side of 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 either by freezing and thawing the blood alternately, or by the passage through it of electric shocks, or by the addition of certain agents such as c hlorof orm, ether, bjle salts, water, tannic or b^ric acids, etc., the haemoglobin becomes liberated from the broken-up cell and stains the naturally yellow plasma of a red colour. Further, the destruction of the corpuscles leads to the blood becoming transparent or, as it is termed. * laky.' The greater part of the red cell consists, as already stated, of hasmoglobin, a substance possessing a remarkable affinity for oxygen ; this it obtains at the lungs and leaves * ' Physiologie der Haussaugethiere.' Digitized by Microsoft® THE BLOOD 7 behind it 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. The process of oxidation in the lungs and reduction in the tissues is constantly occurring at every cycle of the circula- tion, with the ultimate result that the red blood disc 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, and also, probably, destroyed in the spleen and elsewhere. When the red cells die their hsemoglobin is set free, and decomposed into an iron-free residue from which, probably, all the pigments of the body are formed, especially those of the bile. The production of red cells is a matter of extreme rapidity, as may be witnessed,, for example, after haemor- rhage ; the seat of their formation is in the red marrow of bones, where they are formed from certain nucleated cells ; there are several varieties of cells in the red marrow, and it is not quite definitely settled which of these furnish the red blood cells. All other seats of formation are doubtful, though it should be mentioned that the formation of red cells from blood platelets in the blood stream has been put forward as of possible occurrence. In the embryo the future red cells for a certain period are nucleated and contain no hsemoglobin, but these are gradually replaced by non-nucleated haemoglobin-holding corpuscles 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. Blood Platelets are small bodies one-quarter the size of a red cell, which have been observed in the circulating blood, but can al$o be seen immediately after the blood is shed. They have been supposed by some to be the pre- cursors of the red cells, but this point is not as yet settled. We have mentioned that the red colouring substance haemoglobin is retained in the pores of the stroma of the red cells, and with this we must now deal. Digitized by Microsoft® 8 A MANUAL OF VETERINARY PHYSIOLOGY Haemoglobin is a most remarkable substance. It is a proteid, distinguished from the majority of the other mem- bers of its class by the comparative ease with which it may be obtained in a crystalline form, whilst, on the other hand, its behaviour in a dialyser is not that of a crystalloid but of a colloid. It is one of the most complex substances in organic chemistry, containing C, H, 0, N, S, and Fe, and its molecule is an enormous one, the molecular weight being quoted at 13,000 to 14,000. Crystals of haemoglobin when seen in bulk are of a dark-red or bluish-red colour ; they are extremely soluble in water, the solution being dichroic — viz., green by reflected and bluish-red by trans- mitted light. Hsemoglobin is remarkable as being the most important proximate constituent of the body contain- ing iron, the amount being about "4 per cent. The source of the iron is not settled, but there is an organic iron-con- taining substance in food known as licematogen, belonging to the nucleo-albumin group, which possibly furnishes it. Its formation from inorganic iron is probably of doubtful occurrence. The total amount of haemoglobin in a horse's body is about 8'8 lbs. (4 kilogrammes), and the amount of iron contained in this is about 257 grains (17 grammes). This calculation is based on the assumption that the amount of blood in the body is 66 lbs. In the 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, while in the total blood of the horse it forms 13-15 per cent., in the ox 9"96 per cent., sheep 1034 per cent., pig 12-7 per cent., and dog 9"77 per cent. (Ellenberger).* The younger the animal the less haemoglobin ; males have more than females, and castrated animals more than entires (G. Miiller).t Haemoglobin has a remarkable affinity for oxygen, and the ordinary laws relating to the absorption of gases by fluids and solids do not apply— as we shall see later when * ' Physiologie der Haussaugethiere.' -(■ Ibid. Digitized by Microsoft® THE BLOOD 9 dealing with Eespiration — to the absorption of oxygen by haemoglobin. According to Bohr, haemoglobin can absorb carbon dioxide, which combines with the globulin portion of the molecule. We have mentioned that when haemoglobin is charged with oxygen it is spoken of as oxy-haemoglobin ; when it has" discharged its oxygen, which it is capable of doing with considerable facility, it is described as reduced haemoglobin, or simply as haemoglobin. As oxy-haemoglobin it is charged with oxygen in the capillaries of the lungs, brought back to the heart and distributed all over the body ; in the tissues it gives up its oxygen, and as partially reduced haemoglobin is brought back by the veins to the heart for distribution to the lungs, where it renews its oxidized condition. Haemoglobin is never completely reduced in the body excepting in the last stage of asphyxia. Oxy - haemoglobin crystallises in some animals, horse, cat, dog, and guinea-pig, with facility; in others, ox, sheep, and pig, with difficulty. The crystals are generally rhombic plates and prisms, but the form differs according to the animal FlG - 2.— Crystals op (Fig. 2). Beduced haemoglobin can ^^6^0™' only be crystallised with great difficulty rel; c, Guinea- , , „ n pig (Stewart). in an atmosphere free from oxygen. v ' When examined by the spectroscope the two haemoglo- bins produce quite distinctive spectra, by which they may be readily recognised. To put the matter broadly, oxy- haemoglobin 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 nearly the same position (Fig. 3). Oxy-haemoglobin may readily be reduced to haemoglobin by the addition of Stokes's Fluid (an alkaline solution of ferrous tartrate). Oxygen and haemoglobin are so lightly bound together Digitized by Microsoft® 10 A MANUAL OF VETERINARY PHYSIOLOGY that they are readily separated ; oxygen is given off if the blood be placed in a vacuum or boiled, or if it be brought into contact with indifferent gases such as nitrogen and hydrogen ; it is the facility with which haemoglobin parts with it's oxygen which enables the tissues to obtain it. Hemoglobin forms certain compounds with oxygen, carbon monoxide, and nitric oxide : With oxygen it forms oxy-haemoglobin and methssmoglobin. „ carbon monoxide it forms CO haemoglobin. ,, nitric oxide „ NO „ BCD B * a Oxy-hfe- moglobir G ' ! l Haemo- globin (reduced E 1: ,_ J F IG . 3.— Blood Spectka (Waller). Oxy-haemoglobin we have dealt with ; the others, in a work of this kind, can only receive a short notice at our hands, though the subject is full of interest. Methaemoglobin is produced by allowing blood to be ex- posed to the air until it becomes brown in colour and acid in reaction ; or it may be prepared by the action of acids or alkalies on oxy-b.£emoglobin. This substance separates from its oxygen with difficulty, and gives a three-banded spectrum. Methasmoglobin does not occur normally in the body, 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. Carboxy-hsemoglobin. — In this compound the oxygen is replaced by carbon monoxide, which forms a stable com- pound with the hemoglobin and is not displaced on breath- ing oxygen ; hence the rapidly fatal results of this form of poisoning. The blood of people who have died from CO poisoning is of a cherry-red colour, and yields the spectrum of CO-hsemoglobin — viz., two bands very much like those Digitized by Microsoft® THE BLOOD 11 of oxy-hsemoglobin, though somewhat darker and situated slightly nearer to the violet end of the spectrum. These two bands are unaltered by Stokes's Fluid. Nitric oxide haemoglobin in many respects resembles CO-hsemoglobin. Hsemoglobin is easily decomposed either by boiling or the addition of alkalies, acids, or acid salts ; in either case it splits up into a substance containing the iron, known as hsematin, and a proteid substance or substances termed globin. Hsematin in the dry state strongly resembles iodine in appearance ; it has a metallic lustre, a blue-black colour, is not crystallisable, and yields, when pulverised, a dark brown powder which contains 8'82 per cent, of iron. Hsematin is a remarkably stable substance, and the colouring matter presents a distinctive spectrum both in an acid and alkaline solution. Alkaline solutions of hsematin can take up and give off oxygen like hsemoglobin. When hsematin is treated with glacial acetic acid and common salt it yields hsemin, which, when examined microscopically, is found to consist of prismatic crystals, dark, or nearly black in colour. Hsemin crystals may be readily produced by warming the dried blood with a drop of glacial acetic acid on a slide ; this is used as a microscopical test, but it is said that from the blood of the ox and pig hsemin can only be obtained in very irregular crystalline masses. When reduced hsemoglobin is decomposed by acids or alkalies, oxygen being carefully excluded, it yields hsemo- chromogen, a substance presenting a definite spectrum and thus a ready means of detecting old blood-stains. Haema- toporphyrin is obtained by the action of strong sulphuric acid on hsematin, which thereby loses its iron ; hsematopor- phyrin is really hsematin from which the iron has been removed ; it is isomeric with bilirubin. L Hydrobilirubin is obtained by the action of reducing agents on hsematin ; it very closely resembles urobilin, a pigment found in urine. Hsematoidin (Fig. 4) is found in old blood-clots and in the ovary; it is a crystalline iron-free product derived from hsematin, and gives the same reaction with nitrous Digitized by Microsoft® 12 A MANUAL OF VETERINAEY PHYSIOLOGY acid as bile pigment, viz., a play of colours. Hsematoidm is, in fact, chemically identical with bilirubin, and the name is now of interest merely as indicating the close genetic relation- ship of the pigments of bile to the colouring matter of blood. Not- withstanding this close relation- ship, it has not as yet been found possible to convert hsematin into bilirubin. The nearest approach Fig. 4.-CEYSTALS of t bilirubin is i ron , free hfematin ILematoidin (Stewart). (hsematoporphyrin). 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 bilirubin in the digestive canal, under the influence of putrefactive organisms. The White Corpuscles, also termed leucocytes, are found in blood, lymph, pus, connective tissue, etc. ; they exist in blood in the proportion of 1 in 300 to 1 in 700, the propor- tion varying according to the vessel from which the blood is examined. In the splenic artery there are very few, in the splenic vein they are exceedingly numerous. Blood which has been removed from the vessels contains but few, for the reason that they are probably broken down during the formation of fibrin. The white corpuscle is somewhat larger than the red ; it consists of a granular-looking protoplasm, within which is a nucleus ; the nucleus shows no sign of a nuclear network, which is a distinguishing difference between the white cell and its very close ally the lymph cell. The granular condition of the corpuscle is due to minute particles of fat, proteid, and probably other substances, which are on their way either to or from the tissues, probably both. There are at least five varieties of colourless corpuscles : (1) The polynuclear, which are very numerous and consist of a cell containing two or three nuclei united by fine threads ; (2) hyaline leucocytes, relatively few in number Digitized by Microsoft® THE BLOOD 13 containing a single nucleus and more protoplasm; (3) eosinophile cells, consisting of large masses of granular protoplasm with a simple or lobed nucleus : the granules stain deeply with eosin ; (4) lymphocytes derived from the lymphatic glands containing alargg" spherical nucleus and limited protoplasm ; (5) basophile leucocytes, which are very rare and distinguished by staining with basic dyes and a methylene blue (Plate I.). The white corpuscles are capable of undergoing changes in shape ; the movements known as amceboid are exhibited by projections shooting out from the surface and being again retracted (Pig. 5). The amoeboid movements are destroyed by heat or by shocks from an induction coil. These Fig. 5. — Amoeboid Movement. A, B, C, D, Successive changes in the form of an amoeba (Stewart). changes in shape assist materially in the passage of the corpuscles through the walls of the vessels into the tissues- The process is termed diapedesis ; within moderation it is a perfectly normal phenomenon, though under inflammatory and other disturbing influences it becomes greatly ex- aggerated. The white corpuscle has the power of taking up into its interior small particles of colouring matter, bacteria, etc., the importance of which will presently be alluded to. The white corpuscles contain about 10 per cent, of solids. The cell protoplasm consists of proteids belonging to the globulin and nucleo-proteid groups, while the nucleus con- sists of nuclein which is remarkable as being a very stable substance and also as containing phosphorus. The nucleo- Digitized by Microsoft® 14 A MANUAL OF VETERINARY PHYSIOLOGY proteid obtained from the protoplasm is probably the pre- cursor of the fibrin ferment. Besides these we have the complex fatty body lecithin, cholesterin, glycogen (especially in the horse), salts of potash and phosphates, the latter being probably derived from the phosphorus-containing compounds. The origin of the white corpuscles is from the lymphatic system, from which they enter the blood stream through the large lymphatic channels opening into the vena cava at the junction of the two jugular veins. The hyaline corpuscles are derived from the lymphocytes, the polynuclear and eosinophile produce themselves in the blood stream by cell division. The white corpuscles, as well as the red, are constantly being used up and as constantly replaced. They also possess 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 the destruction of which we are unable to account for, 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. 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 under- standing 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 ; it is really a fight between bacteria and leucocytes. The protection afforded to the system by the white blood cells is, therefore, not the least important of the functions performed by them, and whether they accomplish this duty thoroughly or imperfectly depends largely on the composi- tion of the blood plasma (see p. 26). ' Coagulation.— We are now brought to a consideration Digitized by Microsoft® THE BLOOD 15 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 crassamentum, 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. 6). This gradually accumulates, and as it becomes abundant 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 com- pleted. The result of this is that the upper solid layer is considerably decolourised, form- ing the so-called buffy coat, which though natural to the blood of 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 coagula- tion 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 the blood of the horse clot so rapidly as to be almost instantaneous. One thing in connection with horse's blood is undoubted, and that is that coagulation is more easily slowed or prevented by cold and neutral salts than it is in the blood of any other warm-blooded animal. May it not be that some confusion Fig. 6. — Diagram of Clot with Buffy Coat (Stewart). v, Lower portion of clot with red corpuscles ; w, white corpuscles in upper layer of clot ; c, cupped upper surface of clot ; s, serum. Digitized by Microsoft® 16 A MANUAL OF VETERINAEY PHYSIOLOGY 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 coagu- lation is as follows : Kg Sheep Dog Ox - Horse £ to 1J minutes. 1 ,. 8 5 „ 18 „ 5 ,, lo ,, 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 appearance is well described by its name. If instead of allowing the blood to clot spontaneously it be whipped with a rod or bunch of twigs, or, as we say, is ' defibrinated,' 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 pro- duction of fibrin. These changes may be graphically represented thus : Blood. Blood. I Plasma. On Clotting. j Serum, t Fibrin. (Red. I Corpuscles, j White. ' ''"'■ ( Blood platelets. ( Plasma. When Whipped. j Fibrin. I Serum. , | (Red. I ICorpuscles. ■] White. j" Defibrinated blood. (.Blood platelets.) Digitized by Microsoft® THE BLOOD 17 - Fibrin is a yellowish-white, stringy-looking, bulky mass. It may be dissolved by dilute hydrochloric acid, forming acid-albumin or syntonin, also^by dilute alkalies with the production of alkali-albumin, and by the prolonged action of neutral salts, with the formation of globulins. Its bulky appearance would lead to the belief that it exists in blood in large quantities ; it is found, however, to be by weight relatively small. In human blood its proportion is "2 per cent. ; sheep, "2 to "3 per cent. ; ox, "3 to '4 per cent. ; horse, '4 per cent. ; pig, -4 to "5 per cent. ; dog "2 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 — vizi., that clotting is due to the conversion of a fluid fibrinogen into a solid fibrin, under the influence of a ferment. If blood be prevented from coagulating plasma can be obtained, and this plasma, depending upon the agents used in its production, will teach us the main facts of coagulation. If it be obtained by cooling the blood, then the plasma will clot spontaneously by allowing the temperature to rise ; if the plasma be obtained by previously mixing the bid with a definite amount of magnesium or sodium sulphat or common salt, clotting can be obtained by diluting it. If it be obtained by acting on blood with oxalates, then clotting can be brought about on the addition of a lime salt, and if it be peptone plasma (see p. 20), simple dilution will cause it to clot. The clot formed by the plasma coagu- lating 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 saturation, a precipitate of fibrinogen occurs ; it is a proteid belonging to the globulin group, and has previously been alluded to. If this precipitate be re-dissolved by diluting the fluid and allowed to stand, it clots spon- taneously. If a solution of pure fibrinogen be prepared, it does not clot spontaneously, but it may be made to do so 2 Digitized by Microsoft® 18 A MANUAL OF VETBEINAEY PHYSIOLOGY by the addition cf a drop of serum or the washings of a blood-clot. The interpretation of all this is that the substance which brings about coagulation of the blood is contained in the plasma. This substance is fibrinogen, but fibrinogen will not work alone ; it requires a very small quantity of something else, and this something has been termed the fibrin ferment. It is called a ferment inasmuch as a very small amount of it is capable of acting on an indefinitely large amount of fibrinogen, and that its action is closely dependent on temperature. The ferment is known as thrombin ; it does not exist as such in the living blood, but can readily be obtained from blood which is shed. Thrombin can be injected into the general circulation without producing any ill effect. Ex- perimental inquiry shows that there is in the blood a precursor of thrombin spoken of as pro-thrombin, and that under the influence of calcium salts the pro-thrombin is converted into thrombin. In fact, under the influence of calcium salts, any of the tissues of the body, especially lymphatic glands, will provide a thrombin. The substance from which this thrombin is obtained is known as nucleo- albumin, and if nucleo-albumin be injected into the blood- stream clotting at once occurs. Chemically very little is known of the blood ferment beyond the fact that heating to 131° F. (55° C.) destroys it. The substance from which it is formed, pro-thrombin, is rich in phosphorus and contains nuclein. Histologically pro-thrombin appears to be identical with blood platelets, ind the latter may be observed, when repairing the iamaged wall of a bloodvessel, to plug it with a substance resembling fibrin in appearance. It is, in fact, by this means that hEemorrhage gradually tends to cease. Circumstances influencing Coagulation.— It is a matter of jommon observation, that after death the coagulation of blood in the vessels is a slow process, though by exposure ;o air clotting is almost at once produced. At one time it •254 4-351 3-717 Pig - - 5-543 1-504 •273 4-272 3-611 The use of the salts is to assist in secretion, repair, and lisintegration. The growth of the solid tissues of the 3ody absolutely depends on the inorganic material supplied )y the blood. Water free from salts is destructive to pro- Digitized by Microsoft® 22 A MANUAL OP VETEEINAEY PHYSIOLOGY toplasm ; no doubt, therefore, one important function of the salts in the blood is to maintain the vitality of the tissues. Sodium chloride is here especially valuable, and its exten- sive 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 means 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 100° P. to 105° F. (37"8 C. to 40-54° C), 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 this 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 monoxide 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 - -[V s6 ' 6 per cent, of the body weight. Ox - T V = 7-71 „ Sheep - A = 8-01 „ Kg - *=** Dog - tV to T V = 5-5 to 91 per cent, of the body weight. * Ellenberger's ' Physiologie der Haussiiugethiere.' Digitized by Microsoft® THE BLOOD 23 The same observer gives the amount of blood in the body of the horse at 66 lbs., or nearly 50 pints. The Distribution of Blood in the Body (Fig. 8) 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 Fig. 8. — Diagram to illustrate the Distribution of the Blood in the Various Organs of a Rabbit, after Ranke's Measurements (Stewart). The numbers are percentages of the total blood. 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, The Gases of Blood. — The blood gases are obtained by introducing the fluid into a Toricellian vacuum, the in- strument used to obtain it being a mercury pump. In a vacuum the blood froths up and gives off its gases, which are then collected and analysed. The gases are oxygen, carbon dioxide, and nitrogen. The proportion of these found depends upon whether the blood be taken from an artery or a vein ; in the former the oxygen is present in larger amount than in the latter, and the carbon dioxide is less. The nitrogen is in both cases practically the same. Digitized by Microsoft® 24 A MANUAL OF VETEEINAEY PHYSIOLOGY At a pressure of 30 inches (760 mm.) of the barometer and a temperature of 82° F. (0° G-), the following gases are found in 100 volumes of blood : Arterial. Venoi Oxygen 20 12 Carbonic acid 40 45 Nitrogen 2 2 62 59 The exact amount of gas varies ; the above can only be taken as convenient averages. Oxygen exists in arterial blood in the proportion of about 20 per cent, per volume of the blood, whilst in venous blood it is found to vary within wide limits, depending upon the vessel from which it is taken, and the activity of the part at the time of its collection. In the blood of asphyxia it is nearly absent. It will be remembered that by far the greater part of the oxygen was stated to be in combination with the haemoglobin of the red blood-corpuscles ; in fact the pro- portion of oxygen in the blood bears a definite relation to the amount of iron contained by the haemoglobin. It has been determined that 15h grains (1 gramme) of haemoglobin are capable of absorbing "095 cubic inch (l - 55 c.c.) of oxygen. Whatever oxygen the serum of blood contains is simply absorbed ; the amount held in solution is therefore small. Oxygen chemically united with haemoglobin is quite independent of the laws which regulate the absorption of gases (see Eespiration). Besides the vacuum of the air-pump, various chemical substances have the power of deoxidizing the blood-cells ; such reducing substances are ammonium sulphide, sul- phuretted hydrogen, some iron salts, etc. Blood exposed to the air loses oxygen, due to the production of reducing substances formed as the result of decomposition. The Carbon Dioxide in arterial blood is about 40 per cent. ; in venous blood it varies, depending on the vessel from which the blood is drawn. The C0 2 is principally combined Digitized by Microsoft® THE BLOOD 25 with the sodium carbonate in the plasma of the blood, though Bohr considers the haemoglobin is also a carrier. , The Nitrogen in the blood is small in amount, about 2 vols, per cent. ; it does not vary in arterial or venous blood, as in both cases it is simply absorbed by the plasma. Composition of the Blood.— Keviewing the various analyses which have been published of the blood of animals, the following represents the average composition of the fluid : The Plasma. Water - - - 90 parts per cent. Proteids - - 8 or 9 parts. Pats- - - . -l Extractives - - - -4 „ Salts - - - -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, "1 per cent, proteids. Salts - - 1 „ Taking the blood as a whole the following represents approximately its composition in every 100 parts : Water - - 81 parts. {Haemoglobin - 13 parts. Proteids - - 4 „ Salts - - 1 „ Extractives - '6 „ Defensive Mechanisms of the Body. — If the serum of one animal be injected into another of a different species, it may cause the corpuscles to break up {hcemolysis) and haemoglobin to appear in the urine. This destructive effect is found to occur whether the blood be injected into the circulation, or merely added to the foreign blood in vitro. The action is termed globuUcidal, and it can be abolished by previously heating the added serum to 132° F. (55° 0.). Not only will the serum of one blood destroy the corpuscles of another, but it will also destroy Digitized by Microsoft® 26 A MANUAL OF VETERINARY PHYSIOLOGY certain bacteria (bacteriolysis), and the effect on these is greatly increased, if the animal furnishing the serum has previously been treated with an intravenous injection of similar bacteria. These facts have opened a field of therapeutics still in its infancy and endowed with great possibilities. The substance produced in the blood which acts as a protective is known as an antibody ; it is a defensive mechanism of the greatest importance. An antibody is not necessarily the result of bacterial activity, it may be produced in a blood by the injection of almost any proteid, and the serum so obtained is capable of precipitating that particular proteid from solution and no other ; such a body is known as a precipitin. An animal may, by carefully graduated doses of virus, be rendered completely immune to a dose sufficiently large to kill many hundreds of unprotected animals. The blood serum of the one so protected may be employed in the treatment of others naturally infected or unpro- tected ; such a sorum may be both curative and protective, an example of which is rinderpest serum, or it may only be protective, as in tetanus. Another defensive mechanism of physiological value is phago- cytosis. No one can possibly doubt the difference in the resisting power to disease of ' fit ' over ' unfit ' animals, nor the greater protection afforded by maturity as compared with youth. These facts assure the perpetuation of the species and are probably intimately connected with this question. When referring to phago- cytosis (p. 14) we stated that the thoroughness with which the phagocytes did their work depended upon the composition of the blood plasma. It would appear that it does not matter much from what source the leucocytes are derived, they are all capable of turning out equally good work if the blood plasma contains sufficient of a substance which acts upon the bacteria, and renders them more easily eaten by the leucocytes (Wright). The nature of the substance is unknown, but it would appear to act chemically on the bacteria, and render them an easy prey to the leucocytes ; it does not act upon nor stimulate the leucocytes. This substance is known as an opsonin, and it is probable that there are several varieties in the plasma, each having its own particular microbic infection to deal with. Opsonins must not be confused with bacteriolysins, agglutinins (anti-bodies which agglutinate bacteria) or antitoxins, from which they are quite distinct. 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 Digitized by Microsoft® THE BLOOD 27 means of the blood that the final and complete invasion of the body is effected. Nor does the 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- microscopic, and in connection with this question some of the most acute and fatal infectious diseases of animals are caused by organisms of this class, of which rinderpest, foot and mouth disease, rabies, and African ' horse sickness ' are examples. Still, 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 thein are sufficiently large to be caught in the pores of a filter, leaving the filtrate sterile. Other organisms attack the blood cells, for example the important group of Trypanosomes, the malaria parasite, the organism of Texas fever, and such like. In these cases the product of red cell destruction may show itself by the discoloured urine and is evident in the tissues, for example the liver and spleen. Compared with the red corpuscles the white are seldom affected with disease, but there are certain affections of the spleen associated with a great increase in their number. 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 haemoglobinuria, in which the animal in the middle of work suddenly falls [paralysed, the urine becomes coffee-coloured and loaded with methasmoglobin, 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 proteid disintegration, which will be found dealt with in the chapter on digestion. Blood-letting in the treatment of disease was at one time so universal that it came to be regarded as the ' sheet-anchor ' of life, and animals were regularly bled in order to keep them well. ' Blood- letting ' was killed by abuse ; it is a question whether the pendulum has now travelled too far in the other direction, and the employment of a physiological means in the treatment of disease been too long neglected. Digitized by Microsoft® CHAPTER II ' THE HEART The blood in the body has to be kept in constant motion, so that the tissues which are depending upon it for their vitality may be continuously supplied, and also in order that the impure fluid resulting from the changes in the tissues may be rapidly and effectually 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 compartments, 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. 9). So 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 sent into 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, 28 Digitized by Microsoft® THE HEAET 29 where it is purified ; into the left heart this arterial or purified blood is brought back from the lungs for distribu- tion 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 and back to the right side of the heart is called the Systemic circulation (Fig- 10). Mention has been made of valves in the cavities of the heart; they are found on both sides separating auricle Fig. 9. — Diagram of the Circulation through the Heart. 1 and 2, The vense eavee ; 3, right auricle ; 4, right ventricle ; 5, pulmonary artery ; 6, 6, pulmonary veins ; 7, left auricle ; 8, left ventricle ; 9, aorta dividing into anterior and posterior. The arrows represent the direction taken by the blood stream. Erom ventricle, and are known as the right auriculo-ventri- 3ular or tricuspid valve, and the left auriculo-ventricular ar mitral valve. Besides these, valves are found in the vessels arising from the ventricles, viz., in the pulmonary irtery and the aorta ; these valves, pulmonary and aortic, ire known as the semi-lunar valves. No valves are found guarding the entrance of the vessels (veins) into the auricles. Digitized by Microsoft® 30 A MANUAL OF VETEEINAEY PHYSIOLOGY In order to understand the function of these valves, which play such an important part in the physiology 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 Circulation. — The venous blood from the whole of the body flows into the right auricle by means of the anterior and posterior venae cavae ; it passes from here through the tricuspid valve into the right ventricle ; from Fig. 10. — Diagram of the Circulation of the Blood. 1, The heart; 2, 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 ; 11, the circulation through the hind extremities; 13, the circulation through the kidney. tbe right ventricle it travels to the lungs by means of the pulmonary artery, where, having been exposed to the action of the air and become greatly changed in its gaseous com- position, it returns to the .heart by means of the pulmonary veins, emptying itself into the left auricle. It 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. 10). The use of the valves is to allow of and to insure the transference of blood from auricles to ventricles, and from Digitized by Microsoft® THE HEAET 31 e ventricles to the aorta and pulmonary artery without y chance of regurgitation. This they do in virtue of the 3t that they are so constructed and arranged as to open ly in that direction towards which the blood has to be nt. Position of the Heart. — The heart occupies a position in e middle line of the chest, being enclosed in a sac, the ricardium, and suspended from the spine by its aortic ssels. Its base is uppermost, its apex nearly touches the irnum, and the organ occupies in the horse a position (•responding to the third, fourth, fifth, and sixth ribs. It between the fifth and sixth ribs, at their sternal insertion, iere the impulse or ' beat ' of the heart may be felt in the rse. Its other relations are with the diaphragm which just behind the apex, but with which it has no structural mection. On its right side is the right lung, and on left part of the left lung ; there is a triangular notch in 3 left lung of the horse which exposes the left ventricle, d allows it to make its impulse felt against the chest 11. The anterior face of the heart is formed by the right ricle and ventricle, the posterior by the left auricle and itricle. Heart Muscle. — The heart is an involuntary muscle, but 3s not conform histologically to the involuntary muscular res met with in other parts of the body. The muscle is I in appearance; microscopically its fibres are short, iated both in a cross and longitudinal direction, possess sarcolemma, and anastomose freely. The network med by the fibres of the heart is a most distinctive ture of cardiac muscle. The contractile tissue, though •ken of as a fibre, is in reality a quadrilateral nucleated [. In some animals, sheep and ox in particular, cells of teculiar kind are found immediately beneath the endo- dium ; they are polyhedral in shape, containing proto- Bm and a nucleus, and are surrounded by striated fibres ; y are called the cells of Purkinje. ?he arrangement of the fibres of the heart is peculiar; fibres of the auricle are quite distinct from those of the Digitized by Microsoft® 32 A MANUAL OF VETEKINAEY PHYSIOLOGY ventricle, and both are arranged in layers. Two layers exist in the auricle, circular and longitudinal, the circular fibres being continued around the entrance of the veins, whilst in the ventricle several layers exist of oblique, longitudinal, and circular fibres. Owing to the peculiar direction in which the oblique fibres run a somewhat spiral arrangement results. It has been shown that the auricles and ventricles, though separated by a fibrous ring, are yet connected by \ Pig. 11.- -Left Ventricle of Horse exposed to show Mitral Valve. 1, Portion of valve ; 2, columnce carnece, on the upper surface of which are found the muscwli papillares, to which the choreics tendimece are attached. bands of altered muscular tissue which pass through the ring. The cavities of the heart are lined by the endocardium which is reflected over the valves ; this membrane in the left auricle of the horse is of a peculiar grey colour. Certain fibrous rings are found in the heart where the valves are situated, and to which these obtain a firm attachment. The ring surrounding the aortic opening in the ox has constantly in its substance one or more pieces of bony tissue ; this is also common in the horse. Digitized by Microsoft® THE HEART 33 Valves of the Heart. — The auriculo-ventricular valves are nade up of fibrous membrane, in which a small proportion )f muscular fibre is found close to the attached border. Che mitral or bicuspid valve in the horse consists of one arge distinct segment, and several smaller ones united to orm a second ; the tricuspid consists of three segments, me, much larger than the others, being placed opposite to aur. vent Fig. 12. — Diagram to illustrate the Action of the Valves of the Heart (Huxley). n A the auricle is contracting, ventricle dilated, mitral valve open, semi-lunar valves closed. In B the auricle is dilated, ventricle contracting, mitral valve closed, semi-lunar valves open. Aur., auricle ; vent., ventricle ; v., v., vein ; a., aorta ; m., mitral valve ; «., semi-lunar valve. Note the manner in which the papillae have shortened in B, in order to compensate for the ventricular walls approximating. hat portion of the ventricle which leads to the pulmonary rtery. The free edges of all the valves are held in position by irge and small tendinous cords (chordce tendinece) composed f fibrous tissue, which are inserted into musculi papillares Dund on the internal surface of the ventricle; the cords com one papilla do not all pass to one segment of the valve, ut to two or three (Fig. 11). The function of the papillae is 3 restrain the valves from flapping back into the auricle 3 Digitized by Microsoft® 34 A MANUAL OP VETERINAEY PHYSIOLOGY during the contraction of the ventricle, and this they accomplish by gradually shortening as the walls of the ventricle approximate ; compensating by their shortening for the movement of the ventricular wall and thus exerting traction on the cords (Fig. 12). Other bands pass from one side of the ventricle to the opposite wall ; they are called moderator hands, and their function is to restrain the ven- tricular wall from undue distension. 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. 13). This may be readily demonstrated in the dead heart by tying the aorta and pulmonary veins, and introducing into the left auricle a tube which admits a powerful jet of water ; the left side of the heart dis- tends 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 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. The semi-lunar or sigmoid valves, which guard the entrance into the aorta and pulmonary artery, are com- posed of fibrous tissue, and possess at the centre of each segment a small hard body, corpus Arantii, which is particularly marked in the aorta. Fig. 13. — Tricuspid Valve in closed Position seen from the Auricle. Note the cracks in the surface, which represent where the margin of the valves meet and fold in against each other like the lips of a toothless mouth. Digitized by Microsoft® 5" THE HEAKT 35 Movements of the Heart.— If the exposed mammalian heart be watched at work, a great deal may be learned 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 of either auricle or ventricle is spoken of as its systole, while the subsequent relaxation is described as its diastole. Prior to any heart movement occurring the roots of the veins entering the auricles contract through the medium of the circular fibres surrounding them ; this contraction next sweeps over the auricles which are drawn downwards towards the ventricles, the auricular appendage taking an active part in expelling its contents. The ventricles then contract, but more slowly, and as they do so they shorten, become more circular in shape and owing to the oblique direction of the muscular fibres above described, there is a twisting or squeezing of the ventricular walls. The contraction of the ventricles does not begin at the apex, as might be supposed, but at the base, and extends from there to the apex. Further, there is no apex beat, for the apex does not move unless the pericardium be opened. 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. Further, the flow into the right heart is assisted by gravity, particularly the blood in the anterior vena cava, while even in the posterior vena cava this is by no means a negligible quantity. There is likewise an aspiration in the veins produced by a relaxa- tion of the walls of the auricle after the previous contraction, and an aspiration in the thorax the result of inspiration, 3—2 Digitized by Microsoft® 36 A MANUAL OF VETERINAEY PHYSIOLOGY which gives rise to a negative pressure in the veins leading to the heart (see p. 91). The auricles being now full, a wave of contraction which first appears at the vessels leading into them, passes over these chambers, which by a sudden sharp and brief contrac- tion empty their contents into the ventricles. The systole of the auricle produces a backward positive wave in the vessels leading into it, and this wave passing through the anterior vena cava, shows itself in the jugulars of the horse by a distinct pulsation at the root of the neck. The auricular contraction forces the blood into the ventricles, which have been partly filling during the time the auricles were distending, and the final filling of the ventricle by the auricular systole forces up the auriculo- ventricular valves, which bulge into the auricle under the increasing pressure to which the ventricular contents are exposed. The ventricles give a prolonged contraction, and owing to the spiral arrangement of their muscular fibres exhibit a peculiar movement. The pressure which now exists in the ventricles is greater than that in the vessels leading from them, and as the auriculo-ventricular valves cannot be thrown open upwards into the auricles owing to their chordae tendinese, the semi-lunar valves are forced open, and the stream of blood passes into the aorta and pulmonary artery. At the moment the ventricles contract, the heart slightly rotates around its vertical axis from left to right, while the left ventricle hardens and makes its impulse felt against the chest wall. The impulse is syn- chronous with the closure of the auriculo-ventricular valves, and the forcing open of the sigmoid valves. The systole of the ventricle produces a dull, booming, prolonged sound, which is brought about by the muscular contraction of its walls, and probably, at the same time, partly by a vibration of the auriculo-ventricular valves ; the sound is known as the first sound of the heart. The blood now rushes into the aorta and pulmonary artery, and the elastic resistance of these arteries being brought into play, the fluid has a tendency to regurgitate Digitized by Microsoft® THE HEAET 37 towards the ventricles; by this process the semi-lunar valves are closed, the closure being accompanied by the second sound of the heart, which is short and sharp. This sound is due to the sudden tension of the membranous flaps of the valves at the moment of their closure, which gives rise to vibrations. The semi-lunar valves are mechanically most perfect. The thin margins on either side of the corpora Arantii are closely pressed together, the corpora Arantii filling up the centre, and not a drop of blood passes back into the ventricles. These valves do not lie back close against the arterial wall during the exit of blood from the ventricle, but stand out in the stream, probably being kept there by reflux currents. They form a triangular orifice with curved sides. The arterial trunks which during the systole of the ventricles elongate and curve, now at the diastole shrink and shorten, and so bring the base of the heart back to its former place. The force of aortic reflux is not wholly expended on the valves, but largely on the muscular pads to which the valve3 are attached; to admit of this the diameter of the aorta is much greater than the opening out of the ventricle. It is not until the semi-lunar valves are firmly closed that the ventricles begin to relax ; this they now do, and the relaxation of the walls produces a negative pressure, viz., a pressure below that of the atmosphere, which in the dog has been measured at from 1 to 2 inches (25 to 50 mm.) of mercury. This negative pressure opens the auriculo-ven- tricular valves, the blood flows in from the auricle, while the auricle and ventricle, neither contracting nor dilating, assume a passive condition during a period known as the pause. Throughout the pause blood is flowing into the auricles from the pulmonary veins and vense cavse, and into the ventricles from the auricles ; towards the close of the pause the auricles contract, and the whole process is repeated. We have thus the contraction of the auricles, the con- Digitized by Microsoft® 38 A MANUAL OF VETERINARY PHYSIOLOGY traction of the ventricles, and the pause. The time each of these occupies has been determined for the horse, by Chauveau and Marey, by means of a cardiac sound. The value of the periods they give us is as follows : auricular systole, two-tenthB of a second, ventricular systole, four- tenths, and pause, four-tenths of a second. We cannot accept the value of these periods as correct, owing to the fact that they cause the horse to have a pulse of 60 to the minute, which is distinctly abnormal ; 36 to 40 beats per minute is the normal rate. A complete cycle of the horse's heart occurs, on an average, once in every 1*5 seconds ; but the time value of the various phases which make up this period can- not be exactly stated. No matter how fast the heart is beating, the frequency depends not on the duration of 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 contraction of the auricles, the following is a summary of the changes occurring in the heart : 1st Period. — The contraction of the auricles completes the filling of the ventricles. 2nd Period. — The ventricles contract, the auriculo- ventricular valves are closed, 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. 3rd Period. — The aortic and pulmonary valves close, the auriculo-ventricular valves open, the -second sound of the heart is produced, diastole of both auricles and ventricles occurs, followed by a long pause, during which 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 ; Digitized by Microsoft® THE HEAET 39 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. The Action of the Valves of the Heart during a cardiac cycle is peculiar and interesting. We have described how the auriculo-ventricular curtains are floated up as the ventricles fill, and how with increased pressure they come together, being prevented by the chordae tendinese, and the muscular pillars to which these are attached, from being pushed back into the auricle. Further, experimental in- quiry has determined that when the ventricular systole is at its height, these valves bulge upwards into the auricles, assuming a concave surface towards the ventricle ; this appears to be especially the case in the horse. The pulmonary valves, and probably the aortic, not only meet at their free border but actually overlap. Chauveau states that he has tried experimentally to hold back one of the pulmonary valves, but the others have applied them- selves so closely around the finger that it was impossible to produce a patent opening. In the aorta it is probable that )verlapping does not occur to the same extent, and here ihe corpora Arantii are of value. When the sigmoid valves ire not acting they still lie in the blood stream, and not igainst the wall of the vessel as was at one time supposed ; lor do those in the aorta cover the openings of the coronary irteries. It is generally believed that both the aortic and pul- nonary valves are closed by the regurgitation of the blood ; >ut it has been pointed out that as the blood is leaving both ventricles, it is streaming through orifices which at that ime are mere chinks, owing to the pads of muscle which ake their origin from all sides of the mouth of the vessels, /brtices are thus created in the space between the arterial oot and the edge of the valves. These vortices tend to )ress the edges of the valves together, and the valves con- equently close the moment the blood actually ceases to Digitized by Microsoft® 40 A MANUAL OP VETEBINABY PHYSIOLOGY 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 as due to the sudden tension, and not the closure, of the aortic valves at the time of the aortic recoil. - The Cardiac Sounds are really four in number, but as they are in pairs we recognise only two. The first sound is a long booming one, due to the muscle-sound of the con- tracting ventricle, assisted, probably, by the simultaneous vibrations of the auriculo-ventricular valves. The second sound is due to the sudden tension of the aortic and pulmonary valves at the moment of their closure, which gives rise to vibrations. It is a short sound, and its source has been clearly proved by hooking back the valves, which causes the sound to cease. The two sounds are reproduced by the words ' lubb diipp.' Intra-Cardiac Pressure. — The internal pressure exercised by the walls of the heart on the blood, is ascertained by means of an instrument termed a cardiac sound, first used by Chauveau and Marey. It is a double tube having at its extremity two elastic balls ; the air in these is compressed when the cavities contract, and the compression moves a lever placed in connection with a recording surface. The instrument is passed into the right heart through the jugular vein, one ball being in the auricle, the other in the ventricle. It is stated that its presence causes no in- convenience to the animal, due to the fact that no sentient nerves are supplied to the lining membrane of the blood- vessels or even to the heart. A tracing so obtained from the heart of the horse is shown in Fig. 14 ; the curves do not indicate the force of the stroke, but only the differences in intra- cardiac pressure at each instant of one contrac- tion. It is seen from the tracing that the auricles contract first, followed by the ventricles. The contraction of the former is sharper and shorter than of the latter, which is slower, maintained for some time, and then falls. Observations on the intra-cardiac pressure show that it is Digitized by Microsoft® THE HEAET 41 reatest at the beginning of contraction, and then gradu- lly falls; whilst a negative pressure occurs during the eriod of diastole, and is brought about by the relaxation f the walls of the heart during the long pause. This elaxation causes a sucking action which assists in filling be heart with blood. Though both ventricles deliver the same amount of blood he pressure in each cavity is different, owing to the flight iuricle. Right fen- ricle Lpex of ;he leart .. 'ig. 14. — Simultaneous Tracings from the Interior of the Right Heart of the Horse, after Chauveau and Marey (M'Kendriok). Each horizontal line equals t 5 second, the vertical lines indicate ressure ; the vertical dotted lines mark coincident points in the three lovements. 'he auricular curve is a, b, c, the ventricular curve is c', d', e', /'. The auricle contracts sharply, relaxes rapidly, and is followed by the contraction of the ventricle which is maintained with certain oscillations for about three-tenths of a second, and then relaxes ; the pause follows at /, /'. 'he oscillations seen at d, d', d", and e\ e', e", are believed to indicate vibrations of the tricuspid valve. ifferences in the resistance to be overcome in the systemic nd pulmonic circulations. The systolic pressure in the ift ventricle of the horse is equal to a column of blood •om 9 to 14 feet (2 - 4 to 43 metres) in height, or 178 to Digitized by Microsoft® 42 A MANUAL OF VETERINARY PHYSIOLOGY 318 mm. of mercury, and in the right ventricle is equal to 1£ feet of blood (-46 metre), or 34 mm. of mercury in height. In the dog the intra-cardiac pressure has been estimated as follows : Left ventricle - - 7 inches (180 mm.) of mercury. Aorta- - - - 6J „ (160 mm.) Eight ventricle - - If „ (45 mm.) „ The Cardiograph. — The impulse of the heart against the chest wall is graphically obtained by means of the cardio- graph, of which there are many forms. Their essential construction consists of a button applied externally to the chest wall, which is pressed upon by each impulse of the heart, and so conveys the movement to an elastic air- chamber, which transmits it to a recording lever. By this means we obtain a graphic representation of the heart's impulse, but there are many difficulties in obtaining reliable records with this instrument. The cardiograph demonstrates that the aortic valves close slightly before the pulmonary. 'f Capacity of Heart. — The quantity of blood in the heart can only be ascertained approximately ; measuring the capacity of the chambers is no guide. Munk states that the capacity of the ventricle in a horse weighing 880 lbs. is about 1*76 pints (1 litre), equivalent to 2 - 25 lbs. (1 kilo) of blood ; each ventricle contains one-thirtieth of the total blood, so that when both contract one-fifteenth of the total blood is ejected. Colin gives the capacity of the left ventricle of the horse at 1"76 pints, and states that at each systole two-thirds or three-fourths of this quantity are injected into the aorta, viz., ri7 pints (670 c.c.) to 1'36 pints (772 c.c.) ; the left ventricle at each contrac- tion, according to this observer, forces into the aorta about one-twenty-fifth of the total blood of the body. It is said by Colin, that in the horse the ventricles do not empty themselves completely at each systole, and this appears to be the case in all animals. Each side of the heart must hold the same quantity of Digitized by Microsoft® od, for it is evident the amount of blood leaving the irt must be equal to the amount entering it. ^ Work of the Heart.— This may be calculated if we know > amount of blood being discharged from the heart at ih stroke, and the pressure against which it is propelled, e amount pumped out at each systole of the ventricle liable to great variation, at least such are the results of jeriments on the dog. It is obvious that the right ltricle does less work than the left, for the reason that has to pump the same volume of blood against a much aller peripheral resistance ; it has been said indeed that i right heart does one quarter the work of the left. [f we take the amount of blood pumped at each stroke o the aorta of the horse at about 2-25 lbs. (1 kilo) in ight, and the pressure under which it is forced upwards equivalent to a column of blood 10 feet in height, then s work of the left ventricle at each stroke is equal to 5 lbs. raised one foot high, or for 24 hours, allowing the rk of the right heart to be one-fourth that of the left, 39,000 foot pounds. This amounts to about one-thirtieth a horse power per diem ; Munk places it at one-thirty- fch of a horse power. If the amount of blood expelled the left ventricle at each stroke be equal to 2 - 25 lbs., m in a state of repose the entire blood in the body of :orse passes through the heart in about thirty beats, or 45 seconds. Munk says that in the horse the entire od passes through the heart in 50 seconds, in the ox in seconds, and in the dog in 20 seconds. Since the amount of work performed by the heart is reased during exercise, the above calculations are for iorse in a state of repose. Che term Blood Pressure is frequently used in the psiology of the circulatory system. It is one we shall re to consider in detail when we come to speak of the odvessels ; but it should be clearly understood that the .dition is due to the amount of blood pumped into the sels by the heart, and the pressure which results from 5 depends principally on the rate at which that which Digitized by Microsoft® 44 A MANUAL OF VETEETNAEY PHYSIOLOGY is in front of it in the vessels escapes into the veins. If the arterioles are contracted so that the amount passing into the veins is reduced in quantity, then a larger bulk of blood will be between the pump and its outlets, and the blood pressure rises; if, on the other hand, the blood is passing rapidly through the relaxed arterioles into the veins the blood pressure falls. When the amount poured into the venous system in any given time is equivalent to that pumped into the arterial system during the same time (which is the normal condition), the pressure is described as being constant. The above facts may be tabulated as follows : When the heart is more active the blood pressure rises. „ „ less „ „ „ falls. When the arterioles contract the blood pressure rises. ,, „ dilate ,, „ falls. The heaviest work the heart performs is in overcoming the resistance offered by the minute bloodvessels or arterioles ; only a very small part of the heart's work is expended on producing blood velocity. This question of peripheral resistance will shortly be considered in detail. The number of heart beats in different animals, and the conditions influencing it, are more conveniently considered in the next chapter, see p. 70. Nervous Mechanism. — The heart is said to possess no sensory nerves; it 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 conclusive. Not only is it considered that the external Burface is insensible 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. Digitized by Microsoft® THE HEART 45 The nerves supplying the heart are the pneumogastricsj vagus nerves, and the sympathetica; the function of ise is diametrically opposite. The pneumogastric has restraining, or, as it is termed, inhibitory effect over s movements of the heart; the sympathetic has an elevating or augmenting effect. Histologically the two rves differ greatly in structure, the pneumogastric being nedullated, whilst the sympathetic is a non-medullated rve. The inhibitory fibres found in the vagus are derived m the internal branch of the spinal accessory, which tis the vagus within the skull, and travelling with this :ve reaches the heart by its cardiac branches. The lelerator nerves arise from the spinal cord, by the inferior its of the second and third dorsal nerves and probably others ; they pass through the sympathetic ganglia, and ,ch the inferior cervical ganglion, from which they are tributed to the heart. (See Pig. 15.) [f the Vagus Nerve in the neck be gently stimulated the e of the heart beat is slowed and the force of the beat luced. Either of these effects may occur, or they may combined. If instead of stimulating gently a strong nulation be applied, the heart stops in diastole. Strong nulation may be applied to the vagus of the cat without pping the heart, but in the dog even weak stimulation y cause it to cease beating. Phe above action of the vagus is spoken of as inhibitory ; :ontrols or inhibits the heart beat. Experiment shows ,t while both auricles and ventricles are affected by this ion of the vagus, yet the effect appears to be more rked upon the auricles than on the ventricles. The icts of the vagus on the heart are often better demon- ited through one nerve, frequently the right, than its ow, and this is explained by saying there are more ibitory fibres in one nerve than in the other, f one vagus be cut the rate of the heart beat is slightly reased, if both be cut the rate is greatly increased, and blood pressure rises ; the reason why the beats are in-> Digitized by Microsoft® 46 A MANUAL OF VETERINARY PHYSIOLOGY GJ. G.Tr.Vg. Gan- glion on the trunk of the r;T r Y a vagus ; the black ' ' " line through it is the internal branch of the spinal accessory . Vg. The vagus Vn nerve. J ' nc, no. The cardiac bran- ches of the vagus conveying hibitory fibres to the heart. ^ r.Vq. Inhibitory fibres from SpAc spinal accessory entering the vagus. C.Sy. The cervical sym- pathetic nerve. G.C. Inferior ganglion. cervical G.Th.* and G.Th.s Fourth and fifth thora- cic ganglia on sympathetic chain. Asb. Subclavian artery. An.V. Annulus of Vieus- sens. G.St. Ganglion stellatum. D.II., D.III. The inferior roots of the second and third 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 D.III and inferior cervical gang- lion to the heart by nc. D.W The dotted line in certain thoracic nerves, D.I., D.I.V., and D.V., indicate that they may contribute aug- ■^i mentor fibres to the sym- pathetic. D.V Fig. 15.— Diagrammatic Kepresentation of the Cardiac Inhibitory and Augmentor Fibres in the Dog (Foster). The upper portion of the figure shows the inhibitory, the lower the augmentor Digitized ^MSrosoft® THE HEAET 47 eased in frequency is that the inhibitory action of the tgus is removed, and the antagonistic nerve, the sympa- tetic, has things all its own way. If now the cut end of Le vagus be stimulated impulses are sent out which call to existence the inhibitory action, and the heart beats jcome fewer and more feeble. If an artery be placed in communication with a recording oparatus and the vagus stimulated, a tracing such as that ig. 16. — Tracing showing the Influence op Stimulating the Vagus Nerve ; Pall of Blood Pressure due to Arrest of the Heart. Prom a Babbit (Foster). 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 6 occurs a short time after shutting off the current. The notches in the tracing are the beats of the heart. Ben in Fig. 16 is obtained. The inhibitory effect is not btained immediately the stimulus is applied ; at least one eat may occur before the heart stops, and in the same way tie beats do not return immediately the stimulus is with- rawn. The effect of this on the blood pressure is seen in 'ig 16, where the drop in the curve is due to a fall in blood ressure the result of cardiac inhibition, while it rises by japs and bounds shortly after the stimulus is withdrawn. The inhibitory power of the vagus is lost if atropin be pplied to the heart or injected into the circulation, owing Digitized by Microsoft® 48 A MANUAL OF VETERINAEY PHYSIOLOGY to its nerve endings in the heart being paralysed. Minute doses of this alkaloid are sufficient to prevent stoppage of the heart's beat by stimulation of the vagus. The action of atropin is counteracted by muscarin or physostigmin, both of which produce a remarkable slowing effect on the heart, even causing it to stop, behaving, in fact, very much like vagus stimulation. The inhibitory action of the vagus on the heart is under the control of a centre in the medulla ; the exact extent and position of this is not known, but it is situated close to the origin of the vagus. The centre is spoken of as the cardio-inhibitory, it is bilateral, and from it the inhibitory fibres which pass down the vagus obtain their origin. This centre is in action during the whole life of the animal ; its constant action is known as tonic activity, and its function is to keep a rein on the heart ; the tighter the rein is held the slower the heart beat becomes, the slacker the rein the quicker the beat. As to whether the rein shall be tight, moderate, or slack, depends upon the afferent impressions carried to the centre from the periphery, and impulses carried in this way and passed out through another channel are described as reflex impulses. If the central ends of sensory nerves be stimulated the heart may slow down ; painful stimulation of any sensory surface, a blow on the abdomen, an accident, sudden fright, or in the human sub- ject a sickening sight, may reflexly slow the heart through the above centre. The centre is also excited by carbonic acid, since venous blood circulating through it slows the beat. It is probable that the tonic activity of the centre throughout life is a reflex tonus, viz., is not due to impulses originating in the centre, but to the centre always being stimulated through a continuous inflow of sensory im- pressions. A rise in blood pressure causes a slowing of the beat (Marey's law), which is a good example of reflex inhibition effected through the cardio-inhibitory centre. In the dog, cardiac inhibition is slightly increased during expiration, so that in this animal the heart beats slower Digitized by Microsoft® THE HEAET 49 during expiration than during inspiration ; the effect is abolished by section of the vagi (see Figs. 18 and 19). The Sympathetic nerve is the augmentor nerve of the heart ; it accelerates the beat, and is consequently the an- tagonist of the vagus. When stimulated the rate of beat is increased, and in some cases not only the rate of beat but its force. Finally in a third group of cases the force and not the rate is increased. The explanation of these differ- ences on stimulation is considered to be that the sympa- thetic contains two sets of fibres — (1) the accelerators, which increase the rate of beat, and (2) the augmentors, which produce a more forcible beat. These may act sepa- rately or in combination. If the sympathetic nerves on both sides be cut, the heart rate is decreased owing to the influence of the uncontrolled vagus. This view of the effect of the divided sympathetics has not always existed ; at one time it was held that division of the sympathetic led to no effect upon the rate of the beat, from which it was reasoned that the influence of the sympa- thetic, unlike the vagus, was only occasionally in operation. A centre in the medulla controls the operations of the sympathetic ; it is known as the accelerator centre, and it is believed that, like the inhibitory centre, it is in a state of tonic and constant activity. The two antagonistic forces above described are con- stantly at work on the heart : the inhibitory through the vagus slowing the rate, the accelerator through the sympa- thetic quickening the rate. Whichever of these effects is at any given moment most needed, is brought into play by impulses from the centres in the medulla. The vagus is the protecting nerve of the heart. It is eommonly observed after its stimulation and the consequent inhibition, that on recovery there is an improvement either in the rate or force of the heart beats. Gaskell concludes from this that inhibition is due to a building up, anabolism, of the muscular tissue brought about by the vagus, and resulting in an improvement in the condition of the heart. Conversely, he regards the sympathetic as a katabolic 4 Digitized by Microsoft® 50 A MANUAL OF VETEEINAEY PHYSIOLOGY nerve, viz., one bringing about tissue destruction. During muscular contraction, as we shall learn later, there is a breaking down of the complex muscle elements into simpler /bodies, with the production of heat and energy. In the case of the heart muscle this may be hastened through the agency of the sympathetic nerve. The Depressor Nerve. — The nervous mechanisms con- sidered up to this point are concerned in bringing about some modified action of the heart, under the guiding influ- ence 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 through the instrumentality of the vascular (arterial) system. This nerve is the depressor. It originates in the heart, some say in the walls of the aorta, and runs up the neck as a separate branch in the horse, cat, and rabbit, but in other animals it 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 heart in this way is placed directly in communica- tion with the centre which presides over the vascular 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 depressor to the vasomotor centre, resulting in impulses being sent out which cause the abdo- minal arteries to dilate and hold more blood. By this means the peripheral resistance is diminished, 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 Pig. 23, p. 79). Digitized by Microsoft® THE HEART 51 Cause of the Heart Beat. — The nervous mechanisms con- nected with the heart, which we have just considered, only deal with the rate and force of contraction and have nothing to do with causing the rhythmical contraction ; the same may be said of the nervous ganglia which are found in the substance of the heart. The proof of this is very simple, for the heart continues in cold-blooded animals to contract rhythmically when all the nervous connections are divided. Further, a strip of tissue may be so cut from the ventricular wall as to be apparently free of all ganglionic structures, and such a strip may, under suitable conditions, be made to contract automatically and rhythmically. Accordingly, it is in some peculiarity of the muscle tissue that the cause must be sought. As the result of many observations it is laid down as an axiom that the heart is automatic, viz., that the stimulus to activity arises from within and is not brought to it from without. The nature of this inner stimulus cannot be regarded as solved, but it is probable that it is to be sought largely in the composi- tion of the blood or lymph circulating through the heart tissue, and with special reference to the inorganic salts these fluids contain. The automatic rhythmic action of the heart is most highly developed at the venous end and least so at the apex; ifc begins at the veins, courses over the auricles, and runs down the ventricles. Whatever may be the rhythm, the venous end of the heart sets the pace, as it is expressed, for the whole organ. The wave of contraction passes from chamber to chamber through the muscle substance, but the muscular ring between auricles and ventricles has a lower rate of conduction than the general substance of the heart wall. This fact has been utilized to explain the short pause between the auricular and ventricular contraction. Normally, the rhythm of the ventricles follows that set it by the auricles, but this may be destroyed or altered by compressing the connections between auricle and ventricle, either by specially arranged clamps or by ligature. By 4—2 Digitized by Microsoft® 52 A MANUAL OF VETERINARY PHYSIOLOGY varying the compression complete or partial blocking of the normal rhythm occurs, so that the contractions of the ven- tricles become slower than those of the auricles. The nature of the inner stimulus is, as already sug- gested, intimately connected with certain inorganic salts of sodium, calcium and potassium. With suitable arrange- ments for keeping the heart of the frog ' fed ' with a fluid containing in solution chlorides of the above metals, the heart will continue beating for days, and even the mammalian heart may be thus kept alive, provided it be placed in an atmosphere of oxygen while being fed. On this diet of salts the heart finds the material for its inner stimulus. It would appear that not only will no other metals take the place of those named, but that each has a distinct role in the function of nutrition, calcium promot- ing contraction, sodium and potassium bringing about relaxation of the heart. Heart muscle does not behave in its physiological pro- perties the same as skeletal muscle, but possesses certain features peculiar to itself. If a piece of ordinary skeletal muscle be stimulated electrically it responds to a powerful stimulus with a big contraction, and to a weak stimulus with a small contraction. But the heart muscle when stimulated, if it responds at all, gives as big a contraction with a weak stimulus as with a strong one. The heart's motto, it has been said, is expressed by ' all or nothing.' Another peculiarity of heart muscle is that it is only capable of response to stimulation during the phase of diastole ; if stimulated during systole no effect follows ; this is known as the refractory period of the heart beat. The stimulus during diastole produces an extra contrac- tion, but this is followed by a longer pause than usual, so that the extra beat is exactly counterbalanced. The condition of distension of the heart cavities is an important factor in the beat. Within reasonable limits a full heart contracts more vigorously than one less full, though, if too long continued, dilatation and damage of the heart wall follow. We have learnt the provision made for Digitized by Microsoft® THE HEAET 53 correcting this, viz., the depressor mechanism, and the cardio-inhibitory centre. y Coronary Circulation. — The nutrition of the heart muscle is brought .about by the blood supplied to it through the coronary arteries. Unlike any other arteries in the body, the coronaries are filled during ventricular diastole. During systole the muscular pressure on tbe arteries becomes higher than the pressure of the blood in the vessels, and in consequence the vessels are emptied, while during diastole they are filled. Cutting off the coronary circulation rapidly produces fatal results. Action of Drugs on the Heart. — If aconitin, muscarin, phy- sostigmin, and pilocarpin be applied to the heart, they cause a gradual slowing of the heart beat, and finally stop it in diastole as in vagus stimulation. This result is attributable to the stimulating effect these drugs exert on the endings of the inhibitory (vagus) nerves in the wall of the heart. Atropin and nicotin increase the frequency of the heart beats, behaving very much as if the vagus were divided. In fact, if stimulation of the vagus be made after the application of atropin no inhibition follows, the nerve end- ings of the vagus in the heart being supposedly paralysed. If atropin be injected into the circulation the same results are obtained,, including a dilatation of the bloodvessels. Atropin is able to remove the inhibitory action of physos- tigmin and muscarin. Adrenalin applied to the heart augments and strengthens its beat, while if injected into the circulation it causes con- striction of the vessels and a rise in blood pressure. Digitalin reduces the frequency of the heart beat, and later excites the cardiac muscle to a stronger and prolonged systole. It is described as a heart tonic. Wf 'ji Pathological. Disease of the heart of the lower animals is uncommon. It might have been thought that the horse would be exposed to this class of trouble, bearing in mind the enormous strain placed on his heart during labour, and the utter want of consideration shown by the vast majority of those who ride and drive horses. But it is not so. The Digitized by Microsoft® 54 A MANUAL OF VETERINARY PHYSIOLOGY hearts of horses exposed to the greatest strain seldom show any pathological change ; probably the most uncommon lesions found 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. As evidence of the gross strain to which horses are exposed, 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 clause 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 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. 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, and 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. 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 a great thickening of the heart sac, but of more or less extensive effusion into it. The heart then becomes enveloped in a water jacket, which greatly adds to the gravity of the case. In the above acute cases the heart muscle suffers, and hEemorrhages into it are common and widespread. In the dog the heart's action is naturally 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. Digitized by Microsoft® CHAPTBE III THE BLOODVESSELS The use of the bloodvessels is to distribute the blood over the body, to bring it in contact with the tissues, and return it to the heart. To accomplish this purpose there are arteries, capillaries, and veins. The Arteries arise from one common trunk, the aorta, which by the process of dividing and subdividing like the branches of a tree form 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 latter fact 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 its width and length, and at the same time ensures 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, every contraction of the left ventricle, for example, in the horse during rest, throws into them one and a half pints of blood which must be accommodated, and this is provided for by the distension of their walls. For every one and a half pints of blood 55 Digitized by Microsoft® 56 A MANUAL OF VETERINARY PHYSIOLOGY 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. We shall study presently a further use of the elastic arterial wall, when we come to describe the flow of fluid through tubes. Another essential feature possessed by arteries is their power of contractility. Just as we saw the larger arteries were principally elastic, so the smaller ones are principally contractile. This contractility or power of reducing their diameter is produced by the muscular coat previously spoken of. Though the smaller vessels possess this mus- cular 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 diameter of the vessels. In this way the muscular artery may be con- tracted or relaxed dependently upon the set of nerves brought into operation ; and this movement of the smaller muscular vessels acts as a tap and regulates the blood supply to'any given part of the body. Capillaries. — The minute arteries terminate in the capil- laries. 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. It is through this wall that the exchange of material with the tissues occurs. The capillary is capable of expanding and contracting and so accommodating more or less blood as the case may be ; this is brought about by the elastic nature of the membrane composing the capillary 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 are very small. Their size depends upon the amount Digitized by Microsoft® THE BLOODVESSELS 57 of blood which has to pass through them ; in consequence they are larger during active exercise than during rest. If they be observed microscopically in the living animal, the capillaries may be seen as a network enclosing small islands of tissue. These are the areas where the inter- change between the blood and tissues occurs. The Veins receive their blood from the capillaries. They are thinner-walled than the arteries, and their walls collapse when empty. Though some variation exists in their struc- ture, yet, speaking generally, they contain less elastic and muscular material than arteries, and more fibrous tissue. In certain veins, such as the venae cavae and those of the pregnant uterus, there is a considerable development of muscular tissue in their walls. The venous system is larger than the arterial, and its capacity is therefore greater. The abdominal veins are capable of holding the whole blood of the body, as we see 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 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 ensuring the return flow of the blood along the veins to the heart. In certain places such as the bones, intestines, foot, and brain, the veins have no valves. Veins are normally pulseless, but an exception must be made to this statement in the case of the lower extremity of the jugulars, just where they enter the chest. It is quite common in the horse to observe pulsations in these vessels for an inch or so along the neck, due no doubt to the contrac- tion of the right auricle. It is, however, distinctly abnormal for these venous pulsations to extend a great distance up the neck. The presence of a marked jugular pulse is frequently associated with heart trouble. Digitized by Microsoft® 58 A MANUAL OF VETERINARY PHYSIOLOGY Mechanics of the Circulation. — At each systole of the ventricle 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 produced 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 points in 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 pump no fluid issues from the pipe, the jet is only produced 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 now water be 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 it were a rigid tube, 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 Digitized by Microsoft® THE BLOODVESSELS 59 the reason why the elastic recoil of the tube is now brought into play is owing to 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 a clamp ; 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 distension of the tube which produces the recoil is caused by the peripheral resistance. 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 there- fore 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 was met with in the arteries. If, however, the vessels be clamped so as to produce a resistance, the first stroke of the pump causes the arteries to become distended, they then recoil, and while under- going this they receive another stroke from the pump and become still more distended. Once more they recoil on their contents and are once more distended by the action of the pump, and so on. If all 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 only has time to fall a short distance before a second stroke of the pump sends it still higher than before ; this is repeated at every stroke of the pump until the water or mercury refuses Digitized by Microsoft® 60 A MANUAL OF VETEEINAEY PHYSIOLOGY 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, which has been brought about by the peripheral resistance, the elastic recoil of the tube, and the pumping 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, at once the manometer shows a fall in the mean pressure owing to the removal of a certain amount of resistance, 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 possession of the main facts of the circulation, but even up to this point we have not learned all the lessons it is capable of teaching. 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 reduce the resistance and lower the mean pressure, that pulsatile waves will pass over to the venous side of the model, and these can again be obliterated by screwing up the clamp. Lastly, our model if working at mean pressure will show the effect of injury to the arterial tubes; if these 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 Digitized by Microsoft® THE BLOODVESSELS 61 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 smaller arteries with their muscular coat supply the peripheral resistance. Under the influence of this and the contrac- tion 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 system of arteries ensures 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 for- gotten that local effects may and do produce general effects. If, for instance, one artery alone contracts this must lead to an increase of arterial pressure, which produces an increased flow of blood through all the simultaneously un- contracted arteries on 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. We shall meet with a striking instance of this when dealing with the action of the depressor nerve on blood pressure, through Digitized by Microsoft® (I 62 A MANUAL OF VETERINARY PHYSIOLOGY ,t the medium of alterations in the arteries which supply the \A . splanchnic area, by means of the splanchnic nerve. _ / - • : , ...... ■ v \a Fig. 18. — Tracing of Arterial Pressure with a Mercury Mano- meter (Poster). The smaller curves P, P are the pulse curves due to the heart-beat. The space from r to r embraces a respiratory undulation. The tracing is taken from a dog, and the irregularities visible in it are those frequently met with in this animal. The arterial pressure varies, as we have 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 are not due to the heart-beat but are caused by the respiratory movements (Fig. 18). Thus at every inspiration the blood pressure rises and at every expiration it falls. Speaking generally this is true, though the tracing (Fig. 19) shows that the pressure does not rise immediately inspiration commences, nor does it fall as soon as expiration begins. The cause of this will be explained when dealing with respiration. Digitized by Microsoft® 64 A MANUAL OF VETERINARY PHYSIOLOGY The blood pressure in the capillaries is very difficult to ascertain. It is probably £ to | of that in the large arteries or lies between 20 to 40 mm. of mercury. Blood pressure in the veins is ^r or T V 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 - lu,r inch (02 mm.). Facial vein - -J inch (3 mm.). Brachial vein - - - - -h inch (12 mm.). Crural vein - - - - \ inch (14 mm.). Fig. 19. — Babbit. Influence of Bespiratory Movements upon Arterial Blood Pressure (Waller). The blood pressure is the upper tracing, the respiratory movement is the lower tracing. I is inspiration, E expiration. In the large veins just as they enter the heart the pressure is very low, and here the manometer may show even a negative pressure at intervals ; in the anterior vena cava of the dog a negative pressure of \ inch of mercury (3 mm.) may be registered. This is due to inspiration, which by producing a negative pressure in the thorax assists the blood to reach the right auricle ; it is this negative pressure which in the human subject renders operations at the root of the neck dangerous, air being aspirated into Digitized by Microsoft® THE BLOODVESSELS 65 the heart should the vessels be wounded. From observa- tions on the horse the risk of air entering during operation may be neglected. Blowing air into the veins causes no discomfort until a considerable amount has been introduced. Even then only sighing respirations are produced. 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 them- selves to the reduced bulk of fluid in circulation ; this adjustment is effected by means of a nervous apparatus to be dealt with presently, and in this way the blood pres- sure 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 haemorrhage 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. It is astonishing how rapidly a deficiency in the circu- lating fluid is made good. The fact is that the tissues give up their fluid in an endeavour to replace the loss of blood, quite apart from the repair which is being effected through the thoracic duct. It is the loss of fluid by the tissues which causes the thirst of haemorrhage. Circulation in the Living Tissues. —The circulation in the living animal may be readily 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, travers- ing certain sections very rapidly, others very slowly. In the vessels larger than the capillaries, such as the com- mencement of the small veins, the stream of blood behaves somewhat differently ; in these the centre of the vessel is occupied by a column of red cells, whilst between them and the coats of the vessel is a clear layer or zone in which may be seen the white corpuscles strolling lazily along the sides, occasionally stopping, then moving forward once more. This difference in the behaviour of the corpuscles is due 5 Digitized by Microsoft® 66 A MANUAL OF VETERINARY PHYSIOLOGY to the physical fact that the friction against the sides of the vessel is greater than in the centre ; but apart from that, there appears to be an attraction exerted on the white corpuscles by the endothelium, so that they may, as pre- viously pointed out, pass completely through the wall of the vessel into the surrounding tissue. This is especi- ally well marked in inflammation, where the slowly moving white corpuscles become attached, as it were, to the lining of the vessel and collect in masses ; with them also may be seen the blood platelets, which under the normal con- dition of circulation occupy the central zone of the vessel with the red blood cells. Under inflammatory action the white cells pass completely through the vessel wall in large numberB, aided, as previously pointed out, by their amoeboid movements and the spaces existing between the endothelial plates of the vessel. This is known as the migration of white corpuscles. The essential changes taking place in inflammation occur in the wall of the vessel, and the passage of corpuscles through this is not limited solely to the white, but the red may also pass out. That inflammatory changes are essentially due to the con- dition of the vessel-wall and not to that of the blood, is proved by the fact that an artificial corpuscular fluid in- troduced into the inflamed part behaves exactly as does the blood. The Pulse. — It is a fact of common observation that the arteries throb or pulsate whilst the veins do not, and we now have to inquire what really produces this pulsation, and why it stops at the arterioles. When the left ventricle contracts and drives its blood into the aorta, the arteries distend to accommodate it and then recoil owing to their elasticity ; each expansion of the arterial wall coincides with a contraction of the ventricle, and so each beat or throb of the pulse corresponds with a contraction of the heart. This intermittent expansion of the arteries gradually becomes less marked at a distance from the aorta, and dies out at the arterioles. We have pre- viously (p. 58 and p. 61) drawn attention to the fact that Digitized by Microsoft® THE BLOODVESSELS 67 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 con- tinuous 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. In virtue of this property 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 sheltering 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 capillaries. 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 the capillaries, and appear in the veins ; in this way a venous pulse may be produced. An example of this will be given when dealing with the influence of certain nerves on the vessels of the submaxillary gland of the dog, which cause dilatation of the arterioles and throbbing in the veinB. This intermittent expansion of the arteries, called the pulse, produces a wave in the arterial system which is spoken of as the pulse- wave. From what we have said it is evident that the height of this wave is greatest nearest the heart, and falls to zero at the capillaries. The wave travels with considerable velocity ; from 15 to 30 feet per second. This may easily be determined by noting the interval between the commencing successive 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 18 feet. This . 5—2 Digitized by Microsoft® 68 A MANUAL OP VETERINAEY PHYSIOLOGY 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 rate of trans- mission of the pulse-wave and the velocity of the onward flow of the blood. The factors 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 Fig. 20. — Normal Sphygmogram modified from Dudgeon ; Pressure 2 oz. (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 ; /, dicrotic wave ; g, second tidal wave. // ,0 the pulse- wave ; the former travels at a high speed, the latter comparatively slowly, at most some 15 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 sphygmogram, 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 we may at once say that unless the proper degree of pressure is kept Digitized by Microsoft® THE BLOODVESSELS 69 on the vessel, great irregularity in the sphygmograms will be produced ; it is held by some that certain of the tracings obtained are due to instrumental errors, and not to the true pulse-wave. <. The simplest description of a sphygmogram (Fig. 20) is that it consists of a nearly vertical unbroken upstroke (the anacrotic limb), and an oblique downstroke (the catacrotic limb), which is broken by two or three waves known as catacrotic waves. Of these two or three waves /(Fig. 20) is one of the few which occurs with any regularity, and is known as the dicrotic wave. The notch e is described as the aortic notch, and is caused by the closure of the aortic valves. B Fig. 21. — Tracing from the Facial Artery op the Horse (Hamilton). A before, B after destruction 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 pulBe-wave from the periphery towards the heart. That the dicrotic wave is a reflection from the aortic valves, is shown by the tracing in Fig. 21, 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 beat of the pulse for each single con- traction of the heart. 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 Digitized by Microsoft® 70 A, MANUAL OF VETERINAEY PHYSIOLOGY made to distinguish between the pathologist's tension and the physiologist's pressure. If tension be denned as the elastic force exerted by the artery on the blood within, it is evident 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 condi- tions. 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 imary. erotic. \ \ imary: 'al. st- dicrotic. 'r ^ > "? ft, £ AA rv_L \ v^^ / y X vj J ^^^- vA J " "' — LovrTensi on. NormaJ. Jficfli Tension. Fig. 22.— Sphygmograms of Low Tension, Normal, and High Tension Pulses (Waller). arterial wall ; when blood pressure is low and the amount of movement in the artery great, the recoil or dicrotic wave is very marked (Fig. 22). "7"" The pulse varies in character, depending upon 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 - 25 to 28 beats per rairmte 28 „ 32 %i ) J 36 „ 40 )) >) 45 „ 50 »J >i 70 „ 80 )> >i 70 „ 80 >» n 90 „ 100 >) ) Digitized by Microsoft® THE BLOODVESSELS 71 Certain variations 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 in- creases in age the pulse rate drops, and in old age the pulsations are not only reduced in number but weaker. The condition of the arterial wall alters the shape and nature of the pulse tracing in old age. Between size of body and pulse rate there certainly appears to be some connection, and in the human family tall men have a slower pulse rate than short men of the same age. The heart rate is rapidly responsive to all 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 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 in a state of activity, and which consequently are in urgent need both of repair and flushing. In fact, there appears to be little doubt that it is the substance flushed out of the muscles during work which stimulates the heart either directly or reflexly, though other explanations have been offered, such as reflex stimulation of the sympathetics. A relationship exists between heart rate and the con- dition of blood pressure ; when the blood pressure becomes low, the heart rate increases as the result of reflex stimula- tion, 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 111° F. to 113° F. (44° to 45° C). Digitized by Microsoft® ^ 72 A MANUAL OP VETEBINABY PHYSIOLOGY Jf The Velocity of the Blood varies in the arteries, capil- 'ftD laries, 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. The velocity of blood- flow depends on the width of the bed formed by the vessels ; as the arterial system expands, the velocity diminishes ; 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 cause of the flow throughout the entire system is the contraction of the left ventricle, 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 500 times greater than that of the aorta, whilst the passage of blood through it is 500 times slower than in the aorta, owing to the width of the bed. According to the same authority, the velocity of blood in the carotid of the horse is from 11*8 inches to 15'75 inches per second (30 to 40 cm.) ; in the metatarsal artery of the horse 2 - 2 inches (5 "5 cm.) per second, and in the jugular vein 8 - 85 inches (22 cm.) per second. A horse which gave a carotid velocity of 12 inches (30"5 cm.) per second gave a jugular velocity of 9 inches (22 - 75 cm.) per second. Chauveau found in the carotid artery of the horse a velocity of 20'47 inches (52 cm.) per second during systole, 8 - 66 inches (21'75 cm.) per second at the beginning of diastole, and 5'9 inches (15 cm.) per second during the pause. The mean velocity in the carotid of the dog is 10J inches (26-5 cm.) per second: at the end of diastole 8£ inches Digitized by Microsoft® THE BLOODVESSELS 73 (21 - 5 cm.) per second, and at the end of systole 12 inches (30"5 cm.) per second. The velocity of the blood is there- fore increased by each systole of the ventricle, decreased during diastole, and falls still more during the pause. The flow in the arteries is assisted by expiration, while that in the veins is assisted by inspiration. The velocity of the blood is greater in the pulmonary than in the systemic capillaries, while in the venae cavse it is half of that in the aorta. 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 con- clusions, for the velocity in a closed artery and an open one are two different things. In the undivided artery the peri- pheral resistance reduces the velocity, in the divided artery the peripheral resistance 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 nearer 160 inches per second. Or to put it in a practical way, if the carotid of the horse has a sectional area of •2 square inches, the amount of blood passing through the unwounded vessel amounts to 2 oz. per second, while if the same vessel be divided the loss of blood would be nearly 1 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 circula- tion, or on the other hand have taken the longest. In a * The circulation time is determined either by injecting an easily distinguishable salt into the blood, or more precisely by increasing the electrical conductivity of the blood by injecting into it a neutral salt solution. Digitized by Microsoft® 74 A MANUAL OP VETERINARY PHYSIOLOGY horse with a pulse frequency of 42, the average complete circuit is performed in 31"3 seconds (Hering), and is equivalent, according to the latter 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 7*79 seconds, or again in 28 heart beats ; with the dog 16-7 seconds or in 26'7 heart beats. 2.$' / Aids to the Circulation. — The contraction of the left -& ventricle is sufficient to drive the blood all over the body, but in the veins this force is assisted by the muscles com- pressing the vessels from without, while the presence of valves within prevents regurgitation. This is especially the case in the veins, of the limbs where the fluid has to flow against gravity. The circulation in the large veins near the heart is assisted by the process of inspiration and the dilatation of the right auricle, both of which have an aspirating effect on the blood in the larger veins. The sucking action of the left auricle assists also in drawing the blood in the pulmonary veins towards the heart. Influence of the Nervous System. — The bloodvessels are under the control of the nervous system acting on the muscular elements in their walls, by which means a reduc- tion or increase in their size is produced. Such alterations are essential if a mean blood pressure is to be maintained ; a rise in pressure in one part of the system may be com- pensated by a fall in another, and this is entirely brought about by an alteration in the diameter of the small arteries or arterioles. The evidence that the nervous system does possess this power over the bloodvessels, is furnished by the simple experiment of dividing the cord below the medulla, and maintaining life by artificial respiration. The imme- diate effect of division is an enormous fall in blood pressure ; in the dog it will drop two-thirds below the normal, and this is due to the vessels dilating as the result of paralysis, their tone having been lost through the injury to the cord. From this experiment it is quite certain that structures above the section are responsible for the nerve control of Digitized by Microsoft® THE BLOODVESSELS 75 the vessels, and these can be further localized by making a section above the medulla, the influence of which on the blood pressure is nil. It may, in fact, be readily shown that in the medulla, in the region of the fourth ventricle, is a small area the function of which is to produce and maintain the. contracted condition of the bloodvessels defined as tone, and to this area the name vaso-motor centre has been given. By means of it the calibre of the blood- vessels throughout the body is regulated. Experiment shows that in addition to the head centre, the medulla, there are subcentres for vaso-motor action in the cord. If, for example, the cord be divided in the lumbar region, the vessels of the hind limbs dilate, and the blood pressure falls ; but if the animal be kept alive the blood pressure gradually returns to normal, the sub- centre in the cord carrying out the work. This pressure is again at once lost by destroying the already divided cord. Other, and perhaps more obvious, evidence of the influence of the nervous system over the bloodvessels is furnished by the ear of the rabbit, for if the sympathetic be divided in the neck, 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 ; and 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 contract, the flushed appearance disap- pears, and the ear becomes cooler. Since, in the above experiments, mere severance of the nerves which connect the bloodvessels with the central nervous system leads to a dilation of the arterioles, it is evident that impulses are, under normal conditions, being continually sent out along the nerves from the vaso-motor centres. These impulses keep the arterioles normally in a state of medium or partial constriction, and this condition is, as we have already said, known as arterial ' tone.' Now, inasmuch as the function of the vaso-motor nerves is to regulate the blood-supply to any given area of the body in exact accordance with the varying needs ' of that area, Digitized by Microsoft® 76 A MANUAL OF VETEEINAKY PHYSIOLOGY ' tone ' becomes 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 ' ensures that an arteriole may both dilate and contract, according as it receives less or more of the continuous constricting impulses, and thus the regulation of a varying blood-supply is made extremely perfect. Hitherto we have only spoken of the constrictor influence over the bloodvessels, but the nervous system likewise exercises a dilator effect. In contrast to ths constrictor influence, the dilator is not tonic in its action. It might be supposed that a dilat6r effect would naturally follow as the result of removing a constrictor influence from a vessel, 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 perhaps the chorda tympani is the best example. This nerve supplies the bloodvessels of the submaxillary gland with dilator fibres ; if the nerve be cut no evident change in the bloodvessels 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 phenomenon occurs when the dilator nerves to the vessels of the penis are brought into activity. It is by no means certain that all vessels have both a constrictor and dilator nerve supply, in fact there are vessels in the body where no vaso-motor nerves of any kind can be demonstrated, such for example as the brain, heart, and lungs, while it is pretty certain that muscles do not contain any vaso- constrictor nerves. It is here convenient to notice that the vaso - motor supply to muscles, which we have said is essentially dilator in effect, is brought into action reflexly when the Digitized by Microsoft® THE BLOODVESSELS 77 muscle contracts. In this way an extra blood supply is furnished during the period of functional activity only. No special centre has been demonstrated in connection with the vaso-dilator service, though several subcentres in the cord and medulla exist. A consideration of the origin and distribution of the nerves governing the bloodvessels may now be undertaken. The vaso-constrictor fibres for the whole body leave the spinal cord by the inferior roots from the first dorsal to the third or fourth lumbar. They do not at once pass to their destination, but through the medium of the white rami communicantes they enter the sympathetic nervous system, by linking up with the ganglia of that chain lying on either sidj of the spine. Up to the time of entering the ganglia the constrictor nerves are medullated, but after passing through it they lose their medulla, and a fresh lot of fibres, now non-medullated, arise and proceed to the blood- vessels. The fibres for the head and neck are derived from the first four thoracic roots of the spinal cord, and having passed through the vertebral sympathetic ganglia they proceed to the inferior cervical ganglion, and by means of the cervical sympathetic pass to the superior cervical ganglion. From this ganglion fibres are sent out to supply the carotid artery and its branches. The constrictors for the fore limb leave the cord by the fourth to the tenth thoracic roots, they pass into the stellate ganglion, from which, as grey rami, they emerge and join the cervical nerves which contribute to the brachial plexus, and through this supply the vessels of the fore limbs. Those for the hind limb arise from the spinal roots from the eleventh dorsal to the third lumbar, and by white rami join the lumbar and sacral ganglia of the sympathetic chain ; issuing from these as grey fibres, they pass into the sacral plexus, and supply all the vessels of the hind limbs. The abdominal constrictor nerves are the splanchnics, greater and lesser, the former from the last seven dorsal, Digitized by Microsoft® 78 A MANUAL OF VETERINARY PHYSIOLOGY the latter from the first two or thee lumbar roots. The fibres pass to the semilunar ganglion of the solar plexus, and from this they issue as non-medullated fibres, supply- ing all the bloodvessels of the abdominal organs. The splanchnics are the chief constrictor nerves of the body ; section causes a dilatation of the vessels they supply and a considerable fall in blood pressure, especially in those animals where the alimentary canal is largely developed as in herbivora. It will be observed that the essential feature in the distribution of these constrictor nerves is that they pass through the sympathetic system before going to the blood- vessels ; and from being medullated spinal nerves they become non-medullated sympathetic. ^ The dilator nerves in their distribution behave very differently to the constrictor ; they leave the brain or cord by any cerebro-spinal nerve, they may or may not pass into a sympathetic ganglion before distribution, and in contrast to the constrictor fibres they pass direct to their destination, instead of taking a roundabout course ; finally they do not lose their medulla until near their termination. Examples of typical dilator nerves have previously been given, viz., the chorda tympani to the submaxillary gland, and the nervi erigentes, stimulation of which causes erection of the penis ; the former passes in company with a cranial nerve, and the latter with a spinal nerve to its destination. Some spinal nerves contain both constrictor and dilator fibres, for example the brachial and sciatic nerves. On section of these the loss of constrictor influence is at first the most prominent feature, as shown by the hot flushed limb, but later, as the constrictor fibres degenerate, the dilator fibres become apparent, as on stimulation the con- strictor nerves fail to react while the dilator nerves respond. From what has been said, it is evident the nerve supply to the bloodvessels is elaborate and complex. The vaso-motor centre both in the medulla and cord are extremely sensitive to the varying amounts of carbonic. Digitized by Microsoft® THE BLOODVESSELS 79 acid in the blood ; an increased venous condition of blood leads to a constriction of the arterioles and a raising of the blood pressure. In asphyxia the arterioles remain con- stricted under the influence of the now intensely venous blood, as it stimulates the vaso-motor centre to unwonted activity, and though the initially high blood pressure subsequently falls to zero, it does not do so because the arterioles have relaxed, but because the heart has failed. As previously mentioned, there is no single definitely located centre presiding over vaso-dilatation, though such may possibly exist. ^^^^•^^ Fig. 23. — Tracing, showing the Effect on Blood Pressure of stimulating the central end of the depressor nerve in the Babbit (Foster). The time marker below marks seconds. At x an interrupted current is thrown into the nerve, and the blood pressure gradually falls. The vaso-constrictor centre may be influenced reflexly by two distinct kinds of impulse, which pass from the periphery to the centre, the effect of which is either to stimulate or depress the centre ; these are known as pressor and depressor effects, and the fibres which convey them are known as pressor and depressor nerves. Pressor fibres are found in all sensory nerves, and stimulation of them produces an impulse which passes to the vaso-motor centre, from which constrictor impulses pass to the splanchnic area, causing contraction of the vessels and a rise in blood pressure. Digitized by Microsoft® 80 A MANUAL OF VETEEINAEY PHYSIOLOGY The depressor nerve of the heart (see p. 50) is the best example of its class ; by means of it impulses are conveyed to the medulla, and from there transmitted through the spinal cord and sympathetic, system to the splanchnic area. The effect is to depress or lower the blood pressure by causing the abdominal arteries to dilate (Fig. 23). Depressor fibres also exist which cause similar reflex vaso-dilator effects which are, however, too local to pro- duce any general fall of arterial pressure, such as results Fig. 24. — Blood Pressure Curve op a Babbit, 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 of respira- tion ; the large bold undulations being Traube-Hering curves. In each Traube-Hering curve there are about nine respiratory curves, and in each respiratory curve about nine heart-beats. from stimulation of the depressor nerve. Such a mechanism is in operation in erectile and other tissues, -ft , Under certain conditions, such as asphyxia and hsemor- rhage, the vaso - motor centre transmits to the vessels rhythmic constrictor impulses, which result in the appear- ance, on a simultaneous record of blood pressure, of undulations, known as Traube-Hering curves (Fig. 24). Such, of course, can only be detected by taking a tracing of the blood pressure. The existence of these waves is indicative of abnormal excitation of the vaso-motor centre. When vessels are robbed of all nervous connections, they do not necessarily lose their powers of contraction ; the muscular tissue of the arterial wall responds to the exciting Digitized by Microsoft® THE BLOODVESSELS 81 influence of tension, so that increased blood pressure causes contraction and a reduced pressure relaxation of the vessel wall. A very close parallelism exists between the nerve fibres which constrict the vessels and those which cause a more forcible contraction of the heart ; in both cases they are of the non-medullated variety, in each they excite muscular action, and by so doing increase the wear and tear of the tissues involved. In the same way a close similarity exists between those fibres which dilate the bloodvessels and those which slow the heart ; both are medullated and muscle-restraining, and in consequence both excite pro- cesses of repair rather than of disintegration. ■r^,- Peculiarities in the circulation through various tissues occur as the result of their special function ; they 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 ensure the carrying out of its functions. We see this provided for in the frequent arterial anastomoses, for example, the Circle of Willis and the Eete Mirabile of ruminants, which ensures 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 with ruminants. It has probably, therefore, some other function to perform. The pulsations observed in the exposed brain are not due to the pulse in the arteries, but arise from the respiratory movements ; expiration causes the brain to rise by hinder- 6 Digitized by Microsoft® 82 A MANUAL OF VETERINARY PHYSIOLOGY ing the return of blood, whilst inspiration causes it to fall by facilitating the flow. The venous arrangement of the brain is very remarkable ; the walls of the veins are composed of layers of the dura mater, and even portions of the cranial bones may enter into their formation. The veins or sinuses of the brain are large cavities, which from their arrangement are most unlikely to suffer from compression, and from the rigidity of their walls are not capable of bulging as most veins do when obstructed ; in this way the easy return of the venous blood is provided for. The cerebral circulation is considerably assisted by the presence of fluid within the ventricles of the brain. This fluid readily passes from ventricle to ventricle, and from ventricle to spinal cord ; in this way, as the external pressure becomes greater the internal becomes less, and so compression of the brain substance is avoided. It will be remembered that no vaso -motor nerves have been satis- factorily demonstrated in the brain. The singular arrangement of the venous plexuses of the corpus cav'ernosum penis, admits of this organ attaining a vast increase in size, a condition which in the brain every measure is adopted to prevent. The considerable size of the venous plexuses of the penis, their frequent inter- communication, the muscular pressure to which the veins leading from the sinuses are exposed, produce under the direction of the vaso-motor nervous system a considerable increase in the volume of the 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 a remarkably tortuous course ; possibly, in some way or other, it may be con- cerned 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 size of a collapsed and full stomach in the horse, to recognise the necessity for some arrangement existing to Digitized by Microsoft® THE BLOODVESSELS 83 prevent 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. It is a remarkable fact that very little 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. There is, however, one kind of strain which arises in the hunting field, or under similar circumstances, 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 circulation suffices in a state of repose, during which not a sign of any circulatory 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 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 aneurismal, and has its lumen obliterated by Strongylus armatus. It is remark- able 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 point 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. 6—2 Digitized by Microsoft® /r' CHAPTER IV RESPIRATION Section 1. The Lungs. 22 The lungs occupy the whole cavity of the thorax ; during life no space exists between the pulmonary and costal pleura, so that the case is an air-tight one. So long as this air-tight condition is maintained, any movement which tends to increase the size of the case, such as the retreat of the diaphragm and the advance of the ribs, causes a disten- sion of the sacs and the air rushes in ; by a reversed process it is pressed out, viz., by a collapse of the chest wall. If, however, the cavity of the chest be opened to the external atmosphere the lungs collapse owing to their elastic recoil, and the fact that the atmospheric pressure within and without them is now the same. Such a condition would lead in the horse .to asphyxia, as the pleural cavities com- municate, but in those animals where the right and left pleural sacs are distinct, the lung on the wounded side only collapses. The process by which the chest is filled with air, known as Inspiration, is a purely muscular act. The diaphragm as the chief muscle of inspiration contracts, and thereby recedes ; the ribs are rotated, being drawn forwards and outwards, their posterior edges everted, and the intercostal space widened. By this means the capacity of the chest is increased and the lungs tend to fill the space thus created. By doing so they rarefy the air already within them, so that a difference in pressure occurs between the air in the 84 Digitized by Microsoft® KESPIBATION 85 lungs and that outside the body, and air rushes in to restore equilibrium ; this inrush is inspiration. The increase in the size of the chest which occurs during quiet inspiration in the horse is stated by Colin to be as follows : the antero-posterior or longitudinal diameter of the chest is lengthened between 4 and 5 inches (10 to 12'5 cm.), whilst the transverse diameter between the eleventh and twelfth ribs is increased by 1£ inches (4 cm.). Only the last twelve or thirteen pairs of ribs, under ordinary circumstances, take any important share in respiration ; this is due to the true ribs being more or less covered by the scapula and its attached muscles. When, however, a difficulty occurs in the breathing, the elbows are turned out which brings other muscles into play as auxiliaries in respiration, and a certain number of the true ribs now assist in increasing the capacity of the chest. The Movements of the Diaphragm are interesting and peculiar; this large flat muscle with its thin tendon centrally placed works to and fro. In the body it is placed obliquely forwards, extending from the loins to the sternum, and the main to and fro movement occurs in its large upper half rather than in the narrower portion below. Through the centre of the diaphragm the posterior vena cava and oesophagus pass ; it is obvious that any free movement of the diaphragm of this part would cause a ' pull ' on these structures. This is prevented by move- ment occurring principally in the upper half and sides of the muscle, while the lower part and centre take very little share. When the diaphragm is receding, it carries back with it all the structures on the abdominal side which are adjacent to it ; thus the liver, stomach, and spleen are especially affected by this movement. The diaphragm is curved forward. This curve is pro- duced by the pull exerted on the muscle by the air-tight thorax supplemented by the pressure from behind. The diaphragm never becomes flat, even under pathological conditions when the chest cavity contains gallons of fluid. Digitized by Microsoft® 86 A MANUAL OF VETEEINAEY PHYSIOLOGY There is no flattening so long as the thorax is air-tight ; as soon as air enters it would flatten if it were not for the abdominal viscera pressing it forward from behind. In Fig. 25 is a diagrammatic horizontal section of the chest, looked at from above, showing the position of the diaphragm during inspiration and expiration and the dis- placement of the abdominal viscera. Observe the extent Fig. 25. — Horizontal Section op the Horse's Chest, looked at from above, illustrating the movements of the diaphragm (Sussdorf). a, right lung ; b, left lung. 1. Position of the diaphragm during deep expiration ; c, liver during deep expiration ; d, stomach during deep expiration ; e, spleen during deep expiration. 2. Position of diaphragm during deep inspiration ; c', position of liver ; d', of stomach ; e', of spleen during deep inspiration ; /, posterior vena cava as it passes through the diaphragm. to which the sides of the diaphragm move as compared with the centre. The squeezing to which the liver is exposed must be an important help to its circulation, while the movements of the diaphragm must materially assist the flow of blood in the phrenic veins and posterior vena cava. Fig. 26 gives a side view of the horse's chest, the dia- Digitized by Microsoft® RESPIBATION 87 phragm is attached around the margin APE. The dotted line AE indicates the convexity of the muscle and the extent to which it bulges into the chest. The effect of this bulging is that the lungs rest on or wrap around the diaphragm, and, as it were, envelop it. The lungs do not reach as low as the cartilage of the false ribs, but about the Fig. 26.- -Diagram of the Extent of the Chest in the House, and Position of the Diapheagm. The area BCDE is under the scapula and its muscles, and practically not available for auscultation : the surface ABEF is the available area of the chest wall. The lung reaches to within a hand's- breadth of the false ribs. AP represents the last rib ; BE runs parallel to the posterior edge of the triceps. CD 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. breadth of a hand above them. The cut edges of the ribs in the above figure from P downwards indicate the lower and posterior margin of the lung ; the sections of the ribs from A to C indicate the upper border of the lung. It is only the circumference of the diaphragm which is muscular, Digitized by Microsoft® 88 A MANUAL OF VETEKINAEY PHYSIOLOGY and this muscular margin is widest at the sides and runs up to less than half the width above, where it is attached to the last ribs. The central portion of the diaphragm is a felt work of tendinous fibres, running from the muscle to the centre and in other directions. Filling in the upper and central portions of the diaphragm are large muscular pillars, attached to the spine, which support the con- siderable weight hung on the diaphragm, viz., the liver, with the stomach and contents. The diaphragm extends several inches behind its suspending pillars. Neither the muscle of the diaphragm nor the supporting pillars are markedly responsive to electrical stimulation as compared with voluntary muscle. Expiration. — The chest having been filled with air, the next process is its expulsion, and the mechanism here con- cerned is not fully agreed upon by physiologists. Whilst some hold that it is a purely non-muscular act, others con- tend that certain muscles share in the process. All are agreed that the elastic recoil of the lungs is the important factor ; there is also the elasticity of the cartilages of the ribs, which are seeking a return to their position of repose ; and further, the elastic pressure of the displaced abdominal organs acting on the diaphragm ; added to which is the contraction of the abdominal muscles, which presses the viscera still more firmly against the diaphragm. The factors named are sufficient in themselves to ensure that air is expelled from the lungs, though certain muscles attached to the ribs may facilitate their return to the position of repose. 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. Such only occur 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 Digitized by Microsoft® EESPIRA.TION 89 cavity grows, as it were, too large for the lungs that a negative pressure in the thorax is produced. Later on (p. 112) the cause of the first act of breathing will be ex- plained. Thoracic development in young animals is very rapid ; a foal will increase 1J inches (4 cm.) in circum- ference within the first few hours after birth ; when this absolute increase in chest capacity is established, a nega- tive pressure in the pleural cavity is obtained. I 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 inspiratory muscles. The in- ternal intercostals, the fibres of which run in an opposite direction to 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 ex- piratory. The following table shows the inspiratory and expiratory muscles of the chest : Muscles of Inspiration. Muscles of Expiration. Diaphragm. Abdominal muscles. External intercostal s. Internal intercostals. Serratus anticus. Transversalis costarum. Levatores costarum. Serratus posticus. Serratus magnus (during dim- Triangularis sterni cult respiration) . Latissimus dorsi ,, ,, Scaleni ,, ,, In some animals the ribs do very little work and the diaphragm becomes the chief respiratory muscle. In most quadrupeds the sternum is fixed to the ribs and undergoes 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 move- ment between the sternal ribs and the cartilages. During laboured respiration any muscle which can assist in Digitized by Microsoft® 90 A MANUAL OP VETERINAEY PHYSIOLOGY 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 proportion between the two is not invariable. During work the value of the inspiratory and expiratory acts is about equal. During inspiration a slight negative pressure exists in the trachea, and during expiration a slight positive pressure. 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 £ inch (3 mm.) of mercury, and during dyspnoea f inch. In the dog during inspiration the negative pressure in the pleural sac is i inch (6 mm.) of mercury, whilst during expiration ^ inch (3 mm.) has been observed. In the horse J inch (6 mm.) has been registered during a powerful expiration, and 1£ inches (28 mm.) during a powerful inspiration. The negative pressure can be recognised post-mortem by the rush of air immediately the chest is punctured. (y The number 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, Ox - 12 „ 15 „ Sheep and Goat - 12 ., 20 „ „ Dog - - 15 „ 20 „ Pig - 10 „ 15 „ „ Rumination increases the . frequency of respiration, and muscular exertion in all animals at once causes it to rise. In experiments on respiration this is most marked ; walk- ing a horse will nearly treble the number of respirations, but the breathing begins to fall immediately the horse stops, though it does not reach the normal for a few minutes. Digitized by Microsoft® EESPIBATION 91 The ratio of neart-beats tc^ respiration) has been placed at 1 : 4 or 1 : 5. u.tfj^te Effect of Respiration on Circulation. — We have pre- viously alluded to the influence of respiration on the circu- lation, and the assistance this renders in aspirating the blood into both sides of the heart ; further we have drawn attention to the value of the negative pressure in the chest in connection with the diastole of the heart. The whole of the negative pressure found in the heart (p. 37) is not due to the diastole alone, but to the diastole plus the aspiratory movement, for if the chest be opened a smaller amount of negative intra-cardiac pressure is registered. In dealing with blood pressure (p. 63) attention was drawn to certain undulations of respiratory origin. These are produced by the decrease of pressure on the vessels of the thorax during inspiration ; this reduction of pressure is small, but it produces sufficient suction to affect sensibly the thin-walled veins opening into the right auricle, the blood pressure in which is very low. By this suction more blood is aspirated at every inspiration into the right auricle, and consequently more blood is ejected from the left ventricle. In this way we have the arterial pressure raised during inspiration, followed by a fall during expiration (see Fig. 18, p. 63). But 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 Pig. 19, p. 64. The explanation of this is that the pulmonary vessels have a greater capacity during inspiration than during expiration, and the increased amount of blood entering the right heart on inspiration is first used to fill the pulmonary vessels, and this accomplished, the general blood pressure rises by the excess being passed on to the left heart ; similarly the fall does not occur immediately expiration begins, as the pulmonary vessels have not yet returned to their expira- tory capacity. In examining the blood pressure and respiratory curves Digitized by Microsoft® 92 A MANUAL OF VETERINARY PHYSIOLOGY of the dog, it is observed that the pulse frequency is increased during inspiration, and reduced during expira- tion ; this reduction in the frequency of the pulse is due to the stimulation of the cardio-inhibitory centre, whilst the increase is caused by a diminished activity of the inhibitory mechanism (see p. 48). In speaking of inspiration raising blood pressure we must not forget the mechanical advantages of a contraction of the diaphragm compressing the liver and posterior vena cava, and so forcing more blood towards the heart ; this no doubt is another cause of the rise of blood pressure during inspiration. ■^ The Nostrils. — Before the air reaches the lungs it is warmed by passing through the nasal cavities, so that it enters the trachea at nearly the body temperature. The incoming air also becomes saturated with watery vapour ; this saturation likewise occurs in the nasal chambers. In the majority of animals air may pass either through 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 ; we therefore find in this animal 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 capacity of the nasal opening by allowing considerable and rapid dilatation. Another use is in 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, especially the outer segment, viz., that part in communication with the false nostril, 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 Digitized by Microsoft® KESPIRATION 93 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 (Pig. 27). 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. ._-Ex. Fig. 27.— Nostril of Horse. In, The inspiratory portion ; Ex, the expiratory portion. 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 ; the result of this arrangement ensures 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 olfactory. The latter will be dealt with under the Senses. It comprises the upper portion of the superior turbinated bone, ethmoid cells, and a portion of the middle meatus ; the respiratory channel on the other hand lies on the inferior part of the nasal chamber and comprises the inferior meatus, inferior turbinated bone, part of the superior and part of the middle meatus. Digitized by Microsoft® 94 A MANUAL OF VETERINARY PHYSIOLOGY 'J^ The Glottis. — The air having been warmed by passing over the septum and turbinated bones, enters the glottis, the arytenoid 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 powerfully drawn upwards and backwards, and the V-shaped glottis fully opened (Figs. 32 and 33, p. 124). It is a remarkable fact that the laryngeal opening should be so relatively small, con- sidering the diameter of the trachea and the size of the nasal openings. During inspiration the larynx and trachea slightly descend, while they ascend during expiration. This is particularly well seen in horses during the hurried respira- tions of disease, producing 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, occupy 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 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 in or out of them. At first sight it would appear that air ought to enter the sinuses during inspiration, but the reverse is the case ; it is only during expiration that they are filled, whilst during inspiration air is Bucked out. Considering the position of the common inlet to these sinuses, it is difficult to understand why they should fill during expira- tion, though the advantage of hot instead of cold air enter- ing is evident. Respiratory Changes in the Air and Blood. — The changes which the air undergoes on passing into the lungs must now be considered. Digitized by Microsoft® KESPIEATION 95 Atmospheric Air contains in 100 Parts : By Volume. By Weight. Oxygen - - 20'96 23015 Nitrogen - - 79'01 76-985 Carbonic Acid - -03 The above gases are the essential constituents of the atmosphere; the new elements argon and krypton have, so far as is known, no physiological significance, and in the table are included in the nitrogen. The proportion of carbonic acid is small ; it is a natural impurity in the air, though essential to plant life. The atmosphere 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 one per cent, of moisture, and is spoken of as dry if it contains one-quarter 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 carbonic acid, as may be seen in the following table : , T • , „ Carbonic Nitrogen. Oxygen. Acid _ Composition of inspired air - 79-01 20-96 0-03 expired air - 79-01 16-02 4-38 -4;94 +4-36 The volume of oxygen absorbed is slightly greater than that of the carbonic acid 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 inspired (not always in the Tropics) and is saturated with aqueous vapour, the volume expired is actually greater. Digitized by Microsoft® 96 A MANUAL OF VETERINAEY PHYSIOLOGY /l~f The proportion which the volume of oxygen absorbed bears to the volume of carbonic acid given off is termed the CO . respiratory quotient, and is expressed as — ' The quotient O2 varies with different animals, and depends upon the nature of the diet. On a carbo-hydrate diet less oxygen is required, for the reason that the oxygen and hydrogen in the molecule exist in the proportion to form water, so that oxygen is required for the carbon only. On a fat diet oxygen is required for both the hydrogen and carbon in the molecule. In herbivora the respiratory quotient is -9 to 1/0 In carnivora ,, „ „ '75 „ '8 In omnivora ,, „ „ '87 That is to say, for every 1 part of oxygen absorbed by herbivora there is produced '9 to 1 part of carbonic acid, and for every 1 part of oxygen absorbed by carnivora "75 to "8 part of C0 2 is produced. In carnivora it will be observed that the amount of C0 2 produced is considerably less than the amount of oxygen absorbed, for the reason that the latter instead of being devoted to the oxidation of carbon and reappearing as C0 2 , is employed in the oxida- tion of hydrogen and returned as water. The value of the respiratory quotient lies in its being a measure of the com- bustions occurring in the body as a whole ; as a rule the amount of carbon dioxide formed is less than the oxygen absorbed, but there are exceptions, and a respiratory quotient may be above unity as in hibernating animals in storing up fat for the winter, and it is not unknown among other animals after a diet rich in carbo-hydrates. In such cases the CO„ discharged is in excess of the oxygen absorbed, the necessary oxygen contained in the C0 2 being obtained from the intra-molecular store, of which we shall hear presently. These high quotients are only observed where the conversion of carbo-hydrates into fats is occurring, as in the fattening of animals. There are other gases returned from the lungs besides Digitized by Microsoft® BESPIBA.TION 97 carbonic acid and oxygen, but very little is known about them. According to Eeiset, both hydrogen and marsh gas are given off in the expired air of ruminants, in fact, he places the latter at 183 cubic inches in 24 hours. Both are supposed to be derived from the intestinal canal, being absorbed into the blood by the vessels of the intestinal wall. In our observations on the gases of respiration of horses, it was found after deducting the oxygen, carbonic acid, and nitrogen, that a balance remained the nature of which was unfortunately not ascertained ; possibly it was a mixture of hydrogen and marsh gas, but it did not amount to any- thing like the quantity found by Beiset. The nitrogen of the air is returned unaltered. We have previously learnt the changes occurring in the blood during its passage through the lungs ; we have now to study the way in which the interchange of gases between this fluid and the air is brought about. Absorption of Gases in Liquids. — The law regulating the absorption of gases by fluids is very clear. Every fluid in which a gas is soluble absorbs the same volume of gas, no matter what the pressure may be ; but as the number of molecules in a gas depends upon the pressure, it is evident that the weight of the absorbed gas rises and falls in proportion to the pressure; this is known as the law of Dalton and Henry. The volume of gas absorbed by a fluid depends upon the nature of the gas; for instance, 1 volume of water will absorb 1180 volumes of ammonia gas, whilst the same volume of water will only absorb "00193 volume of hydrogen. The temperature of the water is also an im- portant factor, for the higher the temperature the less the gas absorbed. If, now, instead of taking a single gas to be absorbed by a fluid we take a mixture of gases, 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 by itself or whether it form only a proportion of the mixed gases present. 7 Digitized by Microsoft® 98 A MANUAL OF VETERINARY PHYSIOLOGY 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 mixture. 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 and standard barometric pressure (30 inches) 21 volumes of oxygen and 79 volumes of nitrogen ; what is the partial pressure exercised by each gas in this mixture ? 30x21 63 inches of mercury, which is the partial 100 pressure of the oxygen ; and 30x79 23 '7 inches of mercury, which is the partial 100 — • ' pressure of the nitrogen. The term ' partial pressure ' occurs so constantly in the following pages, that the above may help to make the matter clear. If a mixture of gases, say the atmosphere, be exposed over a fluid already containing some of these gases dissolved in it, it is found that if the proportion of dissolved gases in the fluid is less than their proportion in the atmosphere, the latter pass into the fluid until the amounts of gases in the fluid and that in the air are equal. On the other hand, if the fluid contains more dissolved gas than the atmosphere above it, gas will pass from the fluid to the atmosphere until the amounts in each are equal. This is really a process of diffusion, and plays a most im- portant physical part in respiration ; it is one of the means by which the carbonic acid passes from the blood into the air-cells, and the oxygen from the air-cells into the blood. If two different gases be placed in a jar, in a short time a complete mixture will have occurred, as both gases pass each into the other until a thorough and equal mixture has taken place. This is termed the process of diffusion, and is the chief means by which the air in the deeper part of the lungs mixes with the fresh air introduced by breathing. Digitized by Microsoft® EESPIRATION 99 Such are the physical laws which it is necessary to understand before the processes involved in respiration can be fully comprehended. IIZfT^e Respiratory Exchange in the Lungs and Tissues. — -The respiratory exchange in animals is of two kinds : the external respiration, which takes place between the air and the blood through the medium of the lungs, and the internal respiration, which occurs between the tissues on the one hand and the blood and lymph on the other. As we shall see, both are complex questions which are far from settled. The blood having been robbed of about 85 per cent, or more of its oxygen in the tissues, the haemoglobin makes its way back to the lungs in a partly 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 air (alveolar) in the ultimate air-passages. Between it and the air we have only the membrane of the air-sac and the wall of the capillary, both of which are bathed in fluid ; through this wet membrane the oxygen instantaneously passes, being greedily absorbed by the haemoglobin of the red cells ; of necessity the gas must first pass into the blood plasma, and from there it is taken up by the red corpuscles. The oxygen is not simply absorbed by the red cells, but forms with the haemoglobin a weak chemical compound. Experiment has clearly shown that the union of haemoglobin with oxygen is largely inde- pendent of pressure, and therefore does not obey the law of Dalton and Henry, which it certainly would do if simply absorbed. We have yet to learn how it is that the oxygen in the air- vesicles passes into the capillaries to form this chemical union with haemoglobin. Here we have one of the physical laws brought into play which we have previously described. When the venous blood arrives in the lungs it has lost much of its oxygen, the partial pressure of the oxygen is low, whereas the partial pressure of the oxygen in the atmosphere of the air-cells is relatively high ; the result of 7—2 Digitized by Microsoft® 100 A MANUAL OP VETERINARY PHYSIOLOGY this is that practically instantaneous diffusion occurs through the moist membrane separating the gas and the fluid. The oxygen entering the blood plasma unites at once with haemoglobin ; the latter takes up all or nearly all the oxygen it is capable of holding (an amount which is much greater than if simple absorption of oxygen by haemoglobin occurred), and distributes it to the tissues through the medium of the arterial circulation. The tissues are greedy for oxygen ; their oxygen pressure is practically nil; once more diffusion occurs. The high partial pressure of the oxygen in the arterial blood becomes (through loss of oxygen to the tissues) low partial pressure in venous blood, and the partly reduced haemoglobin is carried to the lungs, where the process just described is repeated. But the loss of oxygen to the tissues is not the only change the blood undergoes, for not only is its haemo- globin partly reduced, but as the outcome of tissue activity increased quantities of another gas are added to it. The gas alluded to is carbonic acid ; this is largely taken up by the venous blood and conveyed to the lungs, and the method by which it is got rid of will be explained presently. The fate of the oxygen in the tissues is quite unknown. It is supposed to be stored up in some way or other until required, but in connection with this subject it is necessary that we should glance at internal respiration — viz., the respiration which takes place in the tissues. The most remarkable feature in the respiration of muscle (and we' select this tissue to elucidate the point under consideration) is that although the working of a muscle cannot occur without oxidation processes taking place, yet no free oxygen can be obtained from it; the partial pressure of oxygen in muscle is practically nil. Yet oxidation processes are occurring in muscle, and under such conditions that no free oxygen can reach it, as, for example, when the muscle preparation of a frog is placed in an atmosphere of hydrogen. Such a muscle preparation may be made to contract in an atmosphere of hydrogen Digitized by Microsoft® BESPIEATION 101 and produce carbonic acid, without there being a trace of free oxygen either in the atmosphere surrounding it, or in the muscle itself, and this process may be continued until the muscle is exhausted. The question is, therefore, How does the oxygen-free muscle obtain oxygen for the production of C0 2 ? In other words, what becomes of the oxygen taken to muscles? Pew things in the whole range of physiology are more difficult to understand : oxygen goes to the muscle, it uses oxygen, yet no free oxygen is found in it ! It is supposed that when the oxygen reaches the muscle it is stored up in its substance amongst the muscle mole- cules, hence it has been termed intra-molecular oxygen ; it there forms a complex substance which readily yields carbonic acid and other matters on decomposition, and this passes into the bloodvessels of the muscle and is carried away to be got rid of at the lungs. It has been suggested that the storing up of oxygen in the tissues may be closely allied to the storing up of oxygen by haemoglobin, though with this difference, that the com- pound which holds the oxygen in the tissues is more stable than the oxygen-holding substance in the blood. All we know of the fate of the oxygen is that it eventually deter- mines the production of certain changes in the tissues, which lead to the formation of carbonic acid and other substances ; but the changes which the oxygen undergoes from the time it leaves the blood and passes into the muscle substance, to the moment it issues from the tissues united with carbon as carbonic acid, are completely un- known. The oxidations taking place in muscle and in every other tissue in the body occur in the substance of the tissue and not in the blood or lymph surrounding it. Experiments made to determine whether oxidations occur in the blood have failed, although readily oxidizable substances have been employed for the purpose. We have said the tissues are greedy for oxygen and use it up or store it away as quickly as it arrives ; here is a very good example of their Digitized by Microsoft® 102 A MANUAL OF VETEEINABY PHYSIOLOGY action in this respect. If a comparatively stable oxygen- holding substance such as methylene blue be injected into the circulation and the animal destroyed, it is found that although the blood is dark blue in colour, yet the tissues are normal in appearance until they are exposed to the air, when they turn a vivid blue. The explanation is that the tissues have robbed the methylene blue of oxygen and formed a colourless reduction product, which on exposure to the air takes up oxygen and again forms methylene blue. Fate of the Carbonic Acid. — In the systemic capillaries the partial pressure of the carbonic acid is lower than the partial pressure of this gas in the tissues, the result of which is that it is hurried into the blood by the process of diffusion ; but here, as with oxygen, simple absorption of the gas by the plasma would not be sufficient for the purpose of carrying off the whole of the C0 2 resulting from tissue activity, so that there must be some substance in the blood capable of fixing C0 2 until the lungs are reached. If the serum of blood be exposed to the vacuum of an air- pump, it is found to yield little oxygen but a quantity of C0 2 ; it yields little oxygen because, as we have already learned, this is combined in the red cells ; but the fact that it yields large quantities of C0 2 points to the blood plasma as the chief means by which this substance is carried. It has been determined experimentally that blood plasma will absorb more C0 2 than the same quantity of water, and it is evident, therefore, that there is something in the plasma which assists in carrying it. What this ' something ' may be is doubtful, but it is generally believed that the sodium carbonate of the blood unites with a portion of the carbonic acid, though other substances may assist. Between the amount absorbed by the plasma, and that held in chemical combination by certain salts of the plasma, the total amount is carried along in the venous blood-stream, the partial pressure of the C0 2 in this fluid being high. On arriving at the lungs the venous blood circulates through the Digitized by Microsoft® EESPIEATION 103 . capillary network spread over the walls of the alveoli, the same wet membrane intervening between it and the external air as was described in speaking of the oxygen. The partial pressure of the C0 2 in the air of the air-sacs being lower than that of the blood, diffusion occurs between the blood and the air, the C0 2 passing out until equilibrium is established. The air now in the alveoli of the lungs having lost some of its oxygen, and considerably gained in its carbonic acid— in other words, having the partial pressure of its gases altered— diffusion between the air in the ultimate air-cells and bronchial tubes rapidly occurs until the balance is restored, thus rendering the air in the alveoli fit for further blood-reviving processes. We have dealt with the C0 2 in the blood as if it were entirely carried by the sodium carbonate, but doubt is cast on this view, and Bohr states that C0 2 and haemoglobin form a loose chemical compound, the C0 2 uniting with the proteid portion of the molecule, thus leaving unaffected the iron moiety with which the oxygen is united. The carbon dioxide haemoglobin in this way carries one-third of the C0 2 from the tissues to the lungs, and under the influence of the oxygen in the air of the alveoli the C0 2 is expelled from the corpuscles and discharged into the alveoli. The manner in which the combined oxygen is liberated in the tissues, and the combined C0 2 liberated in the lungs, is explained by saying that certain gases have a tendency to leave the substances with which they are united, when the pressure in the surrounding medium becomes reduced ; this process is termed ' dissociation.' Dissociation liberates the oxygen in the tissues where the oxygen pressure is nil, and assists in liberating the C0 2 in the lungs, where the C0 2 pressure is low, from the substances with which these are chemically combined, viz., haemoglobin and sodium carbonate. In treating of the exchange of gases between the alveolar air and the blood, diffusion has been represented as the only factor at work ; but it is urged by some physiologists, that the cells lining the vessels and alveolar walls cannot Digitized by Microsoft® 104 A MANUAL OF VETEEINAEY PHYSIOLOGY be mere passive witnesses of these remarkable changes, but like the cells in other parts of the body are capable of taking an active share in local matters. In other words, there is a vital aspect to this question, as well as a physical and chemical one. An experiment of Haldane's with carbon monoxide is very suggestive in this respect. He found that though he could, outside the body, get 31 per cent, of haemoglobin to combine with the gas, yet when air, con- taining the same percentage of CO as that to which the haemoglobin had been exposed, was inhaled for even three or four hours, no more than 26 per cent: of the hemo- globin of the blood combined with it. In other words, the cells of the pulmonary alveoli would appear to possess that same selective power which may be seen elsewhere, as, for example, in the kidney, and preferred to allow oxygen rather than carbon monoxide to pass.* The respiratory exchange is influenced by age, being more energetic in young than in adult animals ; this may be due not only to actually increased metabolism, but also to size. It is a well-known fact that the metabolism in a mouse is relatively much greater than in a horse ; it is a question of weight and surface. The larger an animal the smaller the proportion between its weight and the extent of its surface. In other words, body weight and surface do not vary in proportion with each other, and a mouse in relation to its weight has a larger body surface than a horse, and therefore loses, and has to make more heat. Muscular work has an important influence upon the respiratory exchange, and this will be considered in the chapters on Nutrition and the Muscular System. Broadly, it increases both the C0 2 discharged and oxygen absorbed, * The question of the absorption of oxygen and the discharge of carbon dioxide is by no means so simple as might appear from these pages. Physiologists are not agreed as to whether the process is one of diffusion, or a secretion into the blood and an excretion from it. So difficult, indeed, is the problem, that those who have devoted years to its study declare it is beyond explanation in the present state of physical and chemical knowledge. Digitized by Microsoft® RESPIRATION 105 though in experiments on the horse (Zuntz and Lehmann) it did notinfluenee the respiratory quotient. The influence of food on the respiratory exchange is very marked ; during starvation it at first undergoes a marked decrease, and then remains constant. With food the exchange rises, the absorption of oxygen increases, and the output of C0 2 rises. Proteid food is much more energetic in this respect on a fasting animal than is a diet of fat. Temperature has a marked influence on respiratory ex- change, and this will be found dealt with in the chapter on Animal Heat. 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, 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, but that distress appears at 11 per cent., and 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 dissolved by the plasma at an oxygen pressure of two atmospheres being sufficient to carry on the functions. Hyperpnoea is the term applied to the slightly increased amplitude and frequency of the respiratory movements, such as occurs in gentle exercise, as the immediate result of any commencing defective oxygenation of the blood, or other cause which acts as a stimulus to the respiratory centre (see p. 108). When the stimulus is strong or Digitized by Microsoft® 106 A MANUAL OF VETEEINAEY PHYSIOLOGY continued, a further increase in the force and frequency of the respiratory movements takes place, and this condi- tion is known as dyspnoea. The later stage of dyspnoea is characterized by the respiratory movements becoming ' convulsive ' in their activity, and this finale to dyspnoea marks the onset of true asphyxia. ■if 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 inspiratory muscles are less active, the expiratory still powerful, and the convulsions cease. In the third stage the animal lies unconscious, 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. jl , Excess of Oxygen. — When the excess of oxygen is con- siderable, viz., equal to a pressure of five atmospheres, 'warm-blooded animals die with convulsions. By increasing the amount of oxygen above that contained normally in air, the blood cannot be made to take up much more oxygen than if the normal amount only were present ; a pressure of ten atmospheres only causes an increase of *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 Digitized by Microsoft® EESPIEA.TION 107 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 contains 20 per cent, of oxygen which is more than enough for the needs of the body ; even the venous blood is return- ing to the lungs with from ten to twelve volumes of oxygen per cent, unused, while 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 (see p. 99) of the lungs during oxygen inhalation, enables the tissues to obtain their normal amount more easily. By apncea is understood a standstill of respiration, no movement whether inspiratory or expiratory being made. Apnceic pauses may be produced experimentally by blowing air into and sucking it out of the lungs at a more rapid and forcible rate than the ordinary respiratory rhythm of the animal. Something similar in appearance is witnessed in chloro- form poisoning, and the term apncea is frequently used clinically as synonymous with asphyxia. The physiologist uses it in another sense ; apnceic pauses may be produced under conditions absolutely the reverse of asphyxia, as by rapidly and forcibly blowing air into the lungs. Under these conditions some observers suppose that a diminished irritability of the respiratory centre is produced as the result of hyper-oxygenation of the blood, though it is difficult to see how this condition is brought about. There is, on the other hand, good reason to think that repeated expansion of the lungs causes stimulation of the inhibitory fibres of the vagus, and so acts on the centre, and this view is supported by the fact that apncea is difficult to produce after the vagi have been divided. A final view of the cause of apncea assumes the normal stimulus to the respiratory centre to be C0 2 (see p. 112), in which case rapid inflation of the lungs would result in a more efficient removal of this gas, and thus to a diminution Digitized by Microsoft® 108 A MANUAL OF VETEEINARY PHYSIOLOGY in the stimulus to inspiration. Possibly both this view and that of the inhibition of the centre by stimulation of the vagus fibres is correct, the two working con- >\vf currently. />* The Nervous Mechanism governing Eespiration is presided -" over by a centre in the medulla, the position of which in certain animals is very accurately defined, but which in general terms may be spoken of as being situated close to the deep-seated origin of the vagus and in front of the vaso- motor centre. The respiratory centre was at one time con- sidered to consist of an inspiratory and expiratory portion, but the present view is to regard the expiratory centre as doubtful ; at any rate it cannot be localized, and though there are certain facts which suggest its existence, such as the act of straining in parturition,. micturition, or defeca- tion, still as compared with the inspiratory centre it occupies a very subordinate position. Hence it .has been proposed to call the respiratory centre the 'inspiratory centre'; inspiration can only be carried out by rhythmical impulses from this region of the medulla, while there seems no doubt that expiration may be a purely passive act. It is believed that the respiratory centre is connected with every sensory nerve in the body, for the centre may be readily stimulated reflexly through sensory nerves, as an example of which may be quoted the sudden inspiratory gasp given when cold water is dashed on the skin. But besides these there are special motor nerves wholly or almost wholly concerned in respiration with which this centre is closely in touch, for instance, the facial supplying the nostrils, the recurrent of the vagus which dilates the glottis, the phrenics which stimulate the diaphragm, the dorso-lumbar nerves which supply the intercostal and abdominal muscles. All these are interested in the pro- duction of that perfect and orderly sequence of events which beginning at the nostrils pass to the flank, and are so intimately concerned in the production of the respiratory rhythm. The respiratory centre is automatic, that is to say, it is Digitized by Microsoft® EESPIEATION 109 within itself that the discharges are generated which issue forth as inspiratory impulses ; it is, indeed, as automatic as the heart, for if every nerve leading to it were divided the respiratory centre would still go on working. This view is not accepted by all physiologists, many of whom regard the stimulus to respiration as being a reflex one— viz., derived from without the centre. The centre consists of two halves, right and left, both of which work together yet may be shown experimentally to be capable of inde- pendent action. Section of the cord between the medulla and the phrenics leads to immediate cessation of all re- Pig. 28. — Diagram to illustrate the Chief Nervous Connections of the Bespiratory Centre. (After Waller.) spiratory movements, excepting those of the mouth and nostril, which are anterior to the section, and hence to death from asphyxia. The respiratory centre may be stimulated reflexly through any sensory nerve of which an example has previously been given. It is by no means necessary that the sensory nerves carrying impulses to the centre should be .devoted exclusively to this function ; probably the centre is linked up with all the cranial and spinal nerves. . The nature of the impulses issuing from the centre depend upon the character of the impulses which stimu- lated their production; thus the breathing may be con- trolled or even entirely stopped for a few seconds, or it Digitized by Microsoft® 110 A MANUAL OF VETBEINAEY PHYSIOLOGY may be hastened or slowed down, or quickened in rhythm, decreased in depth, or both rhythm and depth increased. From the brain impulses may pass to the centre which may cause an animal to increase its respirations as in sniffing, or to suppress them entirely, as when its head is under water. In the diagram Fig. 28 we have adopted Waller's symbols to signify an increase or decrease of respiration, and it will be seen that the cortex can supply either plus or minus influences. Another reflex path to the respiratory centre is that furnished by the nostrils through the medium of the nasal/ branch of the fifth nerve ; through this channel principally minus influences are transmitted, viz., respiration is diminished. From the skin plus or minus influences may pass to the respiratory centre. A bucket of water dashed against a horse when the breathing is failing in chloroform narcosis will start an inspiration, and painful sensory impressions, as in ' firing,' greatly increase the respiratory movements. From the larynx important impulses pass to the respiratory centre through the superior laryngeal nerve. If this nerve be divided and the end in connection with the brain stimulated respiration is inhibited, in fact, if the stimulation be severe inspiration becomes weaker and weaker, and finally the breathing stops in expiration. This points to the superior laryngeal as stimulating expira- tion and inhibiting inspiration. The same result occurs with the sensory fibres of the glosso-pharyngeal supply- ing the pharynx. Here it is intimately connected with the act of swallowing, producing an inhibition of respira- tion the moment the epiglottis is pressed against the larynx. Influence of the Vagus on Respiration. — The vagus is the most important afferent or ingoing channel to the re- spiratory centre ; it covers the area from the glottis to the alveoli of the lungs. If both nerves are cut the respirations become slower and deeper ; if one nerve only is divided, this effect does not follow. Evidently, therefore, there are tonic impulses Digitized by Microsoft® EESPIEATION 111 passing up the vagus which maintain the normal re- spiratory rhythm, which is lost when the nerves are divided. If the cut vagus be stimulated, using that portion still in connection with the brain, the respiration may be affected in at least two different ways ; either all respiratory movements may partly or completely cease, or the rate of inspiration may be increased, and if the stimulation be powerful the respiration may stop in in -j spiratory tetanus. The interpretation of the results or these experiments is that two kinds of sensory fibres exist in the vagus which act on the respiratory centre ; in what way they act is not fully agreed upon by physiologists. Stating the case broadly the x two kinds of fibres are regarded as inspiratory and expiratory, viz., fibres stimu- lating the inspiratory and expiratory portions of the centre respectively, and this view is necessary if it be held, as some authorities do, that respiration is a reflex act and not, as we have so far assumed, an automatic activity. Both sets of fibres are in alternate activity, and the question is, What is their normal stimulus? This has been determined to be due to the alternate distension and collapse of the air-vesicles, for experiment shows that if air be pumped into the lungs expiration is excited, and if it be sucked out inspiration is excited. Accordingly distension of the air-vesicles by the normal process of inspiration excites expiration, and contraction of the air-vessels in expiration excites inspiration. If, however, the respiratory centre is regarded as primarily automatic, the inspiratory set of fibres may be considered to increase the rate of respiration, the expiratory fibres as inhibiting or con- trolling inspiration, and thus producing expiration. If this view be adopted the act of inspiration proceeds from the automatic centre which requires no other stimulus than that which it generates within itself, while' expiration proceeds from the stimulation caused by distension of the air-vesicles. The nature of the internal stimulus to the respiratory Digitized by Microsoft® 112 A MANUAL OF VETERINAEY PHYSIOLOGY centre has led to much discussion ; it appears now to be generally accepted that the most important stimulus to its automatic action is the gases in the blood. Venous blood circulating through the centre causes the respiration to increase in force and rate, while blood containing a full amount of oxygen lowers the excitability ; the respirations slow down, or even . become suspended. Carbonic acid may be accepted as the chief stimulus to the respiratory centre. Cause of First Inspiration. — At this point it is convenient to consider a question previously deferred (p. 89) — viz., What is the cause of the first act of inspiration in the foetus? When the placental circulation is 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 automatically 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 placental circulation intact may provoke respirations, and in all animals the very first act of the mother is to dry the foetus and stimulate the skin by licking. X? Division of the Phrenic Nerves. — We have referred to the 2p cutting off of the respiratory centre by dividing the cord l(j£ above the phrenic. If the cord be divided below the point of exit of the phrenics, the channel between the re- spiratory 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 paralyzed, if both be divided the whole diaphragm is paralyzed and eventually undergoes- fatty degeneration. Sussdorf states that division of the phrenic nerves in the horse leads to difficulty in breathing, increased heart action, and the collection of faeces in the rectum. In about twenty-four hours these symptoms pass away, and if the animal be worked no appreciable difficulty in breathing is subsequently observed. Digitized by Microsoft® RESPIRATION 113 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 divided the paralyzed flaccid nostrils are drawn inward at each inspiration and so close the opening. The Amount of Air Required. — Numerous respiration experiments have been made on all animals, to determine the amount of air they require and the gases of respiration. The horse is of all others the one to which perhaps the greatest practical interest attaches in this respect, though a knowledge of it in connection with other animals is of value. The lungs of a horse will contain about 1J cubic feet (42*5 litres) of air at the end of a deep inspiration ; during ordinary repose he draws into them between 80 and 90 cubic feet (2,265 to 2,548 litres) 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 250 cubic inches (4-1 litres), 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 reserve is 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 amount 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 disturbing influence alters both the rhythm and depth of the 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; 8 Digitized by Microsoft® 114 A MANUAL OF VETERINARY PHYSIOLOGY but many disturbing factors occur which render experiments on this subject very contradictory, and productive of the greatest variation. During severe work, such as a gallop, a horse is taking air into his lungs to the extent of 850 cubic feet (24,067 litres) per hour at least, and probably more ; the respirations 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 correspondingly less ; immediately the pace slackens or the horse stops the respirations at once fall, and the amount of air inspired becomes reduced. This is one of the great difficulties attending respiration experiments on horses under natural conditions. A horse in a state of repose, according to Zuntz and Lehmann, produces 3 cubic feet (85 litres) of C0 2 per hour, and absorbs nearly 8|- cubic feet (99 litres) of oxygen ; the expired air is found to have lost 4 per cent, of its oxygen and gained 3| per cent, of C0 2 . This is very much more than we found,* but it agrees pretty closely with the observations made on other animals and on man. It may be noted that even in animals which, from their small size or other causes, lend themselves to exactitude in experi- mentation, the most divergent results have been obtained, and the same thing is observed in man. There are certain evident factors which considerably influence the amount of C0 2 produced and 2 absorbed, and of these muscular work and the nature of the diet are the most prominent. As the result of muscular activity * ' The Chemistry of Eespiration in the Horse during Best and Work,' Journal of Physiology, vol. xi., 1890. It is now considered that samples of air are not sufficient to determine respiratory ex. changes, the C0 2 has a tendency to accumulate in the tissues, and an apparatus which admits of prolonged observation is necessary, such as was employed by Zuntz and Lehmann. The apparatus employed in our observations is shown in Figs. 29 and 30, Digitized by Microsoft® RESPIEATION 115 Fig. 29, — Horse in Position on Respiration Apparatus. Fig. 30. 1, The face mask ; 7, inlet tube to bag ; 8, valve box through which the expired air passes to 10, a rubber bag of 20 cubic feet capacity. After an experiment the air is pressed out of the bag, and, passing through 11, 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 . Digitized by Microsoft® 8—2 116 A MANUAL OF VETEEINARY PHYSIOLOGY the production of C0 2 is increased ; in the same way the amount of oxygen absorbed is greater, but experiment has failed to prove a definitely immediate relationship between the amount of oxygen absorbed and the amount of work produced. A diet rich in starch (carbo-hydrates) increases the amount of C0 2 produced, whilst fats have not such a marked effect in this direction. The respiratory quotient (p. 96) approaches unity in animals fed on a diet rich in carbo-hydrates, viz., there is very nearly as much C0 2 given off as 2 absorbed ; this is not the case with animals living on a flesh diet, where the respiratory quotient may fall as low as '7. The following table gives the amount of air respired and the gases of respiration for several animals ; it is an old table by Boussingault. Eecent observations on the horse, which we have previously quoted, give about half the values as compared to those assigned by Boussingault. Amount of Air Amount of Amount of Car- Body Weight. inspired in Oxygen consumed bonic Acid pro- 24 hours. in 24 hours. duced in 24 hours. Cubic feet. Cubic feet. Cubic feet. Horse 990 lbs. 3373 150 151-0 Cow 990 „ 2782 122 122-3 Ass 330 „ 1112 50 50-4 Pig 165 „ 1216 54-7 55-1 Sheep 99 „ 720 32-4 22-6 Dog 44 „ 298 14-0 10-3 Alveolar Air. — We have indicated that the lungs cannot be completely emptied of air ; a column of air even after death exists from the larynx to the alveoli of the lungs. The air in the air-sacs does all the work, and is known as the alveolar ; from the air-sacs to the nostrils is the ' dead space ' of the respiratory tract, and the composition of the air at any two parts of this space is not the same, it grows progressively poorer in oxygen in the direction of the lungs, and richer in carbon dioxide. During expiration the first air to leave is that from the ' dead space,' and the alveolar air follows towards the end of the expiration. Luring inspiration the Digitized by Microsoft® EESPIRATION 117 incoming air is diluted with that already in the lungs, and its chemical composition at once alters. The air which distends the alveoli is therefore a mixture of air already in the alveolus combined with air from the bronchial passage, and the air in the bronchial passage, in its turn, becomes a mixture of bronchial air and air derived from the higher air-passages. How far the air of an ordinary inspiration travels is difficult to determine; under the most favour- able circumstances some of the axial stream of the current might reach the alveoli, especially of the anterior lobes, but the bulk of it will get no further than the bronchi, and by so doing will have displaced and mixed with the air lately occupying the bronchi, now occupying the alveoli. In man it is estimated that in this way about one-eighth of the alveolar air is changed at each respiration. ^ Every endeavour has been made to ascertain the com- position of the air in the alveoli of the lungs ; if this could be put beyond doubt the vexed question of whether the gaseous exchange is due to diffusion or not would be capable of settlement. If, for instance, the pressure of carbon dioxide in the pulmonary capillaries was found to be the same as the pressure of this gas in alveolar air, diffusion would account for the exchange. The question is a very difficult one, but easier to settle from its carbon dioxide aspect than from the oxygen side. Haldane and Priestley, whose work we have closely followed, found in their experiments on alveolar air that under ordinary atmospheric pressure it contains practically a constant percentage of C0 2 , and this is brought about by the influence of this gas on the respiratory centre. Carbon dioxide regulates the ventilation in the lungs, and thus provides the means for getting rid of itself ; the more C0 2 r fc " in the blood the greater the alveolar ventilation. C& Influence of Work on Respirations. — It is by no means jf clear why work causes an increase in the number and depth of respirations. Changes in the composition of the blood-gas stimulating the respiratory centre has been urged Digitized by Microsoft® 118 A MANUAL OF VETEEINAEY PHYSIOLOGY as a reason, but does not stand the test of experimental inquiry, for it is found that the blood leaving the heart during work is normal in composition. On the other hand, the evidence that the panting respirations of work are due to a ' something ' produced in the muscles is very strong, for if the spinal cord of a dog be divided and the hind legs stimulated, increased respiratory movements are caused, just as if the animal had been running some distance. What the substance is is unknown ; some observers have regarded it as sarco-lactic acid or acid phosphates, but nothing is definitely known, though it is interesting to observe that dilute acids injected into the blood give rise to the same condition. But hurried respirations may also be produced 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 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, operates through the circulatory system. Fortunately, the vessels of the lungs are capable of con- siderable adjustment, they hold more blood during inspiration than expiration, and in this way may be re- garded as a safety valve to the heart. The important practical questions of work, ' condition,' and fatigue are 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 to the organic matter mixed up with it. Some modern investigators attribute the ill-effects of vitiated air mainly to the absence of free ventilation and 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 guide to purity; there may be fewer in respired air than in the same air before respiration, in consequence of their being Digitized by Microsoft® KESPIRATION 119 arrested in the lungs. On the other hand, Haldane and Lorrain Smith attributed the ill-effects of respiratory impurity to the excess of C0 2 and deficiency of 2 , hyperpnoea beginning when the C0 2 rises to 3 or 4 per cent. The amount of air required for ventilation purposes is a question of hygiene, and reference should be made to works on that subject. 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 represented 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. 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 expiratory murmur immediately follows the inspiratory without a pause, but there is a marked pause between the end of one expiration and the beginning of the next in- spiration. The ordinary murmur is best heard where the chest wall is thin ; if the ribs be covered with fat or any great thick- ness of muscle the sound may be entirely lost. It is also important to note that there are some chests perfectly Digitized by Microsoft® 120 A MANUAL OP VETERINAEY PHYSIOLOGY healthy where, for no apparent reason, the respiratory murmur is obscure or even undetectable. Q-\0 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 localized 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 n the horse. Both the above pathological conditions and their progress are deter- mined by auscultation and percussion : there are many departures from the normal respiratory murmur, all of which have their signi- ficance. 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 probably rarely a disease 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 increased, the expiratory effort being powerful and prolonged, a chronic typical cough becomes established, and the animal unfit for anything but slow work. Roaring is a nervous affection, to which sufficient allusion is made in the section dealing with the larynx. Spasm of the Diaphragm is another respiratory affection due to disordered nervous supply. The sound emitted is quite unlike that in the human ; it appears to come from within the chest or 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 Digitized by Microsoft® EESPIEA.TION 121 the abdominal viscera are propelled against the diaphragm. The diaphragm rarely gives way below, nearly always above, and in the tendinous substance rather than the muscular. This point is of physio- logical interest. In the nasal passages the only affection of any moment is a collection of pus in the facial sinuses. Laryngitis is frequently the result of strangles infection, or of ordinary cold. It presents no physiological features of interest. In the ox pneumonia is rare, with the exception of the special 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 Position of the Muscles op the Larynx in the Horse. a, Epiglottis ; b, opening leading to the glottis ; c, portion of the aryte- noid cartilage ; d, position of the joint formed between the cricoid and arytenoid cartilages ; e, the trachea. The wing of the thyroid cartilage has been removed so as to expose the constrictor muscles ; 4, 4 represents its cut edge. 1 and 2, Thyro-arytenoideus : 1 anterior, 2 posterior fasciculus. The space between these two muscles indicates the position of the ventricle of the larynx. 3, Crico-arytenoideus lateralis. 5, Crico- thyroid muscle, the bulk of which lies inside the thyroid cartilage, and cannot, therefore, be seen. 6, Crico-arytenoideus posticus. 7, Portion of cricoid cartilage ; the shaded portion in front of the figure represents where it and the thyroid meet. 8, Arytenoideus muscle. Digitized by Microsoft® 122 A MANUAL OF VETERINABY PHYSIOLOGY Section 2. The Larynx. The larynx serves a twofold purpose, viz., respiration and phonation ; in animals the former holds the more im- portant position, the voice-producing function being of a very subordinate character. 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 decreasing in size, and so allowing a larger or smaller amount of air to enter the lungs. Within the larynx are two elastic cords arranged V-shaped, the function of which is connected solely with the production of sound (Fig. 32). Both the respiratory and vocal functions require that the several parts of the larynx should move, viz., that the mouth of the organ should be widened or narrowed, or that the cords should be approximated, drawn apart, tightened or slackened. These movements are brought about by certain groups 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. The Muscles of the Larynx may therefore be divided into those of respiration and phonation (Fig. 31). As the most important feature in respiration is the opening or dilating of the glottis, the term respiratory muscle might be con- fined to the dilator of the glottis, while the constrictors would represent the vocal muscles; but the constrictors are not entirely without a 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. Digitized by Microsoft® EESPIKATION 123 The crico-arytenoideus lateralis and posticus are direct antagonists ; the lateralis depress the arytenoid cartilages and close the entrance into the glottis, the posticus swing the arytenoids upwards and outwards and enlarge the glottis. Fig. 32.— The Laryngeal Opening during Ordinary Bespiration. 1, The epiglottis ; 2, 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 opening it has to cover. PJwnatory Muscles. Muscle which relaxes the vocal Thyro-a/rytenoideus, anterior and cords, especially posterior fasciculus. Muscle which renders the cords Crico-thyroid. tense, Muscles which bring the cords The respiratory adductors. together, Muscle which moves the cords The respiratory abductor. apart, Digitized by Microsoft® 124 A MANUAL OF VETEEINAEY PHYSIOLOGY The entrance to the larynx is formed by the two arytenoid cartilages, the epiglottis, and the aryepiglottic 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 wall of the cavity and render the V-shaped space wider ; when the larynx closes the cords are approximated and the space rendered narrower (Pigs. 32 and 33). During ordinary respiration there is very little if any alteration in the shape and - y 4 V - 1 \\ : 2 Ik^n v ] }' § i ] Jb Fi ■ i i y Fig. 33. — The Laryngeal Opening during Hurried Respiration, seen in a State of Dilation. 1, Epiglottis ; 2, margin of arytenoids ; 3, vocal cord ; 4, pharynx laid open. Note the size and shape of the glottal opening as compared with Fig. 32. size of the glottis ; but during exertion every inspiratory movement is accompanied by an increase in size, every expiration by a decrease. At each expiration the vocal cords pass towards the centre line, and at each inspiration return to the wall of the larynx. The closure of the larynx, such as during the act of swallowing, is a powerful movement, and if the finger at this moment be introduced Digitized by Microsoft® BESPIEATION 125 into the cavity and placed between the arytenoids, it ex- periences considerable pressure. The closure of the larynx is brought about by the depression and approximation of the arytenoid cartilages and the approximation 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 in- tended to seal during a condition of laryngeal repose. It is carried 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 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 experimentally, 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. Sensa- tion to the mucous lining membrane and motor power to the crico-thyroid muscle is supplied in the majority of animals by the superior laryngeal branch of the vagus, this nerve containing both sensory and motor fibres. In the horse the motor fibres running in the superior laryngeal are 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 ? No satis- factory explanation of this fact has been offered. All we know is that both dilator and constrictor fibres run in the recurrent laryngeal nerve and are quite distinct, and that in some animals the different bundles have been experimentally isolated and injured ; injury to the dilator fibres producing Digitized by Microsoft® 126 A MANUAL OF VETERINARY PHYSIOLOGY abductor 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, viz., the abductor muscles are affected. Another curious fact in the history of these 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 pale and wasted the degree of degeneration cannot be compared with that furnished by 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 recurrent has been divided experimentally, that the abductor muscle loses its irritability long before the adductors, and the same fact may be observed in post mortem stimulation of the nerves. If the recurrent laryngeal nerves be divided under ether, and the peripheral ends stimulated, adduc- tion of the larynx is obtained ; but if the ether narcosis be pushed to a dangerous extent and the nerves then stimulated the larynx dilates, that is abduction follows. These and other observations have furnished a law which is of clinical significance, viz., that in functional disturbance 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 on 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 Digitized by Microsoft® EESPIEATION 127 beyond the middle line of the larynx, in its attempt to come into contact with it. The inspiratory distress occasioned in ' roaring ' is not brought about, as has been described, by a paralyzed vocal cord napping about, for the elastic nature of the cord and other reasons negative this. The sound is produced by the paralyzed left arytenoid being drawn into the glottis at each inspiration, which is the explanation why the noise which accompanies the disease is always inspiratory and never expiratory. 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 thin- ness. The position of the resonant chambers such as the mouth, pharynx, posterior nares, and even nasal chambers also importantly affects the quality of the voice. It is obvious that the chief alterations in the larynx during phonation refer to the vocal cords ; these are approximated by the adductor muscles, and separated by the abductor muscles, whilst they are relaxed by the thyro-arytenoideus and tightened by the crico- thyroid. The latter muscle has a peculiar action, it lowers the thyroid cartilage on the cricoid and swings the wing of the thyroid outwards, thus rendering the cords tense. These changes in the vocal cord 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 cavities of the mouth, nose, pharynx, etc., act as resonators. Being filled with air, they effect the needful alterations in the quality of the voice and assist in giving it its distinctive character ; 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, loneliness, excitement, etc. We do Digitized by Microsoft® 1?8 A MANUAL OP VETEEINAEY PHYSIOLOGY 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 class of animal — horse, ass, ox, sheep, and pig — is so distinctive that we may recognise their presence without seeing them; yet though the larynx in all these animals differs more or less, the difference is not sufficient to offer any explanation as to why the sounds it emits 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 they are easy to recognise. The horse is essentially a sociable animal ; when accustomed to be in the company of others he dislikes separation, and shows it by persistent neighing, which is perhaps more noticeable amongst army horses than any others. The neigh of pleasure is often spoken of as the ' whinny ' ; the word rather conveys an idea of the sound made. Sounds which can only be described 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 (which is very rare), 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 prse- crucial and neighbouring gyrus of the dog, and corre- sponding regions in other animals. Stimulation of this region leads to bi-lateral adduction of the cords; it is curious why stimulation of one side of the brain should Digitized by Microsoft® RESPIRATION 129 lead to movements of both vocal cords. There is no region of the cortex of the dog which leads to abduction of the cords, though such a region is found in the cat. The cortical centre communicates with a subordinate centre in the medulla situated in the region of the fourth ventricle, and stimulation of certain parts of this centre leads to abduction and of others to adduction of the cords. 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. 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 bleating of the sheep are expiratory efforts through the mouth. The dog and cat have a 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. Yawning is a deep slow inspiration followed by a short expiration; the air, even in the horse, is taken in by the mouth, which is widely opened and the jaws 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 share in the act. Coughing occurs through the mouth, the long palate in the horse being raised for the purpose. Before coughing can occur the lungs must be filled with air and 9 Digitized by Microsoft® 130 A MANUAL OP VETEETNAEY PHYSIOLOGY 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 has been referred to more fully on p. 120. Digitized by Microsoft® CHAPTEK Y_ DIGESTION Section 1. 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, 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 possessing independent movement ; in the pig the lower lip is pointed and the upper one insignificant. 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 they may bite closer to the ground. In the ox the tongue is protruded and curled around 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 131 9—2 Digitized by Microsoft® I I I 132 A MANUAL OF VETEBINARY PHYSIOLOGY sockets, the object of which is to prevent injury to the dental 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 herbivora 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 1/ V-'Mi/™ MJ-iM rm« / vJI/lm m-\M -LLM RLM-. V -\ -LLM RLM- Pig. 34. — Schematic Tkansverse Section op the Uppek and Lower Jaws of the Horse between the Third and Fourth Molars, showing the Position of the Tables of the Teeth during Best and Mastication. UJ upper jaw, LJ lower jaw, EM 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 right to left ; the tables of both upper and lower molars now rest on each other. 3, The position, half way through the act of mastication ; the outer half of the lower teeth wearing against the inner half of the upper. pushed out of the sockets which hold them ; in this way wear and tear is compensated for, whilst the fang of the tooth becomes correspondingly 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 Digitized by Microsoft® DIGESTION 133 especially well seen in the horse where the cutting surface is chisel shaped, the upper teeth being longest on the out- side, while those of the lower row are longest on the inside (see Fig. 34). 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 difficulty in protruding the tongue and even introducing the tip into the nostrils. It is not a very common habit with horses to protrude the tongue except when yawning, but they have considerable power in withdrawing it in the mouth. A great difference exists between the tongue of the horse and that of the ox ; the former is flabby, broad and flat at the end, constricted opposite the frenum, and swells 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. The inside of the mouth of the ox is covered with long papillae, which look backwards ; these would appear to be of use in preventing the food from falling out of the mouth. In the horse no such papillae exist, in fact the lining mem- brane of the 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 func- tion is probably connected with assisting the tongue to pass the food back in the mouth. Digitized by Microsoft® 134 A MANUAL OF VETBEINAEY PHYSIOLOGY 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. Lapping in the dog is performed 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 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 is performed between the molar teeth ; the movements 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 consider- able amount of play, and this is provided by a disc of cartilage being placed between the articulation, which accommodates itself to the varying movements of the joint in the horse, ox, and sheep, and also saves the part from jar. In herbivora, therefore, we find the cartilage exten- sively developed, whilst in carnivora it is small and simple. The character of the movement occurring in the temporo- maxillary articulation of herbivora during mastication is as follows. During rotatory movement, or lateral displace- ment, one of the articulating heads remains as a fixed point Digitized by Microsoft® DIGESTION 135 simply turning on its centre, whilst its fellow describes an arc ; this is why the movement can only occur on one side at a time (Gamgee). During mastication the contents of the orbital fossae are observed in the horse to be alternately ascending and descending. This movement is due to the coronoid process of the lower jaw, the fossa being pushed up as it comes forward and depressed as it recedes. The muscles which bring about this important lateral move- ment of the jaws, which in the ox, owing to the freedom of the articulation, may be termed rotatory, are the two pterygoids, especially the internal. The herbivora can only masticate on one side at a time ; when tired on one side the process is reversed and the opposite molars take on the crushing. It is surprising the length of time an animal will carry on mastication on one side ; even as long as an hour has been observed in the horse by Colin. Gamgee noticed that in the ox 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 under- stand, 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 masti- cation, explains why the molar teeth of the horse and other herbivora wear with sharp chisel edges (see Fig. 34). In the horse mastication is slow and as a rule well per- formed ; he takes from five to ten minutes to eat one pound of corn, and fifteen to twenty minutes to eat one pound of hay. In the ox mastication is imperfectly performed to start with, but the material is eventually brought back to the mouth by the process of rumination, and undergoes thorough re-mastication. In the dog mastication is imper- fectly performed ; after a few hasty snaps of the jaw the material is swallowed. Digitized by Microsoft® 136 A MANUAL OP VETERINAEY PHYSIOLOGY 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 sterno- 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 impulse 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. The process of Deglutition is usually described as occur- ring in three stages. The first stage practically comprises carrying the food back to the base of the tongue and press- ing it against the soft palate-; it is a simple process and 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 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 ej^glottis and close the larynx, which is further secured by the arytenoid cartilages and vocal cords coming close together. The bolus or fluid can now safely pass towards the pharynx, being grasped tightly by the pharyngeal muscles and pressed into the oesophagus. 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 Digitized by Microsoft® DIGESTION 137 second act of deglutition. The length of the soft palate prevents 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. The action of the epiglottis in the closure of the glottis has been much discussed. We have described it as being 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. With a finger in the larynx it can easily be demonstrated that the part tightly and forcibly closes during the second stage of swallowing, the vocal cords and arytenoids 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 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. Both in eating and drinking the third act of deglutition can occur against gravity ; this is because it is a muscular act. The whole process of deglutition is considerably assisted by the salivary secretion. When this has been experimentally diverted swallowing only occurs with difficulty and very slowly. The oesophagus of the horse is found to differ consider- ably from that of most other animals. It is composed for the greater part of its length of red striated muscle, Digitized by Microsoft® 138 A MANUAL OF VETEEINAEY PHYSIOLOGY 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 cut through close to the stomach no material can escape ; this is one explanation why horses vomit with such difficulty. In the ox, sheep, and dog, the tube is com- posed 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. The first stage of deglutition is voluntary, but the re- maining 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 the superior laryngeal, the centre is made acquainted with the fact that food is present in the fauces. A reflex act is now set up in the centre and an impulse conveyed to the muscles of the part by outgoing or efferent nerves, furnished by the pharyngeal plexus (composed of the vagus and glosso- pharyngeal) to the constrictor muscles of the pharynx, by the hypoglossal to the tongue, and by the recurrent laryn- geal to 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. Swallowing may be induced without the presence of food in the fauces ; touching the rim of the glottis will produce it, as also will pouring fluids into the trachea, or even touching the interior of the trachea as far down as the bronchi. The swallowing centre also presides over the oesophagus, and the peristaltic wave from the pharynx to the stomach is produced by impulses sent out from this centre through the vagus. This wave is, there- fore, not due to the nerve handing on a contraction by direct conduction from one layer of the muscular wall of Digitized by Microsoft® DIGESTION 139 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. 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 upwards from the stomach to the pharynx. 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 under- stand, as much the same process occurs in other secretory glands which we have not the same opportunity of watch- ing during their activity. Classification of Salivary Glands. — The three glands which secrete saliva are the parotid, submaxillary, and sublingual ; these are structurally divided into two groups, mucous and serous (or albuminous) glands, the submaxillary and sub- lingual being types of the first, the parotid the type of the other. The salivary glands in the herbivora are of con- siderable size, the submaxillary and sublingual being well developed in the ox, while in the horse they are rudimentary. According to Colin, there is no relationship between the weight of the glands and the amount of fluid they secrete ; the parotid in all cases secretes more than the others. In the horse it is only four times heavier than the sub- maxillary, but it secretes twenty-four times as much saliva ; in the ox the parotid is not so large as the sub- maxillary, but its secretion is four or five times greater. Amount of Secretion. — Colin places the daily secretion of saliva in the horse at 84 lbs., and in the ox at 112 lbs., though the amount will depend on the dryness of the food consumed ; thus hay absorbs more than four times its Digitized by Microsoft® 140 A MANUAL OF VETERINAEY PHYSIOLOGY weight of saliva, oats rather more than their own weight, and green fodder half its own weight. Physical and Chemical Characters. — Mixed saliva is an alkaline, 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 1005 in the horse, and 1010 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 are probably derived from the soft palate. About "6 per cent, of the saliva consists of mineral matter, and "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 of proteid substances the nature of which has not been exactly determined. Mucin belongs to a peculiar group of proteid bodies combined with a carbo- hydrate, for which see Appendix. Pj^ahji or salivary diastase is the most interesting organic constituent of saliva in man, but it is doubtful if it exists in the herbivora, and under any circumstances its amount has not been de- termined. 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. A substance known as sulphocyanide of potassium has been found in minute quantities in the saliva of the human subject, but is 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 vols, per cent.). The three salivas have different physical properties : — Parotid saliva is watery, clear, and free from mucin, but contains a small quantity of proteid ; submaxillary and sublingual saliva are viscid, especially the latter. In man the parotid saliva is rich in ptyalin. Colin has observed certain peculiarities in the secretion of saliva in herbivora which deserve careful attention. Digitized by Microsoft® DIGESTION 141 He demonstrated that the secretion from the parotids is unilateral, the gland on that side of the mouth on which the animal is masticating secreting two or three times as much as its fellow ; the submaxillary and sublingual glands, on the other hand, secrete equally, no matter on which side mastication is being performed. Further, the parotids secrete during rumination, the unilateral secretion still being maintained, whilst the submaxillary and sublingual Fig. 35. — Apparatus employed by Colin in Experiments on the Secretion of Parotid and Submaxillary Saliva. glands are during this process in a state of rest* In a fasting horse the parotids are quiescent, while in the ox they are active. Observations tend to show that in the former animal during fasting the mouth is kept moist by secretions from the sublingual, palatine, labial and molar glands. The glands of the mouth are extensively developed in the horse, particularly the palatine, and some large ones close to the epiglottis ; their secretion is extremely viscid. Neither the sight of food nor the introduction into the mouth of sapid substances, produces any effect on the salivary secretion from the parotid of the horse ; sapid Digitized by Microsoft® 142 A MANUAL OP VETEKINARY PHYSIOLOGY substances, however, stimulate submaxillary secretion. The apparatus used in these experiments is shown at Pig. 35. The use of the saliva in herbivora is to assist inniastica- tion and swallowing, stimulating the nerves of taste, and in ruminants assisting in rumination. According to our observations on the horse, saliva has no chemical action on the raw starch of its food, and this is not surprising when we remember that the starch grains are enclosed in an enve- lope of cellulose, a substance 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 produced 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 possessing a shape peculiar to the species, these granules are enveloped in a tough envelope 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 cooking, the starch (granulose) is liberated and free to be acted upon ; on the other hand, 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, and a reddish colour is formed on the addition of this reagent, indicating the presence of a substance known as erythrodextrin. At this time the fluid which before was sugar-free, now contains distinct evidence of its presence ; 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 Digitized by Microsoft® DIGESTION 143 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 erythro- dextrin. The sugar formed from starch by the action of saliva is not grape-sugar but maltose ; glucose (dextrose or grape-sugar) only being found 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 which plays the part of a ferment. The process is permanently destroyed by a high, inhibited by a low temperature, re- tarded 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 only produce maltose whereas the acid pro- duces dextrose. ^- The view we hold as to the non-amylolytic action of saliva in herbivora is not supported by other observers ; Ellenberger * distinctly states that both the parotid and submaxillary secretions 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 Smith \ states that the saliva of the horse will convert crushed raw starch into sugar in fifteen minutes, and that the process is continued in the stomach ; he further adds that the saliva of the horse will convert cane into grape-sugar. In ruminants he believes starch conversion takes place both in the mouth and rumen. Though we do not accept these views, we shall shortly endeavour to show how starch is converted into sugar in the stomach of the horse. It is interesting in this respect * ' Physiologie der Haussaugethiere.' t ' Physiology of the Domestic Animals.' Digitized by Microsoft® 144 A MANUAL OF VETERINARY PHYSIOLOGY to note that in man starch conversion, brought about by the action of ptyalin, is also now recognised as taking place in the stomach from the swallowed saliva, in fact, that the bulk of the conversion takes place there, and not in the mouth. Secretion of Saliva. — The mechanism concerned in the secretion 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 afforded the desired information, and there is reason to believe that the same process 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 submaxillary 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. 36). 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 stimulated secretion follows, but if the chorda tympani be previously divided no secretion follows on stimulation of Digitized by Microsoft® DIGESTION 145 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. When the sym- pathetic is stimulated, exactly the reverse is observed — viz., the vessels constrict, in consequence of which the gland V. aym. ch.-t" Fig. 36. — Diagrammatic Eeprbsentation op the Submaxillary "Gland of the Dog with its Nerves and Bloodvessels (Poster). (The dissection has been made with the animal on its back, and is very diagrammatic.) The submaxillary gland (am. gld.) occupies the centre of the figure ; the bloodvessels supplying it, derived from the carotid artery a.car., are seen on the left, whilst the duct from the gland s.md., in which a canula 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 ru nnin g together the two nerves separate, the chorda tympani ch.t. running along the submaxillary duct to the gland. Close to where the two nerves separate is the submaxillary 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.8. and coursing 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. 10 Digitized by Microsoft® 146 A MANUAL OF VETEBlNAEY PHYSIOLOGY 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 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 duet of the gland is higher than the blood pressure within the vessels. Further, if before stimulating the chorda some atropin 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 atropin experiment proves the existence in the chorda of two sets of nerves, viz., secretory and vaso-dilator ; owing to the action of atropin the secretory nerves are paralysed, whilst the vaso-dilators are not. And 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. Pilocarpin is antagonistic to atropin 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. Thi s is known as ' paralytic secretion.' 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, whilst the sympathetic are trophic, hence stimulation of the chorda gives a watery saliva poor in solids, whilst stimulation of the sympathetic gives a scanty saliva rich in solids. Digitized by Microsoft® DIGESTION "147 l ~b The method by which secretion in the parotid gland is carried out differs in no essential respect from that of 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 bloodvessels, while both trunks contain secretory nerves. It will be observed that no reference has been made to the nerve ganglia in connection with salivary secretion. Ganglia are a collection of cells in the course of a nerve. Pig. 37. — Changes in the Cells of the Living Parotid (Serous Gland) during Secretion. A, At rest; B, in the first stage of secretion; C, after prolonged secretion (Poster, after Langley). If these cells be paralysed by nicotine, as was first shown by Langley, stimulation of the nerve does not produce a secretion. The changes occurring in the cells of the salivary glands during secretion depend upon the type of gland. We will therefore describe separately, from Langley's observations, the changes in the cells of a serous gland such as the parotid, and in those of a mucous gland of which the sub- maxillary is a type. 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, the cells are found 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. 37, A). 10—2 Digitized by Microsoft® 148 A MANUAL OP VETERINABY PHYSIOLOGY 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, whilst that edge next the lumen is still granular (Fig. 37, B). In an exhausted condition the cellB are smaller and remarkably clear, only a few granules being left in them on the inner edge, whilst the lumen is now distinct and large, and the nuclei are clearly seen occupying a central position (Fig. 37, C). If a mucous gland at rest be examined under like con- ditions, the cells are found filled with granules much larger Fig. 38. — Cells from Mucous Gland (Submaxillary Gland of the Dog). (Poster.) a, Prom loaded gland ; b, from discharged gland ; a', b', treated with dilute acetic acid ; a', from loaded ; b', from discharged gland. than those of a serous gland, and a nucleus is seen occu- pying one edge of the cell (Fig. 38, 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. 38, b) . If the cells, while in an active condition, be acted upon by water or dilute acetic acid, the granules swell up and become transparent owing to the mucin they contain, and a delicate network is seen to pervade the cell (Fig. 38, a'). A similar appearance is pro- duced in the exhausted cell (Fig. 38, b'), excepting that less transparent mucin is seen and more granular substance, while the nucleus of the exhausted irrigated gland is seen passing towards the centre of the cell instead of remaining Digitized by Microsoft® DIGESTION 149 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 nitration or secretion. Experiments made by Langley and Fletcher go to prove that even water and salts are the result of an act of cell secretion, and not of mere transudation. 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 to meet with a simple stomach in the dog, and complex 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 for a vegetable feeder is by no means a necessity ; the stomach of the ruminant and the simple stomach of the horse could not be in greater contrast, whilst the resulting laboratory processes are practically identical. So far as vegetable food is concerned, it does not matter whether Digitized by Microsoft® 150 A MANUAL OF VETEEINAEY PHYSIOLOGY the solution and absorption of its readily soluble matters comes before maceration, or whether maceration pre- cedes the extraction of the readily soluble substances. If maceration comes first, as in ruminants, bulky gastric compartments are provided for the purpose, and the sub- sequent intestinal canal is small. If the simple stomach comes first, bulky intestines for the purpose of maceration follow ; in both cases ample provision is made 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 under- standing. 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 single-handed by the intestines. 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, whilst between the two comes the stomach of the omnivorous pig, partaking of some of the characters of the carnivora and 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 the distance the stomach lies from the abdominal wall; pure gastric juice has, therefore, never been obtained from the horse. The firBt peculiarity to be noticed in soliped digestion is that the stomach is rarely empty ; it is only when horses have -purposely 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 con- Digitized by Microsoft® DIGESTION 151 siderable. 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 re- action 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 peptonized 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. Let us now suppose that the 'feed' is finished. At once the passage of chyme into the duodenum ceases, or becomes so slowed down that only small quantities of food pass out, and so gradually does this occur that it will be many hours before the stomach is really empty, though had the process continued as it commenced, it would not have contained anything at the end of an hour. This condition of stomach digestion in the horse may be variously modified, depending on 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. AH these are points requiring our attention, but before giving it we must briefly look at the stomach itself. >^= The mean capacity of a horse's stomach is, according to Colin, from 25 to 30 pints, or from "5 to - 63 of a cubic foot ; 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 conditions for Digitized by Microsoft® 152 A MANUAL OP VETERINARY PHYSIOLOGY digestion when it contains about 17J pints, 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 continuation 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 examining the villous membrane it is found to differ greatly in appearance, the PYL. CARD. L.S. CUT. BOU. FUN D . Fig. 39. — Longitudinal Section of the Stomach of the Horse. card., Cardia ; pyl., pylorus ; l.s., left sac ; r.s., right sac ; cut., cuti- cular 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. fundus being channelled, furrowed, and velvety, whilst the pyloric portion is smooth. It is in the fundus only where true gastric juice, viz., pepsin and acid, is secreted; in the smooth pyloric mucous membrane only pepsin is formed. The area of the fundus-secreting surface is about one square foot. Pig. 39 shows the relative position of the various parts of the mucous membrane of the stomach of the horse ; the drawing accurately indicates the shape of the stomach, the position of the inlet and outlet, and the direction and position of the various areas. A very remarkable amount of mucin is secreted by the villous sac of the stomach, Digitized by Microsoft® DIGESTION 153 and forms over the inner surface of the viscus a thick gelatinous 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 usually large and open, and there is a distinct pyloric ring ; behind this the duodenum is dilated, and the gut comports itself in such 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 a letter U ; so much does this F IG -. 40. — Longitudinal Section op the Stomach of the Horse, SHOWING THE SYPHON TkAP OF THE DUODENUM. ce, (Esophagus ; py., pylorus ; d, left sac ; v, fundus ; duo., duodenum. remind one of a well-known form of trap used in drainage, that we have described it as the syphon trap of the duode- num (Fig. 40). The use of this trap appears to be to regulate the passage of material from the stomach into the intestines. Our observations have shown that its presence 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 off. Should fermentation still continue in the stomach, the contents can neither escape into the oesophagus, nor into the bowel, and the coats of the viscus Digitized by Microsoft® 154 A MANUAL OF VETEEINAEY PHYSIOLOGY may be completely ruptured under the intense strain. It was mentioned on p. 138 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 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 contracted ; 5, that only a portion of its surface is capable of secreting a digestive fluid ; 6, that there are remarkable 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 we have shown, mixes in the mouth with four times its bulk of saliva, and after a very perfect grinding passes into the Btomach. If the stomach be empty it is of no size and the material lies in the pyloric region ; as the viscus gradually fills, the gastric juice begins to act, and chyme commences to pass into the intestines probably in a very imperfectly elaborated form. Assuming the animal to have finished eating the hay, we now find the output into the intestine becomes small and slow. The gastric juice has an opportunity- of acting more thoroughly upon the ingesta, which turn yellow on that surface which is in contact with the villous wall, the com- pression 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 Digitized by Microsoft® DIGESTION 155 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 j uice. The entire surface of the stomach and its contents are now acid, excepting at the cardia, where it 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, in spite of its four equivalents of saliva, is due to the com- pression of the material by the stomach walls. The duration of stomach digestion of hay is variable, but we quote one or two of Colin's experiments. A horse received 5 J lbs. of hay which he took two hours to eat ; at the end of that time he was destroyed, and the stomach contained 2*2 lbs. ; thus in two hours he had digested 3"3 lbs. Another horse received h\ lbs. hay, and was de- stroyed three hours from the time of commencing to feed ; in the stomach were found 1 # 54 lbs., so that in three hours this horse had digested 3 - 96 lbs. In the third hour (during which time he was not feeding), judging from the first experiment, he had digested only - 66 lb., whereas the previous rate of digestion for the first two hours was at the rate of T65 lbs. per hour. To return to our previous statement, when the animal is no longer feeding the rate of digestion at once becomes reduced, and it is probable that several hours must elapse, assuming no further food be given, before the stomach completely empties itself. This period may be fifteen, eighteen, twenty -four or even thirty-six hours. We starved a horse for twenty-four hours, and at 6 a.m. gave him 6 lbs. of dried grass ; he was destroyed at 3 p.m., and the stomach still contained 2J lbs. ; in nine hours, therefore, only 3^ lbs. had been digested. In another observation carried out under similar conditions, only 1 lb. had been digested in four hours and three-quarters. Of 4 lbs. hay given only 1 lb. 11 ozs. were digested in six hours ; of Digitized by Microsoft® lbs. lbs. 3-37 ; the second, 3'08 3-83 4-24 4-04 3-56 4-32 5-03 4-10 4-55 4-01 4-35 4-87 4-44 156 A MANUAL OF VETERINARY PHYSIOLOGY 3J lbs. hay, 2£ lbs. were digested in five and a half hours ; while in another observation, of 4 lbs. hay, 2 lbs. 12 ozs. were digested in five hours. Colin's elaborate researches furnish us with very complete data on the question of hay digestion in the horse. He fed fourteen horses on hay, and destroyed two of them at regular intervals ; each animal received 5 - 5 lbs. of hay, and digestion was counted from the time they were fed. Here are the results : Amount of Hay given 5 - 5 Lbs. lbs. After 2 hours, the first horse had digested 3 - 37 3 4 5 6 7 From this it is seen that the rate of digestion during the first two hours is rapid and then falls off, so that even at the end of eight hours there is still something left in the stomach. The second horse in the five hours' observation had very nearly digested the whole of the ration, but this is an exception. There is no doubt that it is extremely difficult to get the stomach to empty itself. We fed a horse on dried grass and destroyed it eighteen hours later ; there was still a small quantity of food in the stomach. In another case the stomach, after fifteen hours, was found empty. In a third case a horse was given grass twice at intervals of twenty-four hours ; he was destroyed eighteen hours after eating his last feed, and a handful of grass was still found in the stomach. Digestion of Oats. — We have now to consider the digestion of oats, and here again we still observe the same fact noted under that of hay, viz., that the stomach commences to pass its contents into the intestine during feeding, and that this slackens considerably when no more food is entering Digitized by Microsoft® DIGESTION 157 the viscus. Colin fed six horses on 5 - 5 lbs. of oats each, and destroyed them at certain intervals. lbs. lbs. After 2 hours, one horse had digested 27 ; a second, 2 - 5 „ 4 „ „ „ 3-1 •„ 3-4 „ 6 „ „ „ 3-5 „ 3'0 We have observed in a horse which had received 2 lbs. of oats, and was destroyed twenty hours later, that the stomach had not completely emptied itself. In another experiment four hours after feeding on one pound of oats, 6 ozs. were recovered from the stomach. A horse received lbs. oats. 4 And was destroyed in hours. 4 Amount digested lbs. ozs. 2 3 3 4 - 4* 4 i n\ 2 4 3 - 3| 2 24 3 4 3 4 - 4 4 6£ 4 - 1 13| 2 6^ 3 4 4 12 The last horse is included to illustrate a point of some importance in the feeding of animals. For eighteen months this horse had never tasted corn, having been fed on a patent food ; a sudden change in diet is the explanation why he only digested 12 ozs. of oats in four hours. It will be observed that the fifth horse in this series digested nothing, even at the end of four hours ; we can only account for this by the fact that the animal was in a strange place where the feeding experiment was carried out, and was of a very nervous disposition. « Arrangement of Food in the Stomach. — An interesting practical and physiological study is the effect of feeding horses on different foods in succession. When hay is given first and oats afterwards, the hay is found close to the greater curvature and pylorus, and the oats in the lesser curvature and cardia ; no mixing has occurred, both foods Digitized by Microsoft® 158 A MANUAL OF VETEEINAEY PHYSIOLOGY are perfectly distinct, and a sharp line of demarcation exists between them (Fig. 41, 1.). During digestion mixing occurs at the pylorus but nowhere else ; no matter what compression the contents have undergone as the result of gastric contractions, the foods always remain distinct. The presence of the oats, however, causes the hay to pass out more rapidly than it would have done had it been given alone. Colin observed that half the hay, but only one- fourth or one-sixth of the oats, would, under these con- ditions, pass into the intestine in two hourB. Ellenberger has shown that when hay and oats are given in this order, a portion of the oats may pass out into the bowel by the lesser curvature without entering either the left sac or fundus of the stomach (see Fig. 41, I.). When oats are given first, followed by hay (Fig. 41, II.), the oats com- mence to pass out before the hay, but the presence of the hay causes the oats to pass more quickly into the intestines than they otherwise would have done. If a horse be fed on three or four foods in succession they arrange themselves in the stomach in the order in which they arrived, viz., they do not mix. The first enters the greater curvature, the last the lesser curvature, and it is only at the pylorus that any mixing occurs under ordinary conditions (Fig. 41, III.). This regular arrangement of the different foods in layers is only disturbed when a horse is watered after feeding ; under these circumstances the con- tents are mixed together and digestion thereby impeded. Apart from this, the influx of a considerable quantity of fluid into a stomach already containing as much as it should hold, means that material is washed out of it into the small and large intestines, and this may set up irrita- tion and colic. By watering a horse after feeding more than half the food may at once be washed out of the stomach. The water which a horBe drinks does not remain in the stomach, but passes immediately into the small in- testines, and in the course of a few minutes finds its way into the caecum ; hence the golden rule of experience that horses should be watered first and iM. afterwards. We / Digitized by Microsoft® DIGESTION 159 may summarise these facts by saying that in a succession of foods the first consumed is the first to pass out. That does not mean to say that the whole of it passes out before any portion of the succeeding food enters the bowel, III. Fig. 41.— Longitudinal Section of the Horse's Stomach, showing the Arrangement of the Food according to the Order in which it was received (Ellenberger). In each case ce is the oesophagus; py, pylorus ; d, the left sac; i),the fundus. I. Hay first, followed by oats : 6, 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, the oats ; 6, the hay. III. The order of three successive feeds ; c, the first feed ; 6, the second ; a, the third. Digitized by Microsoft® 160 A MANUAL OF VETERINAEY PHYSIOLOGY 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 firBt. 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 to dis- tinctly reverse the English practice of giving oats first and hay afterwards, but perhaps only apparently so, for experi- ment 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 to be 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 horses, which have, however, stood the test of prolonged practical experience. 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 been taken the material is found in a finely chopped condition, firm, one may almost say dry, in places, though towards the pylorus it is liquid. This hay contains between four and five parts of saliva ; it is yellow in colour where the gastric juice has 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 Digitized by Microsoft® ' DIGESTION 161 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 liquid, the fluid being creamy in consistency and colour; the oats are swollen, soft, and their 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 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 observations 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 pro- portion of 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' starva- tion, its reaction is neutral or alkaline. We have observed extreme alkalinity towards the pylorus under these con- ditions, due no doubt to the regurgitation of bile and pancreatic fluid. j The Stomach Acids. — It is not necessary here to enter into any detail as to the nature of the gastric acids ; both in the horse and man a considerable amount has been written to prove that the acidity depends upon lactic or hydrochloric acids, and it is possible that both these views may be reconciled. Ellenberger and Hofmeister are of opinion that shortly after a meal lactic acid predominates in the horse's stomach to be replaced by hydrochloric acid 11 Digitized by Microsoft® 162 A MANUAL OF VETEEINAEY PHYSIOLOGY some four or five hours after the commencement of feeding. These observers found that the nature of the acid depended upon the region of the stomach, the period of digestion, and the character of the food; oats induced an outpouring 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, hydro- chloric, lactic, butyric and acetic acids may be found, the two latter in insignificant quantities only. In flesh feeders HC1 predominates, '25 per cent., and lactic acid is found, in small quantities. In vegetable feeders lactic acid at first predominates, "4 per cent., and later HC1 is present in small quantities ; lactic acid exists throughout the whole stomach, but predominates in the right and left sacs, whilst hydro- chloric 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 con- siderable. Having gone carefully into the question of the presence of hydrochloric acid and organic acids in the stomach con- tents, we can only say that, no matter at what period of digestion observations have been made, we have only two or three times succeeded in finding hydrochloric acid in the stomach of the horse, and are 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 impossible 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 extending over a limited area, described on p. 152 as not larger than 1 square foot (Fig. 39). The two kinds of gland employed in the production of gastric Digitized by Microsoft® DIGESTION 163 juice are both found in the villous coat, the one in the fundus, the other in the pyloric portion, though Ellen- berger 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 grourTs, on the surface of Duct. ' Duct. Parietal Cells. . Gland. Gland. SCALE \aojJi Pyloric Gland. Cardiac or Fundus Gland. Fig. 42. — The Gastric Glands after Heidenhain (Waller). the mucous membrane by means of 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 mem- brane is as smooth as that found in the intestine. Each m il " ' . , iii - « /Tl— a. Digitized by Microsoft® / 164 A MANUAL OF VETERINARY PHYSIOLOGY 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. The cells of the fundus gland (Pig. 42) are small, poly- hedral, 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 (Pig. 42) 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 epithelium 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 its principal and parietal cells, and the pyloric gland with only its principal cells, is that the former secretes both the pepsin and acid of the gastric juice (the acid being separated from the blood by the parietal cells), whilst the pepsin only is formed by the principal cells. The pyloric glands, on the contrary, only secrete pepsin and no acid. We have previously mentioned that the cells of the salivary glands undergo certain changes in appearance, the result of rest and activity ; the same remark applies to the gastric follicles, in which the general type of changes during secretory activity is very closely allied to those already described. Langley has found that in the active state the granules decrease in number, the cells 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 became granular. The parietal cells during digestion were found to increase in size but did not characteristically lose their granules. The central cells Digitized by Microsoft® DIGESTION 165 secrete both the pepsin and rennin ferments, but in neither case do these exist as such in the cells, but as a mother substance or zymogen 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 those animals, such as the dog, yielding hydrochloric acid, the cells very possibly form it by an inter-action of the sodium chloride and sodium dihydrogen phosphate of the blood. 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 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 unknown 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 previous secretions examined have been a mixture of saliva, gastric juice, and perhaps other substances. Pawlow devised a method by which the stomach of the dog could be rendered available for physio- logical enquiry, and a pure secretion was obtained (see Figs. 43 and 44). Pure gastric juice in the dog is as colourless as water, thin, transparent, and of strongly acid reaction. Chemically it consists of acid and enzymes, the acidity, which is due to hydrochloric, being about - 46 or '56 per cent. The enzymes fc/are pepsin and rennin; the former is unable to act excepting in an acid medium, and furnishes the only example in the body of this necessary combination. How far the gastric juice of other animals resembles that of the dog in com- position and appearance we do not know owing to the difficulty in obtaining it pure, but in all cases an acid and Digitized by Microsoft® 166 A MANUAL OF VETERINARY PHYSIOLOGY an enzyme are present. The enzyme is invariably pepsin, but the acid is not always hydrochloric. The amount of juice secreted is uncertain, in the dog some 700 c.c. (24£ ozs.) 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. The gastric juice of the dog withstands putrefaction for a long time ; it may be kept for months without undergoing any important change; not so with herbivora; the mixed Pylorus y^ i , N) I i Oesophagus Plexus gastricoi/ vi/ ^^L I Plexus gastricus anterior vagi/ . | , - J Jr\ / Posterior vagi. Fig. 43. — Pawlow's Stomach Podch (Stewart). 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. gastric fluids of the horse rapidly putrefy. The antiseptic properties of the dog's juice are attributed to its hydro- chloric acid ; if this is so it is additional evidence against the acid of herbivora being hydrochloric. There appears to be no reason why lactic acid should not be formed by the marginal cells of the fundus glands, but an important source of lactic supply in herbivora is the carbohydrate of their food. Pepsin is of a proteid nature, though very little is known of it chemically. It best exhibits its action at a temperature of the interior of the body (37° to 40° 0.) ; a low temperature Digitized by Microsoft® DIGESTION 167 retards its activity, while it is destroyed at a high one. The ordinary commercial 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 glycerine extract of the mucous membrane of the stomach suffices, to which is added some dilute HC1. Glycerine has the power of extracting the ferments both Fig. 44. — Pawlow's Stomach Pouch (Stewart). S, the completed pouch ; V, cavity of the stomach ; A, A, the abdominal wall. from the stomach and other portions of the digestive traqt such as the pancreas. The action of pepsin is almost wholly if not entirely confined to the proteid constituents of food. It converts the insoluble proteids into soluble ones not by direct transformation but by several stages. The products intermediate between proteid and peptone have received certain names suggestive of differences in their chemical nature, but as to all of this a good deal of doubt and speculation exists. Digitized by Microsoft® 168 A MANUAL OF VETERINABY PHYSIOLOGY In the following table the various stages of conversion are indicated in the order in which they are found to occur as determined by small differences in the chemical tests, such as solubility or colour reaction, yielded by the peptonized product. The table is the one drawn up by Kuhne. 1. The proteid as consumed, or native albumin. 2. Acid albumin or syntonin. 3. Primary proteoses. 4. Secondary proteoses. 5. Peptones. The proteid having reached the stage of peptones is now capable of being absorbed, but the conversion from proteid to peptone is a most complex one, during which the large proteid molecule is converted into simpler products of an infinitely smaller molecular weight, while so great is the complexity that the resulting product, peptone, is in all probability a group of compounds, rather than a single one, which only resemble each other in their solubility and their definite reaction to Certain chemical tests. Be nnin _ is the second enzyme present in the gastric juice. Commercially it is used in the manufacture of cheese, an infusion of the mucous membrane of the stomach being sufficient to produce the needful change in the milk. There appears no necessity for adult animals to possess this ferment in their juice after weaning, as milk does not form an article of diet unless we except the chemically altered milk given to the pig. In the young animal rennin plus acid causes milk to clot rapidly. The clotting under rennin resembles blood-clotting. The clot contracts after a time, squeezing out a yellowish fluid known as whey, and furthermore it is definitely known that, as in blood-clotting, a calcium salt is necessary to the process of milk-clotting. In fact two distinct steps are recognised as taking place, first the formation of a substance known as paracasein, by the action of rennin on casein, and secondly the action on the paracasein of the lime salts of the milk forming a curd. Digitized by Microsoft® DIGESTION 169 If milk be deprived of its calcium salts, no clotting occurs on the addition of rennin, from which it is considered that the calcium salts are of more importance than the ferment. Eennin takes no part in the digestive process ; once the curd is formed its digestion is carried out by pepsin. Other ferment actions of the agastric juice have been described, such as fat- and starch-splitting, but of their existence there is very little evidence. Proteid digestion is the essential duty of the stomach, while in all vegetable feeders maceration of the vegetable fibres is begun in the stomach as a preliminary measure. Still in all animals a stomach is not essential to life ; in the dog for example it may be removed experimentally, for as we shall see later on, proteid digestion is provided for elsewhere. But in the herbivora, especially 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 preparation of the food for subsequent digestion in the small intestines, for it is quite undoubted that proteid previously acted upon by gastric juice is far more thoroughly handled by the pancreatic fluid than proteid not so previously acted upon. The secretion of gastric juice has but recently been proved to be under the control of the nervous system, and the secretory fibres are contained in the vagus.* Stimulation of the peripheral end of the divided nerve causes after a short delay a flow of fluid. The cause of the latent period is unknown. It can be shown in the dog that mastication, swallowing, taste, odour, etc., are direct excitants of the secretions, for they cause a copious production of gastric juice, though not if the vagus has been previously divided. If the oesophagus of a dog be divided and the upper section brought outside the wound, the animal may be indulged in a meal which never enters the stomach, but which, nevertheless, produces a profuse se^etion of gastric juice. Mechanical stimulation of the mucous membranes of the stomach has no effect in producing secretion. Certain * Pawlow, ' The Work of the Digestive Glands.' Translated by Thompson, 1902. Digitized by Microsoft® 170 A MA.NUAL OF VETEEINAEY PHYSIOLOGY foods in the case of the dog, such as meat extracts, are most effective stimulants, while bread and white of egg are found to have no effect if introduced directly into the stomach, though they operate reflexly through mastication and taste. Finally, Pawlow, to whom all this work is due, believes that the quantity and quality of the gastric juice will be found to depend on the character of the food, so that while in some cases an economical production is arrived at, 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. Such, briefly, is the case as it stands at present. If the above proves to be correct, we have in our hands a most likely explanation of some of the digestive troubles of the horse. There are other changes occurring in the stomach inde- pendently of peptonizing 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 is produced from the starch of the grain, and is not, according to our observations, the result of the action of saliva. Abundant saliva exists in the stomach, but it will be remembered 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 of this formation of sugar ? It has been shown that oats may yield a starch- converting ferment, and the view that the grain provides its own enzyme for the conversion of starch into sugar may be provisionally accepted as the explanation of the pre- sence of sugar in the stomach of the horse. The whole of the Btarch is not thus converted, for distinct evidence of unaltered starch can be obtained in the first portion of the small 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 amylotic action. If oats provide their own starch- converting enzyme, we see the strongest argument against Digitized by Microsoft® DIGESTION 171 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 results from cellulose decomposition. Brown* has shown that the destruction of the cell-wall of oats and barley occurs in the stomach, where it is dissolved by a cyto-hydrolytic ferment ^re- 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. — Stomach digestion in the horse has been divided by Ellenberger and Hofmeister 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 proteid con- version to a limited extent occur. In the third period mixed digestion of starch and proteid occurs, while in the fourth and last period only proteid digestion takes place. The third and fourth periods may together last four hours and upwards. We must be careful to avoid regarding these periods as based on some rigid law ; they are very variable in duration, due to causes we have previously con- sidered, and run imperceptibly into each other. With this caution we give the following periods at which gastric digestion is said by Ellenberger and Hofmeister 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. J. Brown, F.B.S., Jowrnal of the Chemical Society, 1892, p. 352. Digitized by Microsoft® 172 A MANUAL OF VETERINARY PHYSIOLOGY Stomach Digestion in Ruminants. — The Rumen or first gastric reservoir is a viscus of enormous proportions, capable in the ox of holding 60 gallons. It is divided into four sacs by means of very thick muscular pillars, and the whole is lined by a well developed mucous membrane, in part covered by leaf-like papillae. The mucous mem- brane, it is said, contains some small glands which are not considered to provide any digestive secretion. The rumen is in connection with the reticulum, and by means of the oesophageal groove with the omasum. All solid food on first coming from the mouth is received by the rumen and, judging by the contents of this compartment, much of the fluid which is swallowed must also find its way there ; it has been proved by the experiments of Flourens that fluid may find its way from the oesophagus into all four stomachs at one and the same time. The amount of fluid in the rumen is important from a digestive point of view, since rumination is impossible unless a large proportion of water exists in this cavity. The fluid found in the rumen consists of the water which has been consumed, of the amount of saliva swallowed, and of the amount existing in the food ; but much of it is saliva, of which the ox secretes enormous quantities. The contents of the rumen are alkaline, which is pro- bably owing to the saliva ; in appearance they resemble food which has been coarsely ground. This mass is slowly and deliberately, not energetically, revolved within , the stomach, the material at the posterior part being gradually forced upwards and forwards and so a complete mixing occurs. The churning movement is brought about by the extremely powerful muscular pillars of the organ, which are so arranged as to separate it into various sacs ; these pillars, when they contract, shorten the rumen in its two diameters, and press the contents towards the opening of the oesophagus. Fermentation may also assist to mix the contents, owing to the evolution of gas during the process. It is due to the churning movement that the ' hair balls,' found in the rumen of cattle, are formed. Digitized by Microsoft® DIGESTION 173 The essential function of the rumen is to retain the food for remastication, to macerate all fibrous substances and to fit them for cellulose digestion, which here takes place '■ possibly under the influence of ferments contained in the food itself. The amount of cellulose digested in the rumen has been estimated at between 60 and 70 per cent. Ellenberger is of opinion that in addition to the functions named, other digestive changes occur ; he says that carbo- hydrates are digested by means of enzymes contained in the food, and in this way starch and cane sugar are converted Fig. 45. — The Gastric Compartments and True Stomach op Rumi- nants (Colin). C, The oesophagus ; A, A, B, B, the rumen ; D, the reticulum ; E, the omasum ; F, the*abomasum. into maltose. Proteids are also slowly converted into pep- tones, not through any true peptic ferment but by some enzyme provided by the food. The result of the decomposi- tion of cellulose is the production of a considerable quantity of gas. The rumen never empties itself; even after pro- longed starvation it contains food. In young ruminants digpstion occurs principally in the fourth stomach, the other compartments being rudimentary ; when the young animal is placed on solid food it is remarkable how soon these compartments develop, and the process of remastica- tion is established. Digitized by Microsoft® 174 A MANUAL OF VETERINARY PHYSIOLOGY a. The Reticulum or second gastric reservoir is a small one. Its interior is arranged like a honeycomb, in the cells of which foreign bodies such as stones, sand, nails, etc., may be found. 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 contrac- tion of the walls of the viscus ; in order that fluid may be retained in this compartment 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 com- partment when it contracts. Ellenberger is of opinion that the reticulum regulates the passage of food from the first to the third compart- ment, and from the rumen to the oesophagus. In trans- ferring the contents of the rumen to the omasum, the reticulum contracts and forces the material into the open oesophageal groove. That the reticulum is capable of energetic contraction is specially noted by Colin, whose observations on the physiology of the stomach in rumi- nants were mainly carried out by means of an opening in the abdominal wall. 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 peculiar ; its physiology has been elaborately worked out by Ellenberger. | This authority says that it possesses no secreting power ; that its function is to compress and triturate the food which it crushes between its powerful muscular leaves, rasping the ingesta down by means of its papillae. The contents of this sac are always dry, due to the fluid portion being squeezed off and flowing into the fourth stomach by the action of gravity, through a passage formed in the lesser curvature of the organ. The food may find its way into the omasum, either directly from the oesophagus after Digitized by Microsoft® DIGESTION 175 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 oesophageal groove, by which means communication with the rumen and reti- culum is cut off. Normally the reaction of the contents of the omasum is neutral ; if found acid it is due to regurgita- tion from the true stomach. It is peculiar in possessing a separate source of nerve supply, stimulation of the pneumo- gastric producing contraction of all the other compartments but this. The Abomasum is the true digestive stomach, and is the only compartment secreting gastric juice. In the abomasum proteids are converted into peptones, the region of the cardia being in this respect more active than the pylorus. Ellenberger states that starch is also digested, and that this precedes proteid 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 rumi- nant, 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 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 one of starch conversion; the second stage is the same only more pronounced ; the third is one of starch and proteid conversion, both processes occurring at the cardia, but only proteid conversion taking place at the fundus ; lactic acid is present in the former and both lactic and hydrochloric acid in the latter. In the fourth stage starch Digitized by Microsoft® 176 A MANUAL OP VETERINARY PHYSIOLOGY conversion is nearly complete, hydrochloric acid pre- dominates in all the regions, and proteid 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 carried out to elucidate the physiology of the human stomach has been effected on the dog, and has, more or less, been already embodied in the previous pages in dealing with gastric juice. A flesh diet requires very little saliva and practically no mastication, 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 takes 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 their digestion is facilitated by boiling ; liver and flesh are best given raw as cooking interferes with their digestibility. The gastric juice of the dog contains pepsin and hydrochloric acid '46 to - 56 per cent., and it has been shown that it is possessed of considerable activity, and certain peculiarities which have been dealt with on p. 165. Absorption from the Stomach. — The needful changes having occurred in the stomach — and we now refer prin- cipally 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 carbo-hydrates have been converted into sugar, whilst 40 to 70 per cent, of the proteids are converted into pep- tones ; when food has been long in the stomach, not more than 10 per cent, of the proteids escape being peptonized. In ruminants probably the greater part of the food sub- stance is acted upon 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 Digitized by Microsoft® DIGESTION 177 useless to recapitulate all his experiments; they were generally performed with strychnine, 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, no 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 observations. Strychnine experiments are not altogether free from objec- tion, but as matters stand we can only surmise that no absorption of sugar or peptones occurs in the stomach. It is certainly very remarkable what becomes of the pep- tones ; we have never found any in the stomach contents, 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 of the horse to the small area of the mucous membrane, which, he says, cannot be secreting gastric juice and absorbing at the same time. In the empty stomach he attributes the non- absorption of poisons 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 stomach of the dog and pig. Eecent experiments on the dog show that absorption does not take place readily from the stomach. Water taken alone is practically not absorbed at all ; sugars and peptones are absorbed only when in sufficient concentration, while fats are not absorbed. Self-digestion of the Stomach. — A question which for a long time gave rise to an 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 12 Digitized by Microsoft® 178 A MANUAL OF VETERINARY PHYSIOLOGY portions of living material, legs of frogs, ears . of rabbits, etc., if introduced into it are readily digested, also that post-mortem digestion of the stomach in some animals is far from rare. It is believed that the gastric epithelium forms an antibody, known as antipepsin, whiclv neutralizes the digestive action on the living wall. This view is the outcome of recent studies in immunity (see p. 26). 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 productive 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 fermentation of the food, and the action of acids on the saliva, whilst the marsh-gas is obtained by the decomposition of cellulose. 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. Vomiting. — Vomiting amongst solipeds and ruminants is rare, but the act is common in the dog and pig. The reasons 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 duodenum ; (4) the cuticular coat thrown into folds over the opening of the cardia ; (5) muscular loops encircling the cardia, the con- traction of which keeps the opening tightly closed ; (6) the stomach not being in contact with the abdominal wall. Digitized by Microsoft® DIGESTION 179 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 lies in contact with the abdominal wall, the cardia is freely open, the oesophagus is of great size, and, still stranger, the animal possesses the ability, under the control of the will, to bring up food from the stomach as a normal condition, and yet cannot vomit ! It is evident, therefore, that all these theories are not suffi- ciently satisfactory 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 rudi- mentary or very insensitive to ordinary impressions. Vomition in the horse is no doubt seriously interfered with by the thickened oesophagus, contracted cardia, and the arrangement of the muscular fibres. The folds of mucous membrane filling up the orifice could offer no serious obstruction to a distended stomach, for we know that even when this membrane 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. Vomition in the horse is generally indicative of ruptured stomach, and much has been written as to whether vomit- ing occurs before or after rupture. From no inconsiderable experience of these cases, we have arrived at the conclusion that it may occur at either time, and that a horse may vomit though a rent seven or eight inches long exists in the stomach wall. Dilatation of the cardia and oesophagus is essential to the act of vomition 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 (show- ing that it had not a ruptured stomach), and it is not unusual to have attempts at or actual vomition when the small or large intestines are twisted. Vomiting in the 12—2 Digitized by Microsoft® 180 A MANUAL OP VETERINARY PHYSIOLOGY horse is not as a rule attended by any distressing symp- toms ; 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 cerebral centre, have no effect on the horse or rumi- nantB, nor does the horse vomit as the result of sea-sick- ness, though he suffers extremely from it. Why he should vomit more often with a ruptured stomach than a sound one 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 explanation of ruptured stomach in the horse. Rumination. The physiology of rumination has been principally worked out in France by Flourens and Colin, and our knowledge of this singular process is based almost entirely on their observa- tions. (Esophageal Groove. — The oesophagus in ruminants * The only case of vomiting we have 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 nose extended, the ingesta gushed in a stream from both nostrils, and a sound accompanied the effort. Digitized by Microsoft® DIGESTION 181 enters and passes through the rumen, forming a singular groove or channel known as the oesophageal, which on the left communicates with the first and second compartments, and by an opening on the right and inferiorly, with the third compartment (Pigs. 46 and 47). In this way food coming down the oesophagus may enter either of the first three reservoirs, the choice being determined, as we shall presently point out, by the condition in which it is swallowed. The oesophageal groove possesses two lips or pillars, the LR. Pig. 46. — Diagram op the (Esophageal Groove (Carpenter). a;, (Esophagus entering the stomach ; c, its cardiac opening ; rp, right pillar of oesophageal groove ; LP, left pillar of the same ; o, opening into the omasum ; osg, oesophageal groove extending from c to o, about 7 inches in length. To the right of the figure is the rumen, to the left the reticulum. anterior being formed by the reticulum, the posterior by the rumen. The lips are thin above, and thick below where they overlap ; normally they lie in apposition in such a way as to conceal the groove, but in both Figs. 46 and 47 they are intentionally separated in order to show the arrangement. These pillars are composed of involuntary muscular fibres arranged longitudinally and transversely, by which means the groove can be shortened and con- stricted. By a contraction of the pillars the omasum may Digitized by Microsoft® 182 A MANUAL OF VETEEINARY PHYSIOLOGY be shut off from the first and second compartments, and brought nearly in apposition with the eesophagus ; or by their relaxation the first and second may be made to com- municate with the third compartment. When the pillars are relaxed the oesophagus communicates more directly with the rumen and reticulum. Another function of the groove was said to be to cut off a pellet of food pressed into it by a contraction of the rumen and reticulum, the pellet or bolus Fig. 47. — Longitudinal Section of the Bumen and Betictjlum TO SHOW THE POSITION OF THE (ESOPHAGEAL GROOVE IN THE Living Animal. Bw, rumen ; the lettering is placed on the muscular pillars, which are held apart. Bi, reticulum. GE, oesophagus. Bp, right pillar ; Lp, left pillar : both held widely apart to show G, the groove. Om, opening leading to the omasum. being then passed into the oesophagus for remastication. Colin has shown that if the lips of the canal be stitched together rumination may still occur, so the theory that the bolus is formed between these lips is not correct, and this view is supported by the stomach of the llama, which only possesses one pillar. Colin's description of the mechanism of rumination is as Digitized by Microsoft® DIGESTION 183 follows. During the churning movement the food is gently pressed against the lips of the groove, when, by a spasmodic contraction of the diaphragm and abdominal muscles, some of the liquid from the reticulum and some of the solid from ■ the rumen is carried up the oesophagus, while the latter, by the contraction of its funnel-shaped extremity, cuts off the bolus, and by its reversed peristaltic action conveys it to the mouth. In passing under the velum palati the liquid portion is squeezed out and is at once reswallowed, travelling to the third compartment, while the solid mass undergoes grinding. After the bolus is reswallowed it may either return to the rumen, or, if in a finely com- minuted condition, it passes at once from the oesophagus into the third compartment. The reticulum appears to be only a convenient accessory to rumination, for, as previously mentioned, Flourens excised it without interfering with the process of rumination. During the process of rumination the parotid glands secrete, but not the submaxillary or sublingual. Eumination is a reflex nervous act, the centre for which probably lies in the medulla. The process can only be performed by means of the united action of the dia- phragm, walls of the stomach and abdominal muscles. Hence, if the phrenics be divided rumination is carried out with great difficulty, and only by an extra effort of the abdominal muscles; if the vagi be divided the walls of the stomach are paralyzed and the process cannot go on ; if the spinal cord be divided in the mid-dorsal region the abdominal walls are paralysed and rumination can no longer occur. The condition of the stomach and its contents also exercises an important influence ; rumination can only take place when the organ contains a fair amount of food and a considerable quantity of liquid. The ascent of the food in the oesophagus can be distinctly seen in the neck, and sounds may be heard on auscultation due to the passage of the bolus with its fluid admixture, and the friction of the rumen againefc the diaphragm. The amount of each bolus has been estimated by Colin at 3| to 4 ozs. ; Digitized by Microsoft® 184 A MANUAL OP VETERINARY PHYSIOLOGY its formation in the stomach and ascent occupies about three seconds, and its descent after remastication one and a half seconds ; its remastication occupies about fifty seconds. On these data Colin has calculated that at least seven hours out of the twenty-four are required for the process of rumination. £=> Movements of the Stomach begin very shortly after food -^ is received. Waves of peristalsis travel from the middle of the organ towards the pylorus ; these waves become stronger as digestion proceeds, and their function is to press the peptonized food against the pylorus. The >- pylorus is kept tightly closed, and only relaxes to allow a stream of chyme to be ejected, which occurs with con- siderable force. The left or oesophageal end of the stomach - in all animals plays but a passive part, and may be re- garded in animals with a single stomach more in the light of an oesophageal dilatation, a characteristic particularly indicated in the horse. There is very little movement in the left end of the stomach, and this permits starch con- N version to go on undisturbed, especially in the last portions of food swallowed. It is probable that in all animals with a single stomach churning movements are unnecessary, and it is certain they do not occur in the horse, for in feeding on three or four different foods they are all found arranged in strata in the stomach, in the order of their arrival. In ruminants, on the other hand, other movements are clearly indicated ; the immense muscular pillars of the rumen are capable of rotating the contents, and the formation of balls in the rumen, from hair swallowed when licking the body, is most suggestive of rotatory movement. Eber of Dresden says .. that in the ox the rumen normally contracts a little more than three times in two minutes. The relaxation of the pylorus is a distinct mechanism ; it only occurs when material is ready to pass out, and not with every contraction wave which passes over the organ. Yet this statement must be modified in the case of the horse, where, as we have shown, owing to the small size of Digitized by Microsoft® DIGESTION 185 the stomach, and the bulky nature of the food, an amount passes out at the pylorus equal to that received at the cardia. Liquid foods readily pass the pylorus, and prob- ably most liquids pass rapidly out of the stomach. It is especially so in the horse, in which animal the water as consumed sweeps directly through the stomach, and may, on auscultation, be heard passing along the duodenum to the large intestines. The movements of the stomach are excited by the • presence of food, or 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 in fact an automatic organ. Both pneumogastrics supply the stomach, 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. 55, p. 206). In the wall of the stomach are found ganglia with which both the vagus and sympathetic 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 con- traction 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 and 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 intestines, so that in the horse even large poisonous doses of strychnia 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 never absorbed. The secretory Digitized by Microsoft® 186 A MANUAL OP VETERINARY PHYSIOLOGY nerves of the gastric glands have been dealt with on p. 169. The nervous mechanism of the stomach of ruminants is derived mainly from the vagus, excepting for the third compartment, which has a separate and, at present, un- known 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 slower and later-appearing peristaltic contractions of the abomasum, but no contraction of the omasum. Section of both vagi was found to paralyse the cesophagus, rumen, and reticulum, followed by tympany of the rumen. Ellen- berger could not obtain any effect on the stomach move- ments by stimulating the sympathetics. 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 ; in the duodenum the glands of Brunner are found, whilst the follicles of Lieberkiihn are met with throughout the whole of the small and large intestines. Lieberkiihn's crypts supply a considerable proportion of 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 Lieberkuhn are tubular glands, amongst the cylindrical epithelial cells of which numerous mucus-forming goblet cells may be found. 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 had a distinctly digestive effect. It is now known that though a pure secretion of Lieber- kiihn's crypts has little or no digestive action excepting i Digitized by Microsoft® DIGESTION 187 on starch, an extract of, and juice squeezed from the intestinal wall has a most important function. The Lieber- kiihn fluid is quantitatively small in amount, and alkaline in reaction due to carbonate of soda. The intestinal extract, on the other hand, contains three enzymes, and in addition a peculiar chemical substance of remarkable properties. The enzymes are : 1. Enterokinase, which converts the trypsinogen, the mother substance of the pancreatic proteolytic enzyme, into trypsin. 2. Erepsin, also a proteolytic ferment, which supplements the work of trypsin, acting on deutero - albumoses and peptones, breaking them up into amido-acids and hexone bases. 3. Inverting ferments, converting double sugars which cannot be utilized by the tissues into single sugars which can. Of inverting ferments there are three : Maltase, converting maltose and dextrin into dextrose. Inrertase, converting cane - sugar into dextrose and levulose. Lactase, converting milk-sugar into dextrose and galac- tose. Finally, the intestinal fluid contains secretin, which is not a ferment but a chemical substance 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 and secretin will be dealt with in our consideration of the pancreas. Intestinal Digestion in the Horse. — The contents of the stomach are neutralised by the pancreatic and biliary secretions immediately or shortly after they leave the stomach. So much is this the case that on the duodenal side of the pylorus the reaction of 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 Digitized by Microsoft® 188 A MANUAL OF VETBEINAEY PHYSIOLOGY first two-thirds of their length, then neutral as far as the ileum, where they become alkaline ; we have only once found them otherwise than alkaline throughout. He further states that in the fasting horse the contents are alkaline, but that in the digesting animal, whether horse, ox, or sheep, they are acid, the acidity decreasing after passing the common duct, and becoming decidedly alkaline at the posterior portion of the small intestine. This, as we have said, does not agree with our experience in the horse ; it is usual to find the contents of the duodenum next the pylorus neutral, and from this point the bowel is faintly alkaline, 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 through the medium of a very delicate membrane is a construction the ad- vantages 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 latter trouble is tympany. If 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 resembling in appearance, through its precipitated albumin, nasal mucus suspended in fluid. The proportion of mucin must be considerable judging from its ropiness when poured from one vessel to another, and this mucus is probably largely derived from the stomach. Digitized by Microsoft® DIGESTION 189 Throughout the small intestines the character of the chyme is as follows, viz., a yellow, frothy, precipitated, slimy fluid, the material from the anterior part of the intestinal canal having a peculiar mawkish smell, whilst that from the region of the ileum is of a distinctly fascal odour ; the latter is due to indol and skatol formed putrefactively during pancreatic digestion. In the ileum the proportion of fluid material is considerably reduced in amount, and the character of the ingesta may now be recognised, which was previously almost impossible. Function of the Ileum. — As the flow of material into the small intestines is controlled by a sphincter, so is the flow out of it. 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 circulating between the pylorus and ileum, viz., that 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 for this the material could not be acted upon and 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 intestines 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, as it passes under the last rib on the right side, the water which a horse at that moment is drinking may be heard rushing through the intestines on its way to the csecum. One . is always struck < by the fact that the small intestines are never seen full, in Digitized by Microsoft® 190 A MANUAL OF VETERINARY PHYSIOLOGY 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 contents 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 difference in the physical characters of the contents in their several 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 on food is 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 the fact of their enormous development. In many respects they present a consider- able contrast to the small intestines ; for instance, they are always found filled with ingesta, the contents are more solid, the material lies a considerable time in them, and there are no juices other than the succus entericus poured into the bowel. These are conditions exactly the reverse of those found in the small intestines. The bowels which are spoken of as the large intestines are the esecum, double and single colon, and the rectum. 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. 48). To our mind its most remarkable feature is that it is a bag the openings into and out of which are both Digitized by Microsoft® DIGESTION 191 found at 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 brought about by the four muscular bands on the caecum (Fig. 49), which shorten the bowel, forcing the contents upwards towards the 'crook.' The ileum being closed, the only available outlet is into the colon (Fig. 48). Several questions suggest themselves regarding the com- FlG. 48. — C.ECUM OF THE HOESB IN POSITION, ITS INNER FACE BEING SEEN. 1, The first colon ; 2, the ileum. / munication between the large and small intestines. It is certain that in order to get from the ileum into the colon everything must pass into or, at any rate, through the cascum, yet we are assured that material does not remain there long. Could it be possible for the opening of the ileum and that of the colon to be so brought to- gether that material might pass direct from one into the | other? (Fig. 50.) Nothing is returned into the ileum from the caacum ; there must be, in consequence, a sphincter keeping the ileum closed, for when the caecum contracts Digitized by Microsoft® 192 A MANUAL OF VETEEINAEY PHYSIOLOGY material must cross the opening of the ileum in order to reach the colon. This sphincter is furnished by the thickened condition of the wall of the ileum. We see no difficulty in believing that the rigid end of this tube may pass its contents practically 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 csscum are always fluid, some- times quite watery, occasionally of the colour and consist- ence of pea-soup, in which condition they are full of gas Fig. 49. — Schematic Arrangement op the Longitudinal Muscular Bands of the Cmcvu. Bands 1 and 2 are one, and form a complete sling for the bowel ; band 4 runs from the cascum 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 caecum to colon. 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.* The caBcum 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 * We once found the caecum acid. Digitized by Microsoft® DIGESTION 193 that digestion in the horse can only be properly carried out when the contents are kept in a fairly fluid condition. We do not say that the csecum produces no digestive changes in the food, for we have stated that the contents are occasionally of the consistence of pea-soup, but we consider its digestive function subordinate to its water-holding one. Ellenberger views the caecum as a bowel for the digestion of cellulose, where by churning, maceration, and decom- position, this substance is dissolved and rendered fit for Fig. 50. — The Opening of the Ileum and Colon in the Cecum. 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. absorption, and he likens it to the stomach of ruminants and the crop of birds. He further considers that the caecum exists owing 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 ' reached the caecum between 12 and 24 hours after entering the stomach, that it remained 24 hours in the caecum, and that during this time 10 to 30 per cent, of the cellulose disappeared. 13 Digitized by Microsoft® 194 A MANUAL OF VETERINARY PHYSIOLOGY 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 csecum 24 hours, and we believe that cellulose digestion occurs principally, though not entirely, in the colon, and further, that it is not absolutely necessary the material should remain in the csecum, but that it may pass on at once to the colon. Our experiments on digestion have shown that ingesta may reach the caecum 3 to 4 hours after entering the mouth, and we are quite clear on the point that oats may travel some considerable distance along the colon in 4 hours from the time of being consumed, though this is regarded as exceptionally rapid. A horse which had never had maize and had not tasted oats for two or three years, was fed first with 2§ lbs. of maize, and 17 hours later with 4 lbs. of oats. The animal was destroyed 4 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 21 hours the small ration of 2£ lbs. of maize was distributed between the stomach and pelvic flexure of the colon, which is a very large area. In 4 hours the oats reached the same point in the bowel that the maize had arrived at ; this is excep- tionally rapid, but this experiment supports two points it is desired to emphasize, viz., the difficulty in getting the stomach to empty itself completely, and the rapid transit of material through the small intestines. Colin believes that in the csecum starch can be converted into sugar, fats emulsified, and the active absorption of assimilable matters occur. 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 immedi- ately 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 Digitized by Microsoft® DIGESTION 195 bowel becoming very narrow in calibre : the pelvic flexure having been formed, the intestine retraces its steps towards its starting point. Eunning on top of the previously described portion it descends towards the diaphragm, gradu- Fro. 51. — The Double Colon looked at from Above (modified from Muller). 1, The first colon, the csecum being removed ; 2, the pelvic flexure, the bowel being narrow ; 3, the colon suddenly enlarges ; 4, its diaphragmatic flexure ; 5, the single colon. Several of the bands are seen ; note also the sacculated and non-sacculated portions of the bowels. ally growing larger in calibre, and then ascends towards the loin, being here of immense volume — in fact, at its largest diameter ; it then suddenly contracts, and forms the single colon (Figs. 51 and 52). The object of the difference in the volume of the double colon appears to be for the 13—2 Digitized by Microsoft® 196 A MANUAL OF VETEEINAEY PHYSIOLOGY convenience of its accommodation in the abdominal cavity. The double Golon may for the purpose of description be divided into four portions: the ingesta in the first and third descend, in the second and fourth ascend. It is found that the physical characters of the contents are not Fig. 52. — Position of the Csicum and Double Colon on the Floor op the Abdomen seen from Below. The point of the caecum is directed towards the sternum. the same throughout. In the first colon the food is fairly firm, and the particles of corn, etc., can be readily recog- nised ; in the second colon the material is becoming more fluid, whilst 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 Digitized by Microsoft® DIGESTION 197 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 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, rapidly becoming brown or olive-green on exposure to the air ; the colour being due to the chlorophyll i of the food. The contents of the colon are normally alka- I line throughout ; we once, however, found them acid. ^ Digestive Changes. — The changes food undergoes in tbe large intestine have never excited the same interest as those 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 explaining why the large bowels have been regarded in the light of reservoirs for ingesta, rather than as active centres of digestion. As a matter of fact, the large intestines of the horse are actively employed in dealing with cellulose, 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 bacteria may hydrolize cellulose and render it fit for absorption. In the case of oats we mentioned, p. 171, that they probably furnished their own cellulose enzyme, but this has not been proved for all vegetable material. The cellulose of hay is, probably, only extracted after prolonged maceration in the large intestines and the subsequent attack of bacteria. By some, it has been considered that the epithelial cells of the intestine are capable of dealing with cellulose, but on this point no definite statement can be made. Cellulose yields energy to the body on oxidation, but there is another Digitized by Microsoft® 198 A MANUAL OP VETEEINAEY PHYSIOLOGY reason for the extensive preparations made for its digestion in herbivora, viz., the cellulose encloses the proteid, starch, and fat of vegetable substances in a frame-work, and until this is broken down these substances cannot be acted upon. We know that considerable cellulose solution must occur before the material arrives at the large intestines, otherwise neither in the stomach nor small intestine could digestion occupy the prominent position it does. The digestion of proteid, fat and sugar are largely, though not entirely, dealt with in the stomach and small intestine, but there must be a certain amount of these substances so firmly locked up in their cellulose envelope tbat they are not liberated until after prolonged maceration and digestion in the large intestines. We may, therefore, safely assume that proteid, fat, starch, and cellulose are capable of being acted upon and absorbed from the large bowels of the horse. As the result of cellulose digestion carbonic acid and marsh gas are formed in equal volumes. We have in our description of the large bowels drawn attention to the appearance of the caecum and fourth portion of the double colon, with their pea-soup-like contents, on the surface of which gas bubbles are constantly breaking. It may well be that these two places are the active seats of the final transformation of cellulose, the cseeum 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 is rather supported by the remarkably rapid change in the character of the contents in the single colon, the pea-soup-like condition giving way, in the space of a few inches, to the appearance presented 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 proteid products of the small intestines. The small intestine may be regarded as free from putrefactive processes, in fact it Digitized by Microsoft® DIGESTION 199 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 proteid digestion and reducing them to simpler end-products, such as peptones, proteoses, amido-acids, indol, skatol, phenol, phenyl-proprionic, phenyl-acetic and fatty acids, with the evolution of C0 2 , H 2 , H 2 S, and CH 4 . These end-products are got rid of either through the faeces, or they are absorbed into the blood, taken to the kidneys, and combined with sulphuric acid are got rid of through the urine ; especially is this the case with phenol, indol, and skatol. 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 dis- tinctly acid reaction is obtained on the surface of the faeces, though at this time the interior of the ball may be, and often is, alkaline ; the converse of this may also be obtained. In the rectum the single balls collect in masses, to be forced out of the body at the next evacuation. The reaction of this mass is acid, and the colour depends on the food, being, on an ordinary diet, of rather a reddish-yellow or brownish tint due to altered chlorophyll. Absorption from the single colon and rectum is rapid ; the marked change in the physical character of the faeces is evidence of this. Animals may also be killed by the rectal injection of strychnine ; narcosis can be produced by the rectal administration of ether, and life may be sup- ported, at any rate for a short time, by means of nutrient enemata. -^ Intestinal Digestion in Ruminants. — Though intestinal digestion is so important in the horse, it would appear in ruminants to occupy a subordinate position. It is curious why in one animal the changes should occur at the anterior, and in the other at the posterior part of the Digitized by Microsoft® 200 A MANUAL OF VETEEINAEY PHYSIOLOGY digestive tract, but this difference in the arrangement for digesting cellulose depends upon 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 Fig. 53. — Schematic Arrangement of the Intestines of the Ox. 1, The small bowels ; 2, the caecum ; 3, the ' spiral ' colon ; 4, the single colon. narrow in diameter and about 120 feet in length. The large intestines are about 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. 53). It is in this immense length of absorbent surface that the food substances capable of being utilized 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 proteids which have escaped the stomach, and the fats and starches Digitized by Microsoft® DIGESTION 201 are rapidly changed and rendered fit for assimilation ; the altered cellulose in all probability only finds its way here when fit for absorption after its digestion in the rumen. J\ 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 are alkaline in reaction. Munk gives the following statistics respecting the in- testinal canal. In the tiger and lion„the whole digestive tract is 3 times the length of the body, in the dog 5 times, man 9 times, horse 12 times, pig 16 times, and ox 20 times. The comparative shortness of the intestinal canal of the horse is compensated by its enormous capacity, which is 352 pints ; in the ox 140 pints, pig 47 pints, dog 14 pints. The area of the intestinal tract is also given by the same observer — horse 550 square feet, ox 160 square feet, pig 32 square feet, and dog 5£ square feet (M'Kendrick). Movements of the Intestines. — The movements of the intestines are brought about by the involuntary muscle composing its wall. This muscle in the small intestines is arranged in two sheets in a circular and longitudinal manner, while in the large intestines narrow bands of pale muscle of considerable length take the place of the ordinary longitudinal layer, and may be found on all parts where the tube 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. The sacculated condition of the double colon is confined principally to the first and second and fourth portions. The third portion especially at the pelvic flexure is free from Digitized by Microsoft® 202 A MANUAL OF VETERINAEY PHYSIOLOGY sacculations, 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 Fig. 54, also Figs. 51 and 52). In the large intestines the longitudinal layer of fibres is confined to the muscular bands, so that the great bulk of the wall consistB of circular muscle only. The longitudinal bands shorten the bowel, but the main work in pressing the contents along is performed by the circular layer. The bands, in fact, are numerous where the intestine is large, FLEXURES STERNM. PEIAMC DWPHRG^C FROM g, 2 , V" I \ TO 5. SINCLE COLON colon • colon 1 colon '■ colon Fig. 54. — 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 caecum. 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 on to the single colon. and reduced in number where the bowel becomes smaller. This arrangement suggests that they may under suitable conditions produce an irregularity of pull, and we can Bee 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 through the medium 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 for as long as four days. The Digitized by Microsoft® DIGESTION 203 peristaltic movement of the small intestines is quite distinct from that of the large ; the one 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. Belatively quick in the small intestines it becomes slower and more deliberate in the large, but the wave has always the one object in view, viz., to press the ingesta onward. A wave of contraction passing the reverse way, viz., in the direction of rectum to stomach, is known as antiperistaltic : such a movement is considered abnormal, but in the horse, according to the observations 6i Colin, antiperistalsis of the small intestines is a natural con- dition. Some physiologists recognize antiperistaltic move- ments 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 anti- peristalsis is only present under abnormal circumstances. If antiperistalsis be admitted for the large bowels, we see no difficulty in extending it 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 Bmall intestine has been experimentally reversed, so that the portion originally nearest the stomach is made to occupy a position farthest 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. Another movement quite different to the above is the pendular, which shows itself by a gentle swaying to and fro of the different loops of bowel, caused by a simultaneous Digitized by Microsoft® 204 A MANUAL OF VETEEINAEY PHYSIOLOGY contraction of both muscular coats. This movement is not stopped by cocaine or nicotine, from which it is reasoned that the nervous ganglia have nothing to do with it. These pendular movements, which are rhythmical and as regular as the heart-beat, are regarded by Starling and Bayliss, who first described them, as being of the greatest importance, as they cause the material under digestion to be mixed thoroughly with the secretion, and bring it in contact with the wall for absorption. While these rhythmic contractions are in operation the food is not pressed on- wards, but remains in the same region of the bowel, under- going, however, repeated divisions. We have not succeeded in observing the pendular movements in the horse. S In the first and third portions of the colon 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 results follow that material is passing along each section apparently in two 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 open- ing, 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 pre- sents itself. Or take what is far commoner and equally fatal, viz., displacement or actual twist of the large bowel, or a complete twist of the small intestine, leaving the bowels in such indescribable confusion that the parts cannot be unravelled, even when removed from the body ! It is impossible to believe that muscular action of the intestines is free from all blame in the production of these lesions. It is easier to understand a twist of the small intestine apart from muscular action than it is to under- stand displacement or actual twist of the large intestine. A loop or coil of small intestine may be so distended by gas or ingesta as to become twisted, but it is more difficult to Digitized by Microsoft® DIGESTION 205 imagine either of these conditions producing twist or dis- placement of the large intestines, and it becomes.a question, as we have previously said, how far the action of the muscular bands of the bowel may have a contributing influence. That great force is necessary is undoubted, bearing in mind the difficulty, if not impossibility, of restoring the parts to their position post-mortem, or en- deavouring after death to reproduce the lesions experi- . .mentally. These matters will be referred to again. i Z^ Nervous Mechanism of the Intestinal Canal. — Two distinct impulses are conveyed by the intestinal nerves, viz., those for contraction and for 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 in the coats of the large intestines, producing on stimulation much the same results as the vagus, viz., active contraction of both circular and longi- tudinal 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 communi- cantes {re, Fig. 55) to the main sympathetic chain, Sy., and from 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 lumbar cord through re. and Sy. (Fig. 55) to the inferior mesenteric ganglion. From this ganglion inhibitory fibres are given off to both longitudinal and circular coats. 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 be Digitized by Microsoft® 206 A MANUAL OF VETEEINARY PHYSIOLOGY Ret Fig. 55. — Diagram to illustrate the Nerves op the Alimentary Canal op the Dog (Foster). (The figure is very diagrammatic and does not represent the anatomical relations.) Oe. to Bet. The alimentary canal from the oesophagus to the rectum. LV. Left vagus nerve ending on the front of the stomach, rl. Re- current laryngeal supplying upper part of oesophagus. B. V. Eight vagus joining left vagus in the oesophageal plexus Oe. pi., supplying the posterior part of the stomach, continued as B'.V'. to join the solar plexus, Sol. pi., here represented by a single ganglion, and connected through x with the inferior mesenteric ganglion (or plexus). G. m. i. a, a, a, branches from the solar plexus to stomach and small intestines, and b from the mesenteric ganglion to the large intestines. Spl. Large splanchnic nerve arising from the thoracic ganglia of the sympathetic Sy. and rami communicantes r.c. of the dorsal nerves. Spl.mi. Small splanchnic nerve. Both the large and small splanchnios join the solar plexus 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. From this ganglion inhibitory nerves are given off to the large intestines. n.e. Nervi erigentes arising from the sacral cord and proceeding to the hypogastric plexus. PI. hyp. From this plexus impulses of a motor kind are supplied to the large intestines. Digitized by Microsoft® DIGESTION 207 / found in the intestinal wall. The intestinal movements are automatic and self-regulated, though they can be pro- voked by both chemical and mechanical stimuli. The normal stimulus to peristalsis is 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 contraction, which is a fortunate circumstance, as they are normal to the bowel in consequence of bacterial activity. Oxygen gas, on the other hand, inhibits movements, and, as a matter of fact, we know that oxygen gas normally does not exist, or only in traces, in the gaseous contents of the bowels. Cutting off the blood-supply to the bowels causes violent contractions, which occur again when the circulation is re-established; the former is of interest in those cases of twist where the blood- supply is wholly or partly interfered with. Under normal conditions the mind is not conscious of peristaltic movements, but when these become very energetic pain is produced. Under the influence of nervous excite- ment 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, the whole of the rectum and single colon are unloaded. Ordinary exercise is always an important cause of peristalsis, and hence an actual means of unloading the rectum. As previously remarked, the normal stimulus to peri- stalsis is the presence of ingesta in the canal. In the feeding of herbivora bulk is essential, they cannot live in a state of health on concentrated food alone. Their intestines need bulk, if only in order to maintain peri- stalsis. Bunge has shown that if cellulose be withheld from the diet of rabbits they die from intestinal obstruction. It is the cellulose and lignin in the diet of herbivora which largely provide the needful stimulus to peristalsis. Gases of the Intestines. — The largest amount of gas found in the intestinal canal is in the caecum and colon ; the Digitized by Microsoft® II* 208 A MANUAL OP VETERINARY PHYSIOLOGY small intestines 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 bulk 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 — particularly of green food — and the enormous size to which these animals 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 on 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 matters producing sulphuretted hydrogen and hydrogen. ^ tChe Faeces. — The faeces 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 faeces depends largely on the diet. The following table from Gamgee* can only give a general idea of their nature : Approximate Composition of the F*ces op the Water Horse. 76-0 Cow. 84-0 Sheep. 58-0 Pig. 80-0 Organic matter 21-0 13-6 36-0 17-0 Mineral „ 3-0 2-4 6-0 3-0 100-0 100-0 100-0 1000 Considerable differences exist amongst animals in the consistency of the faeces ; they are moderately firm in the * ' Our Domestic Animals in Health and Disease,' p. 253, Digitized by Microsoft® DIGESTION 209 horse, pultaceous in the ox, and hard in the sheep. These differences depend upon the amount of fluid they contain. In the pig they are human-like and very offensive ; in the dog they are soft or hard, dark or light, depending on the diet, the mineral matter of bones producing the light- coloured excreta. It is necessary to remember that ,the proportion of fluid in the faeces does not depend upon the amount of water which is drunk, but rather on the character of the food, the activity of intestinal 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 due to putrefactive processes. Nervous excitement frequently induces a free action of the bowels, accompanied by liquid faeces. Faeces always float in water so long as cohesion is main- tained. The colour of the faeces in the horse is yellowish or brownish-red, in the ox greenish-brown ; they rapidly become darker on exposure to the air. When the animal is grass-fed the faeces 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 faeces of animals re- ceiving hay or grass is due to altered chlorophyll. 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. Faeces contain lignin amongst the indigestible portion of the ingesta, 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 proteid, carbo-hydrate and fatty matters ; products of digestive fermentation, such as lactic, malic, butyric, succinic, acetic, and formic acids ; leucin, tyrosin, indol, skatol, and phenol ; biliary matters 14 Digitized by Microsoft® 210 A MANUAL OF VETERINARY PHYSIOLOGY 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 portions of muscle fibre, fat cells, tendinous and fibrous tissue, are found in animals fed on flesh. Of the inorganic matter silica exists in largest amounts in herbivora, 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 faeces in the form of ammonio-magnesium phosphate. This salt is derived principally from the oats and bran of the food, and it frequently forms calculi through collecting in the colon and becoming mixed with organic substances. Other intestinal calculi are formed from lime deposits in the bowel, while collections of the fine hairs from the kernels of oats become encrusted with ammonio-magnesium phosphate and form oat-hair calculi. In the Persian wild goat and 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 calculi, one 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 their origin from food substances. Stomach calculi have not been unknown in the horse, while in cattle, as the result of licking each other, ' hair balls ' are common objects. The following table by Roger gives the mineral composi- tion of the fseces in every 100 parts of the ash :* Horse. Sodium chloride '03 Ox. •23 Sheep. •14 Potassium 11-30 2 91 8-32 Sodium 1-98 ■98 3-28 Lime 4-63 5-71 18-15 Magnesium 3-84 11-47 5-45 * Quoted by Ellenberger. Digitized by Microsoft® DIGESTION Horse. Ox. Sheep, Oxide of iron 1-44 5-22 2-10 Phosphoric acid 10-22 8-47 9-10 Sulphuric acid - 1-83 1-77 2-69 Silica 62-40 62-54 5011 Oxide of magnesium - 2-13 — — 211 Eoger observes that the ash of the faeces of herbivora contains scarcely any alkaline carbonates. The amount of faeces produced in 24 hours varies with the quantity and nature of the food given. We have observed that on a diet consisting of 12 lbs. of hay, 6 lbs. of oats, and 3 lbs. of bran, the average amount of faeces passed by fifteen horses during an experiment lasting seven days amounted to 29 lbs. 13 ozs. in 24 hours, the faeces being weighed in their natural condition, viz., containing 76 per cent, water ; the dry material of this bulk of faeces is about 7J lbs. More faeces are passed during the night than during the day ; in the above experiment, during the 12 hours (6 p.m. to 6 a.m.), the average amount of faeces per horse was 18 lbs. 3 ozs., whilst from 6 a.m. to 6 p.m. the amount was 11 lbs. 10 ozs. The largest amount of faeces we have known a horse produce was an average of 73'3 lbs. (weighed in their natural state) in 24 hours ; the diet consisted of 12 lbs. of oats, 3 lbs. of bran, and 28 lbs. of hay. In an experiment carried on for several months with different horses all receiving 12 lbs. hay and varying proportions of bran and oats, the average daily amount of faeces was 24 lbs. A horge will evacuate the contents of the bowels about ten or twelve times in the 24 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 70 lbs. and 80 lbs. in the 24 hours. In the sheep it varies from 2 lbs. to 6 lbs. daily ; in swine 3 lbs. to 6 lbs., depending on the nature of the diet. The odour of faeces is distinctly unpleasant, due to the presence of indol and skatol ; in disease they are often extremely foetid, and occasionally horrible. The act of defaecation is performed by a contraction of -^ 14—2 Digitized by Microsoft® / 212 A MANUAL OF VETERINARY PHYSIOLOGY the rectum assisted by the abdominal muscles, the glottis being closed. In the horse the contraction of the rectum alone is sufficient to expel its contents ; this is proved by the fact that this animal can defsecate while trotting, show- ing there is no necessity to fix the diaphragm and hold the breath, though at rest this does occur. In consequence 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 under this force causes the sphincters to dilate,, sometimes to an astonishing degree, and as the last trace of material is exuded, the contraction of the rectum is so great that it forces some of the mucous membrane externally, which may be temporarily imprisoned by the contracting sphincters. The muscle of the rectum receives both motor and inhibitory fibres, as previously described. Its extraordinary power in the rectum in the horse may partly be due to the horizontal position of the body ; no crouching of the body occurs during the act of defalcation, such as occurs, more or less, with all other domesticated animals. The rectum has the whole work to perform single-handed, even as we have shown above, 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 presided over by a centre in the cord. If this is destroyed the rectum remains uncontracted, 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. r Meconium is the dark-green material found in the intes- tines of the fcetus. It consists of biliary acids and pig- ments, fatty acids and cholesterin, while salts of mag- nesium and calcium, phosphates and sulphates, sodium chloride, soda, and potash are also found in it. Meconium is the product of liver excretion. Digitized by Microsoft® DIGESTION 213 Pathological. The diseases of early life in the horse are mainly situated in the chest, while those of the adult period are practically 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 mortal. There are certain obvious explanations of the fact, but neither singly nor combined 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, by far the majority of intestinal cases are of this kind, viz., simple muscular spasms of some part of the digestive tract, but of which part we are certainly ignorant. It is obvious that either 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 it is generally over ■ looked, 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 the 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. 153. It is, however, admitted that stomach spasm is far less common than spasm of the intestinal portion of the tract. We would here emphasize the facts set forth on p. 178, of 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 practically be unknown, and stomach trouble generally a matter of comparatively slight importance. In connection with intestinal trouble, we are unable to say what proportion the cases affecting the small intestines bears to those affect- ing the large. We cannot during life distinguish colic of the one from colic of the other. Still, there are good grounds for thinking that the large bowels are more frequently affected than the small, and for the following reasons : 1. Ingesta pass rapidly through the small intestines — so rapidly indeed that, as mentioned at p. 190, 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 that they remain there long, from what we know of the behaviour of fluids generally in the anterior part of the digestive tract. Digitized by Microsoft® C, 214 A MANUAL OF VETEEINAEY PHYSIOLOGY 2. On the other hand, the large intestines always contain ingesta, for the material passes along it very slowly, so that of the three or four days occupied in accomplishing the journey from mouth to anus, all but a few hours is spent in the large intestines. It is reasonable, therefore, to assume that in cases of pure uncomplicated disordered muscular action of the bowels, the large intestine in the majority of cases is at fault. Colic is not fatal, though Percival described such a case. Our experience leads us to believe that death from pure spasm of the bowels is unknown, and we would emphasize the point not only for the sake of accuracy, but as of value in prognosis. We believe that in any case 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 con- siderably narrows the causes of death from intestinal affections, and groups them mainly under two heads : (a) Inflammation of the bowels, and (6) 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. 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. 231). That the presence of an irritant without a poison has no such effect, is abundantly proved by the pounds of sand and gravel horses may carry in their intestines for months, Digitized by Microsoft® DIGESTION 215 perhaps years, without producing any apparent ill effect, certainly with- out producing enteritis. Similarly, gastritis excepting as the result of poison is practically unknown. Our object in the above remarks is to focus attention on the defects in clinical observation, and to attempt a physiological analysis of the most frequent, the most fatal, and by far the most acutely painful 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, suddenness, and mortality with digestive diseases of the horse. We have attempted to show how physiology is capable of enabling us to steer along a moderately exact course, for it is certain that unless we are agreed regarding the nature of the lesions found at post-mortem examination, we cannot reach that goal which is the object of our existence as a profession, and of which physiology is only the humble handmaid. 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 enquiry 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 unravelling, even when out of the body, but it takes a little careful observation to detect displacements of the large intestine.* The size, weight, and peculiar disposition of the double colon should have secured it immunity from any form of displacement ; looked at in the abdomen, it appears impossible for any force short of some mysterious power to be able to influence the position of the bowels, 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, and that a voluminous bowel like the caecum may become completely inverted, and found within the colon, though to get there it has to pass through an opening only an inch or two wide. So remarkable indeed are these lesions that they cannot always be imitated after death, 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 * From the point of view of equine pathology, one of the most valuable contributions made to veterinary literature by the late Pro- fessor Walley was his account of displacements of the colon in the horse (Veterinary Journal, vol. ix.). It was the first time in this country that the possibility of these immense bowels being twisted and displaced was ever described. Digitized by Microsoft® 216 A MANUAL OP VETERINARY PHYSIOLOGY for telescoping intestines is disordered muscular action, and disordered 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 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 reversing its position. "We cannot attempt to indicate the exact dis- ordered action which occurs ; this question would require to be worked out on the living subject. The colon and caecum are most liberally supplied with bands (Figs. 48, 49, 51, 52, and 54), and it does not appear to us to be beyond the bounds of reasonable probability 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 interest, be briefly dealt with. Apart from such obvious explanations as errors in feeding (see in this con- nection pp. 170, 237), 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, the frequency with which the seizure occurs at or shortly after work, especially that of an exhausting nature, and the practical absence of colic among non- working horses. We have even known a horse in a cavalry 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 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 said on the matter than commonly falls to physiology to deal with, but the basis of exact clinical knowledge is sound anatomy and physiology, and we consider the physiological aspect of digestive disorders has not yet received adequate attention. We must bear in mind that the whole length of the digestive tract is a chemical laboratory concerned in the analysis 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 ? Digitized by Microsoft® DIGESTION 217 We see a faithful reflex of the laboratory processes in the disorders of the canal, the diarrhoea which is full of beneficence, the impaction which indicates a loss of muscular power and physical alteration of the contents, the acute tympany which announces active fermentation, the rupture which indicates the strain on the walls of the apparatus ; these and others too numerous to be dealt with, and which no mere mention explains, 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. The ruminant from the peculiarity of its physiological arrangement is far more liable to stomach than intestinal trouble ; tympany, im- paction, paralysis, and inflammation of one or more of the com- partments are common. In spite of the size of the oesophagus impaction is frequent, in marked contrast to the horse, in which it is uncommon, while calculi, a special feature in the intestine of the horse, are found in the stomach of the ox, though brought about by very different causes. Strangulation of the bowels in the ox is not unknown, but limited to a special variety due to anatomical condi- tions. Parasitic trouble in all animals is a prominent pathological feature, the digestive canal from the mouth to the anus being liable to infection with numerous varieties of parasites, and it also forms the main channel of parasitic entry for other parts of the body. Digitized by Microsoft® ? CHAPTEE VI THE LIVER AND PANCREAS Section 1. C\ The Livfir. I> I V" 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 furnished only with arterial blood for the purpose, 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., constitutes the material with which the liver is flooded. Such a mixture of blood derived from a peculiar and considerable area must be 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 intestinal canal, which are bye-products produced during the gradual breaking-down of the food substances. It is from 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, bitter taste, a specific gravity in the ox of 1022 to 1025, in the sheep from. 1025 to 1031, and in the horse 1005. 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 direct from the liver is relatively watery in 218 Digitized by Microsoft® THE LIVER AND PANCEEAS 219 consistence, that taken from the gall-bladder is viscid, due to admixture with nucleo-albumin during its stay in the latter receptacle. The secretion contains no proteid which is somewhat remarkable ; biliary pigments, bile acids, fats, soaps, lecithin, cholesterin, and inorganic salts are found in varying quantities. By standing in the gall- bladder the solids are 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 Ellen- berger, there is very little mucin in the bile of sheep ; what was believed to be mucin in ox bile, which conferred 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, sheep 5*71 per cent., and pig "33 per cent. The gases found in bile are C0 2 , and traces of 'and N. The chief inorganic salts are sodium chloride and phosphate, besides which are found salts of calcium, magnesium, potassium, iron, with phosphoric and sulphuric acids ; the sodium salts always exist in the largest pro- portion. The iron, which is found as phosphate, is probably derived from the h£emoglobin of the blood during the formation of the bile pigments. The following table, showing the percentage composition of various biles, is mainly compiled from Ellenberger : Horse Bile. Ox Bile. Dog Bile. Pig Bile. Water 95 9291 95-3 88-8 Solids 5 9-6 4-7 11-2 Bile acids - -v Bile pigments 1 Fat - j Mucin - J — 8-3 4-1 10-1 Salts — 1-3 •6 1-1 Percentage Composition of the Ash • of Ox Bile. Sodium chloride - 27-7 Manganese peroxide ■ •12 Potassium - 4-8 Phosphoric ! acid 10-45 Sodium - 36-7 Sulphuric 3> 6-39 Calcium carbonate - 1-4 Carbonic J) 11-26 Magnesium •53 Silica - - •36 Iron oxide •23 Digitized by Microsoft® %v 220 A MANUAL OF VETEKINAKY PHYSIOLOGY The differences found in the composition of bile probably depend upon whether it be taken from the gall bladder or r from a fistula, the former being the more concentrated. S-lh « ^ ne Oholesterine found in bile must be regarded in the light of an excretion; the liver is merely a convenient channel for getting rid of this waste product, which is collected from the many tissues of which it forms a part, brought to the liver, and eliminated through the bile by the bowels. It is found in very regular quantities, and forms the principal constituent of certain gall-stones; it is kept in solution in the bile by means of the bile salts. Lecithin is another waste product of the body excreted from the system through the medium of the bile. ' ' -" 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 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. These 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 crystallize 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 treat- ment may be obtained as a green powder. Both colouring matters of the bile behave like acids, forming soluble com- pounds with metals of the potassium group, insoluble ones with those of the calcium group (Bunge). On the addition of nitric acid (containing nitrous acid) to the bile pigments a play of colour is observed ; this 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 indicative of a different degree of oxidation of the original bilirubin. Biliverdin gives the same play of colours excepting the initial yellowish-red, which is absent. Digitized by Microsoft® THE LIVEE AND PANCREAS 221 Although bilirubin has not been obtained from haemo- globin, there is no doubt that this is the source of the pig- ment, for if haemoglobin be liberated in the blood and enters the plasma, bile pigments appear in the urine ; further, hasmoglobin may be readily decomposed, yielding a proteid and haematin ; and if this hsematin be deprived of iron, the residue thus obtained is not very dissimilar in composition to bilirubin. We have previously mentioned (p. 11) 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 their wear and tear, the liberated haemo- globin is brought to the liver, and under the influence of the liver cells converted into the iron free substance bili- rubin or biliverdin. Part of the iron so liberated escapes from the body through the bile, but the bulk of it is re- tained 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 herbivora, 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 of the ox, and Salkowski states that it is a normal constituent of the urine of the dog during the summer. In the large intestines both bili- rubin and biliverdin undergo reduction resulting in the formation of stercobilin, the colouring matter of the fasces 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 Bile Salts are two in number, glycocholate and tauro- cholate of soda ; they are formed in the liver by the union of cholalic acid with glycine or taurine, and exist in combina- tion with soda. These salts are found in varying propor- tions in different animals; thus, glycocholate of soda is largely found in herbivora, taurocholate principally in carnivora, while in the pig hyoglycecholic and hyotauro- Digitized by Microsoft® 222 A MANUAL OF VETEKINARY PHYSIOLOGY cholic acids are found. Both salts are soluble in water, have a markedly alkaline reaction, rotate the plane of polarized light to the right, and may be obtained in a crystalline form as highly deliquescent acicular needles. Glycocholic acid is the chief bile acid in herbivora, it is produced by the union of glycine with cholalic acid ; it is diminished by an animal and increased by a vegetable diet. Tauroeholic acid is produced from taurine and cho- lalic acid and exists principally in carnivora, though small quantities may be found in the ox. This acid differs from the first characteristically by containing sulphur, by which it shows its proteid origin. Glycine or glycocoll also owes its origin to the proteids of the food, and if administered it reappears externally as urea. It cannot be traced in the free state in the body, but occurs in the urine combined with benzoic acid, in the form of hippuric acid. Petten- kofer'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 cane-sugar, and a similar quantity of strong sulphuric acid ; a beautiful purple-red colour forms. The colour is due to furfurol, and is produced by the action of the acids on the sugar and the subsequent reaction with cholalic acid. The origin of the bile acids is involved in obscurity ; taurine and glycine are probably formed from the disinte- gration of proteid, the precursors of cholalic acid are unknown. Nor do we know why glycine should predominate , in some animals and taurine in others, but 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 intestines into their constituents, the glycine and taurine ! being carried to the liver to be reutilized, while the cholalic acid is excreted. This economical measure has a twofold advantage, for not only can the glycine and taurine be used over and over again, but the bile^acids are the best of cholagogues, and stimulate the production of bile. Digitized by Microsoft® THE LIVER AND PANCEEAS 223 Bile is secreted under a very low pressure, which is the reverse of what occurs in the saliva ; low as the pressure is '('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 a continuous one; whether the animal be in full digestion or fasting, the flow is not intermittent as in the case of the saliva. Though continuous, it is not uniform ; it reaches its 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 curve is given by the pancreatic secretion, and it can be shown that a specific substance, secretin, which stimulates the production of pan- creatic juice, also hastens the secretion of bile. In those animals possessing a gall-bladder this receptacle is filled with bile during abstinence, or if it be empty it is filled even during digestion. The reflux of bile from the biliary duct to the gall-bladder is caused by a sphincter- like contraction of that portion of the duct penetrating the wall of the intestine, by which means the bile is driven back through the cystic duct to the gall-bladder. The bile as formed is propelled along the bile ducts by a contrac- tion of the muscular coat of the tubes, but doubtless both the forcing onward of the bile and the circulation through the liver are largely assisted by the respiratory move- ments, 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 carnivMra. Colin's experiments gave him the following amounts as hourly secretions : Digitized by Microsoft® 224 A MANUAL OP VETERINARY PHYSIOLOGY Horse - 8 ozs. to 10 ozs. per hour (250 to 310 grammes). Ox- - 3 ozs. to 4 ozs. .„ „ (93 to 120 grammes). Sheep - J oz. to 5 ozs. ,, ,, (8 to 150 grammes). Pig - 2 ozs. to 5 ozs. ., „ (62 to 150' grammes). Dog - \ oz. to £ oz. „ „ (8 to 16 grammes). The Use of the Bile from a digestive point of view is disappointing, inasmuch as it does not digest in the sense that pepsin and trypsin do. It is intimately connected with the function of the pancreas, with which object the secretions are poured out either close together in the bowel, or, as in some animals, by a duct practically common to ithe two glands. As the horse possesses no gall-bladder i the secretion is poured into the intestine as fast as it is prepared ; not so with the ox, sheep, pig and dog, where the bulk of it is stored up in a capacious receptacle until required. The reason offered for the horBe having no gall- bladder is that as digestion, under ordinary circumstances, 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 only poured out 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 neutralize the gastric juice and 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. t Bile has a solvent and emulsifying effect on fats, being 1 more active in the presence than in the absence of pan- creaticjuice. Bile cannot split up fats into fatty acids and glycerine as the pancreas does, but if free fatty acids are present the bile salts are decomposed, their soda set free, and soluble soaps formed ; the soaps so formed assist in rendering the emulsifying effect of the bile permanent and the absorption of fat much easier. Fat will not readily pass through a membrane, but if the latter be first Digitized by Microsoft® THE LIVEE AND PANCEEAS 225 moistened with bile the passage is greatly facilitated. 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, the working constituents being the bile salts. Bile has no I action on proteid. According to Hofmeister the bile of the ox, sheep, and horse converts starch into sugar, whilst ! the bile of the pig and dog possesses no such or only to a limited extent. It has been said that bile has an antiseptic effect on the intestinal contents, keeping them from putre- faction and promoting peristalsis, for it has been found that when it is prevented from entering the bowel, con- stipation and extreme foetor of the intestinal contents result. Bile, however, is not a true antiseptic. The clay- coloured faeces obtained in jaundice are probably due to the presence of unacted-on fat ; the fat encloses the proteids which putrefy, hence the odour. The bile acts as a natural purgative and keeps up intestinal peristalsis ; by so doing it hurries the food out of the system before it undergoes ^ putrefactive decomposition. ^ * Glycogen. * )Oj ' It is quite certain that the largest gland in the body // must have some other function than that of the secretion J -* ' of a fluid of comparatively unimportant digestive power, and such is the case ; the liver manufactures and stores up in its cells a peculiar substance known as glycogen or animal starch. Glycogen is spoken of as starch, though it differs from vegetable starch in many important character- \ istics ; thus, it is soluble instead of insoluble in cold water, j and it is stained reddish-brown instead of blue by iodine. The literature of the formation and use of glycogen is extensive, perhaps no substance has given rise to greater controversy ; yet the glycogen story which is accepted to- day is the one originally related by Claude Bernard, who was tbe discoverer of this singular substance. The sugar in the food, and that derived from starch- 15 Digitized by Microsoft® 226 A MANUAL OP VETEKINARY PHYSIOLOGY conversion, finds its way by means of the intestinal vessels into the portal vein, from here it passes into the liver ; under ordinary circumstances it is stored up in the liver as glycogen, being, in fact, reconverted into a kind of starch, and gradually doled out to the system as sugar when required. The liver regulates the amount of sugar which should pass into the blood ; so much and no more is ad- mitted to the circulating fluid, the amount varying between •05 and "15 per cent. The sugar in the blood of the ox was estimated by C. Bernard at "17 per cent., in the calf •1 per cent., and in the horse "09 per cent. When the I liver fails to regulate the amount of sugar in the blood AFTER FOOD. Fig. 56. — 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. 'diabetes is produced, and this occurs when the amount of sugar rises to more than '2 per cent. The glycogen which is thus stored up in the liver for future use may in two 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. If food, particularly carbo-hydrate, be now given the store of glycogen is rapidly replenished, and the sugar-liberating functions once more established (Fig. 56). The storing up of glycogen by the liver and its subsequent utilization is very closely allied to a similar process in the vegetable kingdom ; the starch in the leaves of plants may pass down the stem as sugar for the purpose of nourish- Digitized by Microsoft® THE LIVEB AND PANCEEAS ment 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 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 glucose. This con- I version of glycogen into glucose 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 in the dog does not exceed 17 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 it is deposited in other organs, princi- pally 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 per cent, to 2 - 4 per cent, has been found. Ordinarily it may be stated that the muscles hold 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 are muscles in which no glycogen is found and yet in which active contraction takes place. In the muscles of the embryo, before striation has occurred, the amount of glycogen existing is. something considerable ; as much as 40 per cent, of the dry material of the embryo muscle may consist of this substance. As striation appears the glycogen leaves the muscles to a great extent, and the liver takes on the process of production. The Use of Glycogen. — 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 15—2 Digitized by Microsoft® 228 A MANUAL OF VETEEINAEY PHYSIOLOGY is used for the nourishment of the foetus, and also in rapidly developing cells, such as some found in cartilage and the white cells of the blood; in all these and other places it is simply stored for future requirements. In the adult the chief use of glycogen is to facilitate the metabolic production of muscular energy and animal heat, and this it does in its glucose form as the result of oxidation. The sources of glycogen have been a fertile cause of discussion and object of experimental inquiry. It was natural to consider, as we have so far done, carbo-hydrate material as the chief contributing agent ; it was less certain whether proteids contributed, while the consensus of opinion was against fat taking any share in the process. We must examine each of these in a little 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 dextrose. These sugars are readily converted into glycogen by the liver cells by the process of dehydra- tion. Cane-sugar and milk-sugar are not readily converted into glycogen, but since these double sugars undergo inver- sion in the intestinal canal before absorption-+-cane-sugar into dextrose and levulose, and milk-sugar into dextrose and galactose — they may in this form be readily converted into glycogen. The effect of proteid on glycogen formation is not so easily settled. It is observed that in diabetes, though all carbohydrate food be withheld, yet sugar may appear in the urine on an exclusively proteid diet ; the same thing is observed in the experimental glycosuria which may be produced by the administration of phloridzin, and, further- more, that sugar may be produced even when the animal is i starved. The conclusion appears irresistible that proteid | can produce sugar, and this is explained by saying that : certain proteids split into a nitrogenous and non-nitrogenous portion, the former being converted into urea, while the non- nitrogenous residue is converted into sugar and may thus give rise to glycogen. Proteids, such as casein, which do not contain a carbo-hydrate group, may take no share in the Digitized by Microsoft® /fi THE LIVEE AND PANCEEAS 229 production of glycogen. 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. The question is, therefore, very far from being settled. It The Liver Ferment. — -When a liver is rapidly removed rom the body of a recently killed animal which has been appropriately 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 glycogen disappears. This change is brought about by a diastatic ferment in the liver 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. — Glycogen is a temporary reserve of carbo-hydrate material, which is issued as required to the system in the form of glucose, and by the process of oxidation yields heat and energy. It is readily used up in the interval between meals and readily renewed. The sugar in the blood maintains a remarkably regular percentage, -1 to - 2 per cent., and no doubt this is effected by the gradual supply of this material from the temporary reserve stored in the form of glycogen. Should the per- centage of sugar rise in the blood, the excess is got rid of through the kidneys (diabetes) and lost to the body. The liver itself does not appear to be able to regulate its sugar output to the blood ; this would seem to be one function of the pancreas, the ' internal secretion ' of which, in some way which is not clearly understood, prevents the liver giving off its glycogen as sugar too rapidly. Eemoval of the pancreas, as we shall show later, is followed by diabetes. If expressed Digitized by Microsoft® 230 A MANUAL OF VETEEINAEY PHYSIOLOGY pancreatic and expressed muscle juice be mixed together an active glycolytic substance results, and it is considered that as neither of the above are capable of acting alone, the internal ferment of the pancreas acts upon a ferment in the muscles and makes the decomposition of sugar possible. Diabetic Puncture. — Bernard discovered that if the floor of the fourth ventricle be punctured at a certain definite spot, temporary diabetes resulted, the urine contained sugar, and the liver possessed no glycogen. This spot is known as the diabetic centre, and the effect of the puncture is to stimulate it and temporarily destroy the glycogen-holding capacity of the liver, in consequence of which the material is liberated as sugar. The evidence of this is that if the animal be starved before the puncture is made no sugar appears in the urine. Stimulation of the central end of the vagus or of the depressor nerve produces glycosuria, though stimulation of the first causes the abdominal blood-pressure to rise, and of the second causes it to fall. From this circum- stance it is considered that the effect of the puncture is not to produce mere vascular dilatation, but rather that it stimulates some secretory nerve. The diabetic centre is a reflex one, the afferent or ingoing nerves being most of the sensory nerves, the efferent being the spinal cord, sympathetic and splanchnics. It has been suggested that the muscles at the moment of contraction set up afferent impulses which are carried to the diabetic centre and sugar thus liberated. This would place the muscle in the position of not only using up sugar but of being able to call forth its production as required. If this be proved to be true, it is easy to understand how the heart, the most active muscle in the body, is able to regulate the production of its energy yielding substance. Further Uses of the Liver. ['' 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. Digitized by Microsoft® THE LIVEK AND PANCREAS 231 Another important use of the liver is the formation of urea. The source of urea is the proteid constituent of the food, which in the process of disintegration yields certain amido-acids such as leucine and tyrosine. These substances may be formed in the intestinal canal as the result of pancreatic digestion, or they may be formed in the living cell as the result of the breaking down of proteid. Under any circumstances the leucine undergoes a series of oxidative changes, mainly in the liver, resulting in the formation of urea which is passed on to the kidneys for excretion. The further facts regarding the formation of urea are best dealt with in the section devoted to the kidneys. As the result of proteid 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 proteid 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 neutraliser of poisons introduced into the blood by the intestines. It is a note- worthy fact that many metallic poisons are also arrested in the liver, for example mercury and arsenic. The numerous and complicated changes produced by the liver may thus be summarized : 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 introduction of certain nitrogenous poisons, such as ammonia, by transforming them into urea, and against other poisons of proteid origin by converting them into harmless products, by conjugation with alkaline sulphates. Digitized by Microsoft® 232 A MANUAL OP VETEEINAEY PHYSIOLOGY Section 2. The Pancreas. /0jT 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 biliary apparatus are in possession of, 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 only be obtained 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 has established these fistulas both in the horse and ox, but the profound impression on the nervous system produced by such extensive inter- ference must considerably affect the character of the secretion and the amount manufactured. Pancreatic fluid is an alkaline, clear, colourless fluid like water, and though viscid in some animals is not so in the horse. It has a saltish unpleasant taste, and a specific gravity of about 1010 ; the viscid secretion of the dog has a specific gravity of 1030. The following analysis of the fluid in the horse is given by Hoppe-Seyler : Water - 98"25 Organic matter - '88, containing '86 of fer- ments. : ' ii,! ,; ' Salts - - -86, „ much sodium phosphate. 100-00 Schmidt found the fluid of the dog to have the following composition : Water ■ 90"00 f Organic matter - 9'04 Solids - 9 - 92 -j Salts - - - - 88 containing much sodium chloride. Digitized by Microsoft® THE LIVEE AND PANCEEAS 233 The salts present are sodium chloride in abundance, potassium chloride in traces, sodium carbonate and phosphate, calcium and magnesium phosphates in small quantities. The organic solids are remarkable for the amount of proteid present in them ; they vary in amount in different animals, for example 9 per cent, in the dog and "9 per cent, in the horse. / 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 whether these fibres produce it during the act of digestion. The outpouring of the acid chyme from the stomach into the duodenum at once gives rise to a secretion of pancreatic juice, and it was supposed that the acid acted on the secretory nerves and produced a secretion reflexly. Bayliss and Starling, however, demonstrated the remarkable fact that if an extract of the mucous membrane of the duodenum or jejunum be made by scraping the bowel, and acting on it 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 clearly established, first, that it is not a ferment as it is not destroyed by boiling, and secondly that 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, acting on the mucous membrane of the intestine which produces secretin ; this is absorbed by the blood, and thus produces its specific action on the pancreas. Uses of the Secretion. — The pancreatic juice is poured into the bowel in the horse and 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, and open within a short distance of each other. It is essentially a digestive fluid, and acts on the three Digitized by Microsoft® 234 A MANUAL OF VETERINARY PHYSIOLOGY classes of food stuffs, viz., proteids, fats and carbo- hydrates; to enable this to be effected, it contains three ferments or their precursors, viz. : A Proteolytic Enzyme which acts on proteids (Trypsin). A Diastatic Enzyme which acts on carbo-hydrates (Amylopsiri). A Lipolytic Enzyme which acts on fats (Lipase or Steapsin). Observations appear to show that the proportion of each of these ferments in the secretion depends on the character of the food ; if, for example, the food is rich in fat the secretion would be rich in lipase. It is also probable that not only does the nature of the food determine the pre- dominance of each enzyme, but also the amount of fluid to be secreted. This, as a rule, reaches its maximum in the dog between the second and fourth hour after taking food, and corresponds to the greatest activity of the liver. In dogs which have been starved active secretion of bile, pancreatic juice, and intestinal fluid, takes place, it is said, every two hours, and lasts for twenty minutes. The cause of this is by no means clear. All the fluid thus poured out is reabsorbed. Trypsin. — It has been observed that pancreatic juice taken direct from a fistula in the duct may have little or no action on the proteids of food, but if the same fluid be allowed to become contaminated by the intestinal contents 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 that though the secretion taken direct from the pancreas contains the precursor of trypsin, viz., trypsinogen, yet in the latter form the ferment is unable to act on the proteid 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, 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 trypsin. It is remark- Digitized by Microsoft® THE LIVEK AND PANCEEAS 235 able that of the three ferments secreted by the pancreas, trypsin is the only one which is secreted in an inactive condition. Pawlow 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 obtained usually from a slaughter-house, may be made more tryptically active by the addition of a little dilute acetic acid, does not now imply that the acid has converted the trypsinogen into trypsin, as has usually been supposed. The pancreas used in the preparation of the extracts is already contaminated with minute quantities of enterokinase, whose activity is greatly increased by neutralizing the alkalinity of the extracts. If a pancreas be obtained under conditions which ensure 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 here desirable to draw attention to the fact that secretin and enterokinase are both derived from the mucous membrane of the intestinal canal, and care must be taken to avoid confusing them : the former is not a ferment, the latter is. 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. The action of trypsin on proteids is most interesting. The proteid 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 taken up excepting in its simpler form, and the proteids of oats, barley, hay, or flesh, have to be reconstructed in order to form part of the tissues of the living animal. To enable this to be done trypsin acts on the large proteid molecule and breaks it down in the pro- duction of a number of simpler bodies of smaller molecular weight ; on these the tissue cells set to work, and by a Digitized by Microsoft® s/ 236 A MANUAL OF VETEEINAEY PHYSIOLOGY process of synthesis construct the form of proteid needed by the body. It can be easily shown that the action of trypsin on proteid is much more satisfactory and thorough if the latter has previously been acted upon by pepsin. Trypsin like pepsin produces albumose 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 action of the trypsin is, in fact, to produce a large number of simpler end-products, of which the amido-acids' leucine and tyrosine are the best known and most easily obtained : besides these aspartic and glutaminic acids, tryptophan, and the hexone bases lysine, arginine, and histidine. Should any proteid or peptone have escaped the action of pepsin and trypsin, it may be attacked by another enzyme found in the intes- tinal mucous membrane, known as erepsin, which also has the power of breaking down albumoses and peptones into leucine and tyrosine. .Erepsin is found in most of the tissues of the body, so is not specific to the intestine. Under the influence of bacterial action aromatic bodies are formed — phenol, indol, and skatol, the latter being responsible 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 proteids. Amylopsin, the diastatic ferment, has an action on starchy food similar to that of ptyalin, but more rapid, the final products being maltose and achroodextrin. The hydrolytic action of amylopsin stops at maltose and achroodextrine, but these are in turn attacked by the maltase of the succus entericus and converted into dextrose. Lipase or steapsin, the fat-splitting ferment, acts upon neutral fats, splitting them into free fatty acid and glycerin. The splitting process is followed by saponifica- tion, viz., the liberated fatty acid combines with the alkaline salts to form soaps; as the result of this the production of an emulsion becomes possible. In emulsifi- cation the oil globules are rendered extremely small without Digitized by Microsoft® THE LIVER AND PANCEEAS 237 the power of coalescing, and at one time it was considered that fat in this finely divided state was capable of entering the villi, but there can be no doubt that the minute fat globules are further split into fatty acid and glycerin, and that only the products of this splitting enter the villi. Once, however, within the epithelial cells of the villi, the synthesis of fatty acid and glycerin into fat becomes possible, and recent work indicates that this may be effected by lipase ; in other words, the same ferment which does the splitting is possessed of a reversible action. Lipase is readily destroyed, so that unless quite fresh it does not work in artificial digestions. Under natural con- ditions it is greatly aided, and the process rendered much quicker by the action of the bile. On p. 170 we have alluded to Pawlow's work on the quantity and quality of the gastric juice, being regulated by a specific action on the part of the food itself. Similarly, the same observer has shown that the ferment contents of the pancreatic juice are adapted to the character of the diet ; a definite and constant diet leads to the formation of a pancreatic juice which is unable to deal effectively with a sudden change in diet. The practical bearing of this in the feeding of animals is far-reaching. As a profession we have recognised for years the disastrous effects of sudden changes in diet ; modern science offers the explanation of its action, which in all probability is brought about as the result of an internal secretion. The Changes occurring in the Cells of the gland correspond very closely with 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 in- distinct ; 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 presents a clear or fairly clear zone, while within this there is an intensely granular zone (Pig. 57, A). The minute granules filling the cell are the mother substance of the secretion. When activity commences the granules appear Digitized by Microsoft® 238 A MANUAL OF VETEE1NAEY PHYSIOLOGY to pass centrally towards the alveolus, leaving the cell comparatively clear excepting that portion immediately abutting on the alveolus, which even in the exhausted condition remains granular. These changes result in the cells becoming distinct and clearly denned, 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 Fig. 57. — A Pobtion 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 strise. 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. alveolus is readily defined, whilst the nucleus of the cell, which was hidden in the charged condition, can easily be seen in the exhausted gland (Fig. 57, B). These changes have been worked out 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 maximum is reached between the second and fourth hours after feeding, this maximum Digitized by Microsoft® THE LIVES AND PANCKEAS 239 being followed by a fall, and about the seventh hour by a rise. It will be remembered that the bile gives a similar curve. In the dog it is generally considered there is no secretion during starvation, but immediately food begins to pass out of the stomach the pancreas becomes active. In this con- nection, however, it is desirable to remember that according to some observers a starved dog will actively secrete pan- creatic juice for twenty minutes every two hours. The continuous secretion of the gland in herbivora is provided for by all the lobes not being active at the same time. In the ox the amount of juice secreted is between 7 and 9 ozs. (265 grammes) per hour, in the horse it is much the same, in the sheep \ to J oz. (7 to 8 grammes), pig about ^ to \ oz. (5 to 15 grammes) 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 rela- tively more than herbivora. The pressure under which the pancreatic juice is secreted is low ; it is said to be equal to '67 inch of mercury, which is very little greater than that of the bile. ^[\ K Pancreatic Diabetes.* — If the pancreas of a dog be com- ~%" pletely removed there is a disappearance of all glycogen from the tissues, and the animal dies in the course of a month or less with diabetes, since the power of oxidizing glucose is lost. The glucose 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. If the depancreated animal be placed on a purely proteid diet, no difference occurs in the amount of sugar excreted ; even if no food be given sugar is still formed. If the removal of the gland is incomplete glycosuria still occurs, but it will vary in intensity from fatal to transient effects, depending upon the amount of pancreas left behind, and this is explained by the fact that sugar may be formed * To avoid repetition, this matter should be read in conjunction with the remarks on Glycogen, p. 229. Digitized by Microsoft® 240 A MANUAL OF VETERINARY PHYSIOLOGY from proteid. In fact, it is possible by experience to leave behind just sufficient of the gland to prevent diabetes arising. In any case fatal results may be avoided by graft- ing portions of pancreas beneath the skin, the presence of these preventing diabetes. Evidently, therefore, in some way pancreatic tissue is intimately mixed up with the sugar question, and it has been assumed that the pancreas produces an ' internal secretion ' (see p. 229), the use of which is devoted to pre- serving the organism from an excess of sugar, either by regulating the amount which is to be liberated into the blood from the seats of sugar production (liver and muscles), or by stimulating the sugar- splitting action of the tissue cells (p. 230). Very little is known of the subject; it certainly appears that the acting agent is not an enzyme, for its property is not lost in a pancreas which has been boiled ; further, the internal secretion cannot act alone, it requires the presence of a ferment formed in the muscle, and the combination is then capable of rapidly decomposing dextrose. The blood, as pointed out previously (p. 226), will not tolerate more than '2 per cent, of sugar in circulation, any- thing over this is rejected and got rid of through the kidneys ; in pancreatic diabetes there may be double this amount of sugar in the blood. Histologically, the pancreas is a compound tubular gland like the salivary glands, but there are certain groups of cells peculiar to it which form spherical or oval bodies capable of being seen with the unaided eye. These are known as the Islands of Langerhans ; each is surrounded by a rich capillary network of bloodvessels, and the view has been advanced that these islands are the seat of the internal secretion of the pancreas. Pathological. The most common pathological condition of the liver is Jaundice, and the majority, if not all, cases of jaundice are obstructive, viz., there is some obstruction to the free pouring out of bile ; in con- sequence there is a backward pressure, which being greater than the Digitized by Microsoft® THE LIVER AND PANCREAS 241 low blood-pressure under which bile is secreted, the bile is reabsorbed, and stains the tissues yellow. There is also a form of jaundice affecting the horse and dog in South Africa, due to a parasite in the blood, but in these cases the yellow tint is derived from the destruction of red corpuscles and the liberation of their colouring matter. Biliary Calculi, consisting largely of cholesterin, are not uncommon in ruminants, but rare in the horse. Fatty Liver is common in all animals over-fed and under-worked. In the horse it may lead to Eupture of the liver during work. Enlargements of the liver are 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 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 generally liver disorders are very frequent. 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. Digitized by Microsoft® CHAPTEE VII ABSORPTION Section 1. 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-vascular 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, however, the lymphatic system may be, described as the drainage system of the body, in contra-| distinction to the blood or irrigating system. The Lymph Spaces. — The tissues are bathed in lymph,, which is contained in the lymphatic spaces existing between the capillary blood-vessels and capillary lymph-vessels. There is a constant passage of material from the blood into; the tissues, and from the tissues into the blood. The lymph spaces are irregular passages in the connec- tive tissue, the larger ones being lined by epithelioid plates of a peculiar irregular outline ; these spaces exist outside the bloodvessels, and the lymph finds its way from thes bloodvessels into the lymph spaces. From the lymph spaces the fluid reaches the lymph capillaries, but thJ means by which it gets there is not clear, for it appears certain that excepting in a few caaes there is no direct communication between the space and the capillary. In the vessels of the brain a peculiar arrangement is present| the lymphatic vessel surrounds the artery and obtains its 242 Digitized by Microsoft® ABSOEPTION 243 lymph direct ; such are known as peri- vascular lymphatics. gThe lining of the Lymph Capillary is composed of the same Epithelioid plates with irregular outline which are found in pie spaces, and it is believed that at the junction of the (plates, crevices or intervals 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 epithelioid lining has also a muscular coat, more marked in the large than in the small vessels, -iand also a connective-tissue covering. In the interior of these vessels valves are found which are essentially similar in structure, arrangement, and mode of action to those in the veins. Immediately beyond each valve there is a dila- tation of the vessels which gives them a beaded appearance when the lymphatic is distended. The whole of .the lymphatics of the body converge to- wards 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, unite with 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 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 peri- toneum, have been looked upon as large lymphatic spaces, though even this is now by some considered doubtful. The fluid they contain is lymph, and they are in direct com- 16—2 Digitized by Microsoft® 244 A MANUAL OF VETEEINAEY PHYSIOLOGY munication 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 diaphragm may Fig. 58. — Diagrammatic Section op Lymphatic Gland. ad, Adenoid tissue containing lymph corpuscles, excepting to the left of the figure ar, where they are omitted in order to show the adenoid reticulum. 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 blood- vessel v. Outside the core of adenoid tissue is the lymph sinus or space Is, across which run branched nucleated corpuscles which are simply an open network of connective tissue. These corpuscles are shown on a black ground in order the better to distinguish the lymph space. Surrounding the whole is the trabecular frame- work t. 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 Digitized by Microsoft® ABSORPTION 245 passage through these glands the lymph has corpuscles added to it which ultimately 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 and medulla. The capsule sends in bands of tissue (trabeculce) 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 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. 58). It is through the lymph-sinuses of the cortex that the gland is in direct communication with the afferent lymphatic vessels. In the adenoid tissue of the alveolus is found 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 1012 to 1022, 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 cor- puscles ; it contains, therefore, the proteids of that fluid, viz., fibrinogen, paraglobulin, and serum albumin though in smaller amounts, cells resembling the white cells of the blood, extractives, salts, and gases. The fluid in which Digitized by Microsoft® 246 A MANUAL OP VETERINARY PHYSIOLOGY these are contained is spoken of as lymph plasma. The gases consist principally of carbon dioxide, whose amount is greater than in arterial but less than 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 cow. The salts are distributed much as are those in blood, viz., potash and phosphates 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 move- ments 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 proteids, lecithin, cholesterin and fat, and their nuclei contain nuclein. Owing to their power of move- ment they are able to pass through the bloodvessels into the tissues and vice versa. The proportion of lymph corpuscles to fluid is about the same as the proportion of white corpuscles to blood. The Quantity of Lymph in the body is difficult to arrive at, and varies considerably. Colin obtained from a lymphatic in the neck of horses between 1 to 4 lbs. (J to 2 kilos) in 24 hours; the mean amount was 2 lbs. 6 ozs. (2 kilos) for the same period, but he notes that the variations are very wide, and that herbivora secrete more than carnivora, and young animals more than adults. The amount of material collected from the thoracic duct of a cow in 24 hours has been found to be 209 lbs. (91 kilos), but this is no guide to the quantity of lymph in the body, as the material in the thoracic duct is mixed with the chyle from the intestines. It is usual, however, in this vessel to consider two-thirds of the contents to represent chyle and one-third lymph. The quantity of mixed chyle and lymph Digitized by Microsoft® ABSOKPTION 247 obtained by Colin from the thoracic duct, some hours after the animals had been fed, was as follows : Horse, 30 to 90 lbs. (14 to 40 kilos) in 24 hours. Ox, 46 to 209 lbs. (20 to 91 kilos) „ Sheep, 6J to 10 lbs. (3 to 9-5 kilos) „ „ Dog, 3 to 6 lbs. (1-3 to 2-6 kilos) „ „ The Formation of Lymph. — The theory of lymph formation is by no means settled ; the rival views may be classified as physical and secretory. The physical theory is based upon the laws of filtration, diffusion, and osmosis, while the secretory or vital theory is based upon the activity of the living cells of the body. The physical theory will first claim our attention, and it is the one which at the, present time finds, perhaps, more general acceptance. According to this theory lymph is formed as the result of the operation of the three following factors. Filtration through the walls of the capillaries from the blood to the tissues is always possible when the pressure of blood in the capillaries is higher than that of the fluid in the tissues. It can easily be shown that an increase or decrease in capillary pressure increases or decreases the amount of filtration. Diffusion and Osmosis. — The difference existing between the composition of the blood plasma and the liquid in the tissues outside the capillary vessel is a cause of diffusion and osmosis. Such differences are frequent, as for example, after every period of digestion, and the equilibrium of com- position can easily be restored by the setting in of diffusion and osmotic currents. Differences in composition may arise not only between the blood and the tissues but the tissues and the blood, and in both cases are adjusted in the same way. Diffusion in liquids seems, as in gases, to be the result of the continual movement of its molecules, so that two liquids miscible, but utterly unlike, if brought into contact will gradually form a homogeneous mixture ; or if they be separated by a membrane permeable to the molecules, diffusion will occur through this and a mixture of uniform composition result. Diffusion through a mem- Digitized by Microsoft® 248 A MANUAL OF VETEEINAKY PHYSIOLOGY brane is known as osmosis. Substances which are diffusible are known as crystalloids, those which are non-diffusible are called colloids. Sugar or salt are good examples of diffusible bodies, proteid and starch are examples of colloids, the large size of the molecules of the latter preventing their passage through an animal- or other membrane. This difference in the behaviour of these two classes of substances as regards 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 diffusibility 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. If a strong solution of common salt be injected into the blood, an osmotic current is created proceeding at first from the tissues into the blood ; in course of time the salt will diffuse out of the vessels into the tissues and an osmotic current will then be set up in an opposite direction, viz., from blood to tissues. The diffusibility of proteid substances is very slight or entirely absent, their osmotic properties are correspondingly small and by some have been denied. It is therefore difficult to explain the passage of the pro- teids from the blood to the tissues, excepting on the ground of filtration. A picture of what is occurring in the tissues under ordinary functional activity is probably as follows : The tissue elements are nourished by the lymph thereby effecting an alteration in the composition of the latter, which is made good by diffusion and osmosis from the blood. As the / Digitized by Microsoft® ABSOEPTION 249 result of tissue activity the proteid molecule gets broken down with the production of simpler and crystalloid bodies. These pass into the lymph from the tissues, and raise its concentration ; by so doing they draw water from the blood- vessels to the lymph, or if the concentration in the latter is less than that of the blood, diffusion occurs from lymph to blood. The permeability of the capillary wall as a factor in lymph production has been urged by Starling. He finds that whereas the capillaries in the limbs and connective tissues generally present a very considerable resistance to the filtration of lymph through them, and keep back a large portion of the proteids of the blood-plasma, the intestinal capillaries, on the other hand, are much more permeable, while those of the liver are of the greatest permeability, a very small capillary pressure producing a large transudation of lymph containing as much proteid as the plasma itself. So slight is the effect of capillary pressure on lymph production in the limbs and connective tissues, that in other parts of the body it has been necessary to explain the production as mainly due to diffusion and osmosis. Constant osmotic interchange between blood and tissue-cell occurs through the medium of the lymph, and with remarkable rapidity ; thus, if the osmotic equilibrium be disturbed by injecting a large dose of dextrose into the circulation, within half a minute it is readjusted. The slight influence of capillary pressure in lymph production, mentioned above, only holds good for the normal capillary ; any impaired nutrition of the vascular wall, which may easily arise, increases its permeability, and the slightest increase of capillary pressure then produces an increase in lymph production. Starling records the remarkable fact that no lymph can be obtained from a resting limb, 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 Digitized by Microsoft® 250 A MANUAL OF VETERINARY PHYSIOLOGY changes in the tissue are still occurring, oxygen is being absorbed, carbonic acid and other waste products got rid of, but their channel of excretion is effected by the blood- j) The secretory theory of lymph production is based on the knowledge of the secretory activity of epithelium generally. It was natural, therefore, to regard the endothelial lining of the capillary vessels as the possible seat of secretory activity. Heidenhain, the exponent of this theory, showed that certain bodies {Lymphagogues) when injected into the blood-stream caused an increased flow of lymph, and he regarded these as direct excitants of secretion. The oppo- nents of this theory show that these substances act by their deleterious effect on the capillary wall, or by their causing water to be taken up from the tissues ; the effect of taking up water is to raise the total volume of blood in the vessels, and so cause a general rise in blood-pressure, and in consequence a transudation of lymph. At present it is not possible to decide between the rival theories of lymph formation ; it may be proved that under given conditions both play a part in the process. It seems impossible to exclude the living activity of the cell- body, so strongly urged in the matter of other secretions, while it is equally certain that there are other conditions which are only possible of explanation on a physical basis. We cannot suppose that the condition of the cells forming the capillary wall is invariably the same. This wall is the membrane across which the physical factors have to play their part. Even if the latter are the chief agents in the whole process they must still be more or less subject to the changing states of the cellular capillary wall. And in connection with both these views, it is well to bear in mind that no lymph-secretory fibres have been discovered, though their existence is possible, and further we do not know positively in which way the tissue spaces communicate with the lymphatic vessels, or whether, like the blood- vessels, the latter form a closed system. As fast as the lymph finds its way into the spaces it is Digitized by Microsoft® ABSOEPTION 251 normally passed on to the lymphatic capillaries, so that the rate of output is equivalent to the rate of removal ; when however the output is greater than the rate of removal the lymph accumulates in the tissues and (Edema results. It is conceivable that the rate of removal need not neces- sarily 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. Such an increased secretion of lymph lies on the shoulders of the vascular system, and experience shows that in the majority of cases increased formation of lymph is a more common cause of cedema than defective drainage. It is well known that interference with the venous circula- tion is productive of oedema ; disease of the right side of the heart or portal obstruction is a fruitful source of trouble, the explanation being that there is not only an increase of pressure in the capillaries as the result of the venous obstruction, but also a back flow of venous blood which is kept in contact with the wall of the capillary, and induces changes in the epithelioid cells result- ing in increased lymph formation. The swollen legs so common in horses kept idle in the stable are practically due to the same cause. The venous blood ascends the limbs against gravity and exerts on the capillaries of the legs below the knees and hock a pressure which is nearly equiva- lent 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 removable by exercise. The Movement of Lymph is largely brought about by muscular contractions in the neighbourhood of the vessels, by which means they are compressed and their contents forced onwards, since the valves which the vessels 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 contraction for any resistance in the gland itself. The pressure of the lymph in the lymph spaces is higher than that in the jugular vein, so the flow of Digitized by Microsoft® 252 A MANUAL OF VETERINARY PHYSIOLOGY 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 case of 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 contraction, while a sucking action is produced when it relaxes. This pumping arrange- ment exists in tendons, fasciae of muscles, etc., and is a valuable aid in lymph circulation. Once 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 by gravity, by the muscular contraction of the coat of the duct and by the negative pressure produced in the anterior vena cava vein by the process of inspiration, while the aspiration of the thorax keeps the duct filled; the combined result of these- forces is that the lymph is aspirated out of the duct into the vessel. This aspirating influence has been proved on the horse by experimental inquiry, a negative pressure in the thoracic duct having been observed during inspira- tion, and a positive pressure during expiration. In a manometer placed in the thoracic duct of the ox, Colin states that mixed chyle and lymph rose in five minutes to a height of over three feet ; this pressure is one third of the blood pressure in the aorta, and appears excessive. The lateral pressure in a lymph vessel in the neck of the horse was from one half to three quarters of an inch of a weak solution of soda ; in the dog the lateral pressure was half that found in the horse. The thoracic duct terminates in the anterior vena cava at the jugular confluent in a variety of ways ; its most usual method is that it dilates before entering the vein, and from the dilatation either one or two very short vessels are formed which enter the anterior cava, the entrance being guarded by a valvular arrangement. The Digitized by Microsoft® ABSOEPTION 253 right lymphatic channel also opens into the anterior cava at the jugular confluent, the entrance being furnished with a double semilunar valve. The blood in the vena cava vein is prevented from passing into the thoracic duct - by the presence of these valves, which normally only allow fluid to pass in one direction, viz., from the duct into the vein. Colin has observed that it is not uncommon in the horse to find the lymph in the thoracic duct slightly blood- stained, a slight leakage from the vein into the duct being liable to occur in this animal, though such has never been seen in the ox. The lymph moves slowly in its vessels. Weiss has observed a rate of from 9 to 11 inches (23 to 28 cm.) per minute in a large lymphatic in the neck of the horse, but the velocity in the small vessels is very much less. 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 during digestion a quantity of neutral fat, 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 con- dition of measurably large droplets, such as are seen in milk, but the bulk of it exists as extraordinarily minute particles ; hence the name ' molecular basis,' which is applied to the fat particles in chyle collectively. 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, Digitized by Microsoft® 254 A MANUAL OF VETEEINARY PHYSIOLOGY known as villi and solitary glands. The villi (Fig. 59) are innumerable projections from the inner surface of the mucous membrane shaped like minute fingers; they are only found in the small intestines, and have been calculated by Colin to amount to forty or fifty millions in the horse and 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 a Fig. 59. — Vertical Section of a Villus : Cat. x 300 (Stewart). a, Layer of columnar epithelium covering the villus — the outer edge of the cells is striated; h, central lacteal of villus; c, unstriped muscular fibres ; d, mucin-forming goblet-cells. of capillary bloodvessels, while filling up the finger of the villus not otherwise occupied by vessels is a peculiar structure found especially in lymphatic glands and known as adenoid tissue (p. 245) ; this tissue is relatively larger in amount in the villi of carnivora than of herbivora (Fig. 60) . Covering the entire villus 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 next the interior of the intestine. 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 next the Digitized by Microsoft® ABSOKPTION 255 interior of the bowel is a clear band bearing fine striatums- Lying between the columnar cells are others which from, their shape are spoken of as ' goblet cells ' (Fig. 59) ; by- means of a pore they extrude their contents, 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 con- traction of which, combined with the peristaltic move- ments of the intestine, 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 Epithelium. dog. rabbit. Fig. 60. — Transverse Section op Villi of Carnivorous and Herbivorous Animals (Waller, after Heidenhain). The large cells in the epithelial zone of the dog are the goblet cells. lymph into the lymphatic vessel at the base of the villus. This is known as the pumping action of the villus, and provides an important factor in the furtherance 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 collected into masses where they are known as Peyer's Patches. The Solitary Follicle is essentially a lymphatic structure and is not concerned like the villus in absorbing anything from the food. It consists of a mass of adenoid tissue, the Digitized by Microsoft® 256 A MANUAL OP VETBEINAEY PHYSIOLOGY network of which is filled with leucocytes ; within the net- work 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 corpuscles 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 1007 to 1022. In starving animals it is trans- parent owing to the absence of fat, and it is, in fact, at this time practically 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 reddish, due, no doubt, to a slight leakage from the vena cava, such as has been previously noted (p. 253). 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. 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 muscular contrac- tion of the intestinal villi forcing it onwards, while the valves in the lacteals prevent its return. Digitized by Microsoft® ABSOKPTION 257 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, pilocarpin, physostigmin, etc., are all rapidly absorbed from the air-passages,* and accord- ing to our observations produce their physiological effect in a shorter time than when simply injected under the skin. The lungs also haveHhe power of absorbing certain poisons like curare, which are not absorbed when introduced into the digestive canal. The absorption of water from the bronchial passages is very rapid. Colin introduced six quarts of water per hour into the trachea of a horse ; the animal was destroyed at the end of 3| hours and no fluid was found in the bronchi. He also poured into the air-passages one pint of water at a time ; repeating this without intermission, he poured in 74 pints of water before he caused death. So rapid is absorption from the bronchi, that a horse may be placed under chloroform almost instantaneously by an inlra-tracheal injection of the drug.t The rapidity of absorption is therefore very great, but * 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 swallowing, probably due to stimulation of the recurrent nerve. t It is not intended here to recommend the intra-tracheal adminis- tration of chloroform, which is not only dangerous but produces the greatest excitement in the patient. 17 Digitized by Microsoft® 258 A MANUAL OP VETERINAEY PHYSIOLOGY 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 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 the bloodvessels and lymphatics take part in the process ; ferrocyanide of potassium injected into the face has been detected in a carotid lymphatic in seven minutes. The rapidity of cellular tissue absorption is hastened by muscular movement. ' Absorption from the Conjunctiva is very pronounced for some drugs such as atropin and certain organic poisons, but there are others 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 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 5 hours with a solution of ferrocyanide of potassium ; the salt was detected in the urine in 4| hours, although the skin was quite unbroken. 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 ferrocyanide of potassium ; in 20 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 lymphaties as by the bloodvessels. The mucous membrane of the vagina is found by experi- ment to absorb very slowly. Experiments made on Absorption from the Pleural and Peritoneal Cavities showed that such drugs as strychnin Digitized by Microsoft® ABSOBPTION 259 rapidly produce fatal symptoms when injected into these sacs ; even in such a short time as from 3 to 7 minutes tetanic symptoms supervene. Potassium iodide injected into the peritoneal cavity of a sheep may be detected in the thoracic duct 5 to 8 minutes after the operation. Starling and Tubby have shown, however, that the active agents in absorption from these sacs are the blood- vessels, and that the share taken by the lymphatics is insignificant. 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. Stomach absorption, or rather its absence in herbivora, has been dealt with at p. 176. Even in the dog it is now admitted that absorption 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 only absorbed 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 absorp- tion in the horse and ox points to intestinal absorption as being of considerable importance in herbivora. Tbat this absorption is very rapid is proved by Colin's experiments. Hydrocyanic acid injected into the small intestine of a horse caused death in 1 to 1£ minutes, and potassium ferrocyanide injected into the bowel, after tying the mesenteric lymphatics, was detected in the blood 6 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 possibility that some substances taken up from the bowel may at once pass into the blood via, the thoracic duct (Fig. 61), while others must first proceed to the liver by the portal vessels for further elaboration before entering tbe blood. It will be remembered that the villi are found only in 17—2 Digitized by Microsoft® 260 A MANUAL OF VETEEINAEY PHYSIOLOGY 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 remembering the large chain of glands, along the colon in particular, it is probable that the material absorbed passes through these glands to a greater or less extent, as in the mesentery, before entering the circulation. There is, at any rate, a well-developed lymphatic system in the Fig. 61. — Loop of Small Intestine op the Horse during Active Absorption, with Distended Lacteals. walls of the large intestine, and it is certain that material is taken up from this bowel both by the bloodvessels and lymphatics. The amount of this must be considerable, when the size of these bowels is borne in mind and the character of their contents: Substances can be taken up with extreme rapidity from the large bowels. Colin observed that 18 minutes after injecting a solution of nux vomica into the caecum of the horse convulsions began, and 8 minutes later the animal Digitized by Microsoft® ABSOKPTION 261 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, proteids 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 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. The blood- vessels, therefore, have nothing to do with the absorption of fat, yet from an open thoracic duct not more than 60 per cent, of the total fat given in an experimental diet can be recovered; after deducting that excreted unabsorbed with the fasces, there still remains a balance unaccounted for. The fate of this missing portion of fat is still a matter of conjecture. It has been shown (p. 236) 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 action of the pancreatic juice and bile, and they lead to two possible views as to 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 comparable to the ingestive powers of a white blood-corpuscle. This would readily account for the appearance characteristic of chyle (p. 253), the minuteness of the fat particles it contains being probably intended to prevent embolism by plugging of the capillaries. The view thus indicated was the one Digitized by Microsoft® 262 A MANUAL OF VETEEINAEY PHYSIOLOGY 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 fatty-acid and soap, which then pass in solution into the cells of the villi, as do the proteids and carbohydrates. If the intestinal mucous membrane of an animal in full fat absorption is stained with osmic acid the epithelial cells are found to he crowded with minute particles of varying size, whose blackness shows them to be. fat (Fig. 62). This fact provided the chief support for the view that fat reaches Fig. 62. — Mucous Membrane of Frog's Intestine during Absorption of Fat (Schafbr). ep, Epithelial cells ; str, striated border ; c, lymph corpuscles ; I, lacteal. the lacteals in a state of minute mechanical subdivision 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 border of the epithelial cells, and this is never observed. But if we hence discard the first possibility and accept the more current view that fats are absorbed in solution, we must assume that there is a rapid reconstruction of neutral fats inside the epithelial cells after the absorption of the dissolved soaps or fatty- acids, inasmuch as the cells still always give with osmic acid the appearance characteristic of the presence within them of minute fat-particles. This synthetic reconstruction of fat may possibly be brought about by the reversible activity of the lipase ferment to which we referred on p. 237, Digitized by Microsoft® ABSOEPTION 263 though it is more probably due to the constructive activity of the cell- protoplasm. This second view of the mechanism of fat-absorption further enables us to understand the all- important role of bile in the absorption of fat. Absorption of Carbohydrates, -t- The digestive changes undergone by starch are described on p. 142. The sugar formed from starch by the saliva is maltose, the maltose by the aid of the succus entericus and epithelial cells of the intestine is converted into; glucose, this and its allies being the only form in which Sugar can be utilized by the system ; both cane and milk sugar must be thus converted or else they are excreted in the urine. It is clear from what has been said that the path of absorption for carbohydrates is the bloodvessels. Absorption of Proteid. — If the thoracic duct of a dog be ligatured and a large proteid meal given it is perfectly absorbed, as shown by the increase in urea, while there is no increase in the amount of lymph or of its proteid contents. This clearly shows that the absorbed products pass into the bloodvessels. Proteids before absorption are rendered soluble by conversion into peptones and proteoses, yet there is no blood in the body, including that of the portal area, which is found to contain even a trace of peptone or proteose ; in fact, the presence of these substances in the blood acts as a poison, giving rise to peptonuria. The peptones and proteoses enter the blood as ordinary proteid, so that during their passage through the epithelial wall of the intestines they become regenerated. Beyond the above facts, very little is known of the absorption of proteid. Absorption of Water and Salts. — These are taken up by the bloodvessels and with remarkable rapidity. The amount of water capable of absorption is very considerable. The material passes into the bloodvessels either through the epithelium or between the epithelial cells. Digitized by Microsoft® CHAPTEE VIII DUCTLESS GLANDS AND INTERNAL SECRETIONS The ductless glands of the body are represented by the spleen, thyroid, thymus, adrenals, pituitary, and pineal bodies. The function of these is either imperfectly known or entirely unknown, but within recent years experimental enquiry has thrown some light on their use as glands producing an internal secretion, viz., a something carried away by the blood or lymph stream and utilized elsewhere by the body. Internal secretions are not limited to ductless glands. It is now known that the pancreas, liver, and other glands produce, in addition to the visible secretion passing away by their duct, another or internal secretion passing away by lymph or blood channels, and quite distinct from the ordinary fluid secreted by the gland (see also p. 285). ^7 The discovery of secretin (p. 233) by Starling and Bay- ■ liss opened up a field of the highest importance, possessing possibilities the extent of which cannot be forecast. In secretion we have a specific chemical excitant, or hormone, and it may yet be shown that secretions which have been regarded as due to the influence of the nervous system are in reality produced by a chemical stimulant furnished by the body itself. Edkins, indeed, considers this is so of the gastric juice ; while Starling and Bayliss point to the specific chemical excitant theory as offering some explanation of the sympathy between the uterus and the mammary gland, the occurrence of menstruation, and periodic sexual excitement in the lower animals. The 264 Digitized by Microsoft® THE INTEENAL SECEETIONS 265 ovary has been suggested as the seat of production of such chemical excitant. The corpus luteum is regarded as a ductless gland, its internal secretion being connected ■with the fertilization and implantation of the ovum. The influence of the ovaries on the development of the external genital organs may also in this way be explained, for the arrested development which occurs as the result of removing the ovaries in the young animal is prevented by implanting them in a distant part of the body. The sympathy between ovaries and mammary glands is further shown by the remarkable fact that a cow ovariotomised when in full milk remains in milk for two or three years. The influence of the ovaries on psychic conditions is well recognized : some forms of vice in the mare are cured or improved by removal of the ovaries. It is to be noted that apparently the complete removal of all trace of ovarian tissue in the cat and dog may not invariably prevent periodic sexual excitement (Leeney). It has been stated that the removal of the ovaries from the dog affects metabolism, especially the consumption of oxygen, which falls off, and that this may be neutralized by the administration of an extract of ovary ; this causes the metabolism to rise above the normal, but does not affect the un-operated animal. Similarly, there can be no doubt as to the testicles forming an internal secretion. It is fair to assume that among other functions the implantation of the characteristics of the male, especially the aggressive characteristics, must be regarded as part of its duty. Otherwise it is difficult to account for the alteration in character which occurs as the result of complete castration, and the modifying change which follows from leaving some of the epididymis attached to the cord. The influence of the testicles on the growth of bone is recognized in man ; the long bones continue to grow, due to the delay in the ossification of the epiphyses ; the same is said to have been observed in animals. The effect of castration on the eating properties of flesh is well known. The influence on the thymus gland is very marked ; instead of disappearing at puberty, castration Digitized by Microsoft® 266 A MANUAL OP VETEEINAEY PHYSIOLOGY causes the gland to become larger and more persistent. The effect of removal of the testicles and ovaries, on the dog, cat, deer and birds, may be conveniently considered in the chapter on ' Generation and Development." The adaptation of the digestive fluids to the nature of the food has been referred to (p. 170). This and the influence of a fixed diet in producing a more effective digestive secre- tion, and the harm resulting from sudden changes in diet (p. 237), may possibly be regulated by a specific chemical excitant. These are matters of the highest practical im- portance in the feeding and management of animals. The chief lesson that the present work on internal secre- tions teaches is that an organ apparently functionless may be performing some office of the highest importance, while even those actively employed in the preparation of an obvious secretion may, in addition, be carrying out im- portant chemical activities — the liver, for example, with its external secretion of bile and its internal secretion of urea and glycogen ; the pancreas, with its digestive fluid, and its invaluable internal secretion, which regulates the destruction of sugar. Even the kidney, in all probability, possesses an internal secretion affecting metabolism. The spleen, on the other hand, would appear to possess neither an internal nor an external secretion, for it has frequently been removed without ill effects ; but the question must be dealt with in a little more detail. The Spleen, in spite of the numerous observations to which it has been subjected, is still a physiological enigma. Its vascular arrangement 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 intervention of even a capillary wall. The spleen contains a considerable amount of involuntary muscular fibre and is capable of movement. Thsse movements have been carefully studied, and it is established that they are of two kinds, a slow expanr sion which occurs after a meal followed by contraction, and Digitized by Microsoft® THE INTERNAL SECRETIONS 267 a rhythmical expansion and contraction 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 liberally supplied with motor nerves, and stimulation of these leads to a reduction in the volume of the organ. It is even believed that there may be nerves to the spleen, which 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 function is present 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 on this point 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 regarded as due to the haemoglobin of the destroyed red blood-cells. The theory is very plausible though by no means definitely proved ; at the same time there is great difficulty in getting away from the fact that 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 that of 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 our regarding uric acid as a special product of the spleen. Some physiologists have suggested that the spleen produces an enzyme which converts trypsinogen into trypsin. There is no reason why the spleen might not do so, but it by no means follows that this is normally its Digitized by Microsoft® 268 A MANUAL OF VETEKINARY PHYSIOLOGY function, nor would there appear to be any necessity for this action in face of the fact that it is one of the special functions of the intestinal juice. In connection with all these theories it is well to remem- ber that the spleen may be removed completely and no ill effects follow. Thyroid. — Some of the most interesting work on the ductless glands has been carried out on the thyroid, and it is largely to this body that such little knowledge as we as yet possess of internal secretion is mainly due. For years it had been observed that atrophy or absence of this gland in the human subject was associated with arrested development both mental and physical ; the man so affected remained a child both in , intelligence and appearance. This stimulated experimental enquiry, and the thyroids were removed in many animals, the majority of carnivora dying as the result, while half of the herbivora recovered from the operation. So contradictory were the results obtained by different observers on the gland and its uses, that the whole question was submitted to very close enquiry, which revealed the fact that the ordinary thyroid consists of two distinct portions, one part the thyroid proper, the other the parathyroids. In most animals much the same results are obtained when both parts are removed, but when the parathyroids alone are excised, death rapidly ensues, preceded by convulsions. The removal of the thyroid only gives rise to a train of symp- toms accompanied by chronic wasting, much slower in development than in the case of the parathyroids. Such are the broad lines of distinction between the two portions of the combined thyroid body. The colloid substance con- stitutes the internal secretion of the thyroid, but forms no part of the secretion of the parathyroids ; and histologically while the former consists of vesicles lined by a single layer of cubical epithelium, the parathyroid is composed of columns of epithelium-like cells. The gland contains a nucleo- proteid and colloid substance ; the latter is not a nucleo- proteid, and is remarkable for containing iodine in organic Digitized by Microsoft® THE INTERNAL SECRETIONS 269 combination with the proteid. The iodine- containing sub- stance is termed iodothyrin ; it is a brown amorphous material, containing phosphorus and 10 per cent, of iodine. As to the uses of these bodies little is known. That they produce an internal secretion which finds its way into the system by the bloodvessels or lymphatics is certain; it is probable that this secretion is mainly directed to the nutrition of the body, especially of the central nervous system. Some have considered that the gland produces a substance which neutralizes poisons formed during meta- bolism. The consensus of evidence is that an internal secretion is produced which is essential to the body. It appears beyond all doubt that when from any cause the gland fails to supply the secretion, the symptoms may be relieved by the administration of an extract of the thyroid body, or even by feeding the patient on the prepared gland. Thymus. — This body, composed of modified lymphoid tissue, is mainly of use in fcetal 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, as the process of atrophy is much slower in the castrated as compared with the uncastrated animal, while its early removal has been observed to be associated with a rapid growth of the testicles. 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. Like the thyroids 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 nothing is known of the function of the cortex, the medulla yields under experimental enquiry some remarkable and charac- teristic results. An extract of the medulla of the gland when injected Digitized by Microsoft® 270 A MANUAL OF VETEEINAEY PHYSIOLOGY into the blood causes a marked increase in blood pressure ; even extremely small doses produce this effect. If the vagi are intact the heart-beat is simultaneously slowed, if cut the beat is quickened. The active agent is known as adrenalin, and its effect on the circulation in causing constriction of the small vessels is so marked as to be turned to account in minor surgery. The result of the constriction of the vessels is a rise in blood pressure, and this is not necessarily central in origin, as it may be obtained after the constrictor centre in the spinal cord has been destroyed. Adrenalin acts upon all plain muscle and gland cells which receive sympathetic fibres, and it is distinctly noteworthy that the effects, whether they be augmentory or inhibitory, are identical with those produced by stimulation of the sympa- thetic 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 this gland is con- cerned in the provision of a substance intimately connected with muscular metabolism, especially ' tone,' not only of the skeletal muscles, but also of the muscular fibres of the circulatory system. There is also considered to be some connection between the adrenals and the sexual system. In rabbits the cortex of the gland becomes twice the normal thickness during pregnancy ; and it is believed that in man a connection exists between the adrenals and the growth of the body, the development of puberty, and sexual maturity. Very little is known of the function of the Pituitary Body. The part has been experimentally removed, and in such cases muscular weakness, twitchings, and a lowered tem- perature have been observed. The pituitary is closely allied to the adrenals in the effects on the circulation of extracts made from it, while in its general metabolic functions it is considered to be related to the thyroid. In man the singular disease acromegaly, characterized by an overgrowth of the bones of the face and extremities, is associated with disease of the pituitary body. Nothing is known of the uses of the Pineal Body. It is. regarded as the dorsal eye of a remote ancestor. Digitized by Microsoft® CHAPTEE IX ^ ^ THE SKIN It is obvious that one important function the skin performs is that of affording cover to the delicate parts beneath ; wherever the chance of injury is the greatest the skin is the thickest, while in those parts where sensibility is most required it is thinnest. The skin of the back, quarters, and limbs are good examples of the first type ; on the back especially a protective covering is found which, in some horses, is as much as a quarter of an inch in thickness : the face and muzzle are 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 irrita- tion ; flies will cause horses considerable suffering, and the elephant, with its thick hide, is quite as intolerant of these tor- mentors as is a well-bred horse. The skin 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. 63). The skin is a bad conductor of heat, and this is consider- ably 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 271 Digitized by Microsoft® 272 A MANUAL OF VETERINABY PHYSIOLOGY covering of the skin relieves the parts beneath from exces- sive sensitiveness ; through the sebaceous secretion it assists in preventing loss of heat, while the greasy covering 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. Hair. — Not all parts of the body are 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 Fig. 63. — Section of Mucous Membrane of the Horse's Lip, showing the nerve endings in the touch papilla. 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 considerably with the class of horse : the better bred the animal the finer the coat. Draught horses yield between 7 lbs. and 8 lbs. of mixed hair, dirt, and dandruff by clipping ; in a well-bred horse this would be reduced to 10 oz., or even less ; the amount of hair of the mane and tail is about 1| lbs. It is a well- known fact that, excepting the hair of the mane and tail, that of every other part of the body has only a temporary Digitized by Microsoft® THE SKIN 273 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 condition or stamina. The permanent hair is not entirely represented by that of the mane and tail, the eyelashes are permanent, also the long tactile hairs on the muzzle. The temporary hairs on the horse are of two kinds which can only be dis- tinguished by their rate of growth. If a part be clipped, or, preferably, Bhaved and the growth watched, in a short time it Fig. 64.— Section of Horse's Skin, showing the Casting Off of the Old Haie and Growth of the New. It will be observed that both are emerging from the same follicle. will receive a scanty covering of long rapidly growing hair, followed by a slow growth of ordinary hair. There is no difference in the two hairs excepting the length. The long rapidly growing hairs are known as 'cat hairs'; they are not numerous, being about 27 to the square inch, while the ordinary hairs are about 4,300 to the square inch.* The growth of the hair is regulated by the surrounding * I am'indebted to Major Newsom, Army Veterinary Corps, for the trouble he has taken in making this tedious calculation. 18 Digitized by Microsoft® 274 A MANUAL OF VETEEINAEY PHYSIOLOGY temperature ; if horses in the depth of winter are placed in a heated atmosphere, such as a horse deck on board ship, the majority commence to shed their winter coat in a few days, though the temperature of the outside air may be at freezing-point ; similarly, if taken from a warm to a cold locality the hair responds by becoming longer. Speaking generally the above statements are correct, but there are exceptions and modifications. Some horses do not shed their 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 only grows to a definite length. The full length having been attained 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, everything is present for the needful growth to occur, but there is a restraining influence present which determines the length of hair according to the season. Of the pigment in hair which gives colour to the coat our knowledge, until quite recently, has been of the scantiest kind. The active investigation now being carried out of Mendel's theories of heredity, when applied to the special case of heredity in coat-colour, made it essential 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.* Using the name in its generic sense, three different forms of ' melanin ' are found in hairs — black, chocolate, and yellow. Of these the black is extremely insoluble, and hence very difficult to deal with ; as also is the chocolate pigment, though to a less extent. The yellow, on the other * Florence M. Durham (Proc. Boy. Soc, vol. lxxiv., p. 310, 1904), and further researches as yet unpublished. Digitized by Microsoft® THE SKIN 275 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, which are, however, to be carried out shortly on a large scale, 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 haemoglobin, 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 (see p. 236) in such a way as to give rise to pigment substances. Prom the con- ditions 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 nomencla- ture 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. With the exception of black and grey horses which are liable to turn 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. 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 hence been introduced. * Loc. cit. 18—2 Digitized by Microsoft® 276 A MANUAL OP VETEEINAEY PHYSIOLOGY Of its value there can be no doubt ; it considerably reduces the risk of cold and 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 ' ; our observations have shown that this is due to the proteid lost by the skin, for, as we shall presently see, proteids are regularly found in the sweat of the horse. Clipping largely prevents this loss. The influence of clipping on temperature is dealt with in the chapter devoted to ' Animal Heat.' In some animals, as for instance the dog and 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 in- fluence of cold the hairs on the horse's body may become erect, but there is no indication of this under physical excitement, as in the case of the dog and cat. It is possible that the prescience of a coming storm or change of weather exhibited by cattle may probably 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 y stimulation of the hair-follicle nerves, and so gives rise to l an uneasiness which presages the coming change. 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 in the horse is the first place where sweating begins, the neck, side of chest, and back follow, lastly the hind- Digitized by Microsoft® » V THE SKIN 277 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 occasionally 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 on the general surface of the body. The sweating of the pig is confined to the snout. The Becretion 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 per- spiration.' 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 14 lbs. of water probably represent this loss in the horse for 24 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 amount of sweat secreted daily can only be roughly determined ; there are many conditions which affect it, such as the length of 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 perspiration. The mean amount of water evaporated daily by these two channels under different conditions of work was as follows : At rest - - - - 6-4 lbs Walking exercise - 8-6 „ At work walking - 12-7 „ Trotting - 13-4 „ At work trotting - 206 „ Digitized by Microsoft® 278 A MANUAL OP VETEEINAEY PHYSIOLOGY In each case the distance walked and trotted and the load 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 im- portant source of loss of heat; so marked is this 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 men exists also in the horse, viz., when the skin is acting freely less water passes by the kidneys, 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, which cdntains a pigment, chlorophyll; it possesses a peculiar horse-like odour, and has a specific gravity of 1020. We found horse's sweat to have the following composition :* Water - - 94-38 Containing. [Serum albumin - - 0105 Organic matters - 052 -'. „ globulin - - 0-327 [Fat - - - 0-002 (Consisting principally of potash, and soda, chlorides, some mag- nesia, a little lime, and traces of phosphates. The proteids are thus seen to be serum-albumin and globulin, and their constant presence has been determined by a number of observations ; the mineral matter is very 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 matted hair (which is due to albumin) is often seen covered with saline material, looking like fine * ' The Sweat of the Horse,' Journal of Physiology, vol. xi., 1890. Digitized by Microsoft® THE SKIN 279 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, whilst during work they are assisted by the skin* Urea is also probably present in sweat (see p. 283). It is difficult to see why horses should excrete albumin by the skin ; the loss thus produced accounts for the great reduc- tion of vitality and strength in animals which sweat freely at work, and for which clipping is the only preventive. Nervous Mechanism of Sweating. — A skin may sweat under quite opposite conditions, viz., both with a hot flushed skin and a bloodless cold skin, in others 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 and disproves at once any notion of sweating necessarily depending upon a congested 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. Though we are ignorant of the manner in which the nerves terminate in the sweat glands, still it is certain that there are special branches of nerves, whose function it is to determine the secretion of sweat, and these are quite distinct from those which regulate the vascular supply. If the peripheral end of the divided sciatic in the cat be stimulated the foot-pads sweat ; the proof that this reaction is a specifically nervous one is easy, apart from the fact that stimulation of the sciatic causes a violent constriction of the bloodvessels in the leg, for the sweating occurs when the leg has been cut off or the aorta tied, and it is absent under the influence of atropin. The effect of atropin on the sweat glands is very closely allied to its action on the salivary glands (p. 146) ; it paralyzes the secretory nerves which produce sweat. 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 as the increased secretion takes Digitized by Microsoft® 280 A MANUAL OP VETEEINAEY PHYSIOLOGY 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 utilize 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 pilocarpin. In this case the action is peripheral — that is to say, on the glands themselves — since it occurs when the sciatic nerves are cut previously 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 im- pulses 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 tempera- ture. These facts lead at once to the belief that ' sweat centres ' must exist in the central nervous system com- parable to those of the respiratory and vascular mechanisms, though they have not as yet been so definitely localized. 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 passes out of the cord by means of the rami communicantes of the sympathetic system, and so reaches the brachial and sciatic plexus 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 sweat- ing of the head and neck, limited to the side operated upon ; this may be due to vaso-motor paralysis, though a different interpretation has been placed on it, viz., that the sympa- thetic carries inhibitory impulses to the sweat glands of the Digitized by Microsoft® THE SKIN 281 head, so that on division the secretory fibres act without opposition. In the ox Arloing has shown- that division of the cervical sympathetic 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 wben the nerve degenerates, though even then the glands respond to pilocarpin. 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 stimulating the sciatic than in a limb kept cold, in which latter stimulation of the sciatic may produce no secretion whatever. Further, if a cat in which one sciatic has been divided be placed in a hot chamber profuse secretion will occur on the foot-pads of the limbs not sub- jected to interference, while on the side on which the sciatic has been divided no sweating occurs. This 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 dyspnoeie- condition of the blood and in many cases this may be so, but it is difficult to account for the profuse cold sweating in ruptures of such viscera as the stomach and intestines, or the localized 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 marked 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. 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 different; for instance in the horse pilocarpin produces, as in other, animals, a profuse salivary flow, but, unlike its action on man, the dog, and cat, it has no effect whatever in producing sweating. Digitized by Microsoft® 282 A MANUAL OF VETEKINAEY PHYSIOLOGY 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 4° Fahr. to 5° Fahr., and remains so for some time afterwards. Another peculiarity in sweating of the horse is the patchy perspira- tion observed occasionally, such as a wet patch on the side or quarter which dries slowly, or may remain for days or weeks in a wet or damp condition. 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. The changes occurring in the secreting cells of the sudo- riferous glands of the horse have been described by Kenault. When charged the cells are clear and swollen, the nucleus being situated near their attached ends ; when discharged they are smaller, granular, and their nucleus more centrally placed. 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 metamor- phosis. . The greasy material thus produced saves the epithelium from the disintegrating influence of wet, keeps the skin supple, and gives the gloss to the groomed coat ; from its greasy nature it assists in preventing the penetration of rain, and thereby saves to an extent undue loss of heat. Dandruff. — The material removed from horses by groom- ing consists of a white or grey powder which can readily be moulded by pressure into a dough-like mass. It consists Digitized by Microsoft® THE SKIN 283 of epithelial scales, fat, largely in the form of lanolin, colouring matter, salts, and a considerable amount of silica and dirt, the two latter depending upon the cleanliness of the animal. The amount of dandruff lost in an ordinary grooming varies from 20 to 60 grains for clean horses, and 170 to 200 grains for very dirty animals. An analysis of dandruff from the horse gave the following composition :* Water - - - 17-96 Fat - - - 12-40 Organic matter - - 56 - 22 containing 1'07 of urea. Ash - - - 13-42 „ 2-45 of silica. 100-00 The fatty matter in the skin proves to be lanolin, the same as that found in the fleece of sheep ; it explains the reason why horses living in the open should not be too freely groomed, and supports the prejudice which has always existed against this practice. It is evident that with free grooming the loss in fat alone is something con- siderable, and the animal exposed to chill. The amount of fat depends upon the diet ; on hay alone there is very little in the dandruff, whilst 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 to be chlorophyll, which has undergone modification by passing from the digestive canal to the skin. The use of this pig- ment is unknown, in fact, the horse is the only vertebrate in which chlorophyll has so far been found as a constituent of any cutaneous 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 eyelid secretions are also of a sebaceous nature. > In the sheep a considerable quantity of fatty substance * ' Dandruff from the Horse, and its Pigment,' Journal of Physi- ology, vol. xv., 1893. Digitized by Microsoft® 284 A MANUAL OP VETEEINAEY PHYSIOLOGY 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 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 weather-exposed sheep 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 nithan the whole body of the shorn sheep (Warrington). ' '»- Respiratory Function of the Skin. — Certain vetebrates 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 animals and men have demonstrated that similar changes occur through the skin, but on a very small scale. Varnishing the skin rapidly causes death 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 supposed. Bouley* states that horses shiver when varnished, and the surface of the body and the expired air become colder, the visible membranes respond by becoming violet in tint, and the animals die after several days. According to Ellenberger, if only partly varnished they do not die, but exhibit temporary loss of temperature, 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. 258. Pathological. The chief, pathological conditions of the skin are those due to parasitic invasion; they may produce widespread disease in all animals. * Colin's ' Physiologic.' Digitized by Microsoft® CHAPTEE X THE UEINE The urine is sometimes spoken of as a secretion, but this is not strictly correct; speaking broadly, we may say a secretion is something which is formed in a part for the purpose of being eventually utilized 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 being no longer required, and the retention of which would be harmful. This removal is effected by the kidneys, which may in a sense be regarded as the niters of the body, regulating the composition of the blood by removing from it waste and poisonous products, and maintaining, as will be later explained, its proper degree of alkalinity. In consequence of the discoveries which have been made of internal secretions, physiologists have forecast that the kidneys may yet be shown to take some important part in nitrogenous metabolism. This forecast is based on the fact that in the dog, with only one-quarter the normal amount of kidney substance left, double the normal amount of urea is excreted. We have seen how both nourishment and waste materials are poured into the circulation, and have studied several of the channels by which the latter are removed, viz., by the lungs, skin, and intestinal canal ; we have now to examine the last excretory path, viz., the kidneys. 285 Digitized by Microsoft® 286 A MANUAL OP VETEEINAEY 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 pressure 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 -Boy's oncometer. An oncometer is a metallic capsule in which the living kidney is enclosed, and so arranged that the expansion and collapse of the organ can readily be detected. A tracing given by the use of this instrument shows that the volume of the kidney is affected by every beat' of the heart,' and even the re- spiratory undulations in the blood-pressure. / Structure of the Kidney.- — The kidney 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 balls of capillarieB, the glomeruli, derived from the renal artery ; the artery entering the Malpighian tuft is larger than the vein leaving it, the result is that a high blood-pressure is maintained in the glomerulus. The vessel which supplies these tufts also sends branches to form a plexus around the uriniferous tubules ; these branches do not enter the Malpighian body. The whole glomerulus is contained in a capsule in which it is suspended by its afferent and efferent vessel ; when the vessels are dilated the tuft fills the capsule, when they are collapsed there is a space between them (Fig. 65) . Digitized by Microsoft® THE URINE 287 The minute vein or efferent vessel leaving the tuft breaks up into capillaries around the uriniferous tubule; thus the blood in the plexus of capillaries 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 by cells resembling the epithelioid plates seen in capillaries ; they are flat polygonal cells containing a nucleus. The capsule is practically the dilated beginning of a uriniferous tubule, and the latter is continued from the Fig. 65. — Diagram showing the Eblation of the Malpighian Body to the Uriniferous Tubules and Bloodvessels (Kirke, after Bowman). a, An interlobular artery ; a', branch of artery passing into the glomerulus ; c, capsule of the Malpighian body forming the com- mencement of, and continuous with t, the uriniferous 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. capsule, taking a course of extraordinary complexity in order to reach the pelvis of the kidney ; further, the cells found in the tubule are no longer the flat polygonal cells of the capsule, but a something special to the tubule and even to different parts of it. If we briefly follow the course of a uriniferous tubule (Fig. 66), it is found that on leaving the capsule it becomes twisted in the cortex forming the convoluted tube; it then forms a spiral tube, and leaving the cortex runs straight into the medulla, forming the descending limb of Henle ■, Digitized by Microsoft® 288 A MANUAL OF VETEKINAEY PHYSIOLOGY Fig. 66. — Diagram of the Course of the Uriniferous Tubules (Klein and Noble Smith). A, Cortex of kidney ; a, subcapsular layer not containing glomeruli ; a', inner structure of cortex also without glomeruli ; B, boundary layer of medulla; C, papillary part of the medulla ; 1, Bowman's capsule of the glomerulus ; 2, neck of capsule ; 3, proximal convo- luted tube; 4, spiral tube ; 5, descending limb of Henle; 6, loop of Henle ; 7, thick part of ascending limb; 8, spiral part of ascending limb ; 9, narrow ascending limb in the medullary ray ; 10, the irregular tubule ; 11, distal convoluted tube; 12, curved collecting tube ; 13, straight collecting tube ; 14, collecting tube of boundary layer ; 15, large collecting or discharging tubule of papillary layer. Digitized by Microsoft® THE UEINE 289 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 character 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 resembles 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 tube ; 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 throughout ; broadly, they may be divided into a striated cell staining readily, and a clear transparent cell staining with difficulty. The first epithelium is sugges- tive of secreting cells, the latter, on the other hand, possesses more the characteristics of the epithelial lining of ducts. The amount of blood passing through the kidney is something very considerable ; it has been calculated that in 24 hours 146 lbs. of blood will pass through the kidneys of a dog weighing 66 lbs. Vascular Mechanism. — The vascular arrangements of the kidney are under the control of a rich supply of vaso-con- strictor nerves, while 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, such for instance as when the vaso-constrictor nerves are stimu- 19 Digitized by Microsoft® 290 A MANUAL OP VETEEINAEY PHYSIOLOGY lated, 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 vaso-motor influence over the kidney is of the greatest importance, and largely regulates the amount of urine manufactured. If the renal vein be obstructed, the pressure of blood in the kidney rises, but no urine is secreted ; evidently therefore an increased flow of blood through the kidney is as essential to secretion as is increased blood-pressure. Two theories of urinary secretion have hence been put forward, one being based on the physical conditions which are favourable in the kidney to filtration, while the other is based on the supposition that the cells are secretory. It is obvious that there are two portions of the kidney engaged in the manufacture of urine, viz., the glomerular and the tubular. In the former the conditions for filtration from the bloodvessels of the tuft into Bowman's capsule exist, yet the experiment of obstructing the renal vein, referred to above, has impressed on physiologists the influence of the activity of the endothelial cells of the glomerulus, for if filtration pure and simple could obtain water from the Malpighian tufts, more urine should have been secreted immediately, though not continuously, after ligaturing the renal vein than before. As a matter of fact we know that secretion ceases. The evidence of secretory activity in the tubules of the kidney is based on the following experiment. If sulph- indigotate 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 but the Malpighian bodies are found stained blue. In order to determine what portion of the tubule excretes the dye it is Digitized by Microsoft® THE UEINE 291 necessary to stop the secretion in the glomeruli, otherwise the dye gets carried through the whole length of the tubule. In order to stop glomerular secretion the spinal cord is divided in the neck, the blue colouring matter injected, and the kidney examined. The blue is now found in the cortex only, and within the striated epithelial cells of the first and second convoluted tubes, where the indigo may be seen in granules. From this experiment it is clear that the cortical tubules elected to turn out the indigo, while the medullary tubules were unable to effect this, from which it is judged that a specific secretory activity of these cells 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. Stating these points briefly in connection with secretion they 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 in the tubules the organic matter is excreted as the result of a distinctly secretory activity of their cells. These substances are carried along by the fluid which trickles down the tubules into the pelvis from the kidney and so becomes urine. Under pathological conditions the glomeruli admit of the exit of both albumin and sugar. The secretion of proteid in the tuft and its reabsorption in the tubule was at one time believed to be true, but inas- much as no proteid 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. There are no secretory nerves to the kidney ; the in- fluence of the nervous system is confined to its action on the bloodvessels. The action of diuretics has been studied in connection with the question of urinary secretion, and most observers find that though these determine a greatly increased flow of blood to the kidneys, yet they also exert a directly stimulating effect on the secretory cells. The function of the cells of the tubules does not end with the removal from the blood of the substances presented to 19—2 Digitized by Microsoft® 292 A MANUAL OP VETEEINARY PHYSIOLOGY them ; they are also capable of originating material on their own account. Thus the union of glycine with benzoic acid, resulting in the formation of hippuric 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 providing 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 transformations may occur in the cells leading to the production of colour- ing matters, etc., our knowledge of which is at present obscure. Jj to The Composition of the Urine depends upon the class of 7 animal ; in all herbivora, with certain minor differences, the urinary secretion is much the same : not so with omnivora or carnivora which possess a distinctive urine, especially the latter. When herbivora live on their own tissues, as during starvation, they become carnivora and their urine alters completely in character, corresponding now to the urine of flesh feeders ; the young of herbivora, if still sucking, have a urine possessing much the same characteristics as that of carnivora. But apart from this general statement, it is necessary to point out that in animals of the same class the composition of the urine may vary within very wide limits, depending upon several causes, of which diet is, perhaps, the most important. Urine consists of : Water. {Nitrogenous end-products : urea, uric acid, hippuric acid, creatine, creatinine. Aromatic compounds : benzoic acid, ethereal sulphates of phenol, cresol, etc. Colouring matter and mucus. Salts - - /Sulphates, phosphates, and chlorides of sodium, I potassium, calcium, and magnesium. The Nitrogenous Substances taken up into the blood, either from the disintegration of proteids in the digestive canal or from the metabolism of the tissues, supply the total nitrogen of the urine. A distinction is made between the Digitized by Microsoft® THE UEINE 293 nitrogen from without, viz., that supplied by the food, and the nitrogen from within, viz., that from the tissues, and this is more especially of interest in connection with urea and uric acid. The total nitrogen of the urine consists of : 1. Urea nitrogen. 2. Uric acid nitrogen. 3. Ammonia nitrogen. 4. Creatinine nitrogen. Speaking generally, the nitrogen varies directly with the amount of proteid taken as food. // ; , Urea. — It is by no means decided how urea is produced. It must presumably arise from the disintegration of pro- teids, derived either from proteid food or proteid tissues. As the result of their destruction it is extremely probable that ammonia compounds are formed which are discharged into the blood, and are then subsequently converted into urea in some organ, which is probably the liver. Some suppose that the proteids undergo hydrolytic cleavage with the formation of amido-bodies, such as leucine, tyrosine, aspartic acid, glycocoll, etc., and that these bodies undergo oxidation in the tissues yielding ammonia, carbonic acid, and water. The ammonia and carbonic acid unite to form ammonium carbamate, which is carried to the liver, and by the loss of a molecule of water is readily converted into urea. The oxidation of the amido-bodies is essential as a preliminary step towards urea, in order to get rid of some of the carbon they contain ; in amido- bodies this is in excess of the nitrogen, whereas in urea the reverse is the case. It seems fairly clear that the nitrogenous waste leaves the muscles as ammonia com- pounds, and in this form the nitrogen of the proteid food may be found in the portal vein, the blood of which contains three to four times as much ammonia compounds as does arterial blood. If the blood of the portal vein be experi- mentally compelled to pass into the posterior vena cava without circulating through the liver, the ammonia com- Digitized by Microsoft® 294 A MANUAL OF VETEEINAEY PHYSIOLOGY pounds in the arterial blood become equal in amount to those in the portal blood. The ammonia in the blood is considered by some to be in the form of carbonate ; by others, and perhaps more generally, as carbamate, though ammonium carbamate is readily produced from the car- bonate by the loss of one molecule of water. The sub- sequent disposal of the ammonia compounds is evidently by means of the liver, this gland standing between the portal and systemic circulation, and converting the poisonous ammonia compounds irjto the less poisonous urea. In the urine the urea exists in a free and uncombined Fiq. 67. — Crystals op Nitrate of Urea (Fonke). state, though it is capable of forming salts with acids (Fig. 67). It is a substance very soluble in water. The proportion of urea in urine varies dependently on the nature of the diet. As a rule the larger the amount of nitrogen in the food the more urea excreted, but this is not invariable, for some observers have stated that on a diet consisting principally of hay more urea is excreted than on one of oats and hay. Urea 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 excreted than during rest. Digitized by Microsoft® THE UEINE 295 Judging from our observations on the horse, great varia- tion in the amount of urea may be met with even when the conditions as regards diet, rest and work are identical. It is probable that this applies also to other animals. The percentage of urea present in urine may broadly be stated to vary between 3 and 4 per cent., but it is obvious that the percentage present is influenced by the total Becretion for the twenty-four hours. If this is small in amount, the percentage is higher than when an average production of water occurs. Creatinine has been regarded as another source of urea, but the physiological history of this substance is imperfectly known. In flesh-feeding animals part of it, no doubt, is derived from the food, while another portion is produced within the body, probably originating from the metabolism of muscle. Though the conversion of creatinine into urea may be brought about as a laboratory process, there is no definite proof that the conversion occurs in the body ; if creatine be injected into the blood it does not lead to an increase of urea but of creatinine. It is possible that under physiological conditions creatine before it leaves the muscles may undergo a further change, being decomposed into urea and sarcosine, the latter passing to the liver and there being converted into ammonium carbonate and subsequently into urea. IJT Uric Acid. — The origin of uric acid is not clearly deter- ~3 mined in mammals. In birds it is known to be formed in the liver from ammonia compounds, and probably from lactic acid. In 'mammalia it is known that in herbivora the amount of uric acid is extremely small, or this substance may be even entirely absent ; in carnivora and omnivora it is present, though only in a small proportion of the total nitrogen excreted. The influence of diet in flesh feeders is very marked, meat causing a rise in the uric acid output, while cellular organs, such as liver and sweetbreads, produce a still greater rise ; this fact has afforded a clue to the probable origin of uric acid in the body, viz., from the nucleo-albumins and nucleins, both of which largely Digitized by Microsoft® 296 A MANUAL OF YETEEINAEY PHYSIOLOGY exist in the cellular organs. Pathologically there is an increase in uric acid in the disease known as leuco- cytheemia, in which a great increase in the white blood- corpuscles occurs. These corpuscles contain a quantity of nuclein, and the work of Emil Fischer has shown the close chemical relationship between the nitrogenous bases so easily obtained by the decomposition of nuclein — the purin * bases — and uric acid. The purin bases are hypoxanthine, xanthine, and adenine, and from these uric acid may arise by the process of oxidation. For example, one atom of oxygen allied to purin (C 5 H 4 N 4 ) gives rise to hypoxanthine (C 6 H 4 N 4 0) , two atoms of oxygen to xanthine (C 6 H 4 N 4 2 ), and three atoms of oxygen added to purin lands us finally in uric acid (C 6 H 4 N 4 3 ). If hypoxanthine or uric acid be given to dogs the uric acid is not increased ; if adenine be given the animal dies from suppression of urine and crystals of uric acid block the renal tubules. It is evident that very little is known of the subject of uric acid formation. Even the seat of production is not definitely ascertained, though the liver will probably be found to play no small part in the process, as it is already known to do in the case of birds. The spleen has been pointed to as a probable seat, though possibly it is not relatively more so than other lymphoid tissues. As previously stated, the production of uric acid is affected by diet, being largest on animal food and smallest on vegetable. The acid is therefore present in the dog fed on meat and in the pig, but entirely absent, so far as our observations go, in the horse in health, and probably in all herbivora unless still suckling at the mother. It is important to note that during sickness, especially when there is fever and the animal is living on its own tissues, uric acid may be readily found in the urine of herbivora. The explanation is simple : the animal for the time being is practically carnivorous. * Purin is the name given by Fischer to the nucleus common to the uric acid group of substances from which, by simple transformations, the several members of the group may easily be obtained. Digitized by Microsoft® THE UEINE 297 Uric acid does not occur free in the urine, but in com- bination with soda and potash. Its crystalline formation is shown in Fig. 68 ; it is a substance very insoluble in water, but soluble in alkaline solutions. The ammonia salts present in urine are an index to the neutralization of acids in the body. The acid substances are produced as the result of metabolism ; when they are in excess there is an increase in the ammonia of the urine, the formation of ammonia in the muscles being the natural protection of the body against acid poisoning. When, as occurs in herbivora, there is already an excess of alkali in Fig. 68.— Crystals of Uric Acid (Funke). the diet, a sufficiency of bases is present to neutralize the acid, and ammonia is absent from the urine. With flesh feeders the amount of ammonia is kept at a minimum owing to 'its poisonous nature; on a vegetable diet it all disappears from the urine, being converted into urea. The injection of dilute mineral acid into the veins of a dog does not alter the reaction of the blood, but the ammonia is increased as a natural protection and appears in the urine, with a resulting decrease in the urea. A similar injection of dilute mineral acid in herbivora reduces the alkalinity of the blood, after having used up the store of vegetable alkaline salts. In consequence of the reduced alkalinity the carrying power of the blood for carbon Digitized by Microsoft® 298 A MANUAL OF VETERINARY PHYSIOLOGY dioxide is reduced ; it is retained in the tissues, and gives rise to symptoms which may prove fatal. If ammonium carbonate be given by the mouth, it does not appear as such in the urine, but as urea. Hippuric Acid. — This acid, characteristic of the urine of the herbivora, may arise in two or three different ways. It is known that hay, grass, and grains, contain in their cuticular covering a substance which yields hippuric acid in the body ; if these foods be extracted with caustic potash the hippuric-acid-forming substance is removed, and if animals are fed on forage so treated no hippuric acid is formed in the body ; even, it has been said, if the husk be removed from grain the latter is incapable of giving rise to hippuric acid. The chief source of hippuric acid in the herbivora is from the above hippuric-acid-yielding body. Benzoic acid is derived from various aromatic combinations contained in plants, and this combined with glycocoll, derived from the decomposition of proteid substances, yields hippuric acid. The synthesis occurs in the kidney, and is brought about by the cells of the gland in conjunction with the oxygen of the red corpuscles. Outside the body the synthesis may be produced by using ground-up kidney tissue mixed with blood, and kept at the body temperature. It is probable that the active agent in the synthesis is an enzyme. A second source of hippuric acid is the aromatic (benzoic) products formed in the intestinal canal as the result of the putrefaction of proteids ; lastly, it is believed that hippuric acid may be formed from the aromatic residues of tissue proteids. Hippuric acid never exists in the free state in the urine, but either as hippurate of lime or potash, probably the former. Crystals of hippuric acid are shown in Figs. 69 and 70. The amount of hippuric acid excreted varies with the diet ; it is increased by using meadow-hay and oat-straw, and decreased by using clover, peas, wheat, oats, etc. ; as the urea rises the hippuric acid falls. Liebig many years ago stated that benzoic acid was Digitized by Microsoft® THE UEINE 299 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 found in the urine of horses at rest — in fact, the reverse of Liebig's view. Owing to its easy and Fig. 69. — Crystals of Purified Hippuric Acid (Funke), Fig. 70. — Crystals of Impure Hippuric Acid. rapid fermentative decomposition hippuric acid is rarely to be found in urine twenty-four hours old ; in fifty-four specimens we only found it eight times. This decomposi- tion may be prevented by the addition of a slight excess of milk of lime, and then boiling the freshly voided urine. Digitized by Microsoft® // 300 A MANUAL OP VETEEINAEY PHYSIOLOGY .27 2 Benzoic Acid is the antecedent of hippuric. As just mentioned, it is derived from the benzoic - acid - forming substances in vegetable food ; its crystalline formation is show in Pig. 71. Sulphuric Acid in the urine of carnivora and omnivora is almost wholly derived from the decomposition of proteid bodies undergoing digestion, and its amount is employed as a measure of proteid disintegration in the system. The sulphur is derived from the sulphur of the proteid body, and the acid is united with indol, phenol, skatol, cresol, all of Pig. 71.— Crystals of Benzoic Acid. which are products of proteid disintegration in the intes- tinal canal. In herbivora indol, phenol, and skatol may be derived from the benzene compounds in food, so that the amount excreted is no measure of proteid disintegration. Phenol, skatol, and cresol are poisonous bodies ; part of them are got rid of by the faeces, part are absorbed into the blood, and after oxidation are conjugated with sulphuric acid and eliminated by the urine. By this conjugation the poisonous aromatic compounds are rendered harmless. From the conjugation between indol and sulphuric acid indican is produced, which may be made to yield indigo, a substance common in the urine of herbivora. From the conjugation between phenol and sulphuric acid a colouring Digitized by Microsoft® THE UEINE 301 matter is formed which is found in stale urine; phenol- sulphuric acid undergoes oxidation in the presence of the air, and yields pyrocatechin, to which the brown colour in the stale urine of the horse is due. Some of the indol and skatol may be united with gly- curonic acid, a substance co-related to dextrose, and often present in the urine of the dog. It exerts a reducing action on salts of copper. t Oxalic Acid in combination with lime is constantly found in the urine of herbivora ; its deposit presents a character- istic microscopical appearance (Fig. 72). In dogs it has been produced in considerable quantity by feeding on uric acid ; its origin in the herbivora is doubtless from the oxalates contained in the food. The Colouring Matter of the urine is not yet completely worked out. The chief substance is urochrome ; this is probably an oxidation product of urobilin, as on suitable treatment a pigment is obtained which giveB a spectrum identical with urobilin. Urobilin is not found in normal urine, but there is present a chromogen, or another substance, which yields urobilin. The origin of urobilin is from bile pigment ; the stercobilin formed in the in- testines is identical with it. 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 from the food taken into the body, but mainly from metabolic processes occurring in the tissues. The nature and amount of the salts vary with the class of animal and the character of the food. In the urine of the horse 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 been found that in ruminants the calcium salts are mostly excreted with the fasces, whereas in the horse they principally pass through the kidneys. It is certain that phosphates, which form such a prominent feature in the urine of carnivora and Digitized by Microsoft® 302 A MANUAL OP VETEKINAKY PHYSIOLOGY 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 both suspended and dissolved lime exists ; the former increases with the age of Fig. 72. — Crystals of Oxalate of Limb (Funke). Fig. 73. — Crystals of Carbonate of Lime (Funke). the urine, owing to the development of ammonia, until nearly the whole of the lime is precipitated. The lime exists in combination with oxalic, carbonic, hippuric, and sulphuric acids ; all these combinations do not necessarily exist in one specimen of urine, the salts formed depending Digitized by Microsoft® THE UKINE 303 on the varying relative amounts of the acids formed in metabolism. The amount of lime in the food does not influence the elimination through the kidneys, but more lime is found in the urine of horses at work than of those at rest. Oxalate and carbonate of lime crystals are common microscopic deposits in the urine of the horse (Figs. 72 and 73). Under any condition the urine of a healthy horse is turbid from suspended lime ; this may be got rid of on the addition of acid with profuse evolution of gas, while a clear transparent urine results. Magnesium in the urine is also suspended and dissolved, the amount which is suspended being increased by the ammonia generated in the urine on standing. Potassium exists largely in the urine of herbivora, derived from the potash of the food ; it forms numerous combina- tions, 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 of those at work, which is explained by the considerable amount of potassium excreted with the sweat. Sodium only exists in the urine of herbivora in small quantities, which is due to the fact that very little sodium is found in vegetable food. Sulphuric Acid in its organic combinations has been dealt with previously ; the inorganic sulphur is combined with alkalis as ordinary salts. Chlorine is supplied by the chlorides of the food. The proportion 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 85J grains of common salt. Salkowski places it much higher, viz., about f oz. daily. Phosphoric Acid, though existing largely 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 marked, but never con- siderable. Work does not influence its production. In the urine of carnivora the phosphates are an important con- Digitized by Microsoft® 304 A MANUAL OF VETEEINAEY PHYSIOLOGY stituent. They exist in the urine in two forms, viz., alka- line 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. 74). The phosphates are derived from the food and tissues. According to Munk, if there is an abundance of lime salts in the diet, as in vegetable food, the phosphates are not eliminated to any extent by the kidneys, for the reason that they combine in Fig. 74. — Crystals of Triple Phosphate (Funke). the intestinal 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. Ammonia. — 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 decomposes and carbonate of ammonium is largely formed. The Reaction of the urine of herbivora is alkaline, the Digitized by Microsoft® THE UEINE 305 alkalinity being due to carbonate of potasb. Tbe urine of all vegetable feeders is alkaline, owing to tbe excess of alkaline salts of organic acids contained in tbe food, sucb as malic, citric, tartaric and succinic. During tbeir passage tbrougb tbe body these salts are converted into carbonates, and appear as such in the urine, where they produce con- siderable effervescence on tbe addition of an acid. The nature of the food influences the reaction, for if hay be withheld from the diet, the urine of the horse may be rendered acid by feeding entirely on oats ; this is probably due to the formation of acid phosphates from the food. A considerable quantity of the alkalinity present in the stale urine of the horse is due to the exceedingly rapid fermenta- tive change which occurs in it on standing, leading to the breaking up of part of the urea and the formation of ammonium carbonate. In the dog the urine is acid, due to the acid phosphate of soda, and ,not to any free acid ; no free acids exist in the urine of any animal. In the pig the reaction is either acid or alkaline, depending on the diet : an animal diet producing an acid and a vegetable diet an alkaline urine. TJrine of the Horse. j o Specific Gravity. — This varies considerably dependently on the diet and the amount of dilution. The mean of a large number of observations was 1036, tbe highest regis- tered was 1050 and the lowest 1014. The Quantity of urine is liable to very considerable varia- tion depending on the season and tbe diet; the more nitrogen the food contains the larger the amount of water consumed and the greater tbe bulk of urine excreted. The mean of a large number of observations was 8| pints (4 - 8 litres) in 24 hours, the diet being moderately nitrogenous, but in individual instances very much more than this may be met with, viz., 12, 15, or even 20 pints (ll - 3 litres). Horses at work excrete less urine than those at rest, probably owing to the loss by the skin. In winter, owing 20 Digitized by Microsoft® -Jl 806 A MANUAL OF VETERINAEY PHYSIOLOGY to the reduced action of the skin, more urine is excreted than during summer. The Odour of urine is said to be due to certain aromatic substances 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 sus- pended carbonate of lime and magnesia it contains ; as the urine cools, particularly if it undergoes ammoniacal fer- mentation, the amount of turbidity becomes intense. The Consistence of the fluid depends upon sex, and per- haps 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 it as thick as linseed-oil, and very rare to find it fluid and watery. During oestrum the urine is of the consistence of oil. On a diet of oats and no hay, we have seen the urine so mucinous as to pour like white of egg- The Colour of urine is yellow or yellowish-red, rapidly turning to brown, the dark tint commencing on the surface of the fluid and gradually travelling into its depth. The cause of the colour on standing is due to the oxidation of pyro-catechin (see p. 301). The Total Solids consist of organic and inorganic matter, of which on a mixed diet 5 ozs. are organic and 3 ozs. inorganic ; the quantities are liable to great variation, sometimes being found greatly in excess of that mentioned. The total solids are considerably affected by the diet ; E. Wolff* found that when he reduced the hay and in- creased the corn ration the solids in the urine decreased, whereas on a diet consisting principally of hay and but little corn the solids increased. The composition of the mineral solids is given in the * Ellenberger. Digitized by Microsoft® THE UKINE 807 following table by Wolff. In every 100 parts of salts there are found : Potassium - - - 36-85 >er cent Sodium - - 3-71 ij Calcium - - 21-92 ji Magnesium - - 4-41 >) Phosphoric acid - — Sulphuric 5 J - 17-16 >> Chlorine - - - 15-36 >i Silicic acid . ■32 »» In the following table are given the results obtained by us in the examination of the twenty-four hours' urine of horses at rest and work :* Best. Work. Quantity 8'69 pints 7-88 pints Specific gravity - 1036 1036 " Total solids ... 811 ozs. 8-19 ozs. Organic solids 5-15 „ 5-37 „ Inorganic solids - 2-94 „ 2-82 „ Urea - 3-47 oz Ammonium carbonate as urea - •46 „ Ammonia ■09 „ •19 „ Benzoic acid •23 „ Hippuric acid - •55 „ Phosphoric anhydride - •04 „ •06 „ Sulphuric ,, ■37 „ •54 „ Other sulphur compounds •26 „ •27 „ Chlorine - - - - 1-12 „ •77 „ Calcium oxide - •12 „ •06 „ Magnesium oxide •10 „ ■09 „ Potassium ,, 1-29 „ •95 „ Sodium „ •09 „ •06 „ Salkowskit examined the urine of the horse, and gives the following as the compositi on of one specimen : Water - - 3-5 pints Phenol - - 37-89 grains Organic solids - 6'25 ozs. Organic sulphur - 208-69 „ Ash - - 1-60 „ Inorganic n - 85-77 „ Urea - - 3'25 „ Phosphoric acid - 3-40 „ Ammonia - - 5'53 grains Lime - - 88-50 „ Hippuric acid - -49 oz. Sodium chloride •87 oz. * ' Chemistry of the Urine of th 3 Horse,' Proceedings of the Boyal Society, vol. xlvi., 1889. f Ellenberger's ' Physiologic. ' 20—2 Digitized by 1 Microsoft® 808 A MANUAL OP VETEEINAEY PHYSIOLOGY In the following summary of the urine of animals other than the horse, the main facts are those given by Tereg.* u £ The Urine of the Ox. n J The urine of the ox is much the same as that of the horse, excepting that it is secreted in larger quantities, 10 to 40 pints ; the difference mainly depends upon the amount of nitrogenous matter in the diet, for it has been shown that the more nitrogen a diet contains the larger the amount of water consumed. The fluid is clear, yellowish, and of an aromatic odour ; it is of a lower specific gravity than that of the horse, 1007 to 1030 (in milch, cows, according to Munk, 1006 to 1015), owing to the larger amount of water secreted. The nitrogenous matter found in the urine is mainly represented by urea and hippuric acid, and the amount varies according to the diet. 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 urea is high, 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 on leguminous straw, whilst a medium amount is produced by feeding on hay. The urine of ruminants contains less aromatic sulphur compounds than that of the horse, and more of the in- organic sulphur ; like the horse, the phosphates are either absent or only occur in small amounts. The following table by Tereg shows the composition of the urine of the ox on different diets ; the observations extended over four months : lbs. lbs. lbs. lbs. Total quantity of urine - 26-02 31-17 2998 18-32 „ „ dry matter - 1-71 151 1-40 1-14 „ „ ash - - -88 1-01 1-03 -66 * Ellenberger's ' Physiologie.' Digitized by Microsoft® THE UEINB 309 Calves still suckling excrete an acid urine which is rich in phosphates, uric acid, creatinine, and a peculiar sub- stance known as allantoin ; it is poor in urea, and, according to Moeller, contains hardly 1 per cent, of solids. The Urine of the Sheep. This has an alkaline reaction, a specific gravity 1006 to 1015, and the amount excreted varies from "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 : The inorganic matter contained : Urea 2-21 Chlorine - - 1-05 Hippuric acid 3-24 Potassium chloride - 1-84 Ammonia - •02 Potassium - 2-08 Other organic substances - 2'07 Lime - -07 Carbonic acid ■42 Magnesia - -20 Phosphoric acid - - -01 7-96 Sulphuric „ - -24 Silica - -07 5-56 In sheep urea and hippuric acid stand 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 hippuric acid in the horse is old meadow hay, whilst new meadow hay has this effect on sheep. It will be observed from the table how rich the urine of the sheep is in hippuric acid. In sheep there is very much more magnesia than lime in the urine, consequently the reverse obtains in the fseces of this animal. o > '\ v The Urine of the Pig. This resembles that of carnivora, but its composition depends on the character of the food. The specific gravity is 1003 to 1025, Tt is either acid or alkaline; the Digitized by Microsoft® 310 A MANUAL OF VETEKINAEY PHYSIOLOGY amount excreted varies between 2\ to 14 pints, and it contains uric acid, hippuric acid, xanthine, guanine, and much urea. In the following analysis of the urine the diet consisted of peas, potatoes, and sour milk : Total urine - - - 7 pints Sp.gr. - .- - 1018 Dry substance - - 2-768 per cent. Total nitrogen - - "604 „ Ammonia - - • '024 „ Ash- - - - 1-234 The ash largely consists of phosphates and potassium salts, a moderate amount of magnesium, and very little sodium or calcium. The Urine of the Dog. It is impossible to give the composition of the urine of the dog, as the amount of constituents secreted varies considerably in dependence upon the nature of the diet. 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 f 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 1060 depending on the diet ; the colour is pale yellow to straw yellow ; the urea varies from 4 per cent, to 6 per cent. On an animal diet uric acid is excreted, but dis- appears on giving vegetable food; hippuric acid in small quantities appears with fair regularity ; indican and phos- phoric acid are well-marked constituents, and a substance known as glycuronic acid may be found which exercises a reducing action on salts of copper. The presence of bilirubin in the urine of the dog has been noted by Salkowski (see p. 221). As an illustration of the variation of the dog's urine dependently on the nature of the diet, we may take an example from a long series of experiments by Bischoff and Voit. Digitized by Microsoft® THE UEINE 311 On a diet consisting of meat "57 lb., starch *71 lb., salts 77'5 grains, a specimen of urine gave the following composition : Amount - - - -44 pint Sp. gr. 1049 Urea - - - 326'6 grains Salts - - - 85-6 „ On a diet consisting of meat 2 - 75 lbs. and fat "55 lb., the following was the composition : Amount - - - 1-23 pints Sp. gr. 1054 Urea - - - 1,351 grains Salts - - 189 „ Glycuronic acid exists only in traces, but after the administration of camphor or chloral it is obtained in well-marked quantities. It is a point of practical import- ance to avoid regarding urine which reduces salts of copper as necessarily containing sugar (see p. 301). The Discharge of TTrine. — The urine is constantly being secreted, and it either trickles down or is propelled down the ureters to the bladder by rhythmic muscular contrac- tions. It is quite likely that both movements are employed depending upon the condition of bladder distension ; whereas ' trickling ' is suitable for an empty bladder, some muscular effort on the part of the ureters would be required when the bladder was full. Either drop by drop or by ' spirtB ' the urine enters the bladder, which gradually advances in the pelvis, and rises up so as to touch the rectum. 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 strain the tighter are the ureters closed. If circumstances prevent the evacuation of the bladder con- tents, 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 Digitized by Microsoft® 312 A MANUAL OE VETEKINARY PHYSIOLOGY into the cavity, the weight of the fluid having a tendency to cause the organ to incline towards the floor of the abdomen. 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. Bladder pressure pro- duces a desire to evacuate, an act which may be a purely reflex one, as in the case of the dog, with its spinal cord divided far forward, or, what is more common, as a voluntary act in obedience to the summons issued by the bladder wall. Physiologists are not agreed as to how the act of micturition is carried out, but through the bladder wall impulses are transmitted to the cord, resulting in a con- traction of the organ and a relaxation of the sphincters, though there is some difference of opinion as to this. At the moment the bladder wall begins to contract, it is assisted by the abdominal muscles and a fixed diaphragm, and the flow is never as powerful in the female as in the male, the final expulsion of the last drops from the urethra of the latter being given by the rhythmical contraction of the perineal muscles and accelerator urina. The bladder receives a motor nerve supply through fibres coming off from the lumbar cord, which reach it by the mesenteric ganglion, and fibres coming off from the sacral cord which reach the bladder through the nervi erigentes. It is this latter group which causes an energetic contraction of the bladder. The sensory nerves run in the fibres from the lumbar cord. 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 body ; the male animal also often advances the fore-legs in order to avoid getting them splashed ; in this position the penis is protruded, and the tail raised and quivering. The stream which flows from the two sexes is very different in size, depending on the relative diameters of the urethral Digitized by Microsoft® THE UKINE 313 canal. The mare after urinating spasmodically erects the clitoris, the use of which it 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 circum- stances only pass urine when standing still, though both sexes can defaecate while trotting ; but in a condition of oestrum the mare can empty her bladder while cantering. In the ox the urine simply dribbles away, owing to the curves in 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 the body, at the same time raising her 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 remembered that in an over-distended bladder the fundus hangs into the abdominal cavity, and is thus brought on a lower level than the urethra, both of which contribute to the difficulty of emptying an over-distended organ. As a horse cannot micturate at work, it is obvious that oppor- tunity for this should be regularly afforded, or much suffering results. Pathological. There is scarcely any organ of the horse's body so free from disease as the kidneys. The material in the pelvis which looks like pus is really the natural mucus of the urine, mixed with insoluble lime salts. We have never found sugar in the horse's urine ; proteid is not un- common, but only as the result of inflammatory affection of the lungs and pleura. Digitized by Microsoft® CHAPTER XI NUTRITION /jg Weak and tear is continually taking place in the bodies of all animals, and as fast as destruction occurs repair must follow. We have previously studied the various channels in the body which supply the income and furnish an outlet for the expenditure, but this is only the beginning and the end of the process. To attempt to trace the exact changes which occur, say in the body of a pig, in producing 1 lb. of living material from 5 lbs. of barley-meal, is an impossi- bility. All we can do is to interpret the coarser or more obvious processes which take place, that of the conversion of dead into living tissues being quite beyond our knowledge. Composition of the Body. — The animal body consists of proteids, fats, salts, water, and a very small proportion of carbohydrate. Every food must either contain these principles, or be capable of conversion into them within the animal body. The following table from Lawes and Gilbert shows the relative proportion of these various tissues in oxen, sheep, and pigs, in ' store ' condition : Water Ox. - 59-0 Sheep. 58-9 Pig. 57-9 Proteids - - 18-3 16-0 15-0 Fat - 17-5 21'3 24-2 Ash - 5-2 3-8 2-9 The water is always in the largest and, excluding the carbohydrate, the salts in the smallest proportion. The amount of fat depends upon the condition ; in fat animals it may, roughly speaking, be three times the amount given 314 Digitized by Microsoft® NUTEITION 315 in the above table. The great bulk of the body is repre- sented by the muscles, and these hold half the water and half the proteid found in the system. The following table Bhows the proportion of the chief body constituents of an adult horse weighing 1,100 lbs., and it may be compared with that of a cat : Horse. Cat. Muscles and tendons - 45 per cent. 45 per cent. Bones - - - 12-4 „ 14-7 „ Skin - - - 602 „ 12-0 „ Blood - - - 5-90 „ 6-0 Abdominal viscera - 5'49 „ Thoracic „ - 1-60 „ According to Lawes and Gilbert the following are the relations of parts in the ox, sheep, and pig for every 100 lbs. of living weight : Heart, lungs, liver, blood and spleen Internal loose fat - Stomach and contents Intestines „ ,, - Other offal parts Muscle, bone and surrounding fat - Income and Expenditure. — In order to arrive at a know-^/ ledge of the processes involved in nutrition, tables of the income and expenditure of the body have been drawn up. The Income of the body consists of carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, salts and water; these are contained -in the food, the oxygen being mainly supplied by the air taken in at the lungs. The Expenditure consists of the same elements, which are got rid of by the lungs, urine, and skin. The nitrogen is excreted almost wholly by the urine, excepting in the horse, where there is a loss by the skin. It is usual to regard the urine nitrogen as a measure of the proteid changes in the system, and this is got rid of mainly as urea, and in smaller proportion as uric and hippuric acids, and minor nitrogen compounds. The hydrogen is excreted as water by the lungs, skin, and urine. The Digitized by Microsoft® Ox. 7-0 Sheep. 73 Pig. 6-6 4-6 6-9 1-6 1-6 7-5 1-3 2-7. 3-6 6-2 13-0 15-0 1-0 59-3 59-2 82-6 316 A MANUAL OF YBTEEINAEY PHYSIOLOGY carbon is largely got rid of by the lungs and urine, and in the horse by the skin. The salts are excreted by the kidneys and skin, and in the form of secretions. The sulphur is lost through the kidneys, epithelium, hair, and horn. It is hardly necessary to add that in calculating the true income of the body the fseces may be subtracted without leading to any great error, since they consist chiefly of food which has either escaped being digested or is not digestible. At the same time, they do contain a certain amount of material which represents products of tissue change which have been excreted from the blood into the alimentary canal (p. 208). When the income balances the expenditure the body is in equilibrium : if the expenditure exceeds the income the body loses weight, and if the income is in excess of the expenditure the animal gains weight. Metabolism. — By this term is 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 respiration, the beating of the heart, and the move- ments of the bowels, all mean wear and tear, and as rapidly 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 wrapped in obscurity. The metabolism of the tissues is apparently under the influence of the nervous system. We have previously studied a good example of this in dealing with the secretory nerves of the submaxillary gland, and it is probable, though our information on the point is very defective, that under the guiding influence of the nervous system the nutrition of the body is largely maintained. We constantly observe muscular wasting in some forms of lameness and injury in the horse, which is out of all proportion to the atrophy a part suffers by being simply thrown out of use, and it can Digitized by Microsoft® NUTEITION 317 only be explained by injury to tbe tropbic nerves which regulate the nutrition of tbe part. Even a better example is tbe peculiar changes which sometimes follow direct injury to trophic nerves, as in plantar neurectomy of tbe 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, in both cases being possibly due to tbe loss of tropbic influence, though much may be said in support of the view that the effects observed may be due to failure of tbe mechanically protective arrangements of the parts affected, the failure resulting from section of the merely motor and sensory fibres which the respective nerves contain. But disordered nutrition of a tissue may show itself/ without any obvious injury to trophic nerves, as for example in the phenomenon known as inflammation, or-' the well-known sympathy existing between tbe digestive system of the horse and the laminae of tbe feet. Further evidence of nervous action is afforded in nutrition which is normal in character, such as the change of the coat with the season of the year. The influence of light on meta- bolism is also probably effected through the nervous system ; it appears certain that a connection between visual sensa- tions and the nutrition of the skin occurs in blind men and animals, and the popular belief that a blind horse carries a heavy coat in summer and a short one in winter may be something more than mere superstition. In making these statements we must guard against the error of con- sidering that no growth, repair, or reproduction 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 wbich 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. It is true that metabolism goes Digitized by Microsoft® // 318 A MANUAL OP VBTEEINARY PHYSIOLOGY on during starvation, but even then food is being supplied, inasmuch as the animal is living on its own tissues. The food must contain the elements required by the tissues, viz., water, proteid, fat (or carbohydrate) and salts ; each of these must be in proper proportion, neither deficient nor in excess of the animal's requirements ; each must be present, fat cannot be substituted for proteid, nothing can take the place of salts, and a water-free diet sustains life less long than does the entire absence of food as long as water is consumed. We have, therefore, to in- quire why it is these substances are absolutely essential in q every diet, and how they behave in the system. r Nitrogenous Food. — The history -of proteid in the body j may be conveniently taken up at the point where it was left when dealing with absorption, viz., in the bloodvessels. It will be remembered that as peptone the material passed - from the bowel to the blood, yet no peptone can be detected- in blood, showing that a regeneration has occurred, the peptone being converted back to proteid. We do not know whether the whole of the proteid in its downward course from complexity to simplicity becomes body-proteid, such as would be represented by the serum of blood, or whether a part only undergoes this change while the remaining portion is converted into leucine, tyrosine, arginine, etc., and is not built into proteid, but becomes urea. The interest attached to knowing how the proteid of the food behaves in the body, arises from the remarkable fact that nearly the whole of the nitrogen in it can be recovered from the excretions ; very little, and under some circum- stances none, is stored up. So that the question arises as to whether the nitrogen of the excreta arises from pre-» formed proteid tissue, or from the nitrogen last consumed, and if the latter whether it was from recently formed body proteid, or only material in the leucine or tyrosine condition? It is in an endeavour to answer these questions that the bulk of the work on metabolism has been carried out, and the results group themselves into two theories, Pfliiger's and Voit's. Pfliiger holds that the whole of the absorbed Digitized by Microsoft® NUTRITION 319 material must first be converted into proteid before any destruction of it can occur ; in other words, that there is no short cut to urea excepting through the disintegration of the living cell. Voit contends that the proteid when absorbed is divided into two portions : one, the smaller, repairs wear and tear in the body and is spoken of as tissue proteid ; the other, the larger portion, circulates with the blood and lymph and bathes the body cells, but does not form part of them. This is destroyed by the tissues with the liberation of heat and the formation of nitrogenous end-products, the chief of which is urea ; this portion Voit describes as the circulating proteid. Voit's theory has been subjected to severe criticism, and the experiments on which it is based have been shown to be not entirely free from error, yet there is much in it which explains the observed facts of nitrogenous metabolism. Nitrogenous Equilibrium. — If an animal in poor condition, or a young growing animal, be fed on an ordinary diet, it will be found that the whole of the nitrogen is not recover- able from the excreta, as described above, so that evidently some has been retained in the body and stored up ; further, under the conditions of a liberal diet and active muscular work, the muscles grow and for this purpose they retain nitrogen. But speaking generally all the nitrogen con- sumed is practically recoverable from the excreta. If the nitrogen of the food be increased that of the excreta is increased ; if it be reduced the nitrogen excreted becomes reduced, and this may be maintained through long periods of time. It is certainly a very remarkable fact that the body should be able to work under ordinary circumstances equally well on a moderate as on a large supply of proteid. The influence of this may be still further tested by placing the animal on a diet entirely proteid. The effect of such a diet is to cause at once an increased elimination of nitrogen by the kidneys, so that more is actually being cast off from the body than enters by the mouth. If in order to meet this loss further proteid be given, a larger and still Digitized by Microsoft® 320 A MANUAL OP VETEEINAEY PHYSIOLOGY excessive excretion of nitrogen continues. Experiment, in fact, determines that it is not until three times the usual amount of proteid is. given that the nitrogen entering by the mouth equals that excreted by the kidneys. When this condition is reached the animal is said to be in nitrogenous equilibrium. It is obvious that this is an artificial state and cannot possibly be maintained for long ; further, it is im- possible to bring it about unless the animal starts the experi- ment with some stored-up fat in the body. The diet necessary for the production of nitrogenous equilibrium is seriously deficient in carbon, and the reason why the animal goes on consuming proteid and increasing its excretion of urea is in order to obtain the needful carbon ; proteid contains 54 per cent, of carbon, while fat contains 76 - 5 per cent. It is quite possible on a large nitrogenous diet for the animal to continue to lose weight in consequence of the body carbon being drawn on ; on the other hand, should it gain weight the material which is stored up is not proteid, for we have shown that all the nitrogen appears in the urine. The stored-up substance is carbohydrate and perhaps fat, though this latter point is not yet decided. Proteid in the body splits into a nitrogenous and non-nitrogenous moiety; it is from the. latter that glycogen and perhaps fat are obtained, the former, of course, furnishing the urea. In nitrogenous equilibrium there is carbon starvation, and if the explanation given above is correct, that the increased consumption of proteids is due to the urgent need for their carbon, then the addition of carbon to the diet will cause a reduction in the amount of proteid metabolised. And this is found to be the case. The addition of either starch or fat to the diet at once causes a reduction in the amount of proteid necessarily ingested in order to maintain the condition of nitrogenous equilibrium; this is known as the proteid sparing action of starch and fat, and is one of the few well-established facts in metabolism and the basis of rational dieting. It has long been observed that many diets were exces- Digitized by Microsoft® NUTEITION 321 sively nitrogenous and therefore costly and wasteful; in exact experiments on men it has been shown they can be kept in health for months on a diet far poorer in proteid than what is generally accepted to be necessary. It is extravagance to give 2 pounds of proteid daily to a horse if 8 ounces will meet all necessities. It is at this point the physiologist comes into conflict with practical experi- ence. Theory says the quantity of nitrogen required is nearly independent of muscular work; practice says the harder the machine is worked the more nitrogen must be given. Theory says proteids are not the source of muscular energy, as this is the function of non-nitrogenous food ; practice replies that may be so, but we know from experi- ence that, whether we are getting a horse fit for hard work or cattle and sheep ready for the butcher, the diets given must be strongly nitrogenous and limited only by the appetite. In this matter our personal experience places us on the side of practice and opposed to theories. Why the hard- worked horse needs more nitrogen we are not pre- pared to explain. The suggestion that the machine works more easily and smoothly on a liberal nitrogenous diet does not bring us any nearer to a solution of the problem, but the fact remains that whatever may be the energy obtainable from starch and fat this energy is in some unknown way directed by proteid. All nitrogen over and above that required for repair was considered a wasteful or ' luxus consumption,' 1 a condition to which we by no means sub- scribe. That a wasteful consumption of proteid occurs where horses are not fed in accordance with the work they are performing is undoubted; the excess of nitro- genous material throws an additional strain on the ex- creting channels, and it is certain that, clinically, we are able to recognize the effects of a highly nitrogenous diet in the liver disorders of tropical climates, and lymphangitis and azoturia of the temperate latitudes. Doubtful and difficult of solution as many of the im- portant points are in nitrogenous feeding, they are nothing in comparison with the problem of how the dead food- 21 Digitized by Microsoft® 322 A MANUAL OF VETERINARY PHYSIOLOGY proteid is converted into the living body-proteid, and how the same kind of proteid can be utilized in building up material so different in structure as bone and brain, muscle and fat, liver and skin. It is here convenient to summarize what we have learnt regarding nitrogenous food : 1. The body requires nitrogen ; no diet is complete with- out it, nor can life be permanently 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 mainly in the form of urea. 3. The assumption that the proteids are the source of muscular energy is incorrect, this being the function of non-nitrogenous food, yet increased muscular efforts must be met by an increased nitrogenous ration, the assump- tion being that in some unknown way it directs the produc- tion of energy in the muscle machine, after which it is completely cast off. 4. Proteid is stored up in young growing animals and in those out of condition ; some is also stored up in working animals so long as their muscles are increasing in bulk. It is considered that proteid cannot, under ordinary circum- stances, be stored unless there is accompanying muscular effort, and even then there is a limit to the growth of muscular tissue, just as there is to the skeleton to which it is attached. All true proteids are equally capable of becoming part of the tissues when taken as food, but when albuminoids, such as gelatin, are consumed, they produce the same amount of urea as an assimilable proteid, but the animal loses flesh, viz., none of the material is stored up in the system; when gelatin is mixed with proteid it exercises a sparing action 30^ upon the latter, and less of it is used up in the body. '//> "J-T Non-nitrogenous Food. — The whole of the carbo-hydrate matters found in food, viz., the starch, sugar, gum and cellulose, must, as we have seen, be first rendered soluble before they can enter the system. Further, they can only enter as some form of sugar, and are then stored up Digitized by Microsoft® NUTKITION 323 for future use as fat in certain depots, and as glycogen in the muscles and liver, while for present use they exist as glucose in the circulating blood. The supply of carbo-hydrates is added to by the splitting up* of proteids into a nitrogenous and non-nitrogenous portion (p. 320) ; whether the non-nitrogenous portion of proteid can form fat is uncertain, but it is undoubted that it forms glycogen. Carbo-hydrates are readily oxidized as the molecule pro-' vides sufficient oxygen to oxidize all its hydrogen, and only needs to obtain from the tissues oxygen for the oxidation of the carbon. In this respect they are a great contrast to the fats, in which the proportion of oxygen to hydrogen in the molecule is not sufficient to oxidize all the hydrogen to water, so that fats have to obtain oxygen both for their hydrogen and carbon. In dealing at p. 96 with the ques- tion of the respiratory quotient it was explained that this fraction represented the relative amounts of carbonic acid produced and oxygen absorbed. The theoretical value of' the respiratory quotient on a carbo-hydrate diet is 1, but with fats the volume of oxygen absorbed is greater than the volume of carbonic acid produced, and the respiratory quotient becomes "707. 1 gramme (15£ grains) of carbo-hydrate requires "832 litre (50'8 cubic inches) of oxygen, and produces '832 litre (50'8 cubic inches) of C0 2 . 1 gramme (15£ grains) of fat requires 2 - 8875 litres (176 cubic inches) of oxygen and produces l - 434 litres (87'5 cubic inches) of C0 2 . Great interest attaches to the carbo-hydrates in the feeding of herbivora, as so little fat exists naturally in vegetable food. We have learnt that the carbo-hydrates are one of the sources of muscular energy, and with horses they are the chief source. This material is ' fired off ' by the muscles during contraction, and so markedly are carbo-hydrates the source of muscular work, that the whole store in the body may be used up under the influence of muscular work and starvation. As the result of the oxidation of carbo-hydrates heat is 21—2 Digitized by Microsoft® 324 A MANUAL OP VETEEINAEY PHYSIOLOGY generated, so that these substances supply not only energy but heat to the body. The seat of the necessary oxidation is in the tissues and not in the blood ; the tissues produce enzymes which break up the sugar with the formation of carbonic acid and water; these enzymes are called into activity by the internal secretion of the pancreas. The amount of heat generated by the oxidation of sugar can easily be measured, 1 gramme (15£ grains) yielding 4,100 calories, or 4 - large calories of heat.* Oxidations are constantly going on throughout the life of the animal ; those occurring during rest are providing for the internal work and heat of the body, while during work, in addition to these, they furnish the muscular energy. The influence of carbo-hydrate as a proteid sparer has already been mentioned ; 10 per cent, less proteid is re- quired with the food when carbo-hydrates are present in sufficient quantity. Experiment even shows that under the influence of a considerable quantity of carbo-hydrate, no more proteid is required by the system than is equivalent to the urea excreted during starvation. In spite of the immense value of carbo-hydrates in feeding, a diet of carbo-hydrate without proteid means starvation. The Fats. — As previously noted there is very little fat in the diet of herbivora, in fact the amount is so small that in the fattening of animals fat is always specially added to the diet. It might hence be natural to conclude that the fat in the body is derived from the fat in the food, but this does not cover the whole ground ; great stores of fat may exist in animals receiving a trifling amount of fat in the diet : a cow, for instance, may produce more fat in her milk than she receives in her food, so that it is evident a some- thing not fat is furnishing it. This something is the carbo-hydrate which when in excess of requirements is stored up as fat in the permanent fat reserve depots of the * A large calorie is the amount of heat necessary to raise 1 kilo. (2-2 lbs.) water 1° C. (1-8° P.), and is conveniently named a kilo- calorie. Digitized by Microsoft® NUTRITION 325 body, and subsequently doled out to the system as required. Perhaps also the non-nitrogenous portion of the proteid molecule may contribute to fat formation, though this point is not settled. The storing up of fat is a physiological process, though under certain circumstances it may constitute a patho- logical condition. By its oxidation, which is referred to more fully at p. 332, fat furnishes heat and energy, and in this respect is of higher value than an equal quantity of carbo-hydrate. One gramme (15J grains) of fat yields 9"3 large calories on oxidation. How it is prepared for oxidation is unknown ; the fat as it lies in masses in the body cannot be oxidized until it is brought back into the blood and carried to the tissues, and it is suggested that the fat-splitting ferment, lipase, decomposes the fats into fatty acids and glycerin, in much the same way that the same ferment splits the fat in the intestinal canal before absorption. Should this be the case the lipase regulates the supply of fat to the blood. .' There are certain fat reserve depots natural to the animal, and on which under ordinary circumstances little or no drain occurs ; such are found beneath the peritoneum, around the kidneys, in the mesh of the omenta, and sur- rounding the base of the heart. It is only under the influence of starvation that the fat in these places is drawn on. 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 recognizes also that there are certain breeds which have a distinct predisposition to store up fat. He further 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 carbo- hydrate and fat he also adds proteids liberally. The measure of the diet is that of the animal's appetite ; they can never eat enough to please the feeder, who cheerfully Digitized by Microsoft® II 326 A MANUAL OP VETEEINAEY PHYSIOLOGY accepts the heavy initial outlay, as he knows the sub- sequent saving in time effected. The obesity aimed at with ' show ' cattle, sheep, and pigs is a pathological condition re- pugnant to common sense, and the outcome of a barbarous fashion. The consensus of opinion is in favour of castration as facilitating fattening, though this view has not stood the test of scientific enquiry. It is conceivable that if it has some such effect, it may easily be explained on the ground of greater freedom from excitement. It is quite certain that geldings have no greater disposition to accumulate fat than mares, and if castration favoured fattening there would be no need for that constant striving after fatness instead of 'fitness,' which is so characteristic of all who have charge of horses. 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 in- dividual peculiarity not explained by castration. The fat of horses is soft and of sheep hard ; that of cattle occupies a middle position. Each animal has fat of a certain melting-point to store up, and whether this be derived from oil, carbo-hydrate or food fat, makes very little if any difference. In the fattening of the herbivora it is considered that carbo-hydrates are better fat producers than food fat. The form in which the fats-in food are stored up has been made the subject of many experiments : a dog fed on a hard fat converts it into canine fat which is soft ; cattle fed on fluid fats, like linseed oil, convert them into hard body fats ; still, experiments go to show that foreign fats used for feeding may, if given in sufficient amount, be recognized in the tissues. It is. considered that green food, hay, and carbo-hydrates, produce a hard body fat, while grain feeding, such as oats, conduce to a soft fat. Pats, like carbo-hydrates, exert a sparing action on proteids, and for this reason a fat animal takes longer to starve to death than one which is less fat. Inorganic Food. — The salts in the body perform im- Digitized by Microsoft® NUTEITION 327 portant functions in connection with secretion and ex- cretion ; as Foster expresses it, they direct the metabolism of the body, though how they do so is unknown. To their presence is due the normal composition of the body fluids and tissues, for they regulate the water-flow from blood to tissues and vice versa. Proteids which are free from salts are quite altered in their essential characters, while the part taken by the salts of the body in blood-clotting, rhythmical contraction of the heart, irritability of muscle and nerve, milk-curdling, and growth is of supreme importance. The distribution of the salts throughout the 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 sweat, sulphur in proteid, and lime in bones, etc. Animals fed on a diet which is as far as possible rendered free from salts soon die. When a deficiency in salts occurs, the body apparently for some time draws on its own store, and then certain nutritive changes follow. Cattle in South Africa suffer from inflam- matory conditions of the skeleton (osteo-malacia) in con- sequence of deficiency of phosphate of lime, and the disease can be cured by its administration. Young animals may exhibit nutritive changes in the bones owing to a diet poor in calcium salts. The chief salt used by herbivora is potassium, whilst sodium is used by carnivora. Both carnivora and herbivora obtain in their natural diet a sufficiency of these salts, though the general impression is, that the wild herbivora long for sodium. It is quite certain that under the con- ditions of domestication horses can be kept in perfect health without receiving any sodium chloride, other than that contained in the food, and the amount of this in vegetable substances is small. The iron required by the blood is probably furnished in some organic combination. It is evident that the daily quantity of salts required must depend upon the age of the animal, young growing animals requiring more than adults. Digitized by Microsoft® 328 A MANUAL OF -VETERINARY PHYSIOLOGY a** Storage of Tissue. — Every diet must contain the food principles we have been considering, viz. : Proteid. Fat or carbohydrate, or both. 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. This is shown in the following table from the classical experiments of Lawes and Gilbert : Proportion of Food Principles stored up for every 100 lbs. Increase of Body Weight. Oxen - Sheep - Pigs - Proteid. Fat. Salts. 9-0 58 1-6 7-5 63 2-0 7-0 66 0-8 Amount of Dry Sub- stance in Food re- quired to produce 100 lbs. Increase in Weight. Weekly Increase in Body Weight. 1,109 912 420 1-0 % 1-75 % 6 to 6-5 % The table shows that in all cases the chief increase in body weight is due to the deposition of fat. The ox lays on the most proteid, the sheep stores up the largest amount of salts, the pig puts on the most fat, and fattens, not only on the smallest amount of food, but in the shortest time. Water. — The amount of water found in the tissues of animals is very constant, as may be seen from the table on p. 314, where the body water in different animals varies only from 57'9 per cent, to 59 per cent. The muscles of creatures as far removed as the pig and the snail, the ox and the lobster, contain 78 to 79 per cent., and other tissues are equally uniform. Under the influence 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 Digitized by Microsoft® NUTRITION 329 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 practically that the loss of water is con- siderable. 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 direction, 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 proteid 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 intestinal digestion in herbivora cannot go on; the contents of the colon and Cfficum 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 ; in the first instance the 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 with horses undergoing severe exertion, a very short time 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. The elimination of nitrogen by the starving animal at first falls rapidly, then gradually, and shortly reaches , a fluctuating daily quantity. During starvation the carbonic acid excreted falls in amount, and the oxygen Digitized by Microsoft® 330 A MANUAL OF VETEEINAEY PHYSIOLOGY absorbed becomes reduced, though not in proportion to the fall of carbonic acid. If water be given life is considerably- prolonged ; Colin records a case where a horse receiving water lived thirty days without food. It is notorious that herbivora, though they lose less proteid during starvation than carnivora, do not withstand starvation so well; nor need we go so far as a starvation experiment 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 15 days' starvation, the administration of food after this time will not save them. Colin records an experiment where a horse weighing 892 lbs. died after 30 days' starvation, only being allowed 2£ pints of water per diem. The animal was nourished on its own tissues, the daily loss in weight being 5*9 lbs., which must be considered as exceptionally small. Dewar* 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 13 days. The follow- ing table shows the percentage of dry solid matter lost by the tissues : Fat - 97 per cent. Spleen - - 63-1 „ Liver ■ 56-6 „ Muscles - - 30-2 „ Blood - - 17-6 „ The loss in the glandular organs was very heavy; next * Veterinarian, May, 1895. Digitized by Microsoft® NUTRITION 331 followed the muscles, and then the blood. The central nervous system suffered no loss ; evidently its 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. Cause of Body Waste. — The work of the body may be described as internal and external. By internal work we refer to respiration, the action of the heart, movement of the bowels, animal heat, etc. ; by external work is under- stood those movements of the muscles which transport the body. Every diet given to an animal must take these two factors into consideration ; the ration of subsistence is the minimum diet necessary for the internal work of the body without incurring loss of weight, the animal, of course, doing no work ; the ration of labour furnishes the actual muscular energy employed during work. The changes undergone by food in providing energy as heat and motion fall principally, if not exclusively, on the non-nitrogenous elements ; this has been settled beyond all doubt. Con- sidering that no animal can live on a nitrogen-free diet, and that the harder the work performed, the larger is the amount of nitrogen required, one would have thought, as Liebig did years ago, that the source of energy in food was the proteid substance. This is not so, therefore the urea is no measure whatever of the work performed, in fact is hardly affected by work, though it is largely affected by the amount of nitrogen received in the food. During work the heart and respirations are quickened, the horse sweats, and a larger volume of air is warmed in the lungs ; all this means a loss of heat to the body. In addition the muscles produce heat as the result of contrac- tion, in fact every process seems to tell essentially on the non-nitrogenous elements of the body, which is the explana- tion why carbo-hydrates are so necessary in the diet of hard-worked horBes. The Energy yielded by Food has been ascertained by burning the substance in a calorimeter and measuring the amount of heat given off ; in this way the potential energy Digitized by Microsoft® 332 A MANUAL OF VETEBINABY PHYSIOLOGY of proteid, fat, and carbo-hydrate has been ascertained. Every 1 gramme (15*432 grains) of water in the calorimeter raised 1° C. (l - 8° F.) is called a heat unit; by this method of investigation it has been found that 1 gramme of average proteid evolves, approximately, when oxidized, 5,770 heat units,* or 5'7 large calories. t 1 gramme of fat evolves, when oxidized, 9,300 heat units, or 9*3 large calories. 1 gramme of carbo-hydrate evolves, when oxidized, 4,100 heat units, or 4'1 large calories. Proteids, unlike carbo-hydrates and fats, are not com- pletely oxidized in the body, inasmuch as the nitrogen they cpntain reappears in the excreta in the form of urea. Now the complete oxidation of 1 gramme of urea yields 2,523 calories (or 2 - 5 kilo-calories), which must be subtracted from the value given above for the potential energy of proteids in order to ascertain the energy-value of proteids actually available by the body. Speaking approxi- mately, 1 gramme of proteid gives rise to J gramme of urea, hence the heat of combustion of proteids must be diminished by £ of 2,523 = 841 calories, before we apply the data to the body. This gives us a heat-value for average proteids of 4,929 calories or 4*9 kilo-calories, as based on purely physical determinations. As a matter of fact, all the nitrogen given as proteid does not reappear externally as urea, nor is it all excreted through the urine; some passes off in the faeces. Making allowance for this, it appears, from Eubner's valuable experiments on living animals, that the working value for an average proteid is about 4'1 kilo-calories. The Amount of Food Kequired. — The minimum amount of food required by horses during idleness has been deter- mined experimentally ; the amount required for work can- not be fixed with precision owing to individual variations ; what is sufficient for one is insufficient for another. Still, * One heat unit or small calorie is the quantity of heat necessary to raise 1 gramme of water 1° C. in temperature, f For definition see footnote, p. 324. Digitized by Microsoft® NUTEITION 333 diet tables for working horses have been constructed on the basis of the mean amount found by practical experience to be necessary. Subsistence Diet. — This is the diet necessary for the internal work of the body, the weight of the animal re- maining unchanged ; it represents the minimum amount of food required by horses doing no work. Grandeau and Leclerc kept three horses for a period of from four to five months on a diet consisting of 17"6 lbs. (8 kilos.) of meadow hay. The animals led a life of idleness with the exception of receiving half an hour's walking exercise daily. The 17'6 lbs. of hay furnished as a mean 7*02 lbs. of dry digestible organic matter for every 1,000 lbs. of body weight ; the 7*02 lbs. of organic matter contained - 538 lb. of digestible proteid. The subsistence diet for three horses for 24 hours was, therefore, as follows for every 1,000 lbs. of body weight : Proteid .... -5381b. -244 kilo. Non-nitrogenous - - - 6-482 lbs. 2-946 kilos. 7-020 lbs. 3-190 kilos. This amount of hay (7'02 lbs.) contains the following elements : Carbon - - - 3-563 lbs. 1-619 kilos. Hydrogen - - -385 lb. (6-16 ozs.) -175 kilo. Oxygen - - - 2-986 lbs. 1-357 kilos. Nitrogen - - -086 lb. (1-376 ozs.) "039 kilo. Assuming the correctness of Grandeau's observations, we may accept the above amounts of carbon, hydrogen, and nitrogen, as approximately representing a horse's require- ments for 24 hours during idleness, the animal neither gaining nor losing weight. The ratio of nitrogen to carbon in the above diet is 1 : 41 ; the ratio of the proteids to the non-nitrogenous fats and carbo-hydrates is 1 : 12. From a table furnished by Grandeau and Leclerc, it would appear that no matter what the nature of the diet may be, horses require between 7 lbs. and 8 lbs. of dry digestible organic matter daily for every 1,000 lbs. of body Digitized by Microsoft® 334 A MANUAL OF VETEEINAEY PHYSIOLOGY weight, in order to maintain the nutrition during idleness. The following is the table referred to : Diet. In the Ration. Amount digested. Amount for 1,000 lbs. of Body Weight. Hay alone Maize and oat straw Maize, oats, hay and straw >) ») )i )» Oats alone (crushed) 14-08 lbs. 11-57 „ 9-48 „ 9-49 „ 8-59 „ 6-09 lbs. 8-33 „ 7-30 „ 6-74 „ 6-41 „ 7-02 lbs. 8-22 „ 7-50 „ 7-45 „ 7-02 „ In some German experiments made by Wolff on the sub- sistence ration, 8'3 lbs. of digestible dry organic material were found necessary to maintain the body weight, and from this the digestible fibre, 1'6 lbs., was deducted, as in the experience of Wolff the fibre digested by horses was of no value as sustenance either at work or rest. In our own experiments on the essential diet for horses, we found the body weight could be maintained on 12 lbs. hay. The essential diet presupposes that the food possesses a sufficient proportion of digestible proteids. In one of Grandeau's experiments a horse received 33 lbs. of wheat- straw per diem which furnished 13 lbs. of digestible matter daily (nearly twice the amount actually required), but this diet only supplied "157 lb. of digestible proteids, or less than one-third of the minimum, the result being the horse died from starvation. The essential diet for an ox weighing 1,000 lbs. is, according to the experiments of Wolff, "5 lb. to "6 lb. of proteid, and 7 lbs. to 8 lbs. of non-nitrogenous matter reckoned as starch ; the ratio of nitrogenous to non-nitrogenous matters is as 1 : 14. According to the same authority sheep require a relatively larger essential diet, owing to the growth of the wool and its accompanying fat, viz., for 1,000 lbs. of live weight - 9 lb. of proteid and 10 - 8 lbs. of non-nitrogenous matter, the ratio being 1 : 12. Working and Fattening Diet. — The diet for horses at work, and those for the fattening of cattle, sheep, and pigs, is a question of hygiene, and reference should be made to works on this subject. Digitized by Microsoft® NUTKITION 335 Pathological. > Disorders of nutrition occur with every departure from the normal condition, though much more apparent in some disorders than others. 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 proteids suffer, and in some disorders it is remarkable how rapidly wasting occurs once it sets in. In acute lung cases this is very obvious — in a fortnight the patient may be a wreck. The increased nitrogenous metabolism which this indicates suggests an increased secretion of urea, but exact work in this direction is still much needed. During fever there is an increased excretion of C0 2 , and absorption of oxygen ; uric acid is formed by the herbivora, 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, under-feeding or over-working of animals are obvious causes of metabolic change, while defective teeth are a frequent cause of the same. Actual change in structure as the result of deficiency of a food element is mentioned on p. 327 as a cause of osteomalacia. Osteo- porosis in the horse has also been considered as due to a deficiency of salts in the food, but the weight of evidence is against this view. An excess of salts in the bowel may be productive of considerable trouble. One form of intestinal calculus in the horse is due to the amount of * ammonio-magnesium-phosphate existing in the bowel through feeding too largely on bran. The food-supply may be deficient in proteids or carbo-hydrates or both, or there may be an excess. Disorders from the latter cause are very evident in the horse. Lymphangitis and hsemoglobinuria are diseases of the horse intimately associated with over-feeding and idleness, and have no parallel in any other animal. Broken wind is referred to at p. 120 as having its origin in errors in dieting and management, such as a bulky and innutritious food supply, or heavy work on a distended stomach. Apart from these, there may be other disorders of nutrition responsible, for even under good manage- ment the production of the disease is not entirely controlled. Digitized by Microsoft® CHAPTEE XII . ANIMAL HEAT 11 ./"Oxidations. — In dealing with internal respiration on p. 101 ■we learnt the fundamental fact that the oxidations of the body do not occur in the blood but in the tissues. By means of these oxidations heat is produced, and the substances which are oxidized, viz., proteid, fat, and carbo- hydrate, have already been studied in the chapters on digestion and nutrition. The manner in which oxida- tions are carried out in the tissues is not clearly under- stood, in fact, it is by no means decided how oxidations occur outside the body. The view that oxygen directly unites with the substances oxidized is no longer accepted, for it is known that oxidations do not occur in the absence of watery vapour. In spite of the fact that oxidations within and without the body are very similar, and in their results practically identical, the conditions under which each is effected are not the same, the great dividing line being the relatively low temperature at which oxidations in the body are effected. It is probable that oxidations in the tissues are effected under the influence of enzymes and not directly by the presence of oxygen in the tissues, for it can be shown that, provided sufficient oxygen be supplied, any further increase does not affect the rate of oxidation. We have had before us the evidence of ferments capable of splitting fat, of oxidizing sugar, of converting sugar into glycogen, glycogen into sugar, and of acting on proteids ; all of these may be isolated from the body tissues, and are known as intracellular enzymes. Other evidence can also 336 Digitized by Microsoft® ANIMAL HEAT 337 be adduced of the existence of tissue ferments, by the fact that living tissue removed from the body under suitable conditions will be found to digest itself. It is supposed that the enzymes of the body stimulate the oxygen to activity ; such enzymes have been called oxidases and have been found both in plants and in the animal body. They have not, however, been found in connection with the oxidation of proteid, fat, or carbo-hydrate, though this may yet be demon- strated. An oxidase effects oxidation in the presence of oxygen, but enzymes, which only act in the presence of hydrogen peroxide, are called peroxidases. It is considered probable that the splitting up of food stuffs by ordinary hydro- lytic ferments is the first stage in the process, and this is followed by the action of oxidases ; to the latter is due the formation of carbon dioxide, water, etc., and the production of heat. The heat so formed is derived from the oxidation of food stuffs, as described in the chapter dealing with metabolism, the fats and carbo-hydrate probably yielding in the body the same amount of heat as they do in their combustion outside the body, while the nitrogenous moiety of the proteid is not fully oxidized inasmuch as urea and other waste products carry away with them at least one-third of the available energy of proteid (p. 332). How the heat so formed is distributed, maintained and lost, must now be considered. The Body Temperature. — One important division of the animal kingdom is into warm-blooded and cold-blooded animals. A poikilothermal or cold-blooded animal is one in which the body temperature depends upon its 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. Such a condition exists in reptiles, fish, etc. A homoithermal or warm-blooded animal is one in which the body temperature is independent within wide limits of the temperature of the medium in which they are living : whether this be high or low makes practically no difference. Between these two come a class partaking of the characters of each, 22 Digitized by Microsoft® 338 A MANUAL OF VETEEINAEY PHYSIOLOGY hibernating animals which during the summer are homoithermal, 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 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 which enters 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 considered that the blood in the right heart is the warmest, though Colin found that in the horse the blood of the left side was the hottest. The brain has also a high temperature. The practical aspect of the 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 purposes should be taken in the rectum. With the air at freezing-point there may be as much as 5*4° Fahr. (3"0° C.) difference in temperature between the rectum and the thin skin of the breast in the horse, while at the same external temperature the limbs of this animal, which are naturally cold, in consequence of the underlying tissues having very little vascularity, may indicate 44° Fahr. (25 - 4° C.) difference between the pasterns and the rectum. The Normal Temperature of Animals. — The wide differences which exist in the normal temperature of animals of the same class is remarkable. The following observations were made principally by Siedamgrotzky. Horse : The temperature varies between 100"4° to 100-8° Fahr. (38'0 o to 38-2° C). Age has a slight influence : Prom 2 to 5 years old the temperature is - 100-6° .. 5 » 10 „ „ „ - 100-4° >, 10 „ 15 „ ■„ - 100-8° „ 20 , 98-4 to 100-2° Digitized by Microsoft® ANIMAL HEAT 339 Cattle : The normal temperature is from 101-8° to 102 - 0° Fahr. (387° to 38-8° C.). Wooldridge* places the mean temperature at 101-4° Fahr. (38-5° C), and gives the variations at 100-4° Fahr. (38° C.) to 102-8° Fahr. (39 - 3° C). Compared with the horse the daily variations are small. Sheep : In these animals the greatest variation in temperature occurs, viz., 101-3° to 105-8° Fahr. (38-4° to 41'0° C.) ; probably the majority of temperatures lie between 103-6° to 104-4° Fahr. (39'7° to 40'2° C). The cause of the variation is unknown. Swine : The average temperature is 103-3° Fahr. (39-0° C), varying from 100-9° to 105-4° Fahr. (38-2° to 40-7° C). Dog: The dog is liable to important variations depending on the external temperature; according to Dieckerhoff it varies from 99-5° to 103-0° Fahr., (37 4° to 39-4° C); other observers put it at 100-9°, 101-3,° and 101-7° Fahr. (38-2°, 38-4°, 38-7° C). Variations in Body Temperature. — A rise or fall in body temperature does not necessarily imply an increase or diminution in the production of heat. A rise of tem- perature might be caused by a contraction of the vessels of the skin, due to external cooling, sending a larger quantity of blood into the internal and therefore hotter parts of the body ; or a fall of temperature may be due to the greater cooling which occurs when the vessels are dilated, as by an external rise of temperature. To demonstrate increased heat- production 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, as will be shown presently. Muscular work and the oxidation of food are the chief sources of heat ; during rest the metabolism sinks, the tide is low, while during activity it rises. The * 'The Temperature of Healtby Dairy Cattle.' See Proceedings of the Royal Dublin Society, vol. x., part iii., 1905. 22—2 Digitized by Microsoft® !40 A MANUAL OF VETEEINAEY PHYSIOLOGY emperature of the young animal is higher than that of he adult, while the temperature of animals living in the >pen is lower than those under cover ; in the case of the lorse as much as 1° Fahr. difference in temperature las been registered under this condition. Other causes of variation in temperature will be considered presently. The hermometer does not tell us the amount of heat formed in he body, it only indicates the difference between the heat >roduced and the heat lost. These important points must low be studied. Heat Production. — The chemical action occurring in issues, other than the muscles, as oxidations and eading to the production of heat, have previously ngaged our attention ; the rest of these changes occur aainly in the skeletal muscles, in which four -fifths f the daily heat produced is generated, and in active ;lands such as the liver. The heat furnished by glan- ular activity is amply demonstrated in the liver, though ertainly not in all secreting glands. The tempera- ure of the blood in the hepatic veins is higher than a the portal, higher even than in the aorta. It was hown by Bernard that in the dog while the portal vein ras registering 103-5° Fahr. (39'6° C), the blood in the epatic veins was 106 - 3° Fahr. (41-2° C). Every muscular ontraction leads to the formation of heat in the muscle ubstances. Experiments performed on the external lasseter muscle of the horse showed that during contrac- ion the thermometer registered 5 "0° Fahr. (2*8° C.) higher ban in the same muscle at rest. As the blood streams ut of the muscle its temperature is higher than that in tie corresponding artery, and in this way the whole mass f blood would have its temperature raised, were it not for lechanisms by which the heat is dissipated. But the xcessive production of heat is not always met by a ufficiently rapid compensation by loss, and a high tempera- are may in consequence be produced as the result. This i a most important point in connection with working orses. In the case of man compensation is sufficiently Digitized by Microsoft® ANIMAL HEAT 341 rapid, and little or no rise of body temperature occurs as the result of work. In the horse it is otherwise ; half an hour's trotting may raise the temperature from - 7° to 2*7° Fahr. above the normal, though the amount of rise is largely a question of ' condition '; temperatures of 104° to 105° Fahr. after hard work, especially in a hot sun, are not uncommon. With rest the temperature falls in the course of a few hours, the mechanism for getting rid of heat being able to cope with it. With animals unfit for work through want of condition the temperature may take longer to fall, or even remain above the normal sufficiently long to be designated febrile, and ' fatigue fever ' is not unknown in man. Fever may be due either to excessive production of heat or defective dissipation. In the above case it is probable both factors are at work. The act of feeding, which involves increased muscular activity, not only immediately, but subsequently in the muscles of the whole alimentary canal, raises the tempera- ture of the body. In the dog the maximum is reached from six to nine hours after a meal, during which time from 20 to 25 per cent, more heat is produced. In the horse, according to Siedamgrotzky, the temperature as the result of feeding may rise -4° to 1'4° Fahr. (-2° to "8° C), but, according to this observer, there is no similar rise in the ox, and Wooldridge found not more than '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° Fahr. as the result of feeding must be regarded as exceptional. A liberal diet causes at once an increased production of heat. In the tropics, or with a high external temperature, even a moderate diet may greatly raise the amount of heat . produced. JJ Influence of the Nervous System on Heat Production. — The muscles of the skeleton are not always actively con- tracting, yet heat is always being formed in them. The Digitized by Microsoft® • u $42 A MANUAL OF VETEEINARY PHYSIOLOGY leat is produced as the result of muscle tonus, viz., the iontracted condition of the muscles essential to posture, [here is also in operation, even with the most trifling novement, an antagonism to muscular contraction. For sample, the flexors of a limb cannot contract without the sxtensors being thrown into a condition to oppose the novement. This heat production in muscles is under the iontrol of the nervous system. If an animal be poisoned vith curare the motor end-plates in the muscles are paralyzed, less heat is now being formed in them and the iemperature sinks ; in fact, the animal becomes for the time jeing practically cold-blooded, the body-temperature rising md falling with the surrounding temperature. The same iondition may be produced by dividing the spinal cord )ehind the medulla. In chloroform narcosis heat produc- ion is greatly interfered with, and in prolonged opera- ions this should be borne in mind and the loss of heat >rovided against. Shivering is a physiological process : issociated with the production of heat to compensate for i loss. The shivering which occurs with horses after being vatered during winter is caused by the consumed water ibstracting heat from the tissues in order that its tempera- ure may be raised to that of the body. The ' freshness ' »f a horse on a winter's morning is the outcome of nervous mpulses instinctively started with the object of generating nore heat. Apart from contraction it is believed that muscles are he seat of a quiescent heat production under the influence if the nervous system, and that chemical changes resulting n production of heat are generated as the result of nerve mpulses. Experimental injury to the corpus striatum, he so-called 'heat puncture,' causes an increased produc- ion of heat which may last for some time, without apparently lausing the animal any inconvenience. Heat centres have Jso been located in other portions of the brain, ' optic halamus, septum lucidum, etc., and in the spinal cord. 3y some it is supposed that this extra heat production takes )lace in the liver, but the balance of opinion inclines to Digitized by Microsoft® ANIMAL HEAT 343 locating it in the muscles. No special set of thermogenic nerves has yet been proved to exist, and it is probable that the chemical changes presided over by a central heat centre are reflexly effected through the motor nerve fibres of the muscles. The bearing of this view on the increased production of heat in fevers and rapid muscular wasting in febrile conditions is obvious, and capable of explaining much which has hitherto been obscure. \^_ Heat Loss. — Unless some conditions exist in the body for the regulation of the temperature, the heat resulting from ' metabolic activity would continue to raise it steadily until it accomplished the destruction of the animal, and that this is no mere figure of speech is evident from the fact that a horse produces sufficient heat during idleness to raise the body to boiling point in less than two days. In order to maintain the temperature at a constant point heat pro- duction and heat loss must balance. This balance may be struck either as the result of diminishing the production of heat or as the result of increasing the loss. The tempera- ture of the body may rise either as the result of an actual increase in metabolism or through difficulties in getting rid of heat. The processes by which, within narrow limits, accurate and prompt adjustment is made is known as heat regulation. 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 somewhat similar to these are occurring in the animal body. [ 6 1. By Radiation and Conduction heat is lost to surround- ing bodies, provided, of course, that they are at a tempera- ture lower than the animal's. If the surrounding medium, air, wind, or such like, is hotter than the animal body, then heat is gained instead of being lost. The natural or artificial covering of animals, be it hair, wool, or clothing, checks the loss by radiation and conduction, as Digitized by Microsoft® $44 A MANUAL OP VETEBINAKI PHYSIOLOGY n a dry condition they are bad conductors of heat. When yet, however, they are good conductors and a considerable imount of heat is lost from sweating or rain. Clothing acts )y imprisoning a larger amount of warm air, the air so con- ined being a bad conductor. 2. By Evaporation from the skin the sweat is converted nto vapour and heat is lost, the rapidity of the process lepending on the humidity of the air and its rate of move- nent. The value of this evaporation as a source of heat oss in the horse is considerable, probably higher than the igure fixed for man, viz., 14 - 5 per cent, of the total, but no lata are available. Evaporation is constantly occurring ; vhen the amount of sweat is small it is evaporated as fast is it is produced, and this is referred to in the chapter on he skin as insensible perspiration. The sensible perspira- ion is that which is not evaporated as rapidly as it is )roduced, and is the source of a much greater loss of heat. 3. Evaporation from the mouth and nostrils, warming of nspired air, and vapourizing of water from the lungs. The ormer is a very valuable means of heat loss in those animals vhich do not sweat from the general surface of the skin ; the noist nose and open mouth of the dog are good examples >f the principle, and in a much smaller degree the bedewed nuzzle of the ox. The warming of the inspired air and he vapourizing of water from the lungs are most important lources of heat loss in those animals which do not sweat, [he panting respirations of the dog, and of cattle and sheep n ' show ' condition, are simply a means of cooling the body >y warming a larger volume of air, and so indeed are the mrried respirations of disease. 4. By the urine and faces a loss of heat is incurred in warming the food and water to the temperature of the >ody. The amount of loss thus brought about must be elatively considerable, especially in winter, at which time >f the year, as we have previously seen, the abstraction if heat is so great as to cause shivering ; experiment hows that drinking a pailful of water at 50° F. may cause he body temperature of the horse to fall ;5° to '9° F. A Digitized by Microsoft® ANIMAL HEAT 345 diet of roots, containing as they do 80 per cent, water, is a heavy source of heat loss with cattle in winter, though both in the case of the water consumed and the succulent food ingested, no actual loss of heat occurs until these are excreted as urine and faeces. The heat lost by conduction, radiation, and evaporation, is greater in small than in large animals, as small animals have a relatively greater surface exposed in proportion to their body weight. A dog of 66 lbs. 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 lbs. will lose 91 per cent, by radiation, etc., and 9 per cent, by water evaporation. The skin as a source of loss of heat is largely controlled by the nervous system. Through the vaso-motor nerves the vessels of the skin 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 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 vaso-motor action is an automatic reflex act, as also is the nervous control over the sweat glands, by which more or less water is poured out on the surface of the body and heat lost by its evapora- tion, and 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. The loss of heat from a rabbit after shaving off the fur is one and a half times greater than before shaving. Sheep before shearing excrete less C0 2 and more H 2 than the same sheep after shearing. White rabbits lose 75 per cent, less 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 Digitized by Microsoft® y H 46 A MANUAL OF VETEKINAEY PHYSIOLOGY o the tropics than any other colour, and black horses least if all. The black skin of the negro protects the deeper issues from the sun's rays, from which it might be argued hat black horses in theory should stand exposure to a ropical sun better than grey, but a grey horse has a black ikin and the pigment prevents the rays from penetrating. Garnishing the skin causes a rapidly increased loss of heat, io that the animal dies from cold unless rolled up in jotton-wool (see p. 284). Influence of Heat and Cold. — A moderate degree of cold ipplied to the external surface of the body increases the jroduction of heat, due to increased oxidations. This •esults from reflex motor impulses discharged from the heat- egulating mechanism. At the same time the appetite is ncreased to meet the extra demand, and foods rich in fat ire instinctively sought after by man. The same should be >bserved in the feeding of animals, and an increase allowed n the food to meet the extra oxidations, fat, if possible, 'orming part of it. The body will stand a considerable degree )f cold, but a continuous fall in external temperature cannot >e withstood ; a point is reached where the rate of heat jroductions is below that of heat loss, and the animal dies rom cold. Conversely the body is adjusted to withstand a noderately high external temperature ; the heat of Arabia )r India, which renders surrounding objects such as metal ioo hot to hold, is borne with impunity by the acclimatized aorse; the heat-regulating mechanisms do not allow the external heat to be stored up, but a continuous rise in external iemperature cannot be borne, and a point arrives when the ieat kills, for the discharge of heat from the body ceases, it secomes stored up, and heat-stroke follows. A far higher iemperature can be borne when the air is dry than when noist, as evaporation from the surface practically ceases in i moist atmosphere. "When air has its humidity increased by L per cent, it raises the loss of radiation and conduction 52 per cent., while an increase by 25 per cent, in the humidity )f the air is equal to an increase of 2° C. in the external lir. At a temperature of 88° F. in an atmosphere saturated Digitized by Microsoft® ANIMAL HEAT 347 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 marked rise in body temperature will occur as the result of standing in a hot sun, though doing no work. This passes away with acclimatization. The loss of body hoat among animals lying out at night is partly prevented by the fatty lining to the peritoneal cavity, which saves 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 miserable 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 interest- ing 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 certainly over 100° 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 mere lightness of heart. If healthy, cold has no effect on these young creatures, pro- vided the parent is able to supply sufficient nourishment. There are other animals, such as newly-born pups, kittens, rabbits, and certain birds, such as pigeons, which are born blind, helpless, and more or less naked ; they cannot 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 Digitized by Microsoft® 348 A MANUAL OF VETBEINAEY PHYSIOLOGY Eor regulating body temperature does not develop for some little time after birth, and until locomotion becomes possible. We have seen then that the young of the horse comes into the world prepared by its heat-regulating mechanisms to deal with the question of external temperature, and as time goes on 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 practiced it is un- doubted that no extra covering of any kind is required during the coldest weather, and even where the covering is of the lightest, as with the thoroughbred horse, it is sufficient for the purpose. The thoroughbred brood mares jf this country, once they go to the stud, live in the open for the remainder of their lives and never wear a blanket. A.nd practical experience tells us that this may be gradually imposed on all horses with impunity, even those which have been kept in hot stables. Coughs, colds, and inflam- matory chest affections, usually attributed to cold, are prac- tically unknown among horses living in the open, even Juring the coldest winter, and it is easy to show that these liseases are largely the result of the artificial conditions under svhich working horses have to live. Is it possible for work- ing horses to be clipped and yet wear no blankets ? This question is not only one of hygiene, but also of physi- Dlogy. Practical experience tells us they may be clipped ;wo or three times, even in the coldest winter, and pro- dded they are well fed they take no harm. Colds are ibsolutely unknown, and the explanation of these facts is ihat the horse possesses in a high degree the power of regulating his temperature. The nervous mechanisms we bave been studying are kept in active operation, diminish- ing loss or increasing production as the case may be. A somewhat similar mechanism must exist among the inhabitants of the Polar regions, who live during the winter in their huts, in a temperature which is never ibove freezing-point ; adults, and even children, may expose parts of their bodies to the external air at a temperature at Digitized by Microsoft® ANIMAL HEAT 349 which mercury freezes. Such exposure to the European would certainly result in frost bite. Clipping. — Siedamgrotzky observed the effect of clipping on the temperature of horses. He found that the tem- perature rose after clipping, and fell to normal about the fifth day. It was observed that clipped horses had during exercise a higher rectal temperature by 1"8° Fahr. than undipped horses, and the return to normal temperature was more steady and regular with them than with undipped. The rise in temperature after clipping may be due to vaso-motor 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 has its temperature raised. Another way of accounting for the rise in temperature after clipping is by supposing that an actual increase in the production of heat occurs. This may be due to stimu- lation of the skin influencing the heat-forming mechanism reflexly, either as the result of the mechanical stimulus, or of the increased cooling of the skin due to the removal of the coat. Colin clipped a horse on one side of the body and not on the other ; the subcutaneous temperature in the stable was : Clipped Side. Undipped Side. 86-9° 95° Difference. 8-1° The animal was now taken out into cold air at tb degrees below freezing-point. Clipped Side. Utl Siir d Di # erer In 30 minutes the subcutaneous temperature was - - 85 "1° 94-1° 9-0° 1% hours later - - - 79-9 950 15-1 1 hour „ - - - 83-3 95-5 12-2 1 „ „ - - - 85-1 96-1 11-0 The cooling of the clipped side is very marked, the tem- perature continuing to fall for three hours, while the slight fall in the temperature of the undipped side was restored to the normal in three hours. Hibernation. — The effect of a fall in the temperature of Digitized by Microsoft® 350 A MANUAL OF VETEEINAEY PHYSIOLOGY the bodies of animals is to produce a depression of meta- bolism. 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 depressed metabolism this store is found sufficient to keep them alive, though they wake up at the end of the winter mere skeletons. On waking up the body temperature 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 confined 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. External 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 some- times hibernate in the summer, 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. The Amount of Heat produced by animals depends upon the rate of their metabolism and the surface area of their bodieB as a factor which determines loss of heat, and hence its production if the temperature of the body is to be kept easily 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 more rapid. As heat production must balance heat loss, the small animal must lose more heat, and there- fore produce relatively more heat, than a large animal. The heat produced is measured as heat-units or calories,* * See footnote, pp. 324, 332. The calorie referred to here in the text is the large calorie. Digitized by Microsoft® ANIMAL HEAT 351 and the amount produced per hour for every 2"2 lba. of body weight is given by Colin as follows : Horse - - - - - 2 - l calories. Sheep - - - - - 2-6 „ Dog 4-0 „ A horse loses, according to Colin, 20,684 large calories per diem, or sufficient heat to raise 4,550 gallons of water 1"8° Fahr., or to raise 44 gallons from freezing to boiling point. Wolff, quoted by Tereg, gives a table showing the heat lost per diem by cattle, horses, sheep, and pigs, for every 1,100 lbs. of body weight : Horse at moderate work - - 24,500 calories (large). „ hard work - - - 37,200 „ Ox resting, and on moderate diet - 18,600 „ Sheep, with fine wool - - 27,700 Pigs, fattening - - - 35,000 „ According to Despretz a dog loses 393 calories (large) in 24 hours, and a man 2,700 in the same time. Post-mortem rises of temperature are frequently observed. The explanation afforded of a post-mortem rise in tempera- ture is that metabolism is still occurring in the tissues, but since there is no circulation to carry the heat away the temperature of the part rises. Digitized by Microsoft® CHAPTBE 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 intes- tinal canal is very different from the active skeletal muscles, and these with their long periods of activity and rest are greatly in contrast with the rhythmical movements of the heart. Structure of Muscle. — There are three varieties of muscle in the body : 1. Voluntary, skeletal, striped, or red muscle. 2. Involuntary, pale, or unstriped muscle. 3. Heart muscle. The voluntary muscles are 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 1 inch in length, and of microscopic thick- ness, viz., 5^^ of an inch as an average. The fibre is developed from a single cell, and surrounded by a mem- brane, the sarcolemma. The contents of the fibre are semi-fluid and composed of fibrils, viz., minute thread-like 352 Digitized by Microsoft® THE MUSCULAB SYSTEM 353 masses, each of which is found to be alternately striped with a dark and light band. It is the striping which gives to muscle its characteristic microscopic appearance of striation. Histologists are not agreed as to the detailed structure of the fibrils, but Schafer, whose views are accepted by most physiologists, regards the fibrils, or sarcostyles as he terms them, as divided into a series of masses placed end to end; each mass is known as a sarco- mere, and possesses a dark centre and clear ends. The dark and light stripes which result from this arrangement are composed of different substances, at least they possess different physical properties. During contraction the fluid material in the clear ends flows into the dark centre by means of certain pores. Between the fibrils is a coarse network of material known as the sareoplasm. It is generally believed that the fibril constitutes the contractile portion of the fibre, the sareoplasm being of a nutritive nature. ^ The nerve supply to muscle is both motor and sensory : through the sensory nerves the brain is made acquainted with the position of the body and the condition of muscular tension. This involves the existence of a special muscle sense which plays such an important part in locomotion. In the muscles this sense is represented by special bodies generally found near tendons, called neuro - muscular spindles ; these are from ^ to ^ of an inch in length, and r|-5 of an inch in width ; each spindle is of muscle sur- rounded by a sheath, and has a sensory nerve entering it at one end. Nervous structures, known as the tendon organs of 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 communication with certain areas in the cortex of the brain which are devoted to the ' muscle sense' (see 'Senses,' Section 4). 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 Digitized by Microsoft® ■"" 3$ 354- A MANUAL OF VETERINARY PHYSIOLOGY | supplied with the impulses which bring about contrac- tion, division of the motor nerves, or interference with their function, causing paralysis of the muscle or muscles supplied by them. Each motor nerve enters the primitive fibre about its centre, and terminates in a special organ known as an end plate. By means of curari this end plate may be paralyzed, in which case stimulation of the nerve leads to no muscular response in consequence of the block, though the muscle itself is still irritable and readily responds to direct stimulation. Masses of material built up on the lines described 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 the tendon attachment is the most usual, wherever in fact the parts are exposed to great strain. There are certain muscles in the machine where the strain on them is so considerable that tendinous material is intimately mixed up with the muscular tissue ; this is well seen in the masseters, the muscles of the back, fore-arm, and thigh. In the horse provision is also made for the muscles of the limbs being rested without necessi- tating the animal's assuming a recumbent position, viz., by the check ligaments in the leg ; by means of these an animal can sleep standing, and may remain standing for some weeks without suffering. During progression the entire strain of the body comes on the feet and the muscles of the limbs, and in such paces as galloping the muscular strain is enormous ; for example, during the canter and gallop a weight equivalent to that of the whole body is imposed on a foreleg. But this is a question to be dealt with in the chapter on Locomotion. ?2 Muscle Antagonism. — Every muscle or group of muscles T possesses an antagonist, and though the antagonist may be equal in size this is not always the case, as for example the great difference between the bulk of the muscles which close the jaw as compared with the trifling size of those which open it. The grouping, in co-ordination of action, of volun-: Digitized by Microsoft® THE MUSCULAE SYSTEM 355 tary muscles is a question to be considered later, in the chapter on the Senses. The interest which is here attached to antagonistic muscles is connected with the fact that it is this antagonism which keeps the muscles of the body slightly on the stretch, so that if one be cut across it gapes in consequence. This elastic tension ensures that no time is lost in a muscle coming into action, as there is no slack to take up ; the muscle stands as it were at full cock. Involuntary or pale muscle is not found in masses as is the red, but in thin sheets, which in places such as the blood- vessels 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, and bloodvessels. In none of these places is the sharp, short, active contraction of skeletal muscle required ; slow, steady, deliberate move- ments 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 the part of a tap : it is sufficient if the tap be turned on or turned off slowly and steadily. When we come to study muscular contraction we shall see how rapidly the wave passes along a voluntary and how slowly along an involuntary muscle. In structure pale muscle consists of nucleated spindle- shaped cells, dove-tailed, and held together by a cement substance; it is through the medium of this cement substance that the wave of excitation passes from cell to cell, thus forming a great contrast to red muscle, 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 placed at the junctions of the plexus. It is probably due to these that involuntary muscle continues to contract when all connections with the 23—2 Digitized by Microsoft® 356 A MANUAL OP VETERINARY PHYSIOLOGY centre are destroyed, though some physiologists see reasc for thinking that the contraction of pale muscle may 1 carried out just like that of heart-muscle, viz., as a sel acting mechanism, independent of any nervous connection The nerve supply of involuntary muscle is peculiar an presents a great contrast to red muscle ; whereas the latt< only receives one variety of motor supply, pale muse receives two, viz., one set of fibres which stimulates coi traction, and another which inhibits it. Both sources ai derived from the sympathetic system, which again is i great contrast to the arrangement of the nerve supply t red muscle. Heart muscle is in structure both red and striated, nevei theless it is involuntary ; the fibres are characterized b being formed of branched, nucleated, quadrilateral cells while the sareolemma is absent. As we have already seer the contraction of the heart is primarily dependent on th properties of its muscle-substance, though the automatisr is carefully directed by nervous mechanisms. Muscular Contraction. — This apparently simple act i extremely complex, and will require to be dealt with ii some little detail. Muscles are tissues possessed of irritability and con tractility, viz., they possess the power of responding by i movement to the application of a stimulus. The norma stimulus is effected through the motor nerves under th control of the brain or spinal cord, but of the nature o the stimulus we are ignorant. A coarse reproduction o it can be effected by pinching, pricking, chemical, thermal or electrical stimuli, applied to either the nerve or th muscle itself, and to all these the three varieties of muscl are responsive. When a muscle contracts, in addition ti becoming shorter and thicker, it also undergoes changes ii its extensibility, elasticity, and temperature ; there are alsi alterations in its electrical condition and chemical com position, and these can all be studied by employing i muscle of the frog, which retains its irritability for a lon{ time after removal from the body. Such a muscle suitably Digitized by Microsoft® THE MUSCULAE SYSTEM 357 prepared is known as a muscle-nerve preparation, and with certain modifications what is found to occur in this as the result of contraction, occurs also essentially in the living mammalian muscle. The muscle most usually and conveniently employed for investigating the phenomena of muscular contraction 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 Achillis, intact though free. Fig. 75.-^A Muscle-Nerve Preparation (Poster). m, the gastrocnemius muscle of a frog ; n, the sciatic nerve dissected out back to Sp c, the end of the spinal canal ; /, femur ; cl, clamp ; t a, tendo Achillis with S-hook attached. 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 then attached to the tendo Achillis. This muscle-nerve prepara- Digitized by Microsoft® 358 A MANUAL OF VETEKINARY PHYSIOLOGY tion and its arrangement as above described is shown in Fig. 75. For purposes of experiment the clamp 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 inside 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 horizontal 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 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. The most conveniently controllable stimulus is that obtained as single induction currents or the interrupted current of an induction coil. These have the advantage of being extremely efficient as stimuli and of giving rise in the nerve to impulses which we may regard 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. The complete arrangement of all the apparat us is clearly shown in Fig. 76. An inspection of Fig. 76 shows at once that if the drum of the recording instrument alone rotates, the end of 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 lever will trace a vertical line. If now the muscle is made to contract while the drum is rotating these two lines are compounded into a curve whose ordinates at each point of the curve and whose general shape give us exact information as to the details of the contraction from start to finish. Such a curve is called a ' muscle-curve ' and is typically shown in Fig. 77. Digitized by Microsoft® r& cl t* O o S 1--.S-SI2 > © a o c3 c & ° as a ? .a I « -3 « 3 .. ^ o a -a h 8 O S * 9 Sot >>■%■£ ^£ 3.13 »f I *.§ S' M g 0) B ti i, to o o ■« 3 ft d e demonstrated by the gaping which occurs when they are livided. The use of this elastic tension is to stimulate the hanges which lead to a contraction, also to ensure a rapid ontraction without the necessity of taking in any 'slack,' ,nd further it is essential to the proper action of the ,ntagonistic muscles, which are thus enabled to work gainst an elastic resistance, and so cause a smoothness i motion otherwise unobtainable. The antagonistic action f muscles may be well seen in a rupture of the flexor aetatarsi of the horse ; the unbalanced action of the ;astrocnemius jerks the leg behind the body, and throws he skin over the cap of the hock into folds, while the Lchilles tendon is kinked and bent through slackness. The lastic tension of muscle is not only a valuable stimulus to ontraction in all varieties of muscle, but is also of the reatest value in diminishing shock and strain ; nowhere is his better seen than in the heart and bloodvessels. Muscular tone is the name given to that condition of ontinuous slight contraction present in all the skeletal luscles, and leads to the elastic tension to which we have lready referred. It is due to the continuous discharge f impulses, originated reflexly, by the central nervous ystem ; if the nerves concerned in the production and ischarge of these impulses be divided the tone is lost, and utritive disturbances follow. Tone is also influenced by tie quality of the blood-supply to the muscle and the fficient drainage of the part. Work of Muscle. — If a muscle preparation be loaded with ifferent weights, and the height to which these are lifted bserved, it is found that up to a certain maximum the Dad absolutely increases the amount of work done by the Digitized by Microsoft® THE MUSCULAE SYSTEM 365 muscles. This is considered to be due to the tension exercised on the fibres, as just explained. By gradually increasing the weight the muscle preparation becomes over- loaded, and the muscle may now even elongate. These facts have been shown to be as true for mammalian as for frog's muscle, excepting that human muscle contains twice as much energy for the same volume. If the weight of the load and the height to which it is lifted be known, the work done by a muscle is readily calculable. Work equals the load lifted multiplied by the height through which it is raised, and may be expressed as pounds or tons lifted 1 foot or grammes or kilogrammes lifted 1 metre.* In connection with the work done by muscle it is in- teresting 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' is lost as heat ; in other words, the ' efficiency ' — that is, the fraction of the heat it receives which it converts into work — of a good engine is T V to y. In the animal body various statements have been made as to the proportion of work done to the available energy. Chauveau working with the lip-muscle of the horse placed the work at 12 per cent, to 15 per cent, of the energy liberated, the difference being accounted for as heat. If this were the case the muscle machine would seem to be very little more economical than the steam-engine. Now, Fick showed, some thirty years ago, that the efficiency of an excised muscle of the cold- blooded frog may be as much as \ or even |, and we may not unreasonably expect that mammalian muscle, in the body with its circulation intact, would be still more efficient. And this is borne out by Zuntz's experiments on the dog. He calculated that one-third of the energy liberated appeared as work, while by experiments on men it was found that the proportion was 25 per cent, as external work for the * A 'horse-power,' the unit used in engineering, equals 33,000 foot- pounds of wo'rk per minute. Digitized by Microsoft® 66 A MANUAL OF VETEEINAEY PHYSIOLOGY auscles of the arms (turning a wheel), and 35 to 40 per ent. for the legs (in mountaineering), from which it would ,ppear that the muscles of locomotion are superior as work >roducers. If this be so it gives the animal body an efficiency ' of from ^ to nearly \, which far surpasses that if the best heat-engine. Interesting as the comparison may be, a word of caution s necessary. It is true that an engine and a muscle each ake in energy and utilize a part of it to do external work, rat they work in different ways and along different lines to )roduce the same results. Thus the steam-engine receives ts energy as heat, originated by the combustion of fuel in he boiler-furnace, converts a varying fraction of this into work and discharges the remainder, degraded in tempera- ;ure but otherwise unaltered. A muscle, on the other land, receives its energy in the form of the food it takes irom the blood. This it metabolizes by chemical processes which are ultimately oxidational, converting the potential mergy of the food partly into work and, unlike the engine, partly into heat (see p. 340), and giving off degraded products of its metabolism, of which one is the same as ihat from the furnace of an engine, viz., carbon dioxide. These few remarks must suffice to emphasize the fact that i muscle is a chemical- engine and not a heat-engine. As Pick was careful to point out, if one tries to explain the working of a muscle on the thermodynamic principles which govern the working of a heat-engine, one is landed .n the absurd result that a muscle only converts into work iJo part of the potential energy it receives, the remaining r Vo necessarily being converted into heat, and we have seen that the efficiency of a muscle may be \. In the case sf insects, with their astounding locomotive activities, if the efficiency of their muscles could be determined it would probably be found to exceed that of mammalian muscle. Muscle Currents. — Great controversy has taken place as to whether currents of electricity exist naturally in un- injured muscle. It was found, for instance, that a piece of muscle isolated from the body, and placed in connec? Digitized by Microsoft® THE MUSCULAE SYSTEM 367 tion with a galvanometer, may be made to demonstrate the presence of electric currents which behave in a per- fectly regular manner, viz., under certain conditions they are always weaker, and under others stronger, 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, viz., from the natural surface of the muscle to the cut extremity (Pig. 80). It is now distinctly known that the current Pig. 80. — Diagram illustrating the Electric Currents of Best of Muscle and Nerve (Foster). Being purely diagrammatic, it 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 galvanometer. 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 o 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 6, or from x to y, there is no current, as indicated by the dotted lines. obtained, as just described, is one caused by the injury inflicted on the muscle in its course of preparation for the experiment, the injured (end) point of the muscle being always negative to the less injured (equatorial) points. Digitized by Microsoft® 388 A MANUAL OP VETEBINAKY PHYSIOLOGY Muscle at rest and absolutely uninjured gives no current whatever. If while the galvanometer is registering the direction of this injury current the muscle preparation be stimulated, 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 into contracting activity, it exhibits electrical phenomena, the current of action, which account for the Fig. 81. — Sartorius Muscle arranged to Demonstrate the Diphasic Variation 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 intentionally not placed on the injured end of the muscle as it would be for demonstrating mere ' 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. negative variation of the injury current. The causation of the action current is really 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 galvano- meter swinging first one way and then in the opposite direc- tion. This 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 stimulation, has not yet arrived. This negativity arises during the ' latent period ' (p. 360), and passes along the muscle as a wave which Digitized by Microsoft® THE MUSCULAE SYSTEM 369 precedes the wave of contraction. Thus if, as in Fig. 81, 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 b, which thus becomes negative, and the needle of the galvanometer swings in one direction. Immediately afterwards it passes over the point c, and the needle swings in the opposite direction. Hence the diphasic variation. 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 phenomena in muscle are not an isolated example of electric currents in the body. Closely similar phenomena are demonstrable in nerves, and electrical changes, accompanying their functional activity, occur in secreting glands, in the eye, and to the highest degree in the electric organs of certain fish. The Changes in Active and Resting Muscles. — The changes occurring in muscles are remarkably active. The processes which result in muscular contractions use up at every moment the combustible material of the structure, and the products arising from their 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 characterized by muscle waste, muscle rest is characterized 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 tbe oxygen absorbed, is returned to the venous blood. Whether a muscle be at rest or active, it is always 24 Digitized by Microsoft® W 370 A MANUAL OP VETERINARY PHYSIOLOGY absorbing and storing up oxygen, and giving off carbon dioxide. |j The absolute amount of these varies; during work the oxygen used up and carbon dioxide produced are both increased, and the increase provides some measure of the work performed. Even during rest a muscle is doing work, for we have learnt that it is always in a condition of tonus, viz., of slight contraction. Since a muscular con- traction is essentially the outcome of an oxidational process, the storage of oxygen by the resting muscle may be said, in Pfhiger's words, ' to wind it up ' in preparation for its contracting activity. This accounts for the fact that an excised muscle of the cold-blooded frog can be made to give some hundred contractions when suitably treated. In mammalian muscle, on the other hand, with its much greater metabolic activity, this quiescent storage of oxygen does not suffice to maintain its irritability for more than the briefest interval after its blood-supply is cut off. In an active muscle the bloodvessels are more dilated than in the muscle at rest, and this dilatation provides for the increased quantity of blood now required by the part. By means of the blood the irritability of the muscle, or its power of contraction, is maintained ; whatever leads to a smaller quantity of blood being sent to an active muscle, produces partial or complete paralysis of the group or groups of muscles affected. This is well seen in the horse when 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 called upon to trot muscular cramps occur followed by paralysis. ^ Our study of metabolism has prepared us for the state- ment that the chemical changes occurring during contrac- tion do not normally affect the nitrogenous elements of the muscle. There is probably no increased output of nitrogenous substances such as creatine. The excretion of any increased amount of urea is variable, irregular, or even non-existent, and is in no case even remotely pro- portional to the work done. This is true as long as the body is supplied with a sufficiency of the non-nitrogenous carbo-hydrates and fats. If they are deficient, then Digitized by Microsoft® THE MUSCULAK SYSTEM 371 increased muscular activity does lead to an increased formation of urea, since the muscle now has to metabolize its proteids to provide the energy necessary for the work performed and the heat simultaneously produced. The main products of muscular waste are therefore to be looked for in the destruction of stored-up carbo-hydrate material. Muscles in a state of activity contain less glycogen and sugar than those in a state of rest, due to the amount utilized during muscular activity ; but glycogen is not necessarily the source of the energy, since muscles free from it work normally. 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° Fahr. through feeding. The whole body temperature in the horse is raised during work, and does not fall for some time after. In dogs a rise of temperature of several degrees may be obtained by stimulating the spinal cord, and thus producing muscular contractions. A contracting muscle liberates energy in the form of both work and heat. We have seen reasons for regarding the oxidation of the non-nitrogenous carbo-hydrates as the normal source of this energy, and we have referred to the quiescent storage of oxygen during rest as accounting for the prolonged possible activity of an isolated frog's muscle. In connection with this it is not uninteresting to calculate, as Fick has done, the amount of carbo-hydrate necessarily oxidized to provide all the energy as work + heat furnished by a single maximally vigorous contraction of a frog's muscle. Knowing the heat of combustion of, say, glycogen, and converting the work of the muscle into heat by Joule's equivalent, we find that 1 gramme of frog's muscle can provide all the energy it sets free in a single maximal con- traction by the oxidation of "0006 milligramme of carbo- hydrate. In -the case of fat the necessary amount would be still less, viz., '00025 milligramme. This may serve to diminish our surprise at the working activity of which an excised muscle is capable. 24—2 Digitized by Microsoft® 372 A MANUAL OF VETEEINAEY PHYSIOLOGY ■r/d 'J- Causation of a Muscular Contraction. — This is a problem as yet unsolved ; our previous studies in every way point to the oxidation of carbo-hydrate substance as being the source of energy, and we have seen that it is impossible for a muscle to contract without using up oxygen and producing carbon dioxide. But we are now brought face to face with a paradoxical condition ; if muscle be exposed to the vacuum of a gas-pump no free oxygen can be obtained from it, while if the ordinary nerve-muscle preparation be taken and placed in a jar of hydrogen it continues to contract, and, even still more remarkable, it continues to produce C0 2 , though no oxygen exists in the atmosphere surroundingat ! As C0 2 cannot be formed without oxygen, it is evident the oxygen must come out of the muscle, and to meet this difficulty it is supposed that the muscle molecules store up oxygen during rest in a hypothetical compound of hydrogen and carbon known as inogen ; during muscular contraction this compound breaks down, and the waste products are liberated. The nature of the process by which the impulse conveyed to the muscle along its motor nerve becomes, through the agency of the end-plates, the stimulus which leads to this explosive compound being fired off as a muscular contraction is quite unknown. The nitrogen- holding substance in muscle is only used when the food- supply is insufficient or the work excessive ; it is therefore the 2 intake, and C0 2 output, which have to be examined in dealing with the question of the influence of muscular work on metabolism. Fatigue. — Turning once more to the simple nerve-muscle preparation, it is found that if the muscle be kept at work the first few contractions, as shown by a series of tracings {see Fig. 82), may be progressively more vigorous. But if the stimulations are continued the muscle rapidly becomes fatigued, the latent period is lengthened, the height of each successive contraction becomes less and the duration of each contraction is prolonged, chiefly by a lengthening of the period of relaxation : the muscle, in other words, is in a state of fatigue (Fig. 82). We shall presently study Digitized by Microsoft® THE MUSCULAE SYSTEM 878 fatigue from some other aspects, and will now only point out that the cause is here due to the using up of the con- tractile substance of the fibres and the accumulation in the muscle of the chemical products of contraction. In fact, if a fatigued muscle be washed out with normal saline solution and a little weak alkali circulated through its Fig. 82. — Fatigue Curve of Skeletal Muscle ; Gastrocnemius of Frog (Stewart). Time tracing y^y of a second. The curve is read from right to left. bloodvessels, it becomes restored, and regains its power of contraction. 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. A muscle so fatigued by repeated stimulation may be restored by washing it out with physiological salt solu- tion containing a little alkali ; in course of time mere Digitized by Microsoft® 374 A MANUAL OF VETEKINARY PHYSIOLOGY washing out of the muscle is not sufficient to ensure its recovery, but if serum or blood be transfused it is enabled again to start work. The material in muscles which gives rise to fatigue is probably sarco-lactic acid, and by passing a solution of this acid into muscles the typical phenomena of muscle-fatigue may be artificially induced. The production of potassium salts may also be a cause of fatigue, in spite of the fact that they are usually found in muscle, yet potassium salts in their action on this tissue rapidly destroy its irritability. We have seen (p. 353) that muscles are connected by elaborated nerve-endings with sensory nerves, to whose existence the so-called ' muscular sense ' is due. It is therefore conceivably possible that 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. On the other hand, muscular activity implies the action of central nerve-cells in which the impulses which give rise to the contractions of the muscles are originated, of the passage of these impulses along the motor nerves, and of their com- munication to the contractile fibres by the agency of the end-plates. Hence the phenomena of fatigue, if we regard it as a ' weariness ' of the body as a machine, may be really due to a fatigue of the central cells, of the motor nerves, or of the end-plates. We may at once dismiss the motor nerves from our consideration inasmuch as nerves do not appear to be capable of fatigue (p. 385). Now the blood of a fatigued animal contains fatigue products, and if it be transferred into the circulation of a normal animal, all the symptoms of fatigue are produced. If the spinal cord be divided and the distal end stimulated, the hurried respiration of fatigue may be produced as the result of muscular contractions (see p. 118). Possibly, therefore, the phenomena are due to the injurious action of the products of muscular activity on the central motor nerve- cells, and some experiments on men go to show that the Digitized by Microsoft® THE MUSCULAE SYSTEM 375 central nervous system is readily affected by fatigue products. On the other hand, recent work by Wedenski opens up the possibility that fatigue, if we look upon it as an inability to drive the muscular machinery up to its normal capacity, may be due to the deleterious influence of the products of muscular contraction on the end-plates. This view receives support from the fact that a fatigued muscle will contract by direct stimulation when it refuses to respond to a stimulus brought to it through its nerve. Elaborate experiments on man performing muscular work show how readily the intake of 2 may be increased ; such causes as difficulties in the road, rising ground, increase in pace, change in the load carried, unpractised movements, even a sore foot, may increase the consumption by 18 per cent. Fatigue produces wasteful metabolism, and may increase the C0 2 excreted even as much as 21 per cent. The abnormal use of certain muscles, such as a man with sore feet would employ in order to save himself pain, produces extravagant combustion and fatigue. What applies to man in these matters applies equally to the horse ; ungreased axles, badly-fitting harness and saddlery, badly-made roads, sore backs and lameness, all represent undue muscular wear and tear. 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 highest pitch of perfection into which muscles can be brought. 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 over- step the mark and produce ' staleness,' a result which is usually recovered from by a short judicious rest, to which the system immediately responds. During training all superfluous fat and water are re- moved from the body, the muscle substance is built up, and the respiratory capacity increased ; but it is very necessary to remember that condition, though judged of largely by Digitized by Microsoft® 376 A MANUAL OF VETEEINAEY PHYSIOLOGY the state of the muscles, has a very important claim on the respiratory and circulatory systems. 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 a violent muscular effort is far more dependent on the training of the respiratory and cardiac mechanisms than on that of the 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. As the circulatory pump works at high pressure the bloodvessels must be fit to stand the strain, and to return to the heart at both auricles the amount of blood leaving 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 wherever horses are exposed to long- continued strain, that the chief value in training is located in the functional improvement of the muscular tissue of the heart and in the circulatory system in the lungs ; both of these have to be educated to withstand the extra strain imposed and to work economically. The voluntary muscles have also to be educated to work in the best and most economical manner ; they must be used to advantage, smoothly and in combination ; their response must in- crease in rapidity and power, while their relaxation must not be too prolonged and so cause loss of time. Unpractised movements are a serious source of waste ; by practice the same amount of work can be performed with a reduced expenditure of energy, and this is true for both men and horses. There is additionally another factor of supreme im- portance in training. The respiratory movements, as we Digitized by Microsoft® THE MUSCULAE SYSTEM 377 have learnt, are dependent upon the rhythmic activity of the respiratory centre (p. 108) . Hence this centre must be taught to withstand the extra strain imposed upon it during violent exertion. What the centre has to ' learn ' in respect of this is more or less a matter of conjecture. Bearing in mind the powerfully stimulating influence on the respiratory centre of the waste products of the metabolism involved in muscular contraction, it is conceivably possible that ' wind * is the result of an increased immunity of the centre to the action of these products. However this may be, one thing is certain — namely, that respiratory distress is more potent than most other factors in determining ' staying power,' the one thing to which all long-distance athletes strive to attain. In this connection we may point out that it is said that the deleterious products of metabolism produced during fatigue may be neutralized and immunity estab- lished by giving small doses of extracts of a fatigued muscle ; the question has therefore arisen as to whether _ ' training ' is a process of immunizing against fatigue 3. products ? \\ ^ Chemical Composition of Muscle. — A dead muscle does not possess the same chemical composition as one which is living, and we cannot analyse living muscle 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 care- fully frozen and then very slowly thawed, it does not lose its irritability : 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 1 per cent, of sodium chloride. By this process a viscid liquid is obtained which may be filtered, though with difficulty, at 0° C. The fluid filtrate is opalescent, neutral, or faintly alkaline in reaction, and is known as ' muscle-plasma.' When its temperature is allowed to rise it coagulates in the same Digitized by Microsoft® 378 A MANUAL OF VETEEINAEY PHYSIOLOGY 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 sarco-lactic acid, and the clot con- sists 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 composition of muscle to be approximately as follows : "Water - -75 per cent. Proteids - - 20 Pat - - 3 Carbo-hydrates - •4 to 1 per cent, Nitrogenous waste products - -2 Salts - - - 1 to 1-5 „ Oar knowledge of the nature of the proteids of muscle is a matter of no slight uncertainty, which is not made less by the existing confusion in the terminology employed by various investigators. Into this we cannot here enter. It must suffice to say that the chief and characteristic proteid of dead muscle is the myosin formed in the clotting of muscle-plasma ; it belongs typically to that class of proteids 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 (p. 18). It has not as yet been shown that calcium salts play a part in the coagulative formation of myosin, as they necessarily do in case of fibrin and of the casein-clot in milk. The proteids of living muscle are not entirely myosinogen, nor are those of dead muscle entirely myosin. Other members of the globulin class are present Digitized by Microsoft® THE MUSCULAE SYSTEM 379 in both, as also an ordinary albumin closely resembling serum-albumin. The carbo-hydrate 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 substances have already been fully dealt with in a previous chapter. The nitrogenous waste products or ' extractives ' are creatine, hypoxanthine, xanthine, carnine, taurine (in horse-flesh), uric acid in minute traces (though more abundant in reptilian muscle), and traces of urea, though this is a question still not decisively settled ; of these creatine is by far the most important. It is a substance we have already studied in connection with the production of urea (p. 295). The ash in muscle consists principally of the salts of potassium and phosphoric acid. The gases are carbon dioxide, together with a small amount of nitrogen but no free oxygen. Rigor Mortis. — After death a muscle passes into the condition of rigor or stiffening, by which it changes both in its physical and chemical aspect. 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 sarco-lactic acid. Through the death of the muscle its proteids coagulate, and this process is generally believed to be identical with the clotting of muscle plasma previously described. Eigor mortis and the production of sarco- lactic acid are closely connected, so that if the formation of the acid be prevented by suitable 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 proteids by the products of metabolism in the muscle, and this ex- planation accounts for the rapid setting in of rigor in animals hunted to death. Eigor mortis is delayed in a rabbit in which the labyrinth of the internal ear has been destroyed. This is probably connected with the obscure problem of muscle tonus, with which the labyrinth is connected. The muscles in which delayed rigor mortis occurs are those Digitized by Microsoft® 80 A MANUAL OF VBTEEINAEY PHYSIOLOGY orresponding to the same side of the body as the injured ibyrinth. During rigor mortis C0 2 is produced and ieat evolved ; some after-death temperatures are re- markably high. After a certain length of time rigor mortis lasses off and decomposition commences. It is doubtful whether rigor mortis occurs in involuntary muscle ; the ppearance presented in this variety of muscle may be due o cold, for it has been shown that two or three days after eath smooth muscle may be warmed up so as to be capable f contraction. Phenomena of Contraction in Smooth Muscle. — Though here is a marked difference in appearance between red ,nd white muscle, the actual phenomena of contraction lo not differ excepting in the matter of rate. The itent period, contraction, and relaxation, are present, as a red muscle, but occur more slowly ; they are, in fact, luggish and deliberate. Owing to the existence of lumerous nerve fibres and ganglia, white muscle may be ompletely isolated from all nervous connections, and still ontinue to exhibit the phenomena of muscular contraction. In response to a continuous or induced current pale auscle behaves much as does red, excepting, of course,, hat the response is slower. Summation also is present, hough not identical with that observed in red muscle, for 10 contraction follows the first three or four stimuli ; it is he stimuli which here accumulate before contraction ollows, and after this has occurred the muscle subse- uently responds to further stimuli by an increased height if contraction as does a red muscle. Mechanical stimulation of pale muscle excites a sluggish mt marked response ; pinching the intestines produces leristalsis, and even drawing the finger lightly over the tomach wall may produce ' weals ' of contraction. The auscle is markedly responsive to tension, resembling in his a skeletal muscle which up to a certain ' load ' does aore work the greater the weight it has to lift. Thus a rog's gastrocnemius, loaded successively with 10, 30, and 10 grammes, will do work proportional at each contraction Digitized by Microsoft® THE MUSCULAE SYSTEM 381 to the numbers 106, 312, and 760'5. In the case of the digestive canal this is probably the chief source of stimula- tion, and in this connection we may call to mind the influence of cellulose in the diet of the herbivora (p. 207), the bulkiness of their food providing the mechanical stimu- lation and the distension necessary to produce the tension to which pale muscle is so responsive. The same remark may also apply to the physiological action of the stomach and intestinal gases; nor must the bladder be omitted from this consideration, from the point of view that distension by fluid provides a tension of the walls which acts as a stimulus to contraction. Digitized by Microsoft® CHAPTER XIV THE NERVOUS SYSTEM* Serves. — Various classifications have been adopted for lerves. Anatomically they are known as cranial, cerebro- spinal, and sympathetic, but for physiological purposes hey are classified according to their function. From a structural point of view there are (1) medullated, (2) non- nedullated nerves. Classified according to their function here are (1) afferent, frequently called sensory, and 2) efferent, commonly called motor. The division into notor and sensory is so obviously incomplete, confining the 'unction of nerves simply to the conveyance of motor mpulses, or of those which give rise to sensation, that ;he terms afferent and efferent are better. Afferent nerves are those conveying an impulse from he periphery of the body to a nervous centre, that is to lay conveying centripetal impulses. The centre may be lituated in the brain or spinal cord, and the impulses hus conveyed may be those of (a) special sense such is sight, hearing, taste, smell, etc. ; (b) impulses pro- lucing sensation pleasurable or painful, from skin, muscle, md viscera ; (c) impulses producing the impression of heat md cold, or (cl) impulses leading to a reflex act without iffecting consciousness at all. Efferent nerves are those conveying impulses from a ;entre to the periphery, that is conveying centrifugal * My best thanks are due to Professor Sherrington, F.E.S., for eading this chapter, and kindly supplying that portion of it dealing rith " stepping " and the "scratch reflex." 382 Digitized by Microsoft® THE NEEVOUS SYSTEM 383 impulses ; these impulses in the main are of a motor • nature evoking from the muscles, bloodvessels, and viscera movements and contractions ; they may also be of an inhibitory or controlling character, such for instance as the impulses which slow the heart, dilate the bloodvessels, or restrain the peristaltic contraction of the bowels. But besides these, centrifugal impulses may be of such a nature as to cause glands to secrete, or to regulate the metabolism of a part, or control, stop, or augment other of its actions. Structure of Nerves. — Meduilated nerves, often spoken of as white nerve-fibres owing to their colour, are distinguished microscopically by the fact that their component fibres possess a white fatty sheath enveloping the essential nerve- substance or axis -cylinder, which lies like a core within it. The axis-cylinder is the true nerve-substance, and is in connection with either the brain or spinal cord, depending upon the position of the nerves. In these organs cells with processes are situated ; one of the processes becomes the axis-cylinder of a nerve, so that some of the cells of the brain, and especially of the spinal cord, may be looked upon as possessing processes of immense length. The white fatty sheath, known as the medullary sheath, which covers the axis-cylinder does not extend continuously along the nerve, but is broken at intervals termed nodes ; the portion of nerve-fibre included between two nodes has somewhere in it a nucleus. It is at the nodes, where the fatty sheath is absent, that the material which supplies the nerve with nutrition gains access. Covering the medullary sheath is a delicate membrane which envelops the fatty matter, and is known as the neurilemma. Such is the structure of a single nerve-fibre; bundles of such fibres, enclosed in an appropriate sheath of connective tissue, constitute a nerve. The non-medullated nerves, often spoken of as sym- pathetic or grey fibres, possess no white fatty cover around their axis-cylinder ; they are freely nucleated at intervals, and made up in bundles as are the meduilated nerves. Digitized by Microsoft®, 384 A MANUAL OF VETEEINAKY PHYSIOLOGY The essential feature in the nerve is the axis-cylinder; it is the true impulse-conducting substance, while fatty sheaths can only be looked upon as a means of insulation or support. From what has been stated about the nature of the axis-cylinder, it can be readily understood that every nerve runs direct from its origin to its termination, there is no union of nerve-fibres, each and every one is distinct, though numerous divisions may exist at their termination. In certain cases medullated nerves enter nervous bodies known as ganglia and leave them as non- medullated fibres. All medullated nerves before breaking up in the tissues they are intended to supply lose their fatty sheath and eventually their neurilemma, nothing but the bare axis-cylinder being left. Ganglia on Nerves. — Placed on certain nerves, somewhere in their course, are masses of nervous tissue called ganglia ; these ganglia are composed of an outer covering of con- nective tissue enclosing nerve cells, between which nerve- fibres pass ; the cells are of a particular shape depending upon whether the ganglion examined be from the cerebro- spinal or sympathetic system. In the former the nerve cells are round and possess a projection or pole, which not unfrequently coils before issuing from the cell, and after running a short distance divides T-shaped into two branches which travel in opposite directions ; such a body is known as a unipolar cell. The ganglia belonging to the sympa- thetic differ in the shape of their cells, for these instead of having one pole like the cerebro-spinal, have two, three, or more poles, known as bi-polar or multipolar cells. We may here say, though the subject will be touched on again, that the cells of the brain, spinal cord, and sympathetic system, are mainly multipolar, whilst those of the spinal and cranial ganglia are unipolar. Bi-polar cells may be found in the spinal ganglia of fishes. One process of a nerve-cell is the axis-cylinder of a nerve, the other processes branch, dividing and subdividing like a root, and become primitive fibrils. Nerves are remarkable for their want of elasticity ; they Digitized by Microsoft® THE NEEVOUS SYSTEM 385 do not retract on being divided ; further they are capable of very considerable stretching without breaking. In the human subject the nerves of the limbs require a weight of from 40 lbs. to 120 lbs. to break them. There are nerves supplying nerves, the nervi nervorum, and the rationale of nerve- stretching in painful diseases is probably accounted for by the damage done to these minute nerves during the process. Nerve trunks receive a poor blood supply, though ganglia and grey matter are richly vascular; it is possible that the numbness produced in a sensory nerve by pressure is due to its blood supply being temporarily cut off, the nerve thus losing its irritability. The lymphatics are numerous, 3 •'•> and exist within the lamellae of the perineurium or covering I j -f of the nerve bundle. ' Excitability. — We have no means of distinguishing micro- scopically between an afferent and an efferent nerve ; there is nothing in the structure of a motor, sensory, or secretory nerve, which enables its function to be determined. Further, we know that though in the body impulses pass only in the one direction through a nerve, yet removed from the body and tested electrically it is as easy to pass impulses in one direction as another. Nerves however are excitable, the living nerve can be made to react by means of chemical, mechanical, or electrical stimuli, and when so excited appears to transmit the same impulses as when irritated physiologically, viz., as when the normal body impulses are being transmitted ; thus the stimulation of a sensory nerve gives rise to pain, of a motor nerve to muscular contraction, and of a secretory nerve to secretion. The conductivity of nerves is diminished by cold, compression, or injury, but it is noteworthy that even after long-continued excitation nerves are found practically unfatigued. Impulses are no longer transmitted when nerves are ligatured or divided. //^Electric Phenomena of Nerves. — Some very definite facts /.are known in connection with the electric currents in nerves, and the effect on the excitability of the nerve of transmitting currents through it ; these>facts have been ascertained with 25 Digitized by Microsoft® 386 A MANUAL OF VETEEINAEY PHYSIOLOGY the nerves of the frog, and so far as can be proved apply equally to those of the higher animals. If a nerve be removed from the body and suitably applied to an instrument which is capable of measuring delicate electric currents, the galvanometer, the needle of the in- strument ■will be found to be deflected, showing the passage of a current ; it is spoken of as the current of rest, but it does not exist in uninjured nerves. It is practically identical in direction with the natural muscle current described on p. 367. If while the current of rest is passing shocks be sent into the nerve from an induction coil, the needle of the galvanometer is found to indicate a momentary current in the opposite direction to the current of rest. This momentary opposite current is spoken of as negative variation or the current of action, and the phe- nomenon is essentially the same as in the case of a muscle. The negativity developed at the point to which the stimulus is applied travels as a wave in both directions along the nerve at the rate, for frog's nerve, of about 28 metres or some 90 feet per second. In the case of a muscle the change of shape when stimulated provides the necessary indication of its receipt of an impulse. Not so with a nerve. Here we have no change of shape, no development of heat, to mark its functional activity. The one indication of the passage of an impulse along an isolated 2>iece of nerve is the electrical change taking place in it, and hence the study of the ' current of action ' and the onward rush of the attendant ' negativity ' is here of supreme interest. Turning next to the nerve-muscle preparations suspended in the moist-chamber (see p. 359), and using the contrac- tion of the muscle as an index of the activity of its nerve, we may study other important phenomena of nerve-excita- tion. If a moderately strong constant current be passed into the nerve by connecting it with the poles of a battery, at the moment the connection is made the muscle gives a twitch or contraction, and then remains perfectly quiet though the current is still streaming through its nerve ; if Digitized by Microsoft® THE NEEVOUS SYSTEM 387 the connection be broken as by opening a key in the battery circuit, the muscle gives another contraction. These are termed ' making ' and ' breaking contraction,' viz., a con- traction produced on closing and opening the electric circuit. If instead of a moderately strong constant current a weak or very strong one be used, the results on making and ^ breaking may not be the same. / $7 7j During the period of apparent quiescence following the ,,- ' closing of the circuit, though the muscle is giving no indication of the current, yet changes are occurring in the nerve. If it be tested by stimulating it with an induced current, it is found that its irritability, ' as measured by the greater or less contraction of the attached muscle, is diminished in the neighbourhood of the positive pole (anode) of the continuous current, and increased in the neighbour- hood of the negative pole (kathode). This changed con- dition is known as electrotonus, the diminished irritability being known as anelectrotonus, the increased excitability as kathelectrotonus. Between the increased and reduced irritability is a zone of unaffected irritability known as the neutral point. During the condition of electrotonus there is no interruption to the natural nerve current, which is simply increased in strength if the constant current takes the same direction in the nerve as the current of rest ; but if the constant current be passed in the opposite direction to the nerve current, the latter is diminished. A reference to Fig. 83 will suffice to make the matter clear. At a certain part of the nerve a continuous current of electricity generated at e is passed through it, the application and withdrawal of which gives rise to the making and breaking contraction previously mentioned ; during the passage of the current the muscle is perfectly quiet, in spite of important changes occurring in the nerve. Shocks are now sent into the nerve from an induction coil at a place between the muscle and the points of application of the continuous current ; as the result of the stimulation the muscle either responds more than it should do for the strength of the stimulus employed, viz., there is increased OK o Digitized by Microsoft® 388 A MANUAL OF VETEEINARY PHYSIOLOGY excitability of the nerve (kathelectrotonus), or the muscle does not respond as strongly as it should, viz., there is decreased excitability of the nerve (anelectrotonus). The increase or decrease of excitability in the nerve depends upon whether the continuous current is passed down it, Fie. 83. — Diagram of Electrotonus. N, The nerve running to the muscle m ; e, battery or cell for the produc- tion of a constant current, the positive pole or anode (a) in A being placed furthest from the muscle, the current consequently flowing down the nerve, and in B being placed nearest to the muscle, the current flowing up the nerve. At s the nerve is stimulated by an induced current, and its irritability determined by the contraction of the muscle m ; the irritability is increased in A, kathelectronus, and decreased in B, anelectrotonus. as in A, or up it, as in B ; with a descending current the excitability is increased, with an ascending one it is decreased. The explanation of electrotonus in nerves is that it is a vital phenomenon, viz., the irritability of the nerve is in- creased when its molecules pass from their ordinary con- Digitized by Microsoft® THE NERVOUS SYSTEM 389 dition to one of greater mobility (kathelectrotonus), or it is diminished when its molecules pass from their ordinary condition to one of less mobility (anelectrotonus). Her- mann considers that it is a purely physico-chemical pheno- menon, due to the electric current generating acids at the positive pole, and alkalis at the negative ; the effect of the acid is to lower the excitability of the nerve, and of the alkali to increase it. One practical application of this law is that the excitability of a part, as in pain, cramp, etc., may be removed by passing a current up the nerve, viz., by placing the positive pole nearest the muscle, and producing anelec- trotonus ; or by reversing the process and throwing the current down the nerve, so that the negative pole is nearest the muscle, the irritability of the part may be increased. ^ 7 The Nature of Nervous Impulses is quite unknown, ex- TTcepting that they travel in the form of a wave of electric disturbance, which is shorter, and travels more rapidly than that which traverses a muscle. Impulses are not trans- mitted from one fibre to another in a nerve bundle. Conductivity of Nerpes. — Compared with electricity a nervous impulse travels very slowly, and it is necessary to bear this in mind as comparisons between electric currents and nerve impulses have been made. The velocity of ner- vous impulses in motor nerves has been stated to be between v lll to 140 feet per second, whilst through sensory nerves it is said to be faster, 160 to 320 feet per second ; in visceral nerves the velocity is less. Chauveau ascertained in the pharyngeal branches of the vagus that the velocity amounted to 26 feet per second. Degeneration of Nerves. — When nerves are cut they degenerate, the degeneration always taking place in the portion cut off from its nutrient centre. The nerve fibre, as has been stated above, is but a branch of a nerve cell ; if a portion of a cell be separated from the part containing the nucleus of the cell it soon dies. Thus, when a large amoeba, or a Eadiolarian, is torn up into several pieces, the portions containing no nucleus degenerate and die ; but that portion containing the nucleus repairs itself and Digitized by Microsoft® 390 A MANUAL OF VETEKINAKY PHYSIOLOGY reforms 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 piece 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 (see p. 384) ; the nearer to the Bpinal cord the point at which the section is made the greater the length of nerve which degenerates, the further away from the cord the point at which section is practised the shorter the length which degenerates. When the nerve degenerates the fatty medullary sheath breaks up, forming globules around the axis-cylinder ; the latter also degenerates and ultimately breaks up. The remarkable fact about these changes is the rapidity with which they occur, six days is sufficient to show their com- mencement ; small nerve fibres degenerate more quickly than large. By suturing divided nerves union occurs, and though the act of division causes degeneration, yet when union takes place regeneration of fibres occurs ; a fresh axis-cylinder grows through the length of the degenerated nerve, and. after some weeks and often months motion or sensation is restored, the former always much later than the latter. Even suture of divided nerves is not always necessary for union ; we know clinically that the plantar nerves of the horse will often unite in a few months in spite of a piece being excised, the portion of nerve above sending down an axis-cylinder which soon finds out its divided portion below. Not only iB 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 Digitized by Microsoft® 3f jr THE NERVOUS SYSTEM 391 the fifth is divided ; and many are practically acquainted with the sloughing of the entire foot which sometimes, though fortunately rarely, follows the operation of neurec- tomy. It is undoubted that nerves influence the nutrition of a part ; nowhere is this better demonstrated than in cases of intense muscular atrophy due to nerve injury. Nerve Terminations. — There are some structures such as glands where the nature of the nerve termination is not satisfactorily 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. Spinal Cord. The spinal cord extends from the atlas to about the second or third sacral vertebra, and is completely enclosed in a dense membrane, the dura mater. The canal in which it is lodged is very much larger than the cord, especially at those parts where the greatest amount of movement occurs, as in the neck. The cord is not the same shape nor the same size throughout ; oval in the cervical region, it becomes circular in the dorsal, and again oval in the lumbar portion. It is largest where any considerable bulk of nerves is being given off, and thus there is an enlarge- Digitized by Microsoft® 392 A MANUAL OF VETEEINAEY PHYSIOLOGY ment corresponding to the fore, and another to the hind limbs. On exposing the spinal canal, a large number of nerves are found to be passing through the dura mater either outwards or inwards, and these gain an exit from or entrance to the spinal canal by means of the foramen formed at the junction of the vertebrae. Spinal Nerves. — On opening the dura mater, it is observed that the nerves divide in such a way that one part of each of them runs to the superior, and the other part to the inferior surface of the cord ; these are spoken of as the superior and inferior roots of the spinal nerves. In the horse each superior and inferior root enters the cord not as a single cord but as several. On the superior root, but outside the dura mater, is found a ganglion ; each rootlet of a superior spinal root has a ganglion on it : no such body exists on the inferior root. Both inferior and superior roots unite below the ganglion to form a mixed spinal nerve (see Fig. 84). The function of these two roots is entirely different ; the superior root, possessing the ganglion, conveys centripetal (sensory) impulses only ; the inferior root conveys centrifugal (motor) impulses to muscles and glands. The superior roots are passing into the cord, the inferior roots are passing out of it. Passing out with the inferior root of the spinal nerve, but indistinguishable from it, is a branch of nerve kndwn as the white ramus communicans, which leaves the main trunk after the mixed nerve has been formed, and runs to a distinct system known as the sympathetic. One part of the latter, the gangliated cord, runs under the arches of the ribs and back as far as the loins ; to this cord the white ramus runs, and establishes a communication between the cerebro- spinal and sympathetic system ; in this branch are, among others, the nerves which constrict the bloodvessels. A careful study of Fig. 85 is necessary for the clear elucida- tion of the arrangement of the spinal nerves. e 3 - Arrangement of the Cord. — If a cord be suitably prepared, , a transverse section shows that it consists of two similar halves, united by a comparatively small central mass of Digitized by Microsoft® THE NERVOUS SYSTEM 393 tissue through whose centre a minute (longitudinal) canal runs. The halves are separated by fissures on the superior and inferior surfaces of the cord ; the inferior fissure is wide and does not reach down to the centre of the cord, while the superior fissure is narrow and deeper (Fig. 84). Each half is further seen to consist of a superior, lateral, Fig. 84. — Transverse Section of the Spixal Cord in the Cervical Begion x 8d. The Lines in the Lateral and Superior Columns running from the Outer Margin are Lamina of the Pia Mater (M'Kendrick). a, Processus reticularis; b, superior horn; c, grey commissure; d, superior septum ; e, Goll's column ; /, superior column ; g, point of entry of superior root ; h, substantia gelatinosa ; i, lateral column ; j, large multipolar nerve cells ; Jc, inferior horn ; I, white com- missure ; m, inferior longitudinal fissure ; n, inferior column ; o, central canal ; p, point of exit of inferior roots. and inferior column, 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 placed internally, and forming the medulla, is arranged something like two commas placed back to back, the tail of the comma being uppermost. The tail of the comma corresponds to the incoming sensory Digitized by Microsoft® Fig. 85. — Scheme of the Nebves of a Segment of the Spina Coed (Poster). Gr., grey ; W, white matter of spinal cord ; A, inferior ; P, superior root ; G, ganglion on the superior root ; N, mixed nerve, con- sisting of sensory and motor branches with fibres passing into the sympathetic system at V ; N 1 , spinal nerve, con- sisting of sensory and motor branches, termin- ating in M, skele- tal muscles, S, sensory cell or sur- face, and X, in other ways. V, white ramus com- municans uniting the cerebro-spinal with the sympa- thetic system; it runs out from the cord with the inferior spinal nerve, and is given off from the mixed nerve at V, from whence it passes to 2, a ganglion on the sympathetic chain, and thence on to V 1 to supply the more distant ganglion cim i\ / ;t3 / /^ 1§>- * t ; ^ A\ 7 m.o.< I ^ <& i rG I Tnf Cornu. jiv.c— f j )r^ ^ & ^ m >^-. i s I S l m \^^ •<■'■■ w i>> ra_ \Jy^ ^N^^^^^^^^^^^"^ .' "■ — ■ — ^ ' «% t7- ^^X^T^l ^ ; % Sun:Cornu . / y L X \ "OR L. \ R Fig. 92. — Diagram op the Afferent and Efferent Paths passing to and from the brain by the cord (sherrington). l, Left, r, right ; obm, cerebrum ; cbm, cerebellum ; mo, medulla oblon- gata containing the decussation of p, the pyramidal tract, and of Digitized by Microsoft® THE NEKVOUS SYSTEM 407 /, the fillet; the decussation of /should really be a little higher instead of a little lower than that of p ; na, nucleus gracilis (Goll's) ; ot, optic thalamus ; pvc, the posterior vesicular column, or column of Clarke ; sp g, spinal ganglion ; oa, median posterior column (Goll's) ; do, direct cerebellar tract. The arrows show the direction of the impulses. A centripetal impulse, say from the skin, passes up the afferent nerve, through the spinal ganglion, and enters the superior columns of the cord ; it may pass to the cerebrum direct via the medulla by cg, the median posterior column, which crosses in the bulb and so gains the opposite side of the brain : or the impulse may pass by dc, the cerebellar tract, to the cerebellum, entering it on the same side, and from here crossing over to the opposite cerebral hemisphere. A centrifugal 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 of this as the inferior spinal nerve. of the cord, travelling by the crossed pyramidal tract to the multipolar cells of the inferior cornu of the grey matter, from which the motor nerves arise (Fig. 92). These efferent fibres are the longest in the cord, for unlike the afferent fibres they have few connections with spinal seg- ments, and practically run direct from their origin to their termination. It will be observed that all sensory impulses enter the brain on the side opposite to their origin, whilst all motor impulses leave the brain on the opposite side to that to which they are distributed, so that injury to a motor area of the right brain leads to a left-sided body paralysis. In the lateral columns of the cord both vaso-motor and sweat nerves travel ; decussating in the cord they enter the grey matter of the opposite inferior cornu, and pass out with the motor nerves from the spinal cord. Reflex Action. — Nerve fibres do not under natural circum- stances generate impulses, they transmit them but without modifying them ; modification can only occur in nerve centres, such as the brain and spinal cord, and these centres always consist largely of nerve-cells, of which the nerve- fibres leaving or entering the centre are simply processes or branches. Dealing at present solely with the spinal cord, it may be described not as one long centre, but a series of centres lying end to end, each capable to a greater or less Digitized by Microsoft® K)8 A MANUAL OF VETERINARY PHYSIOLOGY stent of acting independently of its neighbour, and each entre possessing its afferent and efferent roots. In these segments of spinal cord, complex acts can be aitiated by the arrival of simple centripetal impulses ; such ,cts may be carried out without any assistance from the rain, for they can readily be demonstrated in an animal /here the brain has been destroyed. These acts are known y the name of reflex, from which we must not infer that centripetal impulse is simply reflected into an efferent hannel, but rather that a centripetal impulse reaches the ord, and passing into the grey matter stimulates the anglionic cells which generate the efferent impulse. The structures necessary for a simple reflex act are L) an afferent nerve to convey the impression to a nerve entre ; (2) a nerve centre in which the outgoing impulses re generated ; (3) an efferent channel for their trans- lission. More complex acts may need more afferent erves, a larger number of excitable centres, and a greater umber of efferent fibres. A classical example of a reflex act is the drawing up f the leg when the foot is pinched in a frog from which le brain has been entirely removed. Depending upon the egree of pressure applied to the foot, it draws up either ne leg or both, i.e., the reflex movements are unilateral or Symmetrical, according to the number of ganglionic centres 1 the cord which have been stimulated. Still greater iolence applied to the foot of this brainless frog will affect larger number of centres further forward in the cord, so lat the fore-limbs may share in the reflex, and this is uown as irradiation ,- still further excitation may produce mvulsive movements of the entire body, known as general ztion. The brainless frog reacts more regularly to this experi- ;ent than one possessing a brain, which is evidence that le brain is capable of exercising a controlling influence or ihibitory effect over reflex actions. One very prominent feature of a reflex act is its jparently purposeful character ; turning once more to Digitized by Microsoft® THE NEKVOUS SYSTEM 409 the brainless frog, if an acid be applied to the skin of the flank the foot endeavours to remove the source of irritation. In the dog very characteristic reflex actions occur after division of the cord, such as those of walking, galloping, micturition, and defsecation ; tickling the skin causes the animal to scratch the part with the hind foot. The higher we ascend in the animal scale 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. Still, locomotion is often essentially a reflex act, and is very complicated ; for instance the tactile and muscular centripetal impulses required, the exact group- ing of muscles, and the regulation of the degree and rapidity of contraction, would appear at first sight to need the supervision of the highest centres in the brain, but such is not the case ; a pigeon will fly after decapitation. If a horse had to think of every step he had to take he would soon be worn out and blunder. That the higher centres do at times come into play is shown by the judgment which the horse exercises when jumping, viz., the proper distance to take off at, the amount of muscular contraction required to lift the body, and the needful height to which it should be raised, etc., are all evidence of this. J_J 41 By a Co-ordinate Movement is meant one in which the contraction of various related groups of muscles is so 7 adjusted that the extent of their contraction, and every- ^ thing necessary for a perfect movement, is present and faithfully carried out. This co-ordination of movement we have seen may occur even without the assistance of the brain, and we have alluded to the complex co-ordinate movements of locomotion as an example of this. In the spinal cord, therefore, not only reflex but co-ordinate move- ments are generated ; even the crossed or diagonal move- ments 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. Digitized by Microsoft® HO A MANUAL OF VETEEINAKY PHYSIOLOGY The Stepping Reflex. — As an example of reflex action hat of stepping may be considered. When in the dog fhe pinal cord has been severed in the hinder part of the iervical region and the ' shock ' from the transection has )assed off, reflex walking is observable. The walking move- nent includes alternate flexing and straightening of the imb. The forward movement of the hind leg in taking a itep is produced by flexion at the hip, and to prevent the bot brushing against the ground as the leg swings forward lexion occurs at the stifle and hock so as to somewhat aise the foot. The limb is then straightened again, so hat the foot may reach the ground and bear the weight of he body. In order to prevent the limb doubling up under his burden the extensor muscles which support the patella oint and hock from bending have to contract with sufficient lower. Stiffened by the contraction of these muscles the imb serves as a prop to carry the body. While the foot ests on the ground the body moves forward so that in due ourse the hip advances in front of a vertical drawn upward rom the foot. The extended hind limb at this time is loped somewhat backward as well as downward. When his poBture is reached the extensor muscles are thrown nto further action, and give the limb a push off from the [round, propelling the body forward. The hind limb thus nakes its contribution to the progression forward of the >ody. In galloping this extensor thrust is very marked, ,nd is given by both hind legs together, instead of alter- lately as in walking and running. In this reflex, spinal stepping, we may study first the lexion of the limb which occurs in the forward movement f the step. Flexion similar but more pronounced can be iasily excited in the spinal dog* by exciting the skin of the aot electrically. Though the flexion occurs at hip, patella oint, and hock together, it will be simpler to confine our ixamination to the flexion at one of these joints only, for * ' Spinal dog ' is the term used for a dog in which the reflexes are ntirely spinal, owing to the brain having been destroyed, or the cord aving been cut off from the brain. Digitized by Microsoft® THE NEEVOUS SYSTEM 411 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 of 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 Fig. 93.— Diagram of a Reflex Arc (Sherrington). dd, dd', Dendrites ; pk, pk', perikaryon ; ax, ax', axones ; sy, synapse, dd + pk + ax = neurone; (dd + pk + ax) + sy + (dd' + pk' + ax') = con- ductor ; Re, Re = receptors (epithelial) ; Ef, Ef = effectors (muscle). The parts within the dotted line lie in the grey matter of the nerve centre. of the stifle contract, the extensor muscles of the stifle, the vasti and crUreus, are relaxed. In order to understand how this comes about it is necessary to refer to a principle of construction of the nervous system which has been termed ' the principle of the common path.' The structures along which the nervous impulse in a reflex action runs constitute what is called a reflex arc. A reflex arc is a chain of nerve cells (Pig. 93). Each nerve cell consists of three parts : (1) a cell body containing the Digitized by Microsoft® 412 A MANUAL OP VETERINABY PHYSIOLOGY nucleus and called the perikaryon ,- (2) one or more (usually many) branches from the perikaryon, called dendrites, which conduct impulses to the perikaryon ; (3) one branch from the perikaryon which conducts impulses away from the perikaryon, and this branch is called the axone. The whole nerve cell thus composed of these three parts is termed a neurone. The neurones forming a reflex arc follow each other end to end like links in a chain. In the chain the neurones composing it are joined in such a way that the axone of one neurone meets the dendrites of the next neurone, and these junctions of the axone of one neurone with the dendrites of the next are of such a nature that conduction of impulses from one neurone to the next occurs in one direction only, that is from axone to dendrite, and not backwards from dendrite to axone. These junctions between neurones are termed synapses (see Pig. 93). The first link of each reflex chain is a neurone which starts in a receptor organ, e.g., a sense-organ. A receptive field, e.g., an area of skin, is always analysable into receptive points, and the nerve-path of the reflex always starts from a receptive point or points. A single receptive point may play reflexly upon quite a number of different sffector organs. It may be connected through its reflex path with many muscles and glands in various parts. Yet ill its reflex arcs spring from the one single shank, so ;o say ; that is, from the one afferent neurone that conducts :rom the receptive point at the periphery into the central lervous organ. This neurone dips at its deep end into the jreat central nervous organ, the cord or brain. There it snters a vast network of conductive paths. In this net- work it forms manifold connections. So numerous are its jotential connections there that, as shown by the general ionvulsions induced under strychnine-poisoning, its im- )ulses can discharge practically every muscle and effector >rgan in the body. Yet under normal circumstances the mpulses conducted by it to this central network do not rradiate there in all directions. Though their spread over Digitized by Microsoft® THE NEKVOUS SYSTEM 413 the conducting network does, as judged by the effects, increase with increase of stimulation of the entrant path, the irradiation remains limited to certain lines. Under weak stimulation of the entrant path these lines are sparse. The conductive network affords, therefore, to any given path entering it some communications that are easier than others. This canalization of the network in certain direc- tions from each entrant point is sometimes expressed, borrowing electrical terminology, by saying that the con- ductive network from any given point offers less resistance along certain circuits than along others. This recognizes the fact that the conducting paths in the great central organ are arranged in a particular pattern. The pattern of arrangement of the conductive network of the central organ reveals something of the integrative function of the nervous system. It tells us what organs work together in time relationship. The impulses are led to this and that effector organ, gland, or muscle in accordance with the pattern. At the commencement of every reflex arc is a receptive neurone, extending from the receptive surface to the central nervous organ (see Fig. 93). That neurone forms the sole avenue which impulses generated at its receptive point can use whithersoever may be their distant destination. That neurone is, therefore, a path exclusive to the impulses generated at its own receptive points, and other receptive points than its own cannot employ it. But at the termination of every reflex arc we find a final neurone, the ultimate conductive link to an effector organ, gland, or muscle. This last link in the chain, e.g., the motor neurone, differs in one important respect from the first link of the chain. It does not subserve exclusively impulses generated at one single receptive source alone, but receives impulses from many receptive sources situate in various regions of the body (see Fig. 93). It is the sole path which all impulses, no matter whence they come, must travel if they would reach the muscle- fibres which it joins. Therefore, while the receptive Digitized by Microsoft® 14 A MANUAL OF VETERINAEY PHYSIOLOGY leurone forma a private path exclusively for impulses of me source only, the final or efferent neurone is, so to say, , public path, common to impulses arising at any of many ources in a variety of receptive regions of the body. The ame effector organ stands in reflex connection not only yith many individual receptive points, but even with many 'arious receptive fields. Eeflex arcs arising in manifold ense-organs can pour their influence into one and the ame muscle. A limb-muscle is the terminus ad quern of lervous arcs arising not only in the right eye but in the eft, not only in the eyes but in the organs of smell and tearing ; not only in these, but in the otic labyrinth, n the skin, and in the muscles and joints of the limb itself tnd of the other limbs as well. Its motor nerve is a path lommon to all these. Eeflex arcs show therefore the general feature that the nitial neurone is a private path exclusive for a single eceptive point, and that finally the arcs embouch into , path leading to an effector organ, and that this final path 3 common to all receptive points wheresoever they may lie u the body, so long as they have any connection at all yith the effector organ in question. Before finally converg- og upon the motor neurone, the arcs usually converge to ome degree by their private paths, embouching upon inter- mncial paths common in various degree to groups of trivate paths. The terminal path may, to distinguish it rom internuncial common paths, be called the final common >ath (see Fig. 93) . The motor nerve to a muscle is a collec- ion of such final common paths. A result is that each receptor being dependent for com- aunication with its effector organ upon a path not ex- lusively its own but common to it with certain other eceptors, that nexus necessitates successive and not imultaneous use of the common path by various receptors ising it to different effect. The Scratch Reflex. — Good opportunity for study of this orrelation between reflexes is given in the ' scratch reflex.' Vhen the spinal cord has been transected in the neck, this Digitized by Microsoft® THE NEEVOUS SYSTEM 415 reflex in a few months becomes prominent. Stimuli applied within a large saddle-shaped field of skin (Fig. 94) 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 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 faradization. Prominent among the muscles active in this reflex are Fig. 94. — The Scratch Reflex (Sherrington). The ' receptive field,' as revealed after low cervical transection, a saddle-shaped area of dorsal skin, whence the scratch reflex of the left hind limb can be evoked. Ir marks the position of the last rib. the flexors of the hip. If we record their rhythmic con- traction we obtain tracings as in Fig. 96. A series of brief contractions succeed one another at a certain rate, whose frequency is independent of that of the stimulation. The contractions are presumably brief tetani. The stimulus to the hair-bulbs of the shoulder throws into action a lumbar spinal centre, innervating the hip-flexor, much as the bulbar respiratory centre drives the spinal phrenicus centre. In the case of the respiratory muscle the frequency of the rhythm is, however, much less. Digitized by Microsoft® 16 A MANUAL OF VETEEINAEY PHYSIOLOGY The reflex is unilateral : stimulation of the left shoulder rokes scratching by the left leg, not by the right. In the ,teral column of the spinal cord long fibres exist directly mnecting the spinal segments of the shoulder with the tinal segments containing the motor neurones for the exor muscles of the hip, and knee, and ankle. We thus rrive at the following reflex chain for the scratch reflex : .) The receptive neurone (Pig. 95, sa), from the skin to le spinal grey matter of the corresponding spinal segment 1 the shoulder. This is the exclusive or private path of le arc. (ii.) The long descending proprio-spinal neurone ig. 95. — Spinal Arcs involved in Scratch Reflex (Sherrington). iagram of the spinal arcs involved in Fig. 94. L, receptive or afferent nerve path from the left foot ; e, receptive nerve-path from the opposite foot ; sa, s/3, receptive nerve-paths from hairs in the dorsal skin of the left side ; re, the final common path, in this case the motor neurone to a flexor muscle of the hip ; pa, p/3, proprio-spinal neurones. ?ig. 95, pa), from the shoulder segment to the grey latter of leg segments. (iii.) The motor neurone ?ig. 95, fc), from the spinal segment of,_the leg to the exor muscles. This last is the final common path. The tiain thus consists of three neurones. It enters the grey latter twice, that is, it has two neuronic junctions, two ^napses. It is a disynaptic arc. Now if, while stimulation of the skin of the shoulder is voking the scratch reflex, the skin of the hind foot is timulated, the scratching is arrested. Stimulation of le skin of the hind foot causes the leg to be flexed, Digitized by Microsoft® THE NEEVOUS SYSTEM 417 drawing the foot up. 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 (i.) the receptive neurone (Fig. 95, l) (nociceptive) from the foot to the spinal segment ; (ii.) perhaps a short intraspinal neurone ; and (iii.) the motor neurone (Pig. 95, fc) to the flexor muscle, e.g., of hip. Here, therefore, we have an arc which embouches into the same final common path as sa. The motor neurone fc is a 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 effect specific to each differs strikingly in the 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 accom- panying tracing (Fig. 96) shows the result of conflict between the two reflexes. The one reflex displaces the other from the common path. There is no compromise. The scratch reflex is set aside by that of the nociceptive arc from the foot. The stimulation which previously sufficed to evoke the scratch reflex is no longer effective, though it is con- tinued all the time. But when the stimulation of the foot is discontinued the scratch reflex returns. In that respect, although there is no enforced inactivity, there is inhibition. There is interference between the two reflexes, and the one is inhibited by 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 takes its place. Suppose, again, during the scratch reflex, stimuli are applied to the foot, not of the scratching but of the opposite side (Fig. 95, r). This stimulation of the foot causes flexion of its own leg and extension of the opposite. If, when the left leg is executing the scratch reflex, the right foot is stimulated, the scratching, involving as it does the left leg's flexors, is cut short. This inhibition of the flexor scratching movement occurs sometimes when the con- 27 Digitized by Microsoft® A MANUAL OF VETEEINAEY PHYSIOLOGY 2-a a « tvtf "^ "K ,-.■"33+' ■o a o a pja . •„ . „ ^ m en .£P £P Ki ""' .S — ?c^ S«" o ^ c -2 .SP a *" 2 " a '" <3 o o *> n a - « S oig-g ^ a &« J2 o X a j a +» MJ _ a co <§ * s ? * > M 2? ^ll o is cj 7 S fl © ^ > a .s.s o 55 a co o ."S 2* is jS5 o g ; < H a n t^si H 22 d ■<♦-< 45 «. 5 o^_ w 8 >■ " w t3 "^ oj a o O g -SB^ « a P a a W 2 2-3 <2^ f |c!-|oS 5 s H 1 CO 03 rt 2 a o o" » k< y 3 5»* ca ■£ 3 3 '43 « .3-2 "-h* « Si O) d w i-5 is a d + -S .3 _bf e + Digitized by Microsoft® THE NEEVOUS SYSTEM 419 traction of the extensors is minimal or hardly per- ceptible. It is obvious from this that the final common path, pc, 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 (Pig. 95, l) and shoulder skin (Fig. 95, sa, s/3), but also to arcs from the opposite foot (Fig. 95, e), 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 slight, steady extension with some abduction. This extension 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 at the same time appropriately and strongly stimulated. This latter stimulus often inhibits the scratch- ing 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 27—2 Digitized by Microsoft® A MANUAL OF VETEEINAEY PHYSIOLOGY on, but it inhibits the flexor muscles of the opposite leg. hrows into contraction the extensor muscles of that leg. the previous example there was a similar co-ordination. j motor nerve to the flexor muscle is therefore under control not only of the arcs of the scratch reflex from homonymous shoulder, but of those from the crossed ulder as well. But in regard to their influence upon i final common path, the arcs from the homonymous ulder and the opposite shoulder are opposed. Ixperiments show that this inhibition does not take place the motor nerve itself. Many circumstances connect vith the place where the converging neurones come sther in the grey matter at the commencement of the imon path. The field of competition between the rival i seems to lie in the grey matter, where they impinge jther upon the final or motor neurone. That is equiva- ; to saying that the essential seat of the phenomenon be synapse between the motor neurone and the axone- ninals of the penultimate neurones that converge upon There some of these arcs drive the final path into one 1 of action, others drive it into a different kind of on, and others again preclude it from being activated by rest. Ve are now in a posilion to return to the flexion at the e in the reflex act of stepping. We see that the same aulus which excites the motor neurones of the flexors lischarge motor impulses into those muscles, causes the ;or neurones of the antagonistic muscles, the extensors he knee, to cease discharging impulses, and keeps them vented from discharging impulses. The stimulus sets up intraspinal excitation of the motor neurones of the flexor scles and an intraspinal inhibition of the motor neurones Brvating the extensor muscles. Vhen the flexion phase of the act of stepping has been sed through, the leg extends again, perhaps by its own ght, perhaps by return of activity in the motor neurones he extensor muscles which had been inhibited. In due rse the foot reaches the ground. When it does so the Digitized by Microsoft® THE NERVOUS SYSTEM 421 weight of the body gradually comes upon it, and soon presses the sole of the foot with its full force 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 sole of the foot 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 foot is pressed against the ground by the weight of the body in the act of stepping. This extensor thrust gives the propulsive movement of the body forward, 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, and is then given by the two hind-limbs together, and not alternately, as in walking and running. The above comparatively simple acts form certainly only a part of the whole complex reflex which really occurs in stepping. In such complex reflexes many stimuli are at work together, and co-operate harmoniously for a co- ordinate result. In walking, running, etc., probably 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 those joints. These joints and muscles are liberally supplied with afferent nerves conducting centripetal impulses from them. 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 all the nerves of all four of the feet have been severed — not only the skin-nerves of the feet, but all their deep nerves as well. In the second place, when the spinal dog is lifted so that its limbs do not touch any solid support whatever, reflex walking and galloping are performed, although the limbs are stepping wholly in the air. But, for this reflex walk- ing in the air, it is necessary that the limbs hang down. The reflex ceases if the dog be inverted, so that gravity no longer is acting on the joints and muscles as it does in the position usually accompanying acts of stepping. Further, Digitized by Microsoft® A MANUAL OF VBTEEINARY PHYSIOLOGY s probable that the spinal centres which execute reflex king, running, etc., receive much help and direction from rent arcs which arise in the labyrinth of the ear. ?he stimuli, which are the source of reflex walking, etc., 3e, therefore, almost certainly in many receptive organs, [ividually the action of each of these may be quite weak. ay sum up their effect, because their impulses converge the same final common paths, the motor neurones. a summation is probably largely the work of these tor neurones. Their shape bespeaks for them the func- i of an organ for such summation. In each motor irone its dendrites converge to the perikaryon as a eting place, and there the impulses carried to it by the ldrites add their excitatory effects together. As the lapses are places where inhibition and irreversibility of iduction are established in the reflex arc and where lex arcs meet, so the perikaryon seems a place where nmation of impulses from various harmonious sources : added together for a conjoint effect. But the reflex acts carried out by the cord are not dted to those affecting skeletal muscles; the act may a secretory or nutritive one, or involving the contraction relaxation of pale muscle : for example, the contraction I dilatation of the bloodvessels under the influence of j vaso-motor system, the peristaltic movements of the estines, the contraction of the bladder and uterus, and } secretions from the various abdominal glands, are all imples of reflex acts. The time occupied by a reflex act ries dependently upon the strength of the stimulus and 3 nature of the reflex ; the sharper the stimulus the more aid the reflex, the more active the centre the more rapid i response ; impulses which have to cross the cord take lger than those which enter and return from the same le. It is mainly during this appreciable delay, as measured delicate apparatus, that the changes are occurring in the ey substance which lead to an efferent response. In the dog 3 time occupied by a reflex on the same side is estimated "022 up to 2-3 seconds, according to circumstances. Digitized by Microsoft® . THE NEEYOUS SYSTEM 423 // f» Q Tendon Keflexes.— The muscle and tendon reflexes, so y well known in the human subject, have not, so far as we fo L are aware, been studied in the ungulates ; nor do we know £ whether the existence of any reflexes has been demon- strated, if, perhaps, we except the immediate lifting up of the foot, which generally follows pressure on the so-called ' chestnut ' found on the inside of the fore-arm of the horse. 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, and although often called a reflex act cannot truly be so, because the time between the blow and the jerk is too short for any reflex act. It is well seen in the dog, cat, rabbit, etc. Although not a reflex action 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. The jerk is a good index of the condition of the reflex arc, and therefore of the condition of the activity or depression of the segments of the cord by which the extensor muscles are innervated. It is depressed during sleep or anaesthesia, and by ansemia of the cord ; it is intensified when the cerebral restraint is removed from the lumbar spinal segments by diversion or attention to another part, or by severance of the cord in the dorsal region. Another brisk ' jerk ' in the dog is the ischial, obtained from the hamstring muscles by tapping the tuberosity of the ischium. Automatic Action. — Nerve centres are not as a rule capable of issuing impulses which are not the result of an afferent stimulus ; one centre there is however which seems to do so. This is the respiratory centre in the bulb (p. 109). In the same way the tone of the vascular system, or the force which keeps the muscular wall of the vessel in the neces- sary condition of constriction, is in part brought about by automatic impulses. The tone of the muscular walls of the vascular ' system seems to be due to tonic (permanent) Digitized by Microsoft® t A MANUAL OF VETERINARY PHYSIOLOGY ions, either of local nerve apparatus in the sympathetic tern or of the muscular coat itself. ipecial Centres in the Spinal Cord. — In the cord certain itres exist, which though ordinarily under the control of ihief centre in the bulb, yet are capable of carrying on uliar reflex actions even after the cord has been divided, 1 thus separated from the controlling influence of the b. Che cilio-spinal centre lies between the cervical and sal portions of the cord ; in it fibres originate which ough the cervical sympathetic supply the dilator muscle ihe iris. Destruction of the region in question causes a Ltraction of the pupil, whilst irritation of it causes the jil to dilate. Che ano-spinal centre, found in the lumbar portion of cord, controls the act of defsecation ; it would appear be highly developed in herbivora, which possess the ver of bringing it into play not only when the body is at t but during movement. The functions of the ano- nal centre are rather complex, inasmuch as it has not only maintain the tone of the sphincter, but also to relax it ring defalcation, and under the latter condition simul- :eously contracts both the wall of the intestine and the lominal muscles. Che vesicospinal centre also exists in the lumbar portion the cord, and governs micturition ; its action is similar that of. the ano-spinal centre. ja the lumbar portion of the cord other centres are nd, for example, the erection centre, the genito-spinal itre which contains the nervous apparatus employed in emission of semen, and the parturition centre. Vaso-motor centres are found throughout the cord ; they principally under the control of similar centres in bulb, but may act independently. Siveat centres are ibably closely connected with the vaso-motor centres. ophic centres for the nutrition of the tissues also exist the cord; destruction of parts by ulceration, or great scular wasting, may follow injury of the trophic nerves. Digitized by Microsoft® THE NEEVOUS SYSTEM 425 The Functions of the Spinal Cord may be summarized as follows : The cord is the central seat of numerous reflex actions; some of these are intermittent and occasional, others permanent or tonic, such as the maintenance of muscular and arterial tone. There is evidence that it assists in co-ordinating movement, and it is also the path by which the brain and the body are brought into con- nection, both in an upward and downward direction and n from side to side. Cranial Nerves. 49 These are divided into nerves of special sense, sensory ''X nerves, motor nerves, and mixed nerves. Altogether they make twelve pairs, and all but Nos. 1, 2 and 3 arise from the medulla. For nerves Nos. 1 and 2 see Smell and Vision, Chapter XV. Third Pair, or Motor Oculi, is one of the motor nerves of the eyeball ; it supplies with motor power all the muscles (excepting the external rectus and the superior oblique), also the muscle of the upper lid. Through its connection with the lenticular ganglion it supplies fibres to the iris and ciliary muscle ; it is also connected at its origin with two other motor nerves of the eyeball, viz., the fourth and sixth pairs. The deep-seated origin of the third pair is from the corpora quadrigemina and peduncles of the cerebrum. Division of the nerve causes the eye to turn downwards and outward^, owing to the unbalanced action of the superior oblique and external rectus ; there is also depression of the upper lip, immobility of the eyeball, and dilatation of the pupil. The action of the third pair will be discussed again in connection with the physiology of sight. Fourth Pair, or Pathetic. — The motor nerve of the superior oblique muscle of the eyeball; it has a deep- seated origin in the valve of Vieussens. Fifth Pair, or Pars Trigemini, resembles a spinal nerve in having two roots, a motor and sensory ; and the resem- Digitized by Microsoft® 26 A MANUAL OF VETEKINARY PHYSIOLOGY lance is carried still further by the sensory root having large ganglion on it, the Gasserian. The motor root rises from the trigeminal nucleus of the medulla, and is onnected with the cerebral cortex on the opposite side, ^e sensory fibres arise from the sensory trigeminal tucleus, and can be traced downwards into the grey aatter of the cord. This nerve also has connections with be nerves arising from the medulla ; in this way can be xplained the extensive connections and varied reflex acts f the fifth pair. There are three divisions of the fifth pair of nerves, viz., he ophthalmic, the superior maxillary division, and the nferior maxillary division. The ophthalmic division is the smallest of the three urnished by the Gasserian ganglion ; it is exclusively ensory, supplying with sensation the structures over the irow, the eyeball, the lachrymal gland, membrana nictitans, ,nd the pituitary membrane on both sides. The superior naxillary division is wholly sensory and supplies part of he orbit, eyelids, skin, hard and soft palates, pituitary nembrane of the nostrils, and teeth (molars, incisors, and anine), whilst the terminations of the main trunk are extended over the face, upper lip, and nostrils, by means )f a considerable plexus of nerves which issues from the nfra-orbital foramen. The inferior maxillary division is i mixed nerve ; it supplies motor power to the muscles of nastication, viz., the masseters, buccal muscles, internal pterygoid, part of the temporalis, and the mylo-hyoid nuscle of the tongue. By means of its great lingual jranch, which enters the tongue in conjunction with the :horda tympani of the seventh nerve, common sensation s supplied to the anterior two-thirds of the tongue. Besides the above, sensory branches are supplied to the ieeth and lips near the commissures, and filaments to the parotid, molar, and buccal glands. Each of these main divisions of the fifth nerve possesses i ganglion on it, viz., the ophthalmic on the ophthalmic Dranch, the spheno- palatine on the superior branch, and the Digitized by Microsoft® THE NEEVOUS SYSTEM 427 otic ganglion on the inferior branch. All these ganglia receive branches of nerve from the sympathetic and cerebro-spinal system. It is from the ophthalmic or ciliary ganglion that the ciliary nerves of the iris and ciliary muscle arise, the motor root of the ganglion being supplied by the third nerye, and the sensory from a branch of the ophthalmic of the fifth. The ganglion on the superior branch is known as the sphen o- palatine ; it receives its motor supply through the Vidian nerve from the facial, its sensory roots being numerous and supplied by the spheno- palatine branch of the fifth. This ganglion supplies branches to the bloodvessels of the orbit, and others to the palate through which motor power is supplied to the muscles of the soft palate. On the inferior division of the fifth is sometimes found a ganglion known as the otic, the motor root of which is derived from the seventh pair, and the sensory from the inferior branch of the fifth. This ganglion gives branches which supply the tensor tympani of the internal ear, and some branches to the Eustachian tube and tensor palati. In the dog and cat is found the submaxillary ganglion ; it is supplied by the chorda tympani of the seventh pair with secretory fibres for the gland and dilator fibres for the bloodvessels ; to this ganglion also runs a branch of the sympathetic. All the fibres of the chorda do not enter the gland, some supply the tongue. A submaxillary ganglion exists in both dog and cat ; it lies in the hilum of the gland of the same name. Division of the superior maxillary division of the fifth in the horse (Bell's experiment) prevents the animal from grasping food with its lips ; not for the reason that they are deprived of motion, but owing to loss of sensibility the ■ animal is unaware of how to take hold of the food. The relation of the fifth to muscular movements is that it keeps the muscles aware of the position of objects. Complete section of the fifth pair causes loss of sensation to one side of the face, lips, mouth, and temple, part of the ear, cornea, conjunctiva, nasal mucous membrane, and Digitized by Microsoft® 428 A MANUAL OF VETEEINAEY PHYSIOLOGY anterior two-thirds of the tongue. There is paralysis of the muscles of mastication, and the mouth becomes injured by the teeth owing to loss of sensibility ; the food collects on the paralyzed side, where it decomposes and produces local irritation. The animal also frequently bites its tongue, as its position in the mouth cannot be felt. The cornea may become cloudy and ulcerates. As an afferent nerve in reflex acts, the fifth nerve is most important ; without it there would be no closure of the eye nor sneezing, and irritation of the conjunctiva would produce no tears. Sixth Pair, or Abducens, arises from the floor of the fourth ventricle, and supplies the external rectus muscle of the eye with motor power. Paralysis of this muscle causes internal squint. Seventh Pair (Portio Dura), or Facial. — Arises from the medulla, passes through the internal auditory meatus in company with the eighth pair, which it leaves behind in the internal ear, whilst the seventh nerve escapes by the aqueduct of Fallopius, passes beneath the parotid, and finds its way on to the cheek over the external masseter muscle, and is eventually distributed to the upper and lower lips and the alas of the nostrils. It essentially supplies the muscles of expression and not those of mastication. In its course it is joined by branches from the fifth pair and vagus, and gives off to the lingual of the fifth, as pre- viously mentioned, a branch known as the chorda tympani, supplying the front portion of the tongue with taste, and secretory fibres to the maxillary gland and dilator fibres to the bloodvessels. It is really a branch not of the facial but of the nervus intermedins, the little cranial nerve lying between the facial and the nerve of the ear. The facial is a motor nerve to the muscles of the middle ear, external ear, cheeks, lips, nostrils, and orbicular muscle of the eye. Division of the seventh nerve leads to alterations in sight, taste, hearing, smell, and facial expression. As it supplies the muscle which closes the eyelids (the orbicularis palpe- Digitized by Microsoft® THE NEEVOUS SYSTEM 429 brarum), conjunctivitis occurs from exposure of the eyeball; hearing is affected owing to paralysis of the muscles of the internal ear ; smell is impaired due to the paralyzed con- dition of the nostrils ; taste is affected through paralysis of the chorda. The expression of unilateral facial paralysis in the horse is characteristic ; the upper lip drawn to one side, the elongated nostril, the pendulous lower lip, the escape of saliva and food from the mouth, the vacant look, Pkj. 97. Characteristic Facial Expression op the Horse with Paralysis of the Seventh Nerve. the open eye, and the drooping ear, point clearly to the extensive distribution of this nerve. Bernard pointed out that horses were suffocated if galloped after division of both facial nerves, owing to the fact that the nostrils were no longer capable of dilatation. Eighth Pair, or Portio Mollis. — Arises by two roots, one the nerve for the special sense of hearing, the other dis- tributed to the otolith organs and the semicircular canals, and assists through these in maintaining the equilibrium Digitized by Microsoft® t30 A MANUAL OP VBTEEINARY PHYSIOLOGY )f the body. Injury to the semicircular canals produces giddiness, not deafness, and certain movements (termed pendulum-like ') of the head occur ; the direction in which ihese are made depends on the orientation of the canal which has been injured. Ninth Pair, or Glosso-Pharyngeal, arises from the medulla ; t is a mixed nerve, and supplies motor power to the muscles of the pharynx, and sensory fibres to the posterior ;hird of the tongue, soft palate, part of pharynx, and interior surface of the epiglottis. It is also a special nerve }f taste, supplying the posterior third of the tongue, and laving special nerve endings, known as ' taste-bulbs,' in the jircumvallate papillae. Tenth Pair, or Pneumogastric. — This is both a sensory and motor nerve. At its origin in- the medulla it is intimately mixed up with the ninth, eleventh, and twelfth pairs of nerves, and later on with the sympathetic. It is the most sxtensively distributed nerve in the body, supplying the Esophagus, pharynx, lungs, bronchi, trachea, heart, stomach, md intestines ; hence its other name, vagus. The sensory branches of the nerve are not highly andowed with sensation, probably for the reason that their 3hief function as sensory nerves is as afferent channels for reflex action. The motor fibres are derived from the spinal iccessory nerve. In the foramen lacerum the vagus is joined by the jugular ganglion, and for a very short distance it is intimately connected with the accessory nerve ; here it receives filaments from the accessory, sympathetic, hypoglossal, and two first cervical nerves. The vagus now descends behind the guttural pouch and joins the cervical portion of the sympathetic nerve, from which results, in the horse and most other animals, a single cord which passes down the neck above the carotid artery ; as it enters the shest it separates from the sympathetic. The arrangement of the right and left nerves is different ; the right gives off the right recurrent which passes around the dorso-cervical artery, while the main trunk terminates above the origin of the bronchi ; the left gives off its recurrent branch Digitized by Microsoft® THE NEEVOUS SYSTEM 431 opposite to the aorta, and also terminates on the bronchi, forming with the right nerve the bronchial plexus and oesophageal nerves, the latter passing to the stomach and from thence to the solar plexus. The various branches of the vagus may best be studied in the order in which they are given off. The pharyngeal nerve originates at the superior cervical ganglion and passes to the pharynx, where it forms with the ninth pair the pharyngeal plexus. It is a mixed nerve, and supplies the middle and constrictor muscles of the pharynx and the cervical portion of the oesophagus witb motor power. The superior laryngeal nerve supplies the mucous mem- brane of the larynx with remarkable sensibility, and gives a motor branch, the external laryngeal, to the crico-pharyn- geus. In most animals the superior laryngeal supplies the crico-thyroid muscle of the larynx with motor power, but in the horse this is supplied by the first cervical nerve. It is the superior laryngeal nerve which reflexly excites coughing, the coughing centre being situated in the medulla; further, it contains afferent fibres in connection with the respiratory centre, which when stimulated cause arrest of respiration : they are therefore inhibitory fibres. Section of the superior laryngeal causes pain, and produces in dogs a deeper and hoarser voice due to paralysis of the crico-thyroid muscle, which can no longer render the vocal cords tense. The absence of sensibility in the larynx allows food to pass into the trachea, and thus produces pneumonia. The inferior laryngeal, or recurrent, is given off from the main trunk within the chest, on the left side winding around the aorta from without inwards, and on the right side passing around the dorso-cervical artery ; both branches return up the neck and supply all the muscles of the larynx (excepting the crico-thyroid) with motor power. The recurrents are of great practical interest, as they are affected (especially the left) in that common form of disease in the horse known as ' roaring,' which is generally due to paralysis Digitized by Microsoft® !2 A MANUAL OP VETEEINAEY PHYSIOLOGY id atrophy of the muscles which dilate the laryngeal open- g (see p. 126) After division of both recurrent nerves death t asphyxia is likely to follow. We have however observed implete bilateral paralysis of the larynx in horses without iphyxia being produced. In such cases it has been shown tat the age of the horse is the saving factor, the rigidity of ie cartilages preventing the arytenoids from completely illapsing over the opening of the glottis. Division of the recurrent also leads to a partial loss of )ice, and a peculiar cough is produced owing to paralysis the laryngeal muscles. As the recurrent supplies nsory branches to the tracheal portion of the oesophagus id trachea, division causes loss of sensation in these irts. It is curious that the recurrent laryngeal should contain otor fibres, not only for the dilator but also the constrictor uscles of the larynx ; it has been observed that when this 3rve gets out of order, it is the dilator muscles which first scome paralyzed and later the constrictors (p. 125). Irrita- mi of the peripheral end of the recurrent produces spasm the larynx. There are certain poisons, such as that con- ined in Lathyrus sativus and others of the Leguminosse, hich appear to have a special action on this nerve, or at ly rate on the larynx, spasm of the larynx being one of Le earliest symptoms of poisoning. The cardiac branches of the vagus contain the fibres hich exercise a controlling and inhibitory power over the 3art (see p. 45). They also contain the depressor nerve hich is leaving the heart to run up the neck with the leumogastric, entering the medulla by means of the iperior laryngeal branch ; for the indirect action of this jrve on the heart see p. 50. The depressor nerve is •esent as a distinct branch in the rabbit and cat, but in her animals it is mixed up with the vagus. Lastly, the ,rdiac branches contain fibres from the sympathetic which ipply accelerator fibres to the heart (p. 49). The pulmonary branches supply both sensory and otor branches to the trachea and motor fibres to the Digitized by Microsoft® THE NERVOUS SYSTEM 433 bronchi. Through these branches impressions are trans- mitted to the medulla by which the respiratory centre is regulated. Through other branches centripetal impulses are transmitted to the vaso-motor centre by which the general blood-pressure is regulated. The thoracic oesophageal branches supply the oesophagus with motor power, so that division of the vagus causes food to accumulate in the lower part of the tube. The oesophageal nerves, after uniting in pairs in a peculiar manner, run along the oesophagus one superiorly the other interiorly, and passing through the diaphragm they enter the abdominal cavity. The superior nerve supplies the left sac of the stomach and enters the solar plexus, from which it runs to the intestines and other organs (p. 206) ; the inferior nerve terminates in the walls of the stomach at its cardiac or right extremity. Division of both vagi in the horse causes the breathing to become much deeper, more prolonged, and suffocation may result owing to loss of motor power in the larynx. Through the absence of sensation in the larynx, trachea, bronchi, and lungs, food is apt to find its way into the respiratory passages and produce pneumonia. The lungs likewise undergo congestion owing to the laboured and difficult respiration, and the parts become cedematous. In the horse the respirations have been known to fall to five per minute, but the heart beats rapidly owing to the unbalanced action of the sympathetic. Through paralysis of the oesophagus and stomach food collects in the latter, and may extend throughout the entire length of the oesophagus up the neck. Apparently engorgement of the stomach in the horse is not invariably produced as the result of dividing both vagi, for some observers have noted no difficulty in this respect. Experiments made by Colin show that division of the vagi paralyzes the stomach, so that poisons may remain there and cause the animal no inconvenience as they never pass into the intestine, and thus cannot become absorbed (see p. 176). This is a point of practical importance, and warns us how useless drugs 28 Digitized by Microsoft® 54 A MANUAL OF VETEEINAEY -PHYSIOLOGY Iministered by the mouth may be in some digestive oubles of the horse, especially those of the stomach. Eleventh Pair, or Spinal Accessory, arise by two roots, le from low down the cervical portion of the cord, the her from the medulla. It is essentially a motor nerve, it through being intimately connected with the pneumo- istric it also possesses sensory fibres., The use of this 3rve is to supply motor power to the sterno-maxillaris, apezius, and a portion of the levator humeri muscles ; its origin it supplies most of the motor fibres found the vagus, and also furnishes the latter with its cardio- hibitory fibres. The accessory is considered also to issess an influence over the larynx; division of it pro- ices no difficulty in breathing, as in the case of the current laryngeal, but it causes loss of voice due to tralysis of the motor fibres of the vagus. Twelfth Pair, or Lingual. — The branches of this nerve pply the tongue with motor power, and fibres to the uscles which depress the larynx. Section of the nerve i both sides causes paralysis of the organ ; dogs are lable to lap, and injure the protruding tongue with the eth. Medulla Oblongata or Sulb. Situated at the top of the spinal cord, and forming the nnection between it and the brain, is the medulla ilongata. It is composed of white and grey matter, but it arranged with the regularity found in the cord ; the lumns of the latter are continued into it, and give rise to rtain columns in the bulb larger and more prominent an those of the cord. The inferior columns form the ferior pyramids of the bulb, the superior form the iperior pyramids, and the lateral columns dividing into ree parts help to form the restiform bodies. As the main paths or highways in the cord are either >ing to or coming from the brain, it is interesting to study ■iefly their distribution in the bulb. Of all the paths known . the cord only three pass for certain through the bulb to Digitized by Microsoft® THE NERVOUS SYSTEM 435 higher centres in the brain, viz., the pyramidal tract the fibres of which are descending to the cord from their origin in the cells of the cerebral cortex, and the cerebellar tracts which pass upward through the medulla to reach the cerebellum. The tracts passing through the bulb from the cerebrum decussate in the bulb, and in this way account for a right brain lesion producing a left body paralysis. All the other tracts in the cord terminate in groups of cells in the bulb, and act as carriers between it and the cord. The grey matter of the cord does not maintain its charac- teristic appearance in the bulb, the inferior cornua dis- appear, while the superior cornua enlarge. Owing to the decussation of fibres in the inferior pyramid the grey and white matter get mingled up, and nuclei and masses of nerve cells are formed as the result ; from these nuclei the cranial nerves arise. This arrangement leads to con- siderable complexity in the grey matter of the medulla, and a markedly intricate arrangement of the fibres of the white substance. /f Centres in the Medulla. — The various centres found in the bulb are of such importance to life that an injury to this part generally means instantaneous death. The whole of, the rest of the brain may gradually be removed without destroying life, but the medulla itself will not tolerate inter-, ference. The reflex and other centres are so numerous and widespread, that it is remarkable how the varied functions carried out by them can be performed within such a limited area. The centres localized in the bulb are those for mas- tication, swallowing, sucking, vomiting, respiration, phona- tion, coughing, the movements of the heart, bloodvessels, and iris, the secretion of saliva, the glycosuria centre, a centre for the sweat glands and a centre for shivering. Though all these functions have been more or less clearly referred to the bulb, we must avoid falling into the error that a definite representation exists for each of them ; and though the term centre is employed, it is more as a con- venient mode of expression, than as absolutely establishing 28—2 Digitized by Microsoft® 6 A MANUAL OF VETEEINAEY PHYSIOLOGY a fact that any particular group or groups of cells are sponsible for one function more than another. Perhaps 3 only exception to this is the respiratory centre, -which s been denned with a certain amount of exactitude. The mastication and swallowing centres lie in the floor of 3 fourth ventricle ; they have for their afferent nerves the lerior divison of the fifth, glosso-pharyngeal, and the perior laryngeal of the pneumogastric ; whilst the motor inches are in the motor parts of the fifth for mastication, d in the fibres of the pharyngeal plexus of the vagus for allowing. All the muscles of mastication, except the ;astricus, receive motor nerves from the inferior ixillary division of the fifth pair. It would appear that 3 reflex act of swallowing is excited by the presence of )d in the pharynx. k vomiting centre exists in the bulb, which in the horse d ruminants is certainly most imperfectly developed. 3 have previously (p. 180) drawn attention to the fact it there is no drug which has the power of exciting miting in the horse ; tartar emetic has not the slightest ;ion, and the effect of apomorphia is only to produce the ist alarming symptoms of cerebral excitement, but no empt at vomiting. In the dog and pig the vomiting ltre is well developed. The afferent nerves may be those the pharynx, palate, and root of tongue, viz., the glosso- aryngeal, or those from the mucous membrane of the mach, for example, the vagus and sympathetic ; the im- jssion having been carried to the bulb the efferent nerves > the phrenics for the diaphragm, and vagus for the mach and oesophagus. The vomiting centre may be ectly stimulated by irritating the central endof the vagus. Secretion of Saliva. — The centre for this lies in the floor the fourth ventricle at the origin of the seventh and ninth r of nerves. The afferent nerves are those of taste, viz., i gustatory branch of the fifth and glosso-pharyngeal, ilst the chorda tympani is not only afferent from the e part of the tongue but is also the efferent nerve to i submaxillary, and the superficial petrosal that to the Digitized by Microsoft® THE NEBVOUS SYSTEM 437 parotid gland. Other centres in the bulb are the cardio- inhibitory and respiratory centres. For the respiratory centre see p. 108. For the cardio-accelerator and cardio-inhibitory centre see pp. 48, 49. For the vaso-motor centre see p. 75. For the diabetic centre see p. 230. Functions of the Bulb. — The bulb apart from the brain cannot elaborate sensation or voluntary movement. It forms a pathway to the brain for the columns in the spinal cord, and is a conductor of centripetal and centrifugal impulses ; it gives origin to all the cranial nerves but those of smell, sight, and the motor nerves of the eyeball ; finally it is the supreme reflex centre for the nerves govern- ing respiration, circulation, the action of the heart, and the digestive apparatus from the mouth to the intestine. The Pons Varolii conducts centripetal and centrifugal impulses to and fro and up and down ; it connects the cerebrum with the bulb and cerebellum, and several of the cranial nerves arise in connection with the grey matter of the various nuclei found in it. When stimulated, pain and muscular spasms are produced. //} 7 The Thalami Optici are connected with vision, but are mainly supposed to be the centres for tactile impressions which they transmit onwards to the cerebral cortex (Fig. 92, ot). The Corpora Striata are interesting clinically on account of the comparative frequency with which they are diseased in the horse. They are considered to be the centres for co-ordination of motor impulses ; when they are destroyed the animal has an irresistible tendency to move forwards. We have certainly seen this latter symptom shown in the horse in disease of the corpora striata, but it is far from invariable. It is remarkable how extensively the parts may be affected and pressed upon by tumours without symptoms being exhibited : the gradual progress of the pressure or destruction may account for this. The corpora striata are also considered to be concerned in heat production ; there Digitized by Microsoft® ! A MANUAL OF VETEEINAEY PHYSIOLOGY •ears to be no doubt that experimental injury of these lies produces a high temperature. Nothing is known of mechanism, but it is supposed that impulses pass from corpora striata to the muscles, the result being a great rease in the amount of heat produced. It is of interest remember that the corpora striata, unlike the optic lami, are shown by their developmental history to be lly portions of the cortical grey matter. Cerebellum. n the cerebellum is found a collection of fibres and iglion cells in communication with tracts from, the spinal d, bulb and cerebrum. It is the first piece of nervous ue we have studied where the grey matter is externally ced and not internally as in the cord ; the surface being led and doubled in on itself to a considerable extent, s forming convolutions. Che functions of the cerebellum are principally concerned the co-ordination of movement, viz., harmony and r thm in muscular actions; it is enabled to carry out 3 function through its connection with the superior amns of the cord, which keep the cerebellum informed the position of the limbs. There can be no doubt that co-ordinating muscular movement, the cerebellum is isted both by the sense of sight, and by the nerves from otolith organs and semicircular canals of the ear; an mal walks with uncertainty when the eyes are covered and disease of the internal ear is a well-known cause of tigo in the human subject. injury of the cerebellum produces no sensory dis- bance, but entails defects of movement. When sliced ay in birds they lose the power of flying, walking, or iserving their equilibrium ; there is no loss of conscious- is or intelligence, but an inability to co-ordinate the iletal muscles. Injury to one of the crura of the cere- lum produces ' forced movements ' as they are termed, e animal rolls over and over around the long axis of the Digitized by Microsoft® THE NEEVOUS SYSTEM 439 body, or else circus movements or somersaults are per- formed. In dogs superficial injury to one of the processes of the cerebellum causes only temporary disturbance, whilst deep injury or removal of a hemisphere causes rigidity of the legs and shaking of the head ; more extensive injury is followed by disturbance of co-ordination. The entire cere- bellum has been removed in the dog, the animal living for many months ; in the first instance spasms of the muscles of the head, neck, and fore legs, and weakness of the hind legs were present ; when the eyes were closed standing was impossible. These symptoms gradually gave way, and the animal was left with a deficiency of muscular tone, and a tremor in the muscles which increased on the performance of voluntary movement ; it could swim but was muscularly weak, and eventually died from marasmus. The cerebellum influences movement by re-inforcing the activity of the opposite hemisphere of the cerebrum (Luciani), especially of the ' motor area.' The movements produced by the opposite cerebral hemisphere become wanting in steadiness and power when half of the cere- bellum has been removed, and the muscles innervated by that hemisphere are deficient in tone. No direct downward connection of the cerebellum with the cord is known to exist, though, as previously mentioned, the cord in an upward direction is connected with the cerebellum. Mid Brain. There is little of importance to the general student to be said about the physiology of the mid brain. The anterior corpora quadrigemina are concerned with the oldest primitive reflex centre, working from the optic nerve upon the eyeball muscles. 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 ' equilibrating ' ear. The posterior quadri- geminal bodies receive fibres from the cochlea nerve, and have reflex centres connected with lower auditory functions. Digitized by Microsoft® A MANUAL OF VBTEEINAEY PHYSIOLOGY • instance, a cat with the brain cut through just in it of the posterior bodies emits on stimulation of a hurtful d to the skin a long angry vocalized note. But this ses directly the section lies behind the posterior bodies, also the ' chloroform cry ' that animals and men give ler chloroform goes on' when the brain is cut in front the posterior bodies, but not when the section is made dnd them. Cerebrum. Ihe cerebrum is composed of grey and white matter, the y being externally placed and thrown into convolutions. 3se convolutions, though well marked in the lower mals, are by no means so numerous as in the man-like is and man. The use of the convolutions is no doubt to rease the surface of the brain, and the deeper and more aplex they are, the greater, as a rule, is the intelligence ;he animal. In the horse the convolutions are compara- sly very shallow. Jse of the Cerebrum. — In the grey matter of the cerebrum ocated the seat of sensation, reasoning, and will. The ite matter is simply the conducting paths along which i impulses are distributed. ~.t is quite possible for an animal to perform acts which k as if executed by intelligence, or to undertake move- nts which need precision, in spite of the fact that it is hout a cerebrum. Some very curious observations have in made on the frog in which the cerebral hemispheres ve been removed. If stimulated the frog springs, if ■own into the water it swims, if placed on its back it overs its normal position, and if stroked it croaks. All >se actions would indicate the presence of consciousness, t such is not so ; the frog without its cerebral lobes will aain, unless stimulated, in one position until it dies, it pears to possess no power of spontaneous movement, or sver of will. A remarkable experiment performed on a g in this condition consists in placing it on a board ich is gradually brought from the horizontal to the Digitized by Microsoft® THE NEKVOUS SYSTEM 441 vertical position ; during the movement the animal crawls up the inclined plane, and when the board is vertically placed it sits on the top perfectly balanced ; as the board is lowered to the opposite side from which it was raised the creature descends. It is only during the time the board is being raised or lowered that the frog moves, but the movements are executed with precision. It is evident that these acts which strike one as being intelligent are really reflex, and are executed by the spinal cord, the animal being absolutely unconscious of what is going on, and it may be amongst even the higher animals that some acts regarded as volitional are in reality reflex. Motor and Sensory Areas. — So far as we are aware no observations have been made on the motor and sensory areas of the cerebrum of Ungulata, but the dog has on this point been carefully examined. The dog's brain is marked by two fissures known as the sulcus cruciatus (Fig. 98, S), and fossa Sylvii (Fig. 98, F). Between these fissures are arranged four primary convolu- tions!, II., III., and IV. (Figs. 98 and 99). In the anterior part of the fourth or superior convolution are found from before backwards — (a) The motor areas for the muscles of the neck (Fig. 98 — 1). (b) The motor areas for the extensors and abductors of the fore-leg (Fig. 98 — 2). (c) The motor areas for the elevation of the shoulder and extension of fore-limb movements as in walking (Fig. 98 — 8). (d) The motor areas for the flexors and rotators of the fore-leg (Fig. 98 — 3). (e) The motor areas for the muscles of the hind-leg (Fig. 98 — 4). (/) The motor areas for the retrac- tion and abduction of the fore-leg (Fig. 98—7). (g) The motor areas for the lateral switching movements of the tail (Fig. 98 — 6). Close to No. 2 area (Fig. 98) is one d (Fig. 99), stimulation of which causes the eye to turn to the opposite side, opens the eyelid and dilates the pupil. In the third convolution is situated an area 9, 9, 9 (Fig. 99), stimulation of which controls the movement of the orbicu- laris muscle, produces an upward movement of the eyeball, Digitized by Microsoft® A MANUAL OF VETEEINAEY PHYSIOLOGY Fig. 99. Po Illustrate the Motor Areas in the Brain of the Dog (Landois and Stirling). ;. 98, cerebrum of the dog from above. Fig. 99, cerebrum from the side. I., II., III., IV., the four primary convolutions; S, sulcus cruciatus ; F 3 sylvian fosse ; o, olfactory lobe ; p, optic nerve. The positions of the areas are described in the text. Digitized by Microsoft® THE NEEVOUS SYSTEM 443 and a narrowing of the pupil ; behind this is e, e, e, an area which represents vision. In the second convolution is an area a a (Fig. 99), which produces retraction and elevation of the angle of the mouth with partial opening of it. Behind this is c c, stimulation of which retracts the mouth owing to the action of the platysma ; then an area c 1 which like 9 (Fig. 99) causes elevation of one angle of the mouth and of one half the face until the eye is partly closed. Behind this is J f f, which is the auditory centre. In the first convolution is the oral centre {b, Fig. 99), stimulation of which opens the mouth, protrudes and retracts the tongue, while the dog not unfrequently howls. All these centres have been indicated, but it is necessary to remember that though in area they may be as large as a pea, yet to an extent they overlap. The higher the animal is in the scale the greater the complexity observed in the areas, as for instance in the monkey, where the skilled movements of the hands and feet are largely represented in the cortex. The size of an area bears no relation to the size of the part supplied, but does bear a relation to the complexity of movement which the part is intended to produce. Thus the thumb area in the cortex of the monkey is relatively larger than the shoulder or hip area. The effect of removing the motor areas differs according to the animal ; in the monkey it results in permanent motor paralysis of hand or foot, but not of parts with less skilled movement, e.g., shoulder or knee. In the dog paralysis is not necessarily produced, and it has been supposed that the basal ganglia are capable in this animal of taking on 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. 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, Digitized by Microsoft® i A MANUAL OP YETEEINAEY PHYSIOLOGY is possible, certainly in man, to localize with considerable latitude the seat of the trouble. Eemoval of the anterior frontal convolutions in the dog leads to unilateral motor i sensory paralysis, from which the animal recovers ;h the exception that there is loss of muscular sense, the operation be performed on both sides there is an iggeration of the symptoms, and the animal becomes ious. Eemoval of the posterior or occipital lobes leads blindness, no loss of motion or of muscular sense, and ) dog remains obedient but sluggish. Eemoval of a large lbs of cerebral cortex causes the animal to become iensely stupid, it walks slowly, the head hangs down, isibility is diminished ; the dog sees but cannot compre- id, it howls from hunger and eats until its stomach is 1, it exhibits no sexual excitement, and becomes, in fact, eating, complex, reflex machine. 3olin draws attention to the difficulty in producing ralysis experimentally in the horse from lesions of the nispheres. Neither the artificial production of a clot in falciform sinus, nor the introduction of pieces of lead > size of a pea into the convolutions, gave rise to niplegia. This quite bears out what we know to be a lical fact, that it is possible for horses to have in their eral ventricles tumours the size of an egg without pro- jing any disturbance. We have seen such cases, the nours being of variable size, and the clinical history i never given more than a few days' illness, though ) growths must have been forming for a considerable •iod. Che Circulation in the Brain is peculiar ; the veins or called sinuses are enclosed in very rigid membranous [Is formed by the dura mater ; the blood is driven •ough these not only by the force from behind, but by i aspiratory effect produced by inspiration (see also 82). Coverings of the Brain. — The dura mater is a dense rous membrane, which acts the part of a protective r ering for the brain; between it and the arachnoid a Digitized by Microsoft® THE NEEVOUS SYSTEM 445 lymphatic space known as the subdural exists. The arach- noid contains but few vessels and no nerves, and covers the extremely vascular pia mater ; between these is formed the subarachnoid space, which contains the subarachnoid or cerebral fluid. Cerebral Fluid. — The subarachnoid space communicates with the ventricles of the brain, the lymph in it is also sbown to be in communication with the perivascular spaces of the cerebral vessels and the lymphatic spaces in the perineural covering of nerves. Through the fourth ventricle it communicates with the central canal of the spinal cord, and there is also a connection between the cerebral spaces and those formed on the exterior of the cord. The sub- dural and to an extent the subarachnoid fluid communi- cates with the sinuses of the dura mater. The cerebral fluid is secreted by the pia mater and choroid plexus. The use of this cerebral fluid, which normally in horses amounts to 80 or 90 grains, is to equalize the pressure on the brain, afford protection to the latter, and through the manner in which the organ is suspended inside the skull by the dura mater, to save it from jar and concussion; both cerebrum and cerebellum half float on water-cushions. Withdrawal of the cerebral fluid leads to convulsions, and an increase in the amount may cause coma owing to the pressure it exercises. Movements of the Brain. — When the brain is exposed it rises and falls during each respiration, rising with expira- tion and falling during inspiration ; the cause of this is the respiratory rise and fall of blood pressure. Alterations in the volume of the brain have been observed ; the brain expands under a rise in pressure of the systemic arteries, such as is produced by stimulating the central end of the sciatic. Ether and particularly strychnin causes a con- siderable expansion ; chloral hydrate and especially chloro- form cause a marked contraction. No vaso-motor fibres have been discovered acting in the brain. Digitized by Microsoft® i A MANUAL OF VETEEINAEY PHYSIOLOGY The Sympathetic System. m extensive system of nerves exists in the body, the ction of which is mainly to supply the bloodvessels, 3era and glands. At one time, owing to its peculiar dis- mtion, the sympathetic system was regarded as distinct m the cerebro-spinal ; this is now known to be incorrect ; two are intimately connected. Fhe sympathetic is composed of nerves and ganglia ; the ve fibres are remarkable for their fineness and are both dullated and non-medullated ; the ganglia consist of ltipolar cells and nerve fibres. The numerous processes onging to the cells serve to increase the number of cts along which impulses travel, so that these are able pass out in several directions. There is no evidence ,t these ganglia can originate impulses, but they serve transmit nerve impulses. • Until lately there was nothing show that they were capable of performing a reflex act, i this would now appear to be possible, although in a mliarly simplified way. tledullated nerves by passing through a sympathetic lglion lose their medulla, and Langley has shown that irly all the nerve fibres entering a ganglion terminate in i nerve cells of that ganglion, though some pass through bout communicating with the cells. Nicotin applied to ganglion paralyzes the cells but not the nerve fibres. this method of inquiry, which is due to Langley, it is jsible to demonstrate what nerve fibres do and what do ] end in the various sympathetic ganglia. The number fibres in a nerve is increased by passing through a lglion, and further, the ganglion exercises a nutritive set over such of the nerve fibres as are branches from > cells of the ganglion. 3-askell has shown that the extensive sympathetic system japable of classification into three groups : (1) Vertebral iglia, which run on either side of the vertebral column ictically throughout its length. Below and in connection ih these are the large nervous plexuses and ganglia of Digitized by Microsoft® THE .NERVOUS SYSTEM. .. 447 the chest and abdomen, such as the cardiac, solar, and mesenteric plexuses. These are known as (2) the Collateral ganglia ; from these are given off fibres which terminate in the tissues supplied by them, and are known as (3) Terminal ganglia. On reference to Eig. 85, p. 394, this distribution is shown in a diagrammatic form, % being the vertebral, 8 the collateral, and 8' the terminal ganglia. It is through the vertebral ganglia that the sympathetic is mainly brought into connection with the cerebro- spinal system. White medullated nerve fibres run out from the spinal cord, especially in the dorsal and lumbar regions, to join the ganglia on the vertebral chain ; this branch is known as the white ramus communicans (V, Fig. 85). After passing through the vertebral ganglia it loses its medulla, and a branch, the grey ramus communicans (rv, Fig. 85), leaves the ganglion, returns to the spinal cord, and again issues from it in a manner previously described (p. 77), to supply the bloodvessels of the spinal cord, and those of the fore and hind limbs with constrictor influence. Those fibres of the white ramus which do not return pass through the vertebral ganglia, become non-medullated, and join the collateral ganglia. White rami are found running out from the spinal cord of the dog from the second dorsal to the second lumbar nerve; in front and behind these points there are no white but only grey rami. In the cervical region, though there is no white ramus yet fibres run out from the cord by means of the spinal accessory nerve, a division of which enters the vagus and supplies the heart with inhibitory nerves (Fig. 15, p. 46) ; from the second and third dorsal nerves white fibres are given off which pass through vertebral sympathetic ganglia, and finally reach the heart, exercising an augmentor effort (Fig. 15). In the cervical sympathetic fibres are found supplying constrictor influence to the bloodvessels of the head and neck, dilator fibres for the iris, fibres causing the eyelids to open, the eyeball to come forward, and the third eyelid to be re- tracted in the cat, dog and rabbit ; besides these there are sweat fibres for the head and neck, secretory fibres for the Digitized by Microsoft® 8 A MANUAL OP VETEEINAEY PHYSIOLOGY livary glands, and for the glands in the muzzle of the ox. both the ox and dog trophic fibres are found supplying e muzzle, and in the horse there are fibres for the baceous glands of the skin of the ear. Arloing has shown that in both the ox and dog division the cervical sympathetic has been followed by a dry, .pillated, and hypertrophied condition of the skin of the >se and muzzle, due to damage to the trophic fibres. From the spinal cord between the sixth and thirteenth irsal and the first and second lumbar nerves in the dog, e greater and lesser splanchnic nerves are given off, inch run to collateral ganglia, the solar plexus ; from the st to the third lumbar nerves in the dog fibres are also ven off which form the inferior mesenteric ganglion ee Fig. 55, p. 206). From these plexuses fibres are given f supplying the muscles of the stomach and intestines :ih inhibitory power, so that stimulation of the splanchnics uses the viscera to cease moving (see p. 205). The lanchnics are also the chief vaso-motor nerves to the issels of the abdomen ; section of them gives rise to great latation of the vessels of the intestines, liver, kidneys, etc., le to vaso-motor paralysis, and so causes a great fall in ood pressure ; stimulation of the peripheral end of the vided nerve causes the vessels to contract and raises the meral blood pressure. The splanchnics contain sensory )res ; it is through these that abdominal pain is felt, jr further remarks regarding the nerve supply of the scera, see p. 205. The sympathetic system also furnishes e pilo-motor fibres in the cat and dog (see p. 276). The functions of the sympathetic may thus be summar- ed : This nervous system supplies the bloodvessels with mstrictor and dilator fibres, the viscera with motor and hibitory fibres, accelerator fibres to the heart, dilator )res for the pupil, secretory fibres for sweat, salivary, and baceous glands, motor fibres to the muscles of the hair, id fibres which exercise an effect on the nutrition of a part. Psychical Powers. — In attempting to define to what extent Le faculty of reasoning exists in animals, we are treading Digitized by Microsoft® THE NEEVOUS SYSTEM 449 on distinctly controversial ground. Probably this question can only be positively answered in the affirmative for two animals, viz., the elephant and the dog. With the horse the moral sense is very small ; we do not think he knows he is doing anything wrong when he kicks his stable down once or twice a week, or when he ' runs away,' but he does understand 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 indications of want of intelligence, on the other hand they show a determination of purpose and strength of will, which if these animals understood how to combine against man, would obtain for them their complete freedom from civilization. The majority of horses on the other hand have no great strength of will ; they can 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 reason like the dog or elephant, and are more flexible than the ass or mule.* Some horses do 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 from some other cause, has taught him to refuse to work ; to use the familiar expression, he * We are aware that the majority of people will not agree with these views of the defective intelligence of the horse, but we are not alone in our judgment; see 'The Points of the Horse,' by Captain -'Hayes, whose experience amongst horses in all parts of the world was" very' considerable. ■ - l> 29 Digitized by Microsoft® A MANUAL OF VETERINARY PHYSIOLOGY )s,' he has learned to disobey, he has learned his own mgth, and the comparative powerlessness of his master, I this through an exercise of reason. In other words, the se which refuses to wear himself out in the service of n is one possessing too much intelligence and strength will for a slave ; a ' jibber ' is an intelligent and not a pid horse. ^s a rule the intelligence and affection of the horse only st in books and the imagination of those who have the st to do with him ; whatever region of the brain affection ocated in, it does not occupy much space in the equine, king the dog as the standard to judge by, it may be said h the greatest truth that the large majority of horses re no affection whatever, either for their own kind eluding maternal affection) or for human beings. Two ange horses cannot as a rule be put together without agreeing, and no one ever heard of a horse pining away ough the prolonged absence of his master ! The often Dted example of a horse jumping over a man on the rand rather tban treading on him is an act misunder- od ; it is true the horse jumps over the man, but he does because he is taught to jump over every obstacle, and j man on the ground might for all he knows be a bush, other words it becomes largely a reflex action, and only a very limited extent a volitional act. If the horse possesses but little affection it is compensated ■ by cherishing no resentment ; he will kick his friend as idily as a foe, or in many cases his groom with as much eerfulness as a perfect stranger ; to all his hard life and b abominable cruelties of domestication he shows no sign resentment ; water and feed him, and give him a place to in, and he forgets the past in his anxiety for the present. 3 is a peculiar mixture of courage and cowardice ; tysical suffering he can endure, no animal bears pain tter ; when his blood is up nothing is too big or too wide i* him in the hunting field, and he has a keen enjoyment r both chase and race in spite of the punishment they ay entail. But the same horse is frightened out of his life Digitized by Microsoft® THE NEEVOUS SYSTEM 451 by a piece of paper blowing across the road, or at his own shadow, and an unusual sight or a heap of stones on the 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 horse like an electric shock. 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. Eeasoning power in the majority of horses is small ; an animal runs away because he is seized with panic, or his spirits are bubbling over, but with few exceptions dis- tinct acts of reasoning are rare. Of this we daily see examples in our infirmaries; 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 ris,e when down owing to being too close to the wall, or some trifling circumstance of this kind, where if he employed any reasoning powers he would remain quiet until released, instead of which he behaves like a lunatic, inflicting 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 uncontrollable. 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 often learns to ' jib.' The horse is very conservative, he likes nothing new nor any departure from his ordinary mode of life; he will starve himself for days rather than take a new feeding grain, and he dislikes a change of stable or a new place. His gregarious instincts are proverbial; he frets at the 29—2 Digitized by Microsoft® A MANUAL OF VETEEINAEY PHYSIOLOGY 3nce of his companions, and if used to work amongst a y of horses, as in cavalry, he will take any degree of ishment rather than leave them for five minutes, ■ing the absence of his companions he neighs, sweats, 'B with the fore legs, and almost screams with delight rejoining them, not because he loves them, but because lislikes being alone. Anally, his predominant feature, and the feature of all mals below adult man, is the childishness present aughout life ; probably the absence of care, worry and iety may account for this. The horse will play all day b. a piece of rope, or nibble his neighbour persistently ; q the oldest horses when ' fresh ' will perform the antics t foal, and imitation amongst them is so great that if one i string of horses being led along happens to kick out, 3 repeats itself all along the line as if by preconceived mgement. Sydney Smith defined the difference between reason and ;inct as follows : ' If in order to do a certain thing ;ain means are adopted to effect it, with a clear and cise notion that these means are subservient to that end, act is one of reason ; if, on the other hand, means are pted subservient to an end, without there being the jt degree of consciousness that these means are sub- dent to the end, then the act is one of instinct.' rgan* believes that between instinct on the one hand, I reason on the other, we may insert as a middle term telligence,' while Eomanes and others use the word telligence ' as synonymous with ' reason.' Morgan .nes instinct as a motor response to a certain stimulus, , a reflex act, but one accompanied by consciousness, imals come into the world endowed with this innate acity for motor response ; but these instincts are not te perfect, they need training and experience, and their tructor is ' intelligence.' Intelligence, according to this erver, does not imply a conscious knowledge of the Fortnightly Beview, August, 1893. It is from Professor Morgan's er that we have obtained the views of Sydney Smith and Romanes. Digitized by Microsoft® THE NERVOUS SYSTEM 453 relation between the means employed and the end attained ; such a conscious knowledge would be reason. We are asked, in other words, to regard animals as simply reflex machines, their brain being very little higher in the scale than their spinal cord, and for some animals such a position probably meets their case, but certainly not for all. If we accept Morgan's definition of instinct and intelli- gence, it offers no reasonable explanation why dogs fight, and why they worry cats ; why a horse so inclined will turn his quarters towards another as he passes and rapidly let both hind legs fly in the direction of his objective ; nor will it explain why a horse will use his fore legs to strike when he knows his hind legs cannot reach the object of his irrita- tion. It is absolutely impossible to believe that such acts 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 reasoning ; with some it is even well developed, with others it is extremely imperfect. The elephant and dog occupy the top of the scale, the ox and sheep the bottom, the horse comes midway. We do not see how to separate reason from intelligence, but there is no difficulty in separating them from instinct. 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) : foals, calves, lambs, goats, etc., are born prepared to feel their feet at once, they require no teaching and no imitation, their senses are perfect, they can recognize 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 only differ from them in one respect, and that is they are born wild, and so have to learn confidence. Domestication and obedience are not properties transmitted .j^onj^rent^ offspring. CHAPTEE XV THE SENSES Section I. Sight. : delicate structures composing the eye receive a very :ough protection by the anatomical arrangement of the ts. The orbital cavity, for example, is nearly sur- aded by incomplete bony walls, and the layers of fat lin it assist the muscles in protecting the globe and the c nerve. The eyelids sweep the cornea and protect the i from dust and exposure, the tears keep the face of the lea brilliant ; the membrane nictitans moves particles lolid matter which would otherwise produce irritation, the eyeball can be retracted to a considerable extent to st it in withdrawing from injury. The size of the orbit such that ordinary blows inflicted upon the eye are snded on the margin of the orbital cavity, and not on eyeball itself, so that the risk of serious injury is far from large than from small bodies. The shape of the sail is not (in the horse) quite spherical, the vertical ; is greater than the horizontal, and the posterior face tie eyeball is distinctly flatter than the anterior. tructure of the Eye. — Issuing from the back of the eye- , very low down and inclined to the temporal side of globe, is the optic nerve, which after describing a iliar curve upwards, runs in the substance of the actor muscle to enter the cranium through the optic men. This curve in the optic nerve (Fig. 100) is neces- ted by the horizontal movements of the eyeball ; if .the looks backwards the curve is increased, whereas if it 454 Digitized by Microsoft® THE SENSES 455 looks forwards the ' slack ' is taken out of the nerve and the curve entirely disappears. The optic or second cranial nerve has a deep-seated origin in the corpora quadrigemina, and a representation in the occipital region of the cerebral hemispheres (see Fig. 99). The fibres forming the optic nerve decussate, those from the left brain passing to the right eye and vice versa. In some animals this decussation is complete, such as the horse, sheep, and pig ; but in others a certain number of fibres decussate, whilst some enter the nerve on the same side of the brain as that on which they originate ; this is the arrangement in the dog, cat, rabbit, monkey, and man. This partial decussation is considered in man to play an important part in the pro- duction of sympathetic ophthalmia, the inflammatory trouble running along the optic nerve to the commissure, and so finding its way to the other eye. It is quite certain that in the horse, where the decussation is complete, sympathetic ophthalmia from an injury is unknown. Division of the optic nerve causes no pain but only the sensation of light ; stimulation of the nerve causes flashes of light to be perceived by the brain — in fact the optic nerve conveys nothing but impulses which, when they reach the brain, give rise to that altered state of consciousness known as vision. Moreover, as we shall presently point out, the place where the optic nerve enters the eye is blind. j j The globe of the eye is anteriorly made up of a trans- ' parent convex surface known as the cornea, whilst the remainder of its walls are opaque and formed by the sclerotic, choroid, and retina. The sclerotic is the tunic on which the strength of the eyeball depends, the choroid may be regarded as that which principally attends to the vascular supply, while the retina is the sensitive expansion of the optic nerve on which the picture is imprinted, and thus gives rise to sensory impressions. The shape and tension of the eyeball is maintained by means of its humours, which are known as the aqueous and vitreous. The aqueous humour occupies the space between the cornea and the lens. It is a watery fluid, poor Digitized by Microsoft® A MANUAL OF VETERINARY PHYSIOLOGY solids, and is in reality lymph. It is constantly being ■eted, probably by the ciliary processes, and as con- ltly carried away by the lymphatic channels with which jommunicates through the spongy ligamentum pecti- im ; these channels empty themselves into the anterior ;em of veins. If the anterior chamber be experi- ltally evacuated it is refilled in about twenty-four rs. The use of the fluid it contains is to maintain convexity of the cornea ; after death the process of 100. — Vertical Section of the Eye of the House, Natural Size. ornea ; I, lens ; i, iris ; cp, ciliary process ; Ip, ligamentum pec- tinatum ; cl m, position of ciliary muscle ; si, suspensory ligament of lens ; on, optic nerve showing its curve. inage still appears to occur, though, of course, there is reproduction, the result being that in a day or two the lea flattens through loss of the aqueous humour. 'he vitreous humour is a viscid, tenacious material, con- ied within the hyaloid membrane, which permeates its stance. The vitreous contains mucin and a very small sentage of solids. The use of this fluid is to maintain intra-ocular pressure, by which the proper tension of globe is brought about. The whole of the vitreous mber is rendered dark by the liberal application of Digitized by Microsoft® THE SENSES 457 pigment, with the exception of a surface above the optic nerve which is brilliant and iridescent in appearance, and is known as the tapetum lucidum. Between the two humours a diaphragm is situated known as the iris, which regulates the amount of light passing into the eye, and behind this is a focussing arrangement or lens. The cornea, lens, and humours constitute the refract- ing apparatus of the eye. By means of the muscles of the eye the globe is given a considerable range of movement, and in addition it can be retracted within the orbital cavity ; further, these muscles afford some protection to the optic nerve. The similarity in construction between the eye and the apparatus known as a camera is very marked ; both have a refracting surface anteriorly placed, a diaphragm to cut off superfluous rays of light, an arrangement for focussing, a dark chamber in which is placed a sensitized surface, and on which. a reduced and inverted image of the picture is impressed. Though we have thus briefly run over the leading features of the eye, yet there are certain of these structures which need some detailed description if we are to under- stand clearly the phenomena attending perfect vision. The Cornea in most animals is circular in outline, in the horse it is somewhat oval; when viewed from the front and divided into two halves by a vertical line, it is distinctly larger on its nasal than on its temporal side. It is a very tough, non-vascular membrane, richly supplied with nerves, and nourished by the lymph which freely circulates in it. It may be regarded as the chief refractive apparatus of the eye. When viewed from the side the cornea is seen to be convex; measurement shows that in the majority of horses the curvature of the cornea taken in its horizontal and vertical meridians is not exactly the same as it would be supposing its surfaces were accurately spherical. The excess of curvature of one meridian of the cornea over that of the surface at right angles to it pro- duces a defect in vision which is known as astigmatism; Digitized by Microsoft® 3 A MANUAL OF VETEBINABY PHYSIOLOGY i meridian in the horse which is nearly always the .test is the, horizontal. Che Lens is composed of various onion-like layers of 'erent refractive powers. In shape it is bi- convex, the ivexity of its posterior face being greater than that of i anterior. It is held in its place by a capsule which ,lly suspends the lens in the eye, the capsule receiving achment to some long processes behind the iris known the ciliary processes. In the horse the lens is in contact h the ciliary processes, in most other animals there is a all space between the two. The lens possesses inherent sticity, which admits of its surface undergoing an 3ration in shape, so as to be natter at one time, more ivex at another. This alteration in shape occurs through ready manner in which the lens by its elasticity yields the pressure exercised on it through its capsule, so ,t if the tension of the capsule be relaxed the lens ges, or if the tension be increased it flattens. In s way the eye is focussed or accommodated to various tances, a subject which will be dealt with presently, [he Iris is a curtain with a hole in the centre called the til. The shape of the pupil varies in different animals; the dog it is circular, in the horse, sheep, ox and cat ptical ; in the latter animal the elliptical slit is placed tically, in the others horizontally. The iris is mainly a lection of bloodvessels and muscular fibres, the whole ng heavily coated with a brown pigment in the horse, iugh occasionally this is wanting, giving it a bluish- ite streaky appearance, as in the so-called 'wall-eyed' ■se. In the ox and dog the iris is a brighter brown than the horse, while in the sheep it is brownish-yellow. The seular fibres of the iris are commonly described as jular and radiating ; a contraction of the circular muscle itracts the pupillary opening, a contraction of the radia- g fibres dilates it. Langley and Anderson from their ervations on the cat, dog, and rabbit, have proved that ilator muscle to the iris exists ; this question was for a g time in dispute. It is now accepted that dilatation of Digitized by Microsoft® THE SENSES 459 the pupil is due to the influence of a dilator muscle and inhibition of the circular muscle. The nerve supply to these circular and radiating fibres is not the same ; the circular fibres are supplied with motor power through the third cranial nerve, whilst the dilator muscle is supplied by the sympathetic. The latter fibres emerge from the spinal cord at the first three thoracic spinal nerves, from a part known as the cilio-spinal centre ; from here they travel up the neck in the cervical sympa- thetic, and reach the iris through the ciliary ganglion. If the third nerve be divided the radiating muscular fibres of the iris contract under the unbalanced action of the sym- pathetic, and thus dilate the pupil ; if the sympathetic be divided the pupil contracts under the unbalanced action of the sphincter fibres. Stimulation of the retina by light is the natural method by which alterations in the size of the pupil are brought about, the act being reflex ; in a brilliant light the pupil contracts, in a low light it dilates. In the horse this is not strictly true ; in direct sunlight the pupil of this animal is a mere narrow chink, but in ordinary daylight it barely responds, or if it does contract it is so little as not to materially reduce the size of the pupil. Even when ihe light is concentrated on the eye, either by means of a mirror or a lens, the iris practically remains unchanged. Owing to this fact the eye of the horse can be examined by the ophthalmoscope without the use of atropin, or even without artificial light, in fact, under artificial light the pupil dilates. There are certain drugs which dilate the pupil such as atropin and cocain, and others which con- tract it, for example, morphia and eserin. It is curious to observe in the horse that although the pupil, when normally contracted, is elliptical, yet when it is dilated by atropin it becomes circular ; the chief radiating fibres would therefore appear to be above and below and but very few on the sides. Eversbusch* has studied the structure of the iris of the horse, and states that the elongated form * Zeitschrift fur Vergleichen.de Augeriheilkuncle, Heft 1, 1882. Digitized by Microsoft® D A' MANUAL OF VETERINARY PHYSIOLOGY the pupil is due to the presence of an accessory paratus on the posterior surface of the iris, which he Is the lig amentum inhibitorium ; through this ligament ; sides of the iris are not pulled in by the contraction the sphincter muscle. The long axis of the pupil in i horse is always horizontal, or practically so, no matter at the position of the head may be ; this is a point deli will be touched on again in dealing with the muscles the eyeball. The pupil of the horse dilates moderately er the animal has been galloped ; immediately after a ilent death it dilates widely, but in the course of twenty- ir hours or so, it gradually contracts until the pupil jomes a mere slit. In the horse there exists on the edge of the iris, at the ltre and upper part of the pupil, one or more large soot- e bodies known as corpora nigra; a small one may be ind on the lower margin of the iris, but the upper ones j the most prominent. When the pupil is strongly con- ,cted in direct sunlight, the centre of it is entirely blocked t by these pigmentary masses, and divided into an inner d outer portion. It would appear as if this caused an perfect image to be imprinted on the retina, and this iw we at one time held, but on subjecting the question to ;ual experiment no broken image was found to result >m the use of a diaphragm the centre of which was >cked out. The use of these bodies is doubtless to assist absorbing rays of light, but their position in the centre the pupil would not appear theoretically to be the most [table position, and they must have some other function. Le horse, as far as we know, is the only animal possessing 3m. Ligamentum Pectinatum. — Around the attached margin of i iris, viz., at the corneo-scleral border, a peculiar spongy sue exists which gives the iris at this part a distinctly ivated rim ; this is known as the ligamentum pectinatum. mghly speaking it is a rim of spongy iris traversed by rials, crevices, and spaces, which lead into the lymphatic stem of the eye ; the function of this tissue is to carry Digitized by Microsoft® THE SENSES 461 off the aqueous humour as rapidly as it is worn out and replaced, by which means the normal tension of the anterior chamber is maintained. The Choroid coat contains the vessels which nourish the retina ; it possesses innumerable nerves, numerous lymphatics, and further it is an elastic coat. Anteriorly behind the iris it forms the peculiar folded structure known as the ciliary processes, and in front of this it furnishes the tissue which is called the iris ; the iris and ciliary processes are therefore part of the choroid coat. With the exception of one area the whole of the interior of the choroid is covered with pigment, and the same extends on to the processes and iris. The area which is an exception lies on the posterior wall of the eyeball above the optic nerve ; it is of a brilliant colour, being a mixture of green, yellow, and blue, and is known as the tapetum lucidum. This is found in both herbivora and carnivora ; in the former it is due to the interference of light causing iridescence, produced by the arrangement of the connective tissue fibres of the choroid, and not to the presence of any pigment ; in carnivora it is due to minute crystals in the cells of the part, the crystals causing the interference. The use of the tapetum is generally supposed to be to enable animals to see in the dark ; this of course is impossible, but it is probable that the presence of a tapetum may enable an animal to see better in a dim light. The Ciliary Zone is a peculiar and important part of the eye, formed on the one hand by the junction of the cornea and sclerotic, and on the other by the iris and ciliary processes. Between these lies a muscle known as the ciliary, which is firmly attached to the corneo-scleral margin, and runs backwards into the choroid, where it is attached. In man the ciliary muscle consists of both circular and longitudinal (or meridional) fibres ; in the horse, and probably all the lower animals, only meridional fibres exist. The muscle is composed of unstriped fibres, and its use is to pull the choroid forward ; the object of this will be apparent when we discuss the question of accommodation. Digitized by Microsoft® 2 A MANUAL OF VETEEINAEY PHYSIOLOGY The Vitreous humour is enclosed in the hyaloid membrane ; teriorly this membrane, here known as the Zonule of nn, becomes dovetailed into the ridges formed by the iary processes, and enveloping the lens forms its sus- nsory ligament. If the amount of vitreous humour esent is sufficient in quantity, this ligament of the lens ust always be tense, and as it is very inelastic it tends to Rods. Cones. s. 101. — Diagram of Structure of Ebtina (Bowditch, after Cajal). Layer of rods and cones ; B, external nuclear layer ; C, external molecular layer ; E, internal nuclear layer ; F, internal molecular layer ; G, layer of ganglion cells ; H, layer of nerve fibres. ep the lens flattened ; we shall refer to this again in eaking of accommodation. The Retina lies within the choroid and outside the sreous humour ; it spreads out from the entrance of the itic nerve of which it is the expansion. Microscopic amination shows this membrane to be composed of seven pers (Fig. 101), of which the most important is one Digitized by Microsoft® THE SENSES 463 termed from its appearance the layer of rods and cones. It has been shown conclusively that these rods and cones are the essential elements of the retina, and that wherever they are absent the part is insensitive to light, as, for example, at the entrance of the optic nerve which forms the blind spot. Though the layer of rods and cones is the most important it is not placed as one would suppose, next the vitreous humour, but next to the choroid, whilst the layer next to the vitreous humour is composed of nerve fibres and ganglion cells. Bays of light have, therefore, in the first place to pierce the entire thickness of the retina to arrive at the rods and cones ; here they give rise to a nervous impulse which retraces its steps in the retina, until it arrives at the layers next the vitreous humour, from which it is carried off by the optic nerve to the brain. In one sense the most important layer of the retina is the one composed of the rods and cones, since it effects the primary conversion of light-vibrations into visual impulses. Each cone is connected with a single nerve cell, but there may be several rods to one nerve cell ; the cone is, therefore, considered to offer a more direct conducting path than the rods. Visual purple or Rhodopsin is a curious red pigment existing in the eye; it is found in the rods but not the cones of the retina. This colouring matter is readily decomposed by light, and is consequently always being produced. It is possible by keeping an animal in the dark in order to increase the visual purple, to procure then a picture on the retina through its decomposition on exposure to light. It is believed that the vision of night-seeing animals is mainly brought about by the rods in virtue of their visual purple, while the cones are adapted for day- light. Visual purple on the rods increases their irritability in dim lights. At the same time it is quite certain that visual purple is not essential to vision, for there is none in the fovea of the human eye, the area of the most acute vision, and none in certain birds, reptiles, and bats. In people totally colour-blind vision must be carried on by Digitized by Microsoft® A MANUAL OF VETEEINAEY PHYSIOLOGY rods, as it is supposed that the cones are the seat of >ur perception. ^e entrance of the optic nerve within the eyeball is ken of as the optic disc or papilla ; it is a concave oval face surrounded by a white ring formed of sclerotic. It , in the horse, towards the bottom of the eyeball and lined to the temporal side. This region is blind owing ihe absence of rods and cones. ?here is no yellow spot in animals; in man this exists, I the area which it encloses, the fovea, is that of the st acute vision. In the fovea all the other retinal ers but that of the cones have disappeared ; there no rods in the yellow spot or fovea of man. Eep- s possess only cones in their retina, and both, birds I fishes have more cones than rods. A line drawn ough the centre of the. cornea to the yellow spot is led the visual axis of the eye. The visual axis in man is not quite agree with the optic axis, viz., a line drawn ,ctly through the centre of curvature of each refractive dium. In the lower animals we have no means of >wing whether the optic axis is also the visual axis, ; from the absence of the yellow spot it is assumed to be. Chere is, however, an area of acute vision in the horse, 1 the animal bringB it into play by raising the head very ;h, and protruding the muzzle so as to render the face rizontal. The Ophthalmoscope. — We may here describe in outline i theory of this instrument, and the appearance of the ture presented by it. To examine the eye, a mirror ih a hole in the centre is applied to the eye of the server so that he can see through the hole into the served eye ; from a suitable source of light, rays are lected by the mirror through the pupil on to the retina be examined. When light is thrown into the eye, the is are reflected back through the pupil in the direction which they entered, and pass through the hole in the rror into the eye of the observer. On looking at the ina of the horse, a brilliantly coloured surface is Digitized by Microsoft® THE SENSES 465 illuminated, the tints being a mixture of yellow, green, and blue studded with minute dots ; this coloured area is the tapetum (Plate I.). Examination shows this surface to be situated above the optic disc or papilla ; the optic papilla appears of a pinkish colour, with a slightly raised whitish margin. It is very difficult to Btudy the eye of the horse, owing to its frequent movement, so that only occasional glimpses of the papilla can be obtained. From the optic papilla a dense network of vessels may be seen radiating but extending no great distance from it ; this is character- istic of the retina of the horse. The remainder of the Fig. 102. — Direct Method of using the Ophthalmoscope (Stewaet). 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. fundus is purple or brown, but owing to its extent very little of it can be seen. In other animals the vessels radiating from the disc are wider apart and more regular, and several of them have received names ; moreover, the arteries can be distinguished from the veins, which is not possible in the horse. It is to be borne in mind that the view thus obtained of the fundus of the eye is a magnified image, both the lens and vitreous humour making it appear about three times larger than normal. Owing to the presence of the tapetum in the horse, a perfect examination of the lens and fundus may be made without the aid of artificial light ; while under the influence of artificial light 30 Digitized by Microsoft® 6 A MANUAL OF VETERINAEY PHYSIOLOGY 3 pupil dilates so much that there is no need for the use atropin. Accommodation. — All rays of light proceeding from a dis- it object may be regarded as parallel, and all those pro- sding from an object within 20 feet of the eye may be yarded as divergent. A distant object is one situated any- lere between 20 feet from the eye and infinity ; an object >ser than 20 feet to the eye is called near, and this point sreases up to 4 or 5 inches, at which distance no object n any longer be distinctly seen. The nearest distance FAR NEAR 3. 103. — Diagram to illustrate Accommodation (Foster after Helmholtz). ?., Ciliary process ; I, iris ; Sp. 1., suspensory ligament ; l.c.m., longi- tudinal ciliary muscle ; c.c.m., circular ciliary muscle ; c.S., canal of Schlemm. e left half represents the shape of the lens for viewing distant objects, and the right half that for viewing near objects. which objects can be distinctly seen is called the near int. Parallel rays need no focussing on the retina other an that provided by the cornea ; but rays from near jects do require focussing owing to their divergent nature, d it is evident that the nearer the object to the eye the eater the focussing required. This focussing is brought out by a change in the shape of the anterior surface the? lens ; it becomes more convex for near objects, and is increase in convexity is due to the ciliary muscle awing forward the choroid coat, and with it the ciliary ocesses. By this means the tension normally exercised Digitized by Microsoft® THE SENSES 467 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. 103). 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 accommo- dation, and the explanation given above is that of Helmholtz ; it is the one generally accepted. When a candle is held opposite to the eye three images ABC Fig. 104. — Diagram of the Katopteic Test. A, From the anterior surface of the cornea ; B, from the anterior face of the lens ; and C, from the posterior face of the lens. of the flame are seen ; one a very sharp bright one, obviously reflected from the cornea ; a second much duller, but also large, reflected from the anterior surface of the lens ; and a third very small, brighter than the middle one, and inverted, reflected from the posterior part of the lens (Fig. 104). In a normal eye these are seen perfectly and move in a definite direction when the candle is moved, the inverted image passing in an opposite direc- tion to the two erect images, and all are equally visible at any point on the reflecting surfaces. This • phenomenon has been taken advantage of in determining the clearness of the media of the eye, and though superseded by the greater accuracy of the ophthalmoscope, it is still a valuable aid; in cataract one or more of the reflections becomes blurred, and sometimes the image is duplicated. Digitized by Microsoft® 30 — 2 J8 A MANUAL OF VETERINARY PHYSIOLOGY tie first and second images are erect inasmuch as they are fleeted from a convex surface, but the third image is verted, being reflected from the posterior surface of the as which viewed from the front is concave. During the t of accommodation the relative position of these images ters ; the second becomes smaller or larger, and advances sarer to or recedes from the first, as the anterior face of the us becomes more convex or flatter as the case may be. This >servation affords the proof that accommodation is due to :e varying convexity of the anterior surface of the lens. FisheB are normally short-sighted, and accommodation r a distant object is effected with them by moving the lens wards the retina. The ciliary muscle is governed by the ciliary nerves. In ie human subject the constrictor fibres of the iris and the liary muscle are paralysed by atropin, but in the cat (as :st pointed out by Lang and Barrett*), the dog, and cer- Inly in the horse, there is no evidence that any paralysis 1 the ciliary muscle takes place under atropin, though the ipil dilates. Under the full effect of atropin all these limals can see objects quite close to the eye, and this they luld not do if the ciliary muscle were paralysed. Eyes which possess the power of seeing objects distinctly few inches from the eye to infinity are known as mmetropic (Fig. 105 — 1) ; but all eyes do not possess this inge of vision owing to their shape, or more correctly, to ie length of the eyeball. Myopia or short sight is due to the eyeball being too long, hereby the picture is formed in front of the retina, and lly a confused and blurred image falls on the retina ?ig. 105—3). Our observations show that the majority of horses are ightly short-sighted, t Hypermetropia or long sight is due to the eyeball being * ' The Refractive Character of the Eyes of Mammalia,' Boyal mdon Ophthalmic Hospital Reports, vol. xi., part ii. t ' The Eefractive Character of the Byes of Horses,' Proceedings the Boyal Society, No. 334. 1894. Digitized by Microsoft® THE SENSES 469 too short, whereby, though vision may be perfect for distant objects, those near at hand are not distinctly seen, the pic- ture being brought to a focus behind the retina (Fig. 105 — 2). It is obvious that a concave glass which scatters rays is the remedy for myopia, while a convex lens which converges them is the appropriate glass for hypermetropia. Emmetropic 2. Hypermetropia. Myopia. Fig. 105. — Diagram of an Emmetropic, Hypermetropic and Myopic Eye, to illustrate where the Focal Point exists (Kirke). In 2 the short eyeball causes the focus to form behind the retina ; in 3 the long eyeball causes the rays to come to a focus in front of the retina. Astigmatism is another error of refraction, due to irregu- larities in the curvature of the cornea or lens, generally the former. The effect of this condition is that the rays of light passing through one meridian of the eye are brought to a focus earlier or later than those passing through the meridian at right angles to it. The horse is very commonly astigmatic ; the horizontal is generally the meridian of least curvature, and corresponds to the long diameter of the pupil. Digitized by Microsoft® A MANUAL OF VETERINAEY PHYSIOLOGY Irrors of Refraction. — In the following table is given the portion of eyes affected with errors of refraction among horses. E 100 eyes (54 horses) : 51 were myopic and astigmatic. 2 were hypermetropic and astigmatic. 6 were affected with mixed astigmatism. 39 were affected with myopia. 1 was hypermetropic. 1 was emmetropic. he amount of error of refraction is as a rule small, the sf visual defect being myopia with or without astigma- i. The number of astigmatic horses is remarkable, ording to Lang and Barrett's observations,* the cow ild appear to be hypermetropic, and the eye also suffers q astigmatism. In dogs and cats the refraction closely roaches emmetropia. In nearly all the wild animals ex- ned by these observers the refraction was hypermetropic, he Movements of the Eyeball are brought about by means he ocular muscles ; in this way the globe of the eye can rapidly turned in any direction. But the movements somewhat complex, for in some of the lower animals, example the horse, the eyes are laterally placed in the i, so that vision is commonly single-eyed and not >cular as in man. The eye that is viewing an object ated to one side and moving to and fro is being followed his muscular movement by the eye which does not see ; movements are conjugate, but this only occurs so long Qonocular vision is practised. If both eyes be directed n object situated to the front binocular vision becomes uble, and now the movements are no longer conjugate opposite, for while the left eye is inclined to the right right eye is inclined to the left. Another complication he ocular muscles is due to the movement of the head ; as first pointed out by Lang and Barrett, that in the bit and guinea-pig no matter what position the head ipied the pupil was always kept vertical. If the head * Op. cit. Digitized by Microsoft® THE SENSES 471 of the horse or ox be raiaed or depressed to the fullest possible extent, the muzzle being at one time on the ground, at the next high in the air, it will be found that the eye- balls rotate like a wheel, so that the pupil is still kept hori- zontal ; if it were not for this the pupil in the uplifted head would be vertical and in the depressed head oblique. When the head is elevated the eyeball becomes depressed to such an extent that the sclerotic shows largely above, while the cornea partly disappears beneath, the lower eyelid. When the head is depressed to the ground no more sclerotic shows than when it is in the ordinary position ; the probable cause of this will be mentioned presently. Sun- Oblictu\ /Sup,: Rectus. 'Retractor. Extr- Rectus. /nf:Obliaue. Fig. 106. — The Muscles of the Left Eyeball of the Horse viewed from the Temporal Side. The muscles of the eyeball (Fig. 106) are seven in number, viz., four recti, two oblique, and one retractor. The use of the recti is clear enough, they rotate the eye in four direc- tions, outwards, inwards, upwards and downwards. The two oblique muscles rotate the eye in opposite directions around its anterior-posterior axis ; when the superior oblique contracts it pulls the temporal side of the eyeball upwards, and if it were not counteracted by the inferior oblique it would continue to contract until the pupil became vertical like that of the cat ; the inferior oblique pulls the temporal side of the eyeball downwards, in other words Digitized by Microsoft® A MANUAL OF VETEEINAEY PHYSIOLOGY e oblique muscles produce a torsion of the globe or ■el rotation, and their action is regulated from the icircular canals of the internal ear (see p. 502). The ictor partly withdraws the eye in its socket, he nerves supplying the muscles of the eyeball with or power are the third pair to all excepting the external as and superior oblique, the external rectus being )lied by the sixth pair or abducens, and the superior jue by the fourth pair or pathetic. So that we have e pairs of cranial nerves supplying seven muscles, orbicularis palpebrarum, which closes the eyelids, is )lied by the seventh nerve, while the muscle which as the upper lid derives its nerve supply from the third he chief movements of the eyeballs are backwards and r ards, corresponding to the directions described as out- is and inwards in man. During these movements it is ent that the external rectus of one eye is acting in unction with the internal rectus of its fellow, and such ways the case in monocular vision. Animals with the i laterally placed have, however, the power of monocular also of binocular vision, but the latter is only produced m internal squint, and the movements of the muscles now no longer conjugate, for both internal recti are ig together (Fig. 107). Sometimes, then, the group of cles employed in moving the eyeballs is the same in l eye, at other times it is not. The torsion produced fche superior and inferior oblique muscles is of value ;he binocular vision of animals, and in the vertical omenta of the head. When the muzzle is raised, as r iously described, the superior oblique muscle revolves eyeball in its socket until the pupil is horizontal ; the anation of the cornea partly disappearing under the >r lid, and the sclerotic showing extensively above, jars to be due to a conjugate action of the inferior as muscle whenever the superior oblique is so employed. inferior oblique is mainly employed with the internal as in pulling the eyes inwards for binocular vision, Digitized by Microsoft® THE SENSES 47a also, as mentioned above, for maintaining the horizontal pupil when the head is depressed or raised. Monocular and Binocular Vision. — When a horse directs both eyes to the front (Fig. 107) he produces a well-marked double internal squint, and is then capable of binocular vision. The eyes are rotated inwards and slightly upwards by the combined action of the inferior oblique and internal rectus ; the pupils are not perfectly horizontal but nearly so, and the pupillary opening is brought so far to the front that the inner segment of the cornea and iris entirely Fig. 107. — The Position of the Head and Eyes in Binocular Vision. disappear beneath the inner canthus. In no other position than this has the horse binocular vision, viz., single vision resulting from the employment of a pair of eyes, and it is curious to observe that the condition of eye which gives a horse single vision causes in man double vision. Animals with their eyes situated on the lateral side of the head are capable of exercising monocular vision for all objects placed to one side of them and even behind them (Fig. 108) ; Digitized by Microsoft® t A MANUAL OP VETEEINAEY PHYSIOLOGY nocular would appear to be for them as perfect as ocular, but on this point it is difficult to judge. It is tain that in the horse when the attention, either from rm or interest, is particularly directed to an object, it dewed with both eyes, the head being held very high, 1 the ears 'pricked' and turned to the front. In this lition it is evident the most sensitive area of the retina sxposed, but there is no fovea as in man. A horse can an object on the ground immediately under his nose, 1 is able to see when grazing ; this is because his face . 108. — Diagram illustrating the Extent to which a Horse CAN SEE BEHIND HlM. •h. the head straight to the front he can see out of the ' tail ' of both eyes. By the least inclination of the head, as in Fig. 108, a large visual field behind him may be covered. :rows below the eyes. When looking at an object near him on the ground, he prefers to get his head low down order to see it ; but when looking intently at a distant ect, he gets his head as high as possible with the face lining to the horizontal. Ordinary equine vision is monocular, yet the right eye iks when an attempt is made to strike the left, though jannot possibly see what is going on, and in the same y the right pupil contracts when the left is exposed to llight. In man binocular vision is perfect, and the Digitized by Microsoft® THE SENSES 475 explanation afforded is that any part of one retina corre- sponds to the same part of its fellow; so that if the retinas be laid over one another, the left portion of one will lie exactly over the left portion of the other, and their upper and lower parts will equally correspond; but the Fig. 109. — Diagram illustrating Corresponding Points in the Human Eve (Foster). z' x' y' are points in the right eye corresponding to z x y in the left eye; v.l, visual axis. The two figures above illustrate the corre- sponding points on the retina described in the text. temporal side of one eye does not correspond to the temporal side of its fellow, but to the nasal side. In Fig. 109, the two circles represent the two retinas divided into quadrants, L being the left and E the right eye; a and c in the left eye correspond to a' c' in the right eye, Digitized by Microsoft® 3 A MANUAL OF VETEEINAEY PHYSIOLOGY d b and d in the left correspond to b' and d' in the ht eye ; but the optic nerve o is in the left segment one eye, and the right segment of the other. When ) two images of an object fall on corresponding points the retina of man, vision is binocular and only one ect is seen; thus, if the rays fall on the right side of 3 retina, they must fall on the right side of its fellow, is is shown in Fig. 109, v.l from x to x, and x to x' are A B . 110 — Diagram showing Horizontal Section of the Head passing through both eyeballs, to illustrate correspond- ING Points in the Eetina of the Horse. 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. two visual axes ; if the object y x z be looked at, z in h case falls to the left of the visual axis, and y to the it, viz., on corresponding points, by which means the Bet is seen as a single one. Owing, then, to the manner vhich the human eyes are placed in the head, and the vergence of axes of the eyeball, a ray of light from any at is imprinted upon the same side of the retina in both s, and we see the object not as a double image, but Digitized by Microsoft® THE SENSES 477 as a single one. This explanation does not apply to the herbivora ; no matter how greatly the eyes may be con- verged in order to see an object, the rays of light do not fall on the same side of the retina, but on opposite sides of it. The diagram (Fig. 110) will make this point clear. The outer part or temporal side of the retina in the horse corresponds with the temporal side of the opposite eye ; while the nasal side cannot correspond with the nasal side of its fellow, as it is not possible for a ray of light from an object to strike both nasal sides at one time (Pig. 110). Cartilago Nictitans. — The retractor muscle of the eye withdraws the eyeball within the orbit, and the pressure thus produced within the cavity forces the cartilago nictitans forward, so that it may be made to sweep nearly the whole corneal surface- The reason why the cartilage is pressed forwards is due to the fact that though naturally curved, it becomes flattened and straightened out by the pressure caused by retraction and so shoots forward ; when the pressure is removed it retires through its own elasticity, and becomes curved once more. On the cartilage of some animals is a small gland termed the Harderian ; its use is to prepare an unctuous secretion, probably of a protective nature. In the eyelids are found numerous glands, the Meibomian, which furnish an oily secretion, and prevent the overflow of tears. The Tears are secreted by the lachrymal gland which is placed on the upper surface of the eyeball ; they find their way into the conjunctival sac by numerous small tubes. The tears pass through the narrow puncta into the lachry- mal sac, and so into the nostril ; once in the sac the descent to the nostril is readily understood, but it is not clear why the tears prefer passing through a narrow slit in the eyelid to running over the side of the face ; probably the only explanation is the unctuous secretion mentioned above. The use of the tears is to keep the conjunctiva moist and polished, and to wash away foreign bodies. The Eyelashes of the horse are peculiar. Those on the _J lower lid are very few and fine, whilst on the upper lid they Digitized by Microsoft® 478 A MANUAL OF VETEEINAKY PHYSIOLOGY are abundant, and exist not as a single but as a double row ; the rows cross each other like a trellis -work, but without interlacing ; these eyelashes are very long and strong (Fig. 111). A few protective hairs grow from the brow and below the lower eyelid, in some horses they are 4 or 5 inches in length ; they appear to be in connec- Fig. 111. — The Eye of the Hobse. tion with nerve terminations, for their delicacy to the sense of touch is remarkable. The function of these hairs is doubtless protective, and they give the eyes warning of danger. Physiological Optics.— 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 in curvature. As media there are the aqueous and vitreous humours and the crystalline lens ; the latter is further complicated by not being 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 from it 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 deal Digitized by Microsoft® THE SENSES 479 with it as if it consisted 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 in the latter 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. Cardinal Points. — The most simple optical system which can be devised has an optic axis (O A, Fig. 112), viz., a line passing through its centre perpendicular to its refractive surface (a p b) ; 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 retrac- ing Fig. 112. — The Oabdinal Points of a Simple Optical System (Foster). A, Optic axis ; a p b, a curved spherical surface ; n, nodal point ; F 2 , principal posterior focus ; F 1( principal anterior focus ; e f, rays proceeding from F x , rendered parallel to the optic axis ; p, the principal point ; the rays m d, p, and m' e, pass through the nodal point n and undergo no refraction ; the rays c d, parallel to the optic axis, are refracted and meet at F 2 . tive surface perpendicularly, such as 0, m', pass through the nodal point and are not refracted; all rays of light parallel to the optic axis, such as c d, strike the refractive surface obliquely and are refracted, and the point where they meet is called the principal posterior focus, F . On the optic axis, in front of the refractive surface, is situated a point F x known as the principal anterior focus ; rays proceeding from this point strike the surface obliquely, and are so refracted as to be rendered parallel (e f) to the optic axis (0 A). 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 optical system we have imagined. For a more complex system such as Digitized by Microsoft® 80 A MANUAL OF VETERINARY PHYSIOLOGY ie eye, even when simplified, there are two nodal points, two principal )ci, and two principal points ; but with the reduced eye where we have ut one surface and two media, the two nodal points become one, and le two principal points one. Dioptrics. — In order to be able to calculate the position of the irdinal points of the eye certain data must be known, such as the jfractive index of the media, the radius of curvature of each refracting ?ig. 113. — The Cardinal Points of the Eye or the Horse (Berlin ?, is the first principal focus, situate "7244 inch in front of the cornea. ~j is the anterior principal point. 3, is the first principal point, distant from the cornea - 3201 inch. 3-n is the second principal point „ „ „ '3641 „ I, is the first nodal point „ „ „ -6693 „ L„ is the second nodal point „ „ „ "7157 „ £„ to a is the distance of the retina from the second nodal point, •8000 inch. 3 to "¥„ is the distance from the cornea to the second principal focus (which Berlin shows to be behind the retina), 1'7594 inches. lurface, the distance from the cornea to the lens, and the thickness of he latter. A very slight error in the determination of these may produce a considerable error in calculation, so that all measurements nade by us on the frozen eyes of horses are rejected as wanting in iccuracy, but as an illustration of the measurements of the actual and ■educed eye, those furnished by Berlin* are here given, though even * Zeitschrift fur Vergleichende Augenheilkunde, Heft 1, 1882. Digitized by Microsoft® THE SENSES 481 these are not free from error. According to Berlin the horse is normally long-sighted, the retina being in front of the second principal focus. What may have been true for the eye he examined is not universally true, for as we have previously stated the majority of horses are slightly short-sighted, therefore the point F„ will fall in front of the retina. The simplified or reduced eye (Fig. 114), consisting of one surface and two media, gives for the horse, according to Berlin, the following values : Passage of Light through Lenses. — In nature all rays of light are diverging, but so slight is the divergence of the rays from distant Fig. 114. — The Cardinal Points of the Bedtjced Eye of the Horse (Berlin). F, the first principal focus is situated l - 063 inches in front of the cornea. F„ the second principal focus is'' situated 1'427 inches behind the cornea (in the diagram it falls outside the eye, but this is not normal ; see above remarks). K to a, the distance from the nodal point to the retina, 1004 inches. H to a, the distance from cornea to retina, 1"3683 inches. objects, that for the purposes of the eye they are practically regarded as parallel. All rays proceeding from an object situated at from 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 proceeding from a body 1 foot from the eye than in one 10 feet from the eye ; conversely, the further the object is from the eye the less divergent the rays, until we reach that point beyond 20 feet where the rays may be regarded as parallel. 31 Digitized by Microsoft® S2 A MANUAL OF VETERLNABY PHYSIOLOGY A convex lens has two curved surfaces, and a line drawn through e centre of these two surfaces is known as the principal axis of the qs (Fig. 115, m m). The essential idea of a double convex lens is that is thicker at the centre than at the edges. Situated on the principal ds of a biconvex lens at a point in its interior is the optical centre fig. 115, O) ; any straight line passing through the optical centre is rmed a secondary axis (Fig. 115, n n). Fig. 115. Fig. 116. igures illustrating the action of lenses upon bays of llght passing through them (landois and stirling). ig. 115. — Biconvex lens ; 0, optical centre ; mm, chief or principal axis ; n, n, secondary axis. ' When parallel rays of light (Fig. 116, a} pass through a convex lens ley are refracted and brought to a point / on the opposite side of the ns known as the principal focus ; the only rays not refracted are lose passing through the centre of the lens, viz., those coinciding with le principal or secondary axes. The converse of this is also true, viz., ivergent rays proceeding from the principal focus of a lens / pass irough and are rendered parallel (Fig. 116). 'ig. 117. — Kays of light passing through a convex lens from I at a point beyond the focus /, cross at some point v, and invert the image (Landois and Stirling). The distance from 0, the optical centre of the lens, to /, its principal acus, is known as the focal length of the lens. If the divergent rays astead of proceeding from the foous of the lens (Fig. 117, /) proceed com a point I beyond the focus, then the rays on passing through the ans are not rendered parallel but convergent (as the refractive power 3 more than sufficient to render them parallel), and they come to a ocus again on the other side of the lens at the point v. The distance Digitized by Microsoft® THE SENSES 483 from the lens at which they come to a focus depends upon the distance of the luminous point from the lens on the opposite side; thus the nearer the luminous point I to the principal focus /, the further will the focus on the opposite side recede, and vice versa. The two foci I and v are termed conjugate foci, and as we have shown they have a definite relationship. If the rays of light proceed from a point L (Fig. 118), 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 Fig. 118, d d). Parallel rays of light passing through a concave lens, instead of being refracted to a focus are bent so as to become divergent, so that a concave lens has no real focus ; but if the divergent rays be produced Fig. 118. — Rays of light from a point L, between the focus F and the lens, diverge when passing through a convex lens. 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 is further pre- vented 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 31—2 Digitized by Microsoft® 34 A MANUAL OF VETER1NAEY PHYSIOLOGY to its primary colours by passing through a prism or a convex lens, z., a spectrum is formed. The colours of the spectrum are differently fracted, the red being the least bent, the violet the most ; when there- re we can see the red distinctly the eye is not focussed for the violet. bromati6 aberration is prevented in the eye by the unequal refractive >wer of the various media, and the action of the diaphragm or iris. Formation of a Retinal Image. — Eays of light falling on the jre, as from the arrow X Y (Fig. 119), issue as a pencil i rays from every point of the arrow, the pencil containing central ray known as the principal ray. All principal lys a a pass through the nodal point n without undergoing afraction, while the rays b c, and V c are refracted to a iG. 119. — Diagram of the Formation of a Retinal Image (Foster). 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 b, c and V, c' are refracted. In this way the arrow X Y forms a smaller inverted image of an arrow on the retina Y X. reater or less extent, so that in this way the retinal image Bcomes inverted, and very much smaller than the object represents ; it is a miniature though perfect representa- on of the object presented to the eye. The chief refrac- on undergone by these rays is at the anterior surface of le cornea ; doubtless the other media also refract, the ins for example, but an eye can have very good distant ision without a lens, whose important function is to rovide the means for accommodation. Theory of Vision. — The change which occurs that enables le vibratory ether to start a nerve impulse by its action u the retina is unknown. A photochemical theory based a the ready decomposition of visual purple has been pro- Digitized by Microsoft® THE SENSES 485 posed. In this view it is suggested that the action of light on the visual purple is allied to that of light on the photo- graphic plate, and that the chemical change thus set up in the retina excites a nerve impulse which is transmitted by the optic nerve and tract to the visual centre in the cortex of the brain. 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. Turning once more to Fig. 119, we observe that the angle X n Y is equal to the angle Y n X. The angle X n Y 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, provided we know the distance of the nodal point from the retina ; thus at the distance of a mile, a man six feet high is represented on the retina of the horse by an image shr 0I an mcri m height, in the human eye at the same distance the picture of the man would be T tjtu of an inch, or about the size of a red blood-corpuscle. The nearer the object the larger the image ; taking the six-foot man again at a distance of 10 yards, his height on the retina of the horse would be \ of an inch, whilst on the retina of a man it would be rather over \ of an inch. Section II. Smell. 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 arrange- ment of these bones the nasal passage may be divided inte two channels, one which lies next the floor of the chamber, which from its obvious communication leads directly to the Digitized by Microsoft® 6 A MANUAL OF VETERINARY PHYSIOLOGY spiratory passages, and another channel which lies above and leads to structures situated very high in the face d nose, but with no outlet save what is furnished it from low. But apart from this there are differences in the ysical characters of the mucous membrane which divide 3 nasal chamber into a lower part through which the air ,vels, and an upper part which is devoted to the sense of tell ; the one is known as the respiratory and the other 3 olfactory portion. Both the respiratory and olfactory rtions of the nasal chambers are supplied with sensation the fifth pair of nerves. In the horse the nasal chambers are of extreme import- ce, inasmuch as it is the only animal we are called upon deal with which is unable under ordinary circumstances breathe through the mouth ; the majority of animals a breathe through both nose and mouth, but owing to b extreme length of the soft palate in the horse this is der ordinary circumstances impossible. So far as respira- m is concerned the question of the nostrils has been alt with (p. 92), but the arrangement of that portion voted to the sense of smell has yet to be considered. From the olfactory tracts in the brain are formed the actory lobes, which in some animals possess a well- irked cavity, in others only a canal ; in the cavity some id is contained which communicates with the cerebro- inal, and notably in the horse with that contained in the eral ventricles. From the olfactory bulbs nerve-fibres 3 given off which penetrate the cribriform plate of the imoid, and ramify over the mucous membrane covering e upper portion of the septum, the superior turbinated ne, and the upper third of the superior and middle satus. The mucous membrane of the olfactory region jfers from that of the respiratory portion in being thicker d of a yellowish tint; it is in this membrane that the ires of the olfactory nerve are distributed. This nerve is m-medullated, and in the surface of the membrane where terminates two or three different kinds of cells are to be and. One known as a rod cell is generally believed to Digitized by Microsoft® THE SENSES 487 be the terminal cell of the olfactory nerve, though this has also been attributed to a cylinder cell which is likewise found in the membrane ; other observers consider that both cells are the terminal organs of the olfactory nerve. No definite statement can be made on this point, but perhaps the balance of opinion is in favour of the rod cell being the chief agent whereby odours give rise to nervous impulses which result in smell. 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. We have no idea of the nature of the particles which constitute an odour, but it is certain that before they can make any impression on the olfactory nerve endings, they must become dissolved in the fluid covering the nerve terminations, for a dry olfactory surface is insensible to smell. There are certain odours which excite the olfactory organs more readily than others ; thus flesh, blood, and offal have a remarkably stimulating effect 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 we have known a horse eat meat with evident pleasure. Some of the herbivora have a remarkably keen scent, antelopes and deer have the power of detecting the presence of man even a considerable distance away, and it is evident that in most animals the sense of smell plays a more important part in their daily lives than with ourselves. 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. The organ of Jacobson, which is well marked in herbivora, is said to have some connection with the sense of smell. Cuvier regarded it as the means by which the herbivora distinguished between poisonous and non- Digitized by Microsoft® 3 A MANUAL OF VETBRINAEY PHYSIOLOGY ionous plants ; this is not correct, for cattle-poisoning comparatively frequent, and in certain parts of the rid, for instance, South Africa, is extremely common ong horses and cattle. Experience is a valuable factor ; mals brought up on a pasturage containing poisonous nts frequently learn to disregard them. The odour of a body can be detected with greater accuracy ' sniffing '; by this inspiratory act no time is lost in diffu- n occurring between the respiratory and the olfactory ;ion, as the odoriferous particles are forcibly drawn up- rds. The sense of smell rapidly becomes blunted, at 7 rate in ourselves ; any offensive odour is always most rked when first detected. 3y the sense of smell animals have the power of recog- ing their own offspring ; a cow which has lost her calf 1 yield milk for weeks to a ' dummy ' clothed in the skin ;he dead calf, and she can recognise the difference between ' ' dummy ' and that belonging to another cow. If the ti of a young animal, kid for instance, be dressed with an silt which disguises the body smell, the mother is unable recognise her young. The odour of food is readily ognised by the herbivora, though to the human senses the grains are equally free from any odour but that of sack which contains them. Without tasting it, a horse I refuse a grain he is not familiar with. It is possible t everything and everybody has a distinctive odour, at 3t it would appear to be so from the remarkable manner inds will follow a scent, or a dog recognise his own ster in the dark from amongst a crowd of other persons, the case of hounds, the amount of odour required to nulate the olfactory organ must be something too nitesimal for expression. Digitized by Microsoft® THE SENSES 489 Section III. \?- Taste. 1 The sense of taste is nearly 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 qualities exist, viz., sweet, bitter, acid, and salt. Animals are certainly capable of dis- tinguishing 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 of the tongue is sensitive to bitter tastes, the tip to sweet and saline tastes, the sides to acid tasteB, while the middle portion of the tongue is insen- sitive 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. On the tongue certain papillae are found which are inti- mately connected with the sense of taste, viz., the filiform, fungiform, and circumvallate; the latter are probably the most important in connection with the sense of taste, but the others are most numerous. In both circumvallate and fungiform papillae, but especially the former, structures are found known as taste buds, bulbs, or taste goblets. They 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, whilst above is formed the gusta- tory pore, or opening into the interior of the body of the cell by which fluid finds its way in. Within the goblet or barrel are other cells, processes from which may be pro- jecting at the pore. It appears to be essential to taste that fluid should readily find its way into the pore, and as a provision to ensure this the papillae containing the buds are situated close to glands. M'Kendrick states that in a single Digitized by Microsoft® i A MANUAL OP VETEKINAKY PHYSIOLOGY jumvallate papilla of the ox 1,760 taste-goblets have n counted, in the papilla foliata of the sheep and pig 00, and in that of the ox as many as 30,000 goblet- s. The nerve supplying these taste-buds is the glosso- iryngeal, which is essentially the nerve of taste, and inly distributed to the posterior part of the tongue ; if 3 nerve be divided the taste-bulbs degenerate. The sso-pharyngeal nerve consists of a medullated and non- dullated portion ; the former terminates in the tongue and bulbs, whilst the latter proceeds to the taste-goblets. s goblet cells are not strictly limited to the tongue, but re been found in the palate, and close to the epiglottis ; y have not been found on the anterior two-thirds of the gue, a region which we know to be also possessed of the se of taste, and one not supplied by the glosso-pharyngeal ve. This area of the tongue is supplied by the gustatory nch of the fifth, and it is to this nerve (which probably eives its taste fibres from the chorda tympani of the enth) that the sensation of taste is here imparted. Sensa- l to the tongue is supplied by the lingual branch of the h pair, while motor power is furnished by the hypoglossal twelfth pair. t is necessary for the purpose of taste that the substance mid be dissolved ; this is one of the functions of saliva, I experiments on herbivora show that taste produces an indant secretion from the submaxillary and sublingual ads, though not from the parotid. Section IV. The Cutaneous Senses and Muscle Sense. Phese are pressure, warmth, cold and pain, and nerves ough which these qualities are conveyed are known in human subject to be remarkable for the fact that they distributed in ' spots ' throughout the whole cutaneous face. Whether, as some suppose, there are special nerves ich convey these sensations is not definitely known, but Digitized by Microsoft® THE SENSES 491 it appears to be proved that each of these senses has its own spots of distribution in the skin, those for pain being probably the most superficially seated as well as the most numerous, while the warm spots are the fewest in number and the deepest seated. Temperature Senses. — Cold spots are more widely dis- tributed than warm, and exist in largest number in the clothed parts of the body. The cold spots are sensitive to cold, the warm spots to warmth ; the fact that the former exceed the latter in distribution and number suggests that it is more necessary the body should be made acquainted with the fact that it is cold than that it is hot. In fact the feeling of warmth or cold does not depend upon the temperature of the body but the temperature of the skin. It is obvious that the observations made in the investigation of a temperature sense could only be carried out on man. There is no reason to think it does not equally apply to all animals. Pressure Sense. — This has also a punctiform distribution, the spots being more numerous than those of the tempera- ture sense. The special nerve endings connected with this sense are found in a ring around the hair follicle, in which position they are obviously most favourably situated for stimulation through the hair itself. In the hairless parts of the skin special tactile corpuscles are found, and in the horse special nerve endings are found 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, are located the touch organs proper (p. 272) ; the parts are endowed with exquisite sensi- bility, which enables the animal to be kept acquainted 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 pro- tective purposes. The tactile sensibility of the foot, by informing the animal of the character of the ground it Digitized by Microsoft® I A MANUAL OP VETEEINAEY PHYSIOLOGY travelling over, is useful though not absolutely essential locomotion ; nor is the tactile sensibility in the foot the horse absolutely essential to its safety in pro- ission, as is clearly proved by the results of plantar irectomy. Pain Sense is the most widely distributed of the cutaneous ises. It is distributed in spots, probably supplied by icial fibres, though no special nerve endings have been ermined. Pain confined to the surface of the body can readily located, but the localisation of interior pain is ficult ; that of colic for example is referred to the abdo- aal wall. It is considered in man that the explanation the difficulty in localising interior pain is, that the ;ment of the spinal cord supplying the affected organ, ars the pain to the skin region of the same spinal ;ment instead of to the organ. Painful sensations are of various characters, hence such ms as stabbing, boring, burning, throbbing, etc., to ex- sss the impression imparted. It is presumed that :ongst the lower animals these different qualities of pain st ; it is quite certain, for instance, that the pain exhibited a horse during an attack of colic is very different from it shown when pus is forming in the foot. Pain may be lveyed by channels which under ordinary conditions lvey no sensation, especially is this the case in disease, e normal heart, liver, muscles, bones, etc., may be tidied, pinched, wounded, and cauterized, without causing ich or any sensation, but under the condition of inflam- ,tion they become acutely sensitive, and the same applies such viscera as the intestines, kidneys, bladder, etc. Of ) nature of pain nothing whatever is known. Muscle Sense. Sensory nerve endings have been found in muscle and idon (p. 353). In the former they are spoken of as neuro- scular spindles; these from their construction are readily acted by variation in the tension of contracting muscles, Digitized by Microsoft® THE SENSES 493 and in this way they keep the central organism informed of their condition. They,- •with the tendon endings, are employed in judging active muscular movements ; passive movements are determined by impulses passing to the centre from the sensory nerve endings in joints, while the position of the limbs is known by sensory impressions which arise in the skin and subcutaneous tissue of joints. When muscle sense is lost inco-ordinate muscular contractions occur ; well seen in a dog in which the sensory roots leading to the hind limbs are divided. There is, of course, under these conditions, no loss of motor power, yet the animal, through m.h.6. Pig. 120. — Muscle Spindle (Halliburton, after Buffini). c, Sheath of the spindle; n.tr., trunk of nerve, which sends fibres through the sheath into the spindle, where they form endings (pr.e., s.e.,pl.e.) of various kinds ; m.n.b., bundle of motor fibres (Stewart). a loss of muscle sense, drags the limbs as if they were paralysed. Later on the sensory impressions which should pass from muscle, joints, and skin, but are unable to reach the cord through the roots being divided, are now replaced by visual impressions, and the dog learns to walk through the medium of his eyes, but if placed in the dark the whole of the pseudo- paralytic symptoms return. From this it is evident that one of the necessary conditions for perfectly controlled muscular movements is a muscle sense pouring in sensory impulses into the central nervous system, and by imparting a continuous knowledge of the condition of the muscles effecting their control. Digitized by Microsoft® [ A MANUAL OF VETEEINAEY PHYSIOLOGY Thirst. Fhirst is referred to the pharynx ; observations show that , istening the palate allays thirst, while on the other , id, the filling of the stomach with water through a ula does not immediately allay the desire for fluid. Che loss of water caused by sweating, purging, etc., is de good to the blood by taking up water from the iues ; in this way the drain on the lymph may be con- arable. It has been supposed that the sensation of rst referred to the palate may be brought about by a iciency of water or lymph in the part. Little or nothing known of the nervous apparatus involved in thirst, nor y dryness of the tissues should be referred to the irynx and palate. The sense of thirst is generally y lost in one particular group of affections — viz., acute orders of the digestive system in the horse. No horse fering acute intestinal or stomach pain will, as a rule, nk, yet the dry condition of the mouth suggests that rst should be present. Hunger. Hunger is referred to the stomach. The close approxima- n of the stomach walls is not necessary for the production all animals of the sensations of hunger, for some of the rbivora may be very hungry even when the stomach itains a moderate amount of food, the horse and rabbit ■ example ; further, the sensations of hunger may be noved though the walls of the stomach remain in opposi- n — viz., by the introduction of nutritive enemata. The ison why the sensations of hunger are referred to the >mach wall is unknown. An animal deprived of its cerebrum shows all the usual ;ns of hunger, though obviously in this case it is an un- uscious exhibition. Digitized by Microsoft® THE SENSES 495 Section V. , „ Hearing. The Nature of Sound. — When a body is made to vibrate its vibrations are communicated to the adjacent air and give rise in this to waves which travel at a definite rate, and when they reach the ear so act upon its structures as to lead to the sensation of sound. The vibrations which constitute the waves take place to and fro along the direc- tion in which the wave is travelling ; in this sound differs from light, whose vibrations are transverse to the direction of propagation. In comparing one sound with another we are conscious of only three possible differences between them ; they may differ in loudness, 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. Now, 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 vibra- tion. 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 partial tones ; where they are absent the tone is thin, where they are present they give richness, and confer on it that ' character ' which enables us to recognise one musical instrument from another by the mere sound it emits. Those sounds which we group under the general term of ' musical ' result from the regularity of their causative vibra- tions and the definiteness in wave-length of the latte'r. Digitized by Microsoft® 496 A MANUAL OF VETBEINAEY PHYSIOLOGY Noise is essentially the result of the absence of this regularity and definiteness. We have 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 we are near it, and the musical hum- ming 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 are about thirty per second, while the average 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 shows that the cat is capable of recognising sounds in- audible to the human ear. External Ear. — The vibrations of sound are collected by a freely moving funnel-shaped body or external ear; it is composed mainly of cartilage, which is curved and hollowed out in such a way as to form a good collector, while several muscles enable it to assume considerable changes in direc- tion. The two chief directions taken by the ears are back- wards and forwards ; judging from the behaviour of many horses in carrying one ear backwards and the other forwards, it would appear that they are capable of hearing and appre- ciating sound in two opposite directions at one and the same time ; we say appreciating, inasmuch as something more than mere hearing is required for auditory judgment. The funnel formed by the external ear leads somewhat indirectly to a canal known as the external auditory meatus ; in and around this is found an unctuous secretion, and above it, in the funnel of the ear, are many hairs which evidently are for the purpose of protection. The movements of the ears give evidence of what is passing through the mind of an animal. The ears of the horse are turned well to the front and closely pricked — viz., the points approximated, when he is attentive, whether the attention be devoted to a something he is alarmed at or pleased with. The ears are laid back on the poll in sour- ness of temper and in vice ; they are moved rapidly to and Digitized by Microsoft® THE SENSES 497 fro when a horse is anxious either from impending danger or other cause; one ear carried forward and the other backward or both turned backwards are considered the sign of a good stayer and willing worker, while drooping ears are indicative of muscle fatigue or debility. "Whatever part those remarkables sacs, the guttural pouches (confined solely to solipeds), are intended for, it is probable, from their anatomical connection, that they take some share in the sense of hearing, perhaps that of supplying the need- ful amount of air to the middle ear. The actual use of the guttural pouches is involved in obscurity, but we may pro- visionally consider them as part of the middle ear. 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 beneath the skull ; in other words, they are probably associated with acuteness of hearing. At one end of the external auditory canal is a piece of membrane stretched completely across it known as the Tympanum, it separates the external from the middle ear (Fig. 121). The Middle Ear is on the opposite side of the tympanum to the external ear ; it consists of a cavity containing a chain of very small bones, known as the malleus, incus and stapes, which stretch like a bridge across the space from the tympanum to the third or internal ear. The middle, like the external ear, is in communication with the external air, but by means of a passage known as the Eustachian canal which opens into the pharynx. The tympanum has, therefore, air on both sides of it, the object of which is to ensure that the atmospheric pressure on either side is equal, and in this way ensure its free swing. The air finds its way into the Eustachian tube during the act of swallowing, and by the same channel it is conveyed to the guttural pouches. The Tympanum 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 sound, causes the bridge of bones to move ; further, their movement is assisted by some small muscles which are attached to them. The Internal Ear, known as the labyrinth (Pig. 122), is composed of the semicircular canals, the vestibule, and the cochlea ; these are con- tained in a solid piece of bone in which two small foramina or windows 32 Digitized by Microsoft® 498 A MANUAL OP VETEEINARY PHYSIOLOGY 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. Fig. 121. — Diagrammatic Section of the Horse's Ear. 1, External auditory canal ; 2, the tympanum ; 3, chain of bones across the middle ear ; 4, the Eustachian tube ; 5, the internal ear. All three parts of the labyrinth communicate, but it is quite certain that all three do not take an equally active part in hearing. The evidence on this point is clear so far as the semicircular canals are Pig. 122. — The Labyrinth (Edmunds). The semicircular canals are to the right, the cochlea to the left ; both windows may be seen, the fenestra rotunda being the lowermost. The groove across the body of the organ lodges the auditory nerve. The figure is enlarged. concerned, and some have even included the vestibule, regarding the cochlea as the essential organ of hearing. The whole of the internal ear 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. Digitized by Microsoft® THE SENSES 499 Within this membrane is a membranous labyrinth, the counterpart of the semicircular canals and vestibule, and this also contains fluid known as endo-lymph. The membranous labyrinth is composed of two pouches, the saccule and utricle lying in the vestibule ; with the latter the membranous semicircular canals are connected, while the former communicates with the middle canal of the cochlea. On both utricle and saccule is an area known as the macula acustica, on which branches of the auditory nerve are distributed to cells known as hair cells ; similar areas exist in the semicircular canals. The hairs on the cells project into a mucoid mass frequently containing crystals of carbonate of lime ; these crystals are known as otoliths. leissner's membrane Scala Vestibuh Fihresof Auditor^ Sea J a Tymnani r analis cochleae i-Membrana tectoria .Hair cells ■-Memb r an a baSilaris ^■Pillars of Corti Fig. 123. — Diagrammatic Transverse Section of a Turn of the Cochlea (Stewart). The two windows existing in the bony labyrinth have been men- tioned above. 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 tympanum 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. 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 travels. If a spiral of the cochlea be cut across (Fig. 123) 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 32^2 Digitized by Microsoft® 500 A MANUAL OF VETERINAEY PHYSIOLOGY shape and called the cochlea/r 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 Beissner, whilst 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. The cochlear canal is the continuation of the membranous labyrinth. The upper passage of the cochlea, viz., 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 Pig. 124. — Organ of Coeti (Barker, after Eetzids). nib, Basilar membrane ; re, nerve fibres passing in to arborize around the hair cells ; p, inner pillar of Corti, with its basal cell, 6 ; p', outer pillar, with its basal cell, b' ; 1, 2, 3, supporting cells of Deiters ; H, Hensen's supporting cells ; i, internal hair cells with its hairs; e, external hair cells; e', hairs of three external hair cells ; n, n 1 , to n*, cross-sections of the spiral strand of cochlear nerve fibres (Stewart). window, the fenestra rotunda, which separates the scala tympani from the cavity of the tympanum. The cochlear canal terminates suddenly at the summit of the cochlea, and at this point the two seals, which in their windings have been decreasing in size from base to apex, meet and communicate by a small opening, the helieotrema, 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 (Pig. 124), 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 of Corti, fit into each other in a peculiar manner. Flanking either side of the tunnel are certain cells Digitized by Microsoft® THE SENSES 501 of two distinct kinds ; those nearest to the tunnel are somewhat flask- shaped, and having hairs growing from their summit, are spoken of as the inner and outer hair cells ; external to the outer hair cells are some tall conical cells known as Hensen's cells. It will be remem- bered that the auditory nerve ascends the axis of the cochlea, giving off fibres 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. This description of the organ of Corti is as it presents itself in tranverse section ; if, how- ever, we look at the tunnel from above where the rods from either side meet, it is observed that in their union the rods of the outer wall of the tunnel fit into the heads of the rods of the inner wall, and the square- ness of their heads is such that the arrangement is very like the keyboard of a piano. 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 one as readily as another. At the same time it is certain that animals can learn to recognise sounds and associate them with certain ideas, as for instance the commotion and excitement amongst the horses of a regiment when the trumpet sounds 'feed,' and again the recognition by a dog of its master's voice. 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, though their capacity for the enjoyment of music is absent or extremely small. The vibrations set up in the tympanum are, as we have seen, communicated to the chain of bones, the stapes of which, through the fenestra ovalis, imparts a push to the peri- lymph of the labyrinth ; this fluid transmits the impulse through the vestibule, and from here into the scala vestibuli Digitized by Microsoft® 502 A MANUAL OF VETERINARY PHYSIOLOGY of the cochlea. The vibrations ascend the spiral staircase, and set in motion the membrane of Eeissner, which causes the lymph in the cochlear canal to vibrate ; when these vibrations reach the summit of the cochlea they enter the scala tympani through the helicotrema. The lymph in this canal is now set in motion, with the result that the basilar membrane, on which the organ of Corti rests, is affected, and the vibrations are ultimately lost at the blind extremity of the canal, whose membrane is pushed outwards at the fenestra rotunda. Every push inwards at the fenestra ovalis causes, therefore, a push outwards at the fenestra rotunda. During the time the vibrations are crossing the cochlear canal from one scala to another the organ of Corti is affected, and by means of the auditory nerve the impulse is conveyed to the brain. It is in this organ of Corti, with its nerve endingB, that the complex sounds which make up even a single note of music are analysed, and this analysis was at one time supposed to be effected by the rods of the organ, which were believed to vibrate to their own particular tone, in the same way as a tuning-fork will pick out its own tone from sounds in its vicinity and vibrate to it. This view, tempting as it is, is negatived by the fact that the rods of Corti do not exist in birds, and it has therefore been supposed that the vibrations to the nerves terminating in the organ are set up by the vibration of the basilar membrane on which the organ is built, but the question is far from settled. Even the function of the vestibule is disputed ; while some hold from analogy that it is connected with auditory sensations — through the nerves terminating in hair-cells which are found on the areas previously described as the macula acustiea — others believe that it is wholly devoted to the perception of movements of the body, by which means the animal is informed of the extent and direction of its own movements. The Semicircular Canals and Labyrinth, though connected with the internal ear and sharing in common with it the nerve of hearing, are yet devoted to functions of quite Digitized by Microsoft® THE SENSES 503 another kind. To the labyrinth is assigned the control of the ocular muscles and the maintenance of the horizontal position of the pupil (see p. 472). It is also engaged in the obscure problem of muscle tonus (p. 364), and this is gathered from the fact that destruction of it delays the appearance of rigor mortis on the same side. Finally, the labyrinth is the means by which the body learns, or is made acquainted with its position and movements, and this is effected by impulses proceeding from it to the cerebellum. To quote the words of Sherrington, the Fig. 125. — Diagram showing the Position Occupied by the Semi- circular Canals. After Ewald (Stewart). H, P, S, are three mutually rectangular planes which indicate the position of the canals. In the horizontal plane, H, are found both external canals; in a vertical longitudinal plane, S, are found both superior canals; in a vertical transverse plane, P, both posterior canals are placed. The plane of the superior vertical canal of one side is parallel to the plane of the posterior vertical canal of the opposite side. labyrinth keeps the world right side up for the organism, by keeping the organism right side up to the external world. This function falls to the semicircular canals, the arrange- ment of which is peculiar and interesting. There are three bony semicircular canals, so arranged that their three planes are placed at right angles to each other, two being vertical and one horizontal (Fig. 125) ; within each bony canal is one of membrane, the two being separated by a fluid known as the peri-lymph. Within the membranous canals is also a fluid known as the endo-lymph, and at certain parts of the canals Digitized by Microsoft® 504 A MANUAL OF VETERINARY PHYSIOLOGY ;he vestibular branch of the auditory nerve has special lerve endings known as hair-cells. Impulses set up in ;hese hair-cells are brought about either by alterations in ihe pressure of the peri-lymph, such as occur in consequence )f movement, or by mechanical stimulation produced by grains of calcium carbonate (otoliths) found in the labyrinth, ;losely associated with the nerve endings. Impulses so set ip are conveyed to the cerebellum, which is the centre lominating the co-ordination of muscular movements, and ihe movements necessary for equilibration. The semicircular canals are arranged as above described, io that movements of the body in the three dimensions of space may produce their respective effects on the brain. It s by means of them that an animal is made acquainted ;vith the direction in which its body is travelling, forward >r backward, right or left, up hill or down, or in the move- nents which occur in jumping. It is no wonder, consider- ng their extraordinary importance, that these canals are securely lodged within the substance of the hardest bone n the body. If the semicircular canals be injured the resulting menomena depend upon the position of the canal which las been destroyed. If the horizontal canal, the head )scillates in a horizontal plane ; if the vertical canals be lamaged forced movements occur in a vertical plane; standing and locomotion become impossible. If all three :anals are destroyed violent inco-ordinate movements result, the animal turns somersaults, the head is twisted, ;he eyeballs roll from side to side, and special measures lave to be taken to prevent the creature killing itself ihrough its own violence. Digitized by Microsoft® CHAPTEE XVI THE LOCOMOTOR APPARATUS The muscles are attached to bones, and these, by their movements, may be inclined to each other at angles of varying size. These angles are opened and closed during progression, and the mechanical aid which is introduced to effect this is that of the lever. The lever is com- posed of a power, fulcrum, and weight, and according to the relative positions which these occupy, a lever is spoken of as being of the first, second, or third order. In a lever of the first order the power is at one end, the weight at the other, and the fulcrum between the two. The muscles which extend the head act as a lever of this order, the head being the weight, the occipito-atloid articu- lation the fulcrum, and the muscles of the neck the power. In extension of the hind-leg the gastrocnemii muscles are the power, the hock -joint the fulcrum, and the leg below the hock the weight. A lever of the first order is principally a lever of extension, and exists all over the body ; it is also a lever of power, for if the long-arm be 5 feet, and the short-arm 1 foot, a power of 1 lb. at the long-arm will support a weight of 5 lbs. on the short-arm. It is to be noted that as a lever increases in power it loses in speediness of action. In the lever of the second order, which is a rare one in the body, the weight is placed between the fulcrum and the power as in a wheelbarrow, the wheel being the fulcrum. When the leg is fixed on the ground and the body passing over it a lever of the second order is formed, the ground 505 Digitized by Microsoft® 506 A MANUAL OF VETEEINAEY PHYSIOLOGY being the fulcrum, the triceps or gastrocnemii the power, and the body through the elbow or hock joints the weight. The third order of lever is the lever of flexion. The power is placed between the fulcrum and weight ; the nearer the power is to the fulcrum, the greater the flexion obtained for a given expenditure of muscular force. This lever is one for speed, and what it gains in speed it loses in power ; it is therefore a wasteful lever, but an essential one in the limbs. Examples of it in the body are numerous ; in the flexion of the elbow- joint, the weight is the leg below the elbow, the power is the flexor brachii muscle at its insertion into the radius, whilst the elbow-joint forms the fulcrum. In the flexion of the hock the power is the flexor metatarsi, the fulcrum is the hock-joint, the weight being represented by the limb below the hock. The reason why the third lever is more frequent than the others, is due to the fact that the chief movements of the limbs are directed to moving comparatively light weights through a great distance, or through a certain distance with great pre- cision, rather than moving heavy weights through a short distance (Foster). As to the weight to be carried, we may say that the weight of the fore-leg of a cavalry horse cut off at the elbow was found to be 17 lbs. 8 ozs. ; cut off at the knee, through the upper row of bones, it was found to weigh 7 lbs. 10 ozs. ; one fore-foot with corona weighed 2 lbs. 3 ozs., and the hind-leg, cut off at the hock-joint, weighed 10 lbs. 9 ozs. Stillman* points out that the terms flexor, extensor, adductor, and abductor, cause the chief function in muscles to be lost sight of, viz., the power of propelling ; it is necessary, however, to remember that propelling is not a power apart from flexion and extension, but the result of them. Co-operative Antagonism. — As a rule, to which there are certain exceptions, the contraction of any group of muscles is attended by a contraction and not a relaxation of their antagonists. This is described by Waller as ' Co-operative * ' The Horse in Motion.' Digitized by Microsoft® THE LOCOMOTOR APPARATUS 507 Antagonism.' The amount of contraction thus exhibited by antagonistic muscles is insufficient to neutralize the effect of the direct motors, but it would appear that for the due performance of such movements as flexion, extension, etc., the antagonistic group of muscles should offer some slight opposition. This can readily be demonstrated by flexing the fingers and grasping the arm with the opposite hand ; both extensor and flexor muscles will be felt to harden. Moreover the opposition of antagonistic muscles appears in many cases to be essential to the due performance of movement ; Waller quotes as an example of this the fact that in lead palsy only the extensor muscles of the arm are affected, yet the flexors are powerless to act. The difference existing between the articulation of the fore and hind limbs with the trunk has until recent years been the cause of considerable error being promulgated. It was previously supposed that the muscular attachment of the fore-leg to the trunk indicated that the body was simply slung between the fore-legs, the latter acting as props whilst the hind-limbs did the work. Instantaneous photography has shown us that the fore-limbs not only act as props but as propellers of the body ; especially is this seen in the gallop, where by measurement it has been shown that one fore-leg will propel the body a distance of 10 feet, and in so doing will raise it 4 inches in height in a vertical direction.* By means of the fore-legs also, the horse is enabled in draught to assist its hind-legs in stopping weights. Joints are formed wherever two bones come into contact. Dealing only with those joints in the limbs which are of the most practical interest, it is observed that ball-and- socket joints (as in the hip), hinge-like joints (as in the hock), and gliding joints (as in the knee), are found; all these are coated with articular cartilage and lubricated with synovia. Synovia is a viscid, yellow, alkaline fluid con- taining proteids, mucin, and salts. The viscidity of synovia is due entirely to the mucin it contains, and this confers on * Stillman, ' The Horse in Motion.' Digitized by Microsoft® 508 A MANUAL OF VETEEINAEY PHYSIOLOGY it its slippery nature. There is no difference between the synovia of joints and that of bursas. It is said that the amount of synovia in a joint is greater in animals at rest ihan in those at work, but the extra bulk appears to be due ;o an increase in the watery material, whilst the proteids ire decreased ; the salts, on the other hand, especially those }f sodium, exist in a larger proportion than in the synovia )f working animals. The bursas in the limbs of the horse are very important structures ; they are placed where the tendons pass through Dony channels, and without them the rapid movements of ;he limbs would be impossible ; that the strain on them irom wear and tear is considerable we know from practical jxperience. Hock-joint. — Solipeds appear to stand alone in having she ridges of the astragalus placed obliquely, instead of /ertically as in other animals ; the oblique ridges in the lorse occasion some considerable difference in the action of :he limb. It is usual to speak of a screw action of the hock Droduced by the oblique ridges of the astragalus ; this screw action, we believe, is an entire misconception. The idges on the astragalus do act as a screw but not on the lock ; the effect is on the stifle, and produces that remark- tble stifle action particularly well seen in trotters. If the •idges on the astragalus turned the hock outwards, every lorse would travel as if it were ' cow-hocked.' The leg )elow the astragalus is carried directly forwards ; when, " lowever, it comes to the ground, and the body passes over t, it is not uncommon in some horses to observe a con- iderable twist outwards of the hock-joint, the toe being urned in ; this is due to the ascent of the 1 lower end of the ibia on the astragalus, leading to the upper end of the ibia turning in the stifle-joint, the result of the leg being ixtended. The object of the stifle being turned outwards luring the flexion of the leg is to clear the abdominal wall, ,nd the reason why solipeds have oblique ridges on the iStragalus and ruminants and carnivora vertical ones, is hat the ribs of the latter class are short and do not come Digitized by Microsoft® THE LOCOMOTOR APPARATUS 509 near the pelvis (as in the horse), and therefore the abdominal wall is not in the way. A spring or automatic flexion action in the hock has been described, such as may readily be observed in the dead leg, when if the hock be flexed slightly it either flies back or completes its revolution with a jerk. This condition does not exist during life, nor after death until rigor mortis occurs ; it is produced by the lateral ligaments of the hock- joint, and is purely a post- mortem condition. The flexor metatarsi muscle is remarkable in having a tendon running its whole length, so that from the origin at the femur to the insertion at the front of the hock there is a stout tendinous cord. A somewhat similar arrangement exists in connection with the gastrocnemii muscles. When the flexor metatarsi acts the hock is flexed, but the use of the tendon running from origin to insertion is not at first sight quite clear. Chauveau considers that it automatically flexes the hock, but tendons are devoid of any such power ; it would appear that its function is to relieve the muscle when the animal is standing, or sleeps standing. When muscles which perform flexion and extension are acting together with equal force no movement results ; such is the case when the weight is on the limbs and the animal at rest. When a horse is at rest his gastrocnemii muscles and flexor metatarsi are acting in opposite directions and equally ; the one is trying to close the femoro-tibial angle, the other keeping it open. It is the function of the tendinous portion of the flexor metatarsi and gastrocnemii muscles to assist in keeping the leg fixed without any great muscular effort. The chief movement of the hock occurs between the tibia and astragalus. Though the range of motion between these bones is considerable, yet it is not fully exercised in all paces ; it is only in the jump and gallop that the angle formed between the tibia and metatarsal is closed to any great degree. When the joint is completely flexed in the dead dissected limb, if we look at the posterior part, viz., the now uncovered ridges of the astragalus, we find that when the joint is flexed to the utmost the tibia and Digitized by Microsoft® 510 A MANUAL OF VETEEINAEY PHYSIOLOGY astragalus are no longer in apposition, the tibia has partly left the astragalus and a small space exists between them. To prevent flexion to a dangerous degree two stops are placed on the anterior face of the inferior extremity of the tibia, one outside, the other inside, the outside being the larger of the two ; these stops come into contact with two rests on the astragalus, and in this way we think a certain amount of jar may be imparted to this bone. As the inside stop comes into contact with the astragalus slightly before the outside stop, we conceive it possible that the inside of the astragalus receives more concussion than the outside. Can this help to offer any explanation of the position of spavin ? Of the ridges on the astragalus, one is narrow, the other broad ; the narrow one is the inside ridge, and it runs completely down to the surface which articulates with the magnum, and sometimes considerably overlaps it. The movement in the true hock-joint is very simple as well as extensive ; but the movements between the small bones composing the joints are complicated. In the first instance they are very limited ; the astragalus moves on the magnum, the magnum on the medium, and the medium on the large metatarsal ; but the amount of movement in these is not the same, the movement between the astragalus and magnum being the greatest. One might suppose that the movement in this part was. rather of a front to rear, viz., to and fro character, though the fact that the ligamentous attachment between the bones is situated at the central part suggests that this is probably not the case. Pathology proves the correctness of the latter supposition. An examination of the face of these bones when affected with articular disease exhibits well-marked, sharp, and rather deep grooves, which run obliquely across the surface of the bones, and are better seen between the astragalus and magnum than elsewhere. The grooves are the result of friction during the movement of the joint, and they indicate that the motion of these bones on one another is more of the nature of a rotation. Again, these grooves show Digitized by Microsoft® THE LOCOMOTOR APPARATUS 511 where the greatest amount of pressure normally comes on the bones ; it will always be found that the most extensive damage in disease is on the anterior and internal surface, and this rule holds good whether it be the astragalus, magnum, medium, or head of the large metatarsal which we are examining. If a longitudinal section of the leg from the thigh to the fetlock be made, it is observed that the line of weight on the bony column mainly falls through the anterior part of the hock-joint. There can be no doubt that this pressure is removed by resting the leg, viz., flexing the hock, and this is probably the reason why no horse ever stands for any length of time resting equally ^ on both hind-legs. ft' The Stifle is the largest joint in the body ; the cause of its rotation has been previously described. One function of this joint is that of rendering the limb firm and rigid when the foot is on the ground, and this it does by the con- traction of the muscles inserted into the patella; if the latter bone be kept fixed on the upper part of the trochlea of the femur, no flexing of the hock or stifle can occur. This experiment can be readily tried on a horse just destroyed; the limb having been extended, the simple pressure of the hand on the crural muscles is sufficient to prevent the bending of the hock unless considerable force be employed. No bending of the hock during life can occur if the foot be kept extended ; the first movement in the advance of the leg and the flexing of the hock and stifle is that the foot is flexed. In a certain surgical condition, com- monly known as dislocation of the patella, the limb is rigid from the femur to the metatarsus ; but, though the foot may be flexed, neither hock nor stifle responds, owing to the patella being. fixed. We believe that in the majority of these cases the patella is not fixed from dislocation, but from cramp of the vasti muscles. The amount of move- ment in the stifle is considerable, and to admit of it being carried out with perfect freedom, the convex condyles of the femur play in cups formed of cartilage on the upper surface of the tibia. Digitized by Microsoft® 512 A MANUAL OF VETEKINAKY PHYSIOLOGY The Hip is a cup and ball joint ; the range of outward movement obtained by it in the horse is limited by the insertion of the ligamentum teres (and pubio-femoral liga- ment) into the inner side of the head of the femur, and not into its centre as in most other animals. This is said to be the reason why the horse rarely ' cow-kicks.' The lengthening of these ligaments accounts for ' cow-hocks ' in horses. The Shoulder-joint is characterized by the considerable surface of movement afforded by the humerus and the small surface of the scapula, the object being to obtain a large range of motion for the humerus. The Elbow presents an articulation with ridges which influence the turning outwards of the knee in progression ; if the knees are turned out too much the leg below is thrown in as it is brought forward, and in this way one cause of ' brushing ' and ' speedy cutting ' is produced. The Knee consists of three main joints and numerous minor ones ; the upper joint possesses the largest range of motion, whilst the lower joint practically does not open. Probably such defects as ' speedy cutting ' and its opposite condition, ' dishing,' are influenced not only by the elbow, but by the shape of the articular surfaces between the radius and upper row of bones. The radius is peculiar in presenting on that articular surface next the knee a concave surface anteriorly and a convex one posteriorly ; these form two condyles, of which the inner is more curved than the outer. The outer condyle plays on the trapezium, cuneiform, and lunar; the inner condyle plays solely on the scaphoid. When the knee is flexed the influence of the condyles is seen ; the concave articular surface of the radius is removed from the surface of the bones of the knee, and the convex articular surface appears as the joint grows wider. The inner condyle being larger than the outer depresses the scaphoid, so that a very important movement occurs between the scaphoid and lunar. This action of the radius on the scaphoid throws the foot slightly outwards, probably with the object of enabling it Digitized by Microsoft® THE LOCOMOTOE APPAEATUS 513 to clear the opposite limb. We believe that an examina- tion of the knees of ' dishing ' horses will show that extreme curvature of the inner condyle of the radius is the cause of the action, in the same way that turned-in elbows, and alterations in the curvatures of the radius and humerus, will probably account for horses throwing the foot inwards, and thus ' brushing ' or ' speedy cutting.' \<£ The Fetlock Joint, owing to the presence of the sesamoid bones, forms a yielding articulation. In a state of repose the greater part of the horse's weight is borne on the posterior half of the metacarpal articulation and the articular surface of the sesamoids. One great advantage gained by the articulation of the fetlock being yielding is the destruction of the concussion of impact when the body comes to the ground. A similar condition is observed in the joint of the foot, for which see the chapter devoted to that subject. The Function of the Suspensory Ligament has been a fruitful source of discussion. Its chief use, no doubt, is to support the fetlock; in no other way could a joint be supported which is placed in this part of the limb, pos- sessed of so much motion, and exposed to such concussion. Though ligaments and tendons are held to be non-elastic, yet we must claim for the suspensory ligament a little more elasticity than would be obtained if the sesamoids were united by bony tissue to the metacarpal, and the pleasant- ness and freedom from jar experienced in the riding-horse are in part due to the suspensory ligaments. Stillman claims for the suspensory ligament a function which he believes to be demonstrated by instantaneous photo- graphy, viz., that it acts the part of a spring, flexing the fetlock sharply when the weight is taken off it, and explains why the dirt is thrown out of the feet of a galloping horse. We have no evidence of the correctness of this statement ; the sharp picking up of the foot from the ground in walking (a movement so rapid as almost to defy detection) must rest with the flexor muscles. Besides these functions, the suspensory ligaments assist the horse to stand while 33 Digitized by Microsoft® 514 A MANUAL OF VETEEINAEY PHYSIOLOGY sleeping. If the suspensory ligament be divided, the fetlock sinks but does not come to the ground; if the perforans be divided a slight sinking of the fetlock is the only change. To bring the fetlock to the ground, both flexors and suspensory ligament must be divided, which demonstrates that all three support the weight while standing. Function of the Check Ligaments. — Horses are enabled to sleep while standing, and remain for some considerable time without lying down, by means of a singular arrangement of so-called ' check ' ligaments which exists in both fore and hind limbs ; we have previously touched on this question so far as the hind-limb is concerned. The flexor tendons of the fore-limb support the weight, the extensors keep the limb rigid. In order that the strain of supporting the weight may not be placed solely on the muscles of the arm, both flexor and extensor tendons receive branches of ligament from the radius and metacarpus. These are attached to the tendons in such a way as to cut off the muscles, at any rate, to a considerable extent, from the strain of standing in one position for any length of time. In the act of standing the rigidity of the bony column of the leg is maintained by the extensor tendons, each phalanx having an extensor attached to it, viz., the extensor meta- carpi leading to the large metacarpal bone, extensor pedis to the corona and pedis (receiving also a slip from the sus- pensory ligament), and extensor suffraginis to the suffraginis. This latter receives a strong slip of ligament from the out- side of the carpus, which entirely takes off the strain from the muscle, and keeps the tendon taut during sleep. Further, the horse is enabled to stand whilst sleeping by means of the fascia of the arm and thigh ; both of these are attached to the muscles and tendons of the part, affording them considerable support of a non-muscular nature. Centre of Gravity. — The centre of gravity at rest is fixed, but during motion it oscillates from front to rear, depend- ing on the position of the body and the pace. Owing to Digitized by Microsoft® THE LOCOMOTOE APPAEATUS 515 the fact that more weight is carried on the fore than on the hind legs, the centre of gravity lies nearer to the elbow than the stifle. If a vertical line be dropped just behind the ensiform cartilage of the sternum, and intersected by a horizontal one passing through the lower part of the middle third of the body, the point of intersection is the centre of gravity of the body at rest ; this is the rule given by Colin. We may say, speaking roughly, that the vertical line passes about 6 inches behind the elbow, the horizontal just below the shoulder-joint ; the centre of gravity is where these intersect. It is obvious that the position of the centre of gravity will vary with different horses, but not to such an extent as seriously to affect the truth of the above statement. During locomotion the centre of gravity moves to the front and rear of the normal at rest ; for example, in jumping it is in front of it when the fore-legs are coming to the ground, behind it wben the hind-legs are leaving the ground ; it is in front of it during draught, behind it during backing. Distribution of the Weight of the Body. — The fore-legs carry more weight than the hind, which is perhaps the reverse of what might be expected ; but if a horse be carefully weighed, it is found that the fore-legs take more than one-half the body weight. The position of the head (which may weigh as much as 40 and 50 lbs.) considerably affects the weight on the legs. Thus, if the head be raised up when the fore-legs are weighed, the latter will be found to be carrying over 20 lbs. less weight than if the head were dependent. The practical application of this fact is obvious — keep a stumbler well in hand. When a man is on the horse's back, it is found that 66 per cent, of his weight is carried on the fore-legs, and 34 per cent, on the hind ; the amount of weight on the fore-legs is increased by leaning forward in the saddle, and decreased by leaning back. An explanation why fore-legs are worn out earlier than hind is afforded us by what we now know of the physiology of locomotion — viz., the fore-legs act as propellers of the body, and owing to their being nearest to the centre of 33—2 Digitized by Microsoft® 516 A MANUAL OF VETERINARY PHYSIOLOGY gravity, they also bear the largest share of the weight of the body and the weight of the rider. The Structure and Function of the Limbs in Relation to the Production of Lameness. — As high up as the shank we may say that there is no practical difference in the anatomical arrangements of the fore and hind limbs, and yet we know how commonly the foot and coronet of the fore-leg are affected with lameness, and how rarely in comparison the hind one. In comparing the knee and hock great differences are observed ; it is true that in both a number of pieces of bone enter into their formation, but here the likeness endB ; the small bones of the knee have considerable movement, the small bones of the hock only a trifling amount. The lower row of knee bones, so far as movement is concerned, are the nearest approach to the movement of the small bones of the hock, yet the latter are frequently diseased, the former rarely affected. Evidently, then, the presence of small and comparatively immobile bones in the hock cannot constitute an explanation of the frequency of hock disease. Does the manner in which the joints are flexed throw any light on the acknowledged fact that knee disease is rare' and hock disease frequent ? It will be observed that these two joints bend in opposite directions ; the knee opens in the front when flexed, the hock opens at the back ; we have already given reasons for believing that some injury may be inflicted on the hock-joint by its method of closing. Continuing this comparison of the fore and hind leg, it may be remarked that the stifle corresponds to the elbow, and the patella to the ulna ; during flexion of these joints the elbow opens at the back whilst the stifle opens in front ; in other words, though corresponding joints — the elbow and the stifle, the hock and the knee — they do not agree in the direction in which their movement is made. The hip- joint corresponds to the shoulder- joint, and though in the hip all the movement is done by one bone instead of two, yet the to-and-fro movement is practically the same in each. When the fore-leg comes to the ground, no matter what Digitized by Microsoft® THE LOCOMOTOR APPARATUS 517 the pace may be, the limb must be straight in order that the foot may be placed down flat, or, as in the faster paces, heel first. This straightening of the knee renders the bony column of the leg rigid for the time being ; the shock of impact is therefore greatest at that part of the column nearest to the point of impact, and decreases as it passes up the leg. It would be anticipating our subject to attempt to deal with the various means which exist in. the foot to render this shock as little destructive as possible ; we can only allude to the weight being supported on the laminae, to the presence of a foot articulation which is yielding posteriorly, the existence of an elastic movement of the posterior part of the foot, and the presence of an elastic and indiarubber-like cushion, the foot-pad. There are, however, two distinct strains imposed on a limb — viz., the shock or concussion when the foot comes to the ground, and the strain or compression occasioned when it is leaving the ground ; one is the concussion of impact, the other the compression of propulsion. The hind-leg differs from the fore-limb in its method of providing for the concussion of impact ; here we find that the limb instead of being straight — as the fore-leg is from the elbow to the foot — is bent, and it is bent at the hock, at a point which we may take to be midway between the stifle and the ground. The shock of impact comes, therefore, largely on the hock. The fore-leg in providing for propulsion rotates over the foot, the limb still being straight from the elbow to the ground, and the shock of rotation is mainly confined to the lower end of the bony column. In the hind-leg propulsion is obtained not only by the foot remaining fixed on the ground, but also at the same time by a straightening or unbending of the hock, which gradually opens until the tibia forms with the metatarsal bone the nearest straight line it is capable of making. In this way we may say that the hock performs twice as much work as the knee, and such a statement throws some possible light on the frequency with which this joint is affected with disease. Digitized by Microsoft® 518 A MANUAL OF VETEEINAEY PHYSIOLOGY The Anti-concussion Mechanisms existing in the limb are, roughly speaking, of two kinds — viz., (1) those for receiving the weight of the body on the leg when the foot comes to the ground, without the part suffering from the concussion ?f impact, and (2) those which admit of propulsion by one iore-limb without the parts suffering from the compression )f propulsion. The first is principally provided by the yielding joints formed in the pedal and fetlock articulations, 3y the arrangement of the foot, and by the tendinous and igamentous material at the back of the limb ; the second s furnished by the column of bones forming the limb being sroken up from the scapula to the pedis, and progressively ncreasing in size from the seat of the largest amount of sompression — viz., the foot — to the least amount in the shoulder. Probably the coronet and pastern represent the weakest part of the fore-limb, and their small size in comparison with the weight they have to support is jvidence of this. To ease the skeleton from concussion the muscles and ;endons are brought into play and rendered taut ; we know, 'or instance, how much better a limb is prepared to stand i sudden shock if sufficient warning is given through the sense of sight. The tendons and muscles of the limbs help to take the shock. So long as the muscles maintain their elasticity the vork done by their tendinous attachments is comparatively slight; as the muscles tire the strain on the tendons ncreases, and in consequence they may give way, and this vill occur at their weakest part. In this tired condition of imb the skeleton also suffers, the bones forming the column eceive more shock than normal, and the smallest and hortest bones situated nearest to the seat of concussion, r iz., the ground, may even fracture under the strain, and mder any circumstances run a grave risk of becoming nflamed. This argument is based on clinical observation ; ve do not believe that any riding horse sprains its back endons or suspensory ligament until the muscles tire, and ire no longer capable of exhibiting that perfect elasticity Digitized by Microsoft® THE LOCOMOTOR APPARATUS 519 inherent in muscular tissue. We do not, however, say that no horse suffers in its pastern bones until the muscles tire (for example the cart horse), though the strain on them is undoubtedly greater at this time than any other. The strain on the pastern bones during draught depends upon the force exerted, viz., the compression of propulsion, and that this is something considerable may readily be seen in any heavy draught work. Fractures of the pastern also teach us some useful lessons ; we may regard them for our purpose as experi- mental evidence of the shock inflicted on the lower bones of the limb. This shock is caused when the foot comes to the ground, not when it leaves it, and it may occur on hard ground or on sand; in the former case the cause of the concussion is obvious, in the latter at first sight it is not so clear, yet when we remember how rapidly horses tire when working at any fast pace over sand, and, owing to the nature of the ground, the manner in which they must misjudge the application of that muscular bracing which saves the skeleton from concussion, it is not difficult to explain the well-known fact that pasterns frequently fracture on sandy soil. Direct concussion in a horse which is not tired and is not working on sand may also produce a fracture of this region. Fractures of the pastern may occur from galloping horses on the wet sand of a seashore ; this is the result of concussion : the next hardest thing to a macadamized road is a wet seashore. Our only object in dealing with a subject which appears to be foreign to the one under consideration is to bring some light to bear on the strain to which the skeleton is exposed. This strain would appear to be greatest on the suffraginis in the fore-limb, for fracture of this bone is incomparably more common than fracture of the corona, though this might be accounted for by the density of the latter and the absence of a medullary canal. In concluding these remarks on fracture of the pastern, we would draw attention to the fact that the strain imposed on the bones in all cases is probably nearly identical in direction, for Digitized by Microsoft® 520 A MANUAL OF VETEKINAEY PHYSIOLOGY there is a remarkable similarity in appearance presented by fractures of either the corona or suffraginis, the fractured portions agreeing in shape and size, in some cases almost piece for piece. In spite of what we have said about direct concussion affecting the pastern bones, we do not think that this is necessarily the only factor present in the production of ringbone. The compression of propulsion must take a part; we mean by this, the shock imparted to the pastern bones while the foot is on the ground and the body is passing over it. The fore-leg from the knee to the foot is only intended to open and close in one direction ; we can readily make the foot touch the point of the elbow, but we cannot make it touch the front of the fetlock. Now if we study the movement the limb makes from the time the foot comes in contact with the ground, we observe that the fetlock at first descends and then ascendB, and having reached the desired point the limb passes over the foot which remains fixed on the ground, and at this moment an important movement occurs in the pastern, viz., its rotation from rear to front. While the fetlock is ascending the meta- carpal is moving on the suffraginis, the suffraginis on the corona, and the latter on the pedis ; but as soon as the limb becomes vertical (in the rotation of the body over the foot), the movement between the suffraginis and corona becomes exceedingly limited, and for all practical purposes, owing to their immobility, the two may be regarded as one bone ; thus the remaining rotation of the body occurs between the corona and pedis. It is only possible to understand this by following it out on the dead limb, the leg being upright and the foot fixed. The important point is this — during the rotation of the body over the foot considerable compression and strain must be experienced in the pastern ; this strain is most severely felt at the articulation between the suffraginis and Gorona, owing to the fact that these are locked together during the main extension of the limb. Further, as the upward and forward propulsion to the body is given as the Digitized by Microsoft® THE LOCOMOTOE APPAKATUS 521 foot is leaving the ground, much of the shock resulting from it must be expended on the pastern bones. Irregularities in the ground surface are a severe strain on the coronets, especially of a horse out of condition. The study of hoof prints will readily demonstrate the fact of uneven tread ; the lateral deviation of the coronet and pedal joints is something very small, yet every uneven tread, be it caused by a rut or a pebble, throws a strain on these parts, and there can be no doubt that many cases of ringbone originate in strains of the lateral ligaments of these joints. In the Act of Standing the body is supported on four props; two of them have only a muscular attachment to the trunk, the other pair are united by a ball and socket , joint. It is unnecessary to allude by name to the muscles connecting the fore-leg with the trunk, excepting the serratus magnus through the medium of which the body is principally slung on the scapulae. No matter what the position of standing may be, the horse never, in a state of health, keeps its fore-feet in any other position than together ; . one fore-limb advanced in front of the other is abnormal excepting when grazing. On the other hand, it is very rare to see a horse standing squarely on both hind-legs, he is invariably resting the limbs alternately. Some years ago we drew attention to this as being an explanation of the exemption of the hind-limbs from navicular disease ; by this process of resting, the compression of the navicular bone (through the body weight above, and the perforans tendon below) is relieved. The horse only learns to rest the fore-feet when too late. In Lying Down the animal brings the four legs together under the body, and bends both knees and hocks, the knees and chest touching the ground before the hind-quarters. "When down he either lies extended on one side or seated on the chest, two lateral legs being under the body and two outside it. If resting on the chest he inclines to one side or the other ; he cannot like a ruminant lie plumb on the keel of the sternum owing to its sharp ridge. If Digitized by Microsoft® 122 A MANUAL OF VETEEINAEY PHYSIOLOGY nclined to the near side, the near fore-foot is placed close o the breast-bone, the elbow touching the ground, the lear hind-foot is unde* the abdomen and he lies on the >utside of the hock and shank ; the off fore-foot lies close o the off elbow but as a rule outside it, and the point of he off hock touches the ground. A horse does- not lie long n one position owing probably to the enormous weight of lis body. It will be observed that the animal lies on the joint of the elbow which is underneath the body. This is ihe cause of ' capped elbow,' and not that usually assigned, riz., resting on the heel of the shoe. "When down the mimal is either sitting on its chest or lying on its side, but n any case no position is maintained for any great length )f time. He may sleep sitting on his chest, in which case tie rests the chin on the ground with the lower lip frequently everted and so rests on the incisor teeth. The ayes are never completely closed, and he is the lightest sleeper imaginable. Cattle repose on the breast with the head turned round to the side. In Rising the horse can only get up by extending both fore-feet in front of the body ; the hind-quarters are now pressed upwards, the animal securely fixing his toes in the ground, and assisted by the muscles of the back, the animal is immediately on his feet, the fore-part always rising before the hind. The ruminant rises quite dif- ferently, in fact the reverse of the horse, the hind-quarters being the first to ascend. Locomotion. — We have now to study the question of locomotion in the horse, and describe how the legs are moved during the different paces. It will be remembered that our knowledge of this subject chiefly depends upon graphic records and instantaneous photography, the pioneers in the field being Marey* in France, Stanford, Stillman, and Muybridge in America, t We have selected typical studies in order to elucidate the text, but it must * ' Animal Mechanics,' International Scientific Series. t ' The Horse in Motion.' Digitized by Microsoft® THE LOCOMOTOE APPAEATUS 523 be remembered that no hard-and-fast attitudes can be adopted. The sharp quick clever horse whose muscular response is rapid does not move hie legs quite in the same way as the slow lethargic indifferent type ; and similarly a tired horse moves differently from one whose muscles are fresh and responsive. The Walk (Pig. 126) is the slowest pace, the movements are somewhat complex, and may roughly be divided into Fig. 126. — The Walk (Ellbnbbkger). four stages. In the first the body is balanced on three legs, in the second stage on two diagonal legs, in the third on three legs, in the fourth on two lateral legs, and the next movement brings it back to the first stage, only with different legs employed. Tracing the movements in each stage, the horse advances one fore-leg — say, the off (Fig. 126, 1) — and is left standing on the near fore, near hind, and off hind ; in the second stage the near hind is Digitized by Microsoft® 524 A MANUAL OF VETEEINAEY PHYSIOLOGY picked up, and the animal is standing on the near fore and off hind, viz., on diagonal legs (Fig. 126, 2) ; in the third stage the off fore has come to the ground, and the animal is balanced on both fore and the off hind leg (Fig. 126, 3) ; in the fourth stage the near hind is advanced to be placed over, or in advance of, the track af the near fore ; to make room for it the near fore is advanced, and the horse is left standing on two lateral legs, viz., off fore and off hind (Fig. 126, 4). The next movement brings the animal into the first position, with the near fore leading instead of the off fore. The fore- leg remains on the ground for a longer time than it takes in passing through the air, and comprises the period during which the body is passing over the limbs. The movement in the air both of 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. Stillman refers it to the spring or rebound of the suspensory ligament, but it is doubtless due entirely to the flexor muscles. 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 leg, and the foot is placed down flat or heel first. 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 foty^fflU* In heavy draught work it is no uncommon thing to se^roe toe put down first, but here the conditions are very different. It appears to be a matter of indifference with which fore-leg an animal starts the walk ; under some conditions he may indeed make the first step with a hind leg, in which case the next to move is the corresponding fore-leg in order to make way for the hind foot. The Trot (Fig. 127) is a very simple pace to analyse ; the body is supported on diagonal legs (Fig. 127, 1), which by their propulsion drive it off the ground, during which period all the legs are in the air (Fig. 127, 2) ; when the Digitized by Microsoft® THE LOCOMOTOR APPARATUS 525 Fig. 127.— The Teot. From instantaneous photogra2>hs by 0. Anschiltz. (Elleribergcr.) Digitized by Microsoft® 526 A MANUAL OF VETEEINAEY PHYSIOLOGY body comes to the ground again the next pair of diagonal Legs receive it (Pig. 127, 3), and once more propel it. There are thus three stages to the trot ; the body in two of them is supported by diagonal legs, and in one of them it is in the air. The trot appears to be the only pace in which instan- taneous photography has supported the conventional aotions of this movement. We can see the trot, first Decause it is a simple pace, and secondly because the body is comparatively long in the air. When a horse falls at ;he trot, he does so either through not flexing his knee sufficiently before bringing the leg forward, or the extension )f the knee is not perfect, and in consequence the limb is unfit to stand weight. The knee should be sufficiently but not unduly bent and the leg brought rapidly forward, the imb then sharply extended, well braced, and the foot placed firmly on the ground heels first. In the Amble the horse, instead of using diagonal legs uses the lateral limbs, so that off fore and off hind are on ;he ground instead of off fore and near hind. An animal may amble both at the walk and trot, h\ this respect resembling a camel. There is no doubt that it is a per- fectly natural pace for some horses ; others are taught it, is it is a particularly pleasant one for the. rider and less fatiguing for the horse. In the Canter (Pig. 128) the body is pushed upward off the ground by one fore-leg — we will say the off fore [Fig. 128, J) — the near fore and both hind being off the ground ; in the next stage all the legs are off the ground though the feet are no great distance from it (Fig. 128, 2) ; n the third stage the body returns to the ground, alighting Dn the near hind-leg, which is not placed under the centre )f gravity as in the gallop, but behind it, the animal being 3alanced on one limb only (Fig. 128, 3) ; in the fourth stage the off hind and near fore come to the ground iogether, so that the body is now balanced on three legs — riz., near fore and both hind (Fig. 128, '4); in the fifth stage the off fore comes to the ground, but as it does so Digitized by Microsoft® THE LOCOMOTOR APPARATUS 527 Fig. 128.— The Canter. From instantaneous photographs by 0. Anschutz. {Ellenbcrger .', Digitized by Microsoft® 528 A MANUAL OF VETEEINARY PHYSIOLOGY ;he - near hind rises ; the animal is still left on three legs — viz., both fore and off hind (Fig. 128, 5) ; in the sixth stage ;he near fore and off hind leave the ground, the horse being balanced on the off fore only (Fig. 128, 6) ; the next movement is a repetition of the first, the off fore pressing ;he body upwards. In the example quoted the off fore is ;he leading leg, and it will be seen that it is this which 'ives the final propulsion to the body. This is the explana- ion of why the leading leg tires so early. Though it is a natter of indifference which leg a horse leads off with in he walk and trot, this is not the case in the canter or ;allop. There are some animals which, so long as they are eading with the leg of their own choice, are pleasant in heir paces, but if forced through fatigue or other cause to ead with the opposite fore leg, their movements are rough md clumsy and wanting in co-ordination. It should form )art of the training of every horse to teach him to change lis leading leg in the canter or gallop with facility ; this sducation would prevent many cases of sprain. The Gallop is a very difficult pace to describe, and the malysis I give of it here is from one of Muybridge's mmerous instantaneous photographs. The gallop (Fig. 129) consists of seven stages ; for limplicity we will elect to describe it from the time the mimal is in the air, with no legs on the ground, but all our of them brought well under the body ; this is the first itage (Fig. 129, I) ; in the second stage one hind-leg, say he off, comes to the ground, the foot being placed down lose under the centre of gravity and not behind it as in the anter (Fig. 129, 2) ; in the third the near hind comes to he ground, the horse now being balanced on two hind- egs, both fore being in the air (Fig. 129, 3) ; in the fourth tage the off fore comes to the ground, but the animal is lot balanced on three legs as in the canter, for at the aoment the off fore comes to the ground the off hind is xtended, leaving the horse on diagonal legs — viz., off fore ,nd near hind (Fig. 129, 4) ; in the fifth stage the near ind leaves the ground, the animal being balanced on the Digitized by Microsoft® THE LOCOMOTOE APPAEATUS 529 6 h 7 Fie. 129.— The Gallop. After Stamford, Muyhridge, and Stillman. (' The Horse in Motion.') Digitized by Microsoft® 3d. 530 A MANUAL OF VETBEINAEY PHYSIOLOGY off fore-leg (Pig. 129, 5) ; in the sixth stage the near fore comes to the ground (Fig. 129, 6a), and the off fore leaves it (Fig. 129, 6b)— the body is still supported on one fore- leg; in the seventh stage the body passes over the near fore-leg (Fig. 129, 7), and by a contraction of its muscles the entire weight is lifted off the ground, and propelled forwards and upwards (Fig. 129, 1). The simplest descrip- tion of the gallop is that the horse takes a stride with the hind-legs which then leave the ground ; he next strides with the fore-legs, and at the end of this propels the body for several yards through the contraction of the muscles of the fore-leg on which he was last bearing. During the true gallop he never has more than two legs on the ground at the same time, and they are always pairs, excepting in position 4, Fig. 129. The points of importance in both the gallop and canter are that the heel of the foot comes to the ground first, that the hind-legs break the shock of the falling body, and that the fore-legs take the largest share in propelling the weight. Two of these facts were described years ago by Lupton, but were not accepted.* In examining the track of a galloping horse it is remarkable to observe what a very straight line the hoof-marks leave, showing that each foot is brought well under the middle line of the body. When a horse gallops, no matter how fast the pace, the fore-feet never extend beyond a vertical dropped from the muzzle. In the Jump (Fig. 130) the horse rises to it by the pro- pulsion upwards which the fore-legs give to the body (Fig. 130, 1), the knees at the same time being flexed to enable the feet to clear the obstacle. Both hind-legs being fixed on the ground, the body is through these propelled forwards (Fig. 130, 2). In alighting the animal does so through the medium of the fore-legs, either together or one following the other but always straight (Fig. 130, 3). In- stantaneous photography disproves the theory that in the jump a horse naturally alights on the hind-legs, ' though it is true that some clumsy horses do. * See footnote, p. 559. Digitized by Microsoft® THE LOCOMOTOR APPARATUS 531 Fig. 130.— The Jump. (Anschutz-Ellenberger. ) Digitized by Microsoft® 532 A MANUAL OP VETERINARY PHYSIOLOGY In Rearing the hind-legs are brought well under the )ody, the head and neck are thrown up, and the propelling Dower of the fore-legs directs the body upwards, where it is sustained by the muscles of the back and loins. So long is the centre of gravity falls within the base formed by the lind-feet, the body is in a position of stable equilibrium ; rat if it passes outside this, the horse comes back on to ihe point of both hocks, and may either roll over on its side or go directly backwards. If the latter, the ■ first part )f the body to strike the ground is the occiput ; in this way racture of the base of the skull may occur. In Kicking with both hind-legs the head is depressed, md a powerful contraction of the muscles of the quarter md back throws the croup upwards, and at the same time >oth legs are violently extended. Kicking may be practised lither with one hind-leg backwards or one hind-leg for- vards. The latter is very dangerous ; fortunately only in accomplished horse can effect it ; it is known as cow-kicking.' Owing to the pubo-femoral ligament a lorse can only kick outwards with difficulty. Striking vith the fore-feet is not common, and is not character- stic of British horses, nor are ' cow-kicking ' or ' buck- umping.' In Buck-jumping the animal springs bodily off the ground, he head being suddenly depressed between the fore-legs md the back violently arched. The Normal Daily Work of Horses, the rate at which they tre capable of performing it, and the power they exercise n doing so, must now be briefly considered. Rankine has aid down that mechanical daily work is the product of three luantities : (1) the effort ; (2) the rate ; (3) the number of mits of time per day during which the work is continued. )ur only difficulty is in obtaining the value of the effort, vhich it is clear must depend upon the nature of the work, he character of the ground, the weight carried or drawn, md the physical fitness of the animal. The normal work if horses would appear to be 3,000 foot-tons per diem ; a mrd day's work is equivalent to 4,000 foot-tons, and a Digitized by Microsoft® THE LOCOMOTOB APPARATUS 533 severe day's work is 5,000 foot-tons. Redtenbacher* places the daily work of a horse for 8 hours at 6,700 foot- tons, and Rankine's tables f show that a draught horse exercising a force of traction of 120 lbs. for 8 hours a day, performs 6,200 foot-tons of work. I think both these estimates are without doubt too high. The co-efficients of resistance employed in our calculations were those determined for man by the Rev. Professor Haughton ; we know of none specially calculated for the quadruped. Assuming the weight of the animal, plus the weight carried or drawn, to be equal to 1,000 lbs., then 3,000 foot-tons of work will be obtained by the following work : Walking at 3 miles per hour for 8'7 hours. " n ^ ,, ,, O'o ,, ») )> ^ I) >> o'l „ Trotting : , 8 „ „ 1-5 „ Cantering „ 11 „ „ 1 This table is only given as a means of conveying to the mind the value of 3,000 foot-tons of work, though trotting 12 miles or walking 18^- miles are commonly done in practice as a day's work. The Velocity of the gallop has been variously stated, but it is certain that no horse has galloped 1 mile in 1 minute as is reported of Flying Childers. A horse named Salvator in 1890, carrying 7 stones 12 lbs., was galloped on a straight course against time, and did a mile in 1 minute 35£ seconds. The most severe galloping ever recorded was performed by Quibbler in 1786, who galloped 23 miles round the flat at Newmarket in 57 minutes 10 seconds. The .fastest pace at which trotting has been performed is 1 mile in 2 minutes 8 J seconds. The celebrated American trotting-horse Tom Thumb trotted 100 miles in 10 hours 7 minutes, including a stoppage of 37 minutes ; an English mare did the same distance in 10 hours 14 minutes, including a stoppage of 13 minutes, while Sir E. Astley's Phenomenon trotted * Quoted by M'Kendrick. t ' Encyclopedia Britannica,' art. ' Animal Mechanics.' Digitized by Microsoft® 4 A MANUAL OF VETERINARY PHYSIOLOGY ' miles in 53 minutes. All the old performances here ioted are from Youatt's work on ' The Horse.' Turning now to what may be expected of ordinary >rses, it may be noted that the average walk of a cavalry irse is 3'75 miles per hour ; the average trot is 7'5 miles ir hour, or a mile in 8 minutes, and a fast trot is miles per hour. A gallop is from 12 to 14 miles per mr. The stride of horses at various paces was measured a very ingenious manner by Stillman and Muybridge. ley give the stride at the walk as 5 feet 6 inches ; at the at between 7 feet and 8 feet ; at the canter about 10 to 1 feet ; and the gallop between 16 feet and 20 feet — they en speak of a stride of 25 feet. An American pacer has ien known to cover 21 feet in a stride. The question of the Weight which a horse can carry is ie affecting the vital interests of the cavalry service; ere is a great difference between the actual or total weight horse can carry and the effective weight he can carry. The question of weight is greatly influenced by the pace which it has to be carried, and under any circumstances largely governed by the weight of the animal's own idy. We have shown that horses should not be asked carry more than one-fifth of their body-weight, and is conclusion will doubtless apply to all riding horses.* tie-fifth of the body- weight of a cavalry horse is roughly LJ stones ; instead of this they carry about 20 stones. The physiological features of Draught can only be glanced . The subject of draught is a complex one, and our in- rmation is still very incomplete. Quadrupeds appear to ) designed for the purpose of draught, a horizontal spine not intended for carrying weight ; such can only be itisfactorily met by an upright column, as in man, who om his conformation is essentially devised for carrying a irden ; the horse, on the other hand, is constructed for luling or draught. Brunei, in his article on ' Draught,' f * ' The Effective Weight Horses can Carry ,' Journal of Comparative xthology and Therapeutics, vol. xi., No. 4. t ' Book of the Horse,' Youatt. Digitized by Microsoft® THE LOCOMOTOR APPAKATUS 535 points out that the reason why a horse is more suited for draught than for carrying weight, is that he can throw his weight considerably in front of his centre of gravity, the feet forming the fulcrum, and ' allowing the weight of the body in its tendency to descend to act against the resistance applied horizontally and drag it forward ; as the resistance yields the feet are carried forward and the action continued.' Such is the theory of draught. The nature of the vehicle, the condition of the roads, the angle the trace forms with the horizontal, the presence or absence of springs, four wheels or two, high or low front wheels, and the width of the track, are features which singly or combined greatly complicate the question. The force exerted in draught depends upon the load and the pace ; in the light or mail stage-coach, where 10 and 11 miles an hour were attained, the strain or force of trac- tion employed by each horse was only 40 lbs. ; in the heavy coach it was 62| lbs. for each horse. The higher the velocity the less the force of traction which can be em- ployed, and the shorter the duration of labour. For slow draught work at 2£ to 3 miles per hour, and for 8 hours a day (which appears to be the most suitable pace and dura- tion of labour), a force of traction of from 100 lbs. to 125 lbs., or 150 lbs., is quoted by Brunei as being the most suitable. But a force of traction of 120 lbs. for 8 hours a day is too much to expect. Watt found that a horse could raise a weight of 150 lbs. passed over a pulley, 220 feet per minute. This, as applied to engines, is termed 'horse power,' and is equal to 33,000 lbs. lifted 1 foot high per minute, 33,000 foot-pounds per minute. This standard of comparison cannot be generally applied to horse labour, as it is far too high. An animal could only perform this amount for 3£ hours per diem, whereas its most useful work is performed in 8 hours. The actual dead pull which a horse can exert depends upon his body- weight ; no animal tested by me against a dynamometer has pulled his own weight, nor should we expect it. From 65 to 78 per cent, of the body-weight was Digitized by Microsoft® A MANUAL OP VETERINARY PHYSIOLOGY by us to represent the maximum muscular effort of )rse.* The animals tested were grouped according to lirit they put into their work : roup ' excellent ' pulled 78'5 per cent, of their body-weight. „ 'good' „ 77-6 „ 'fair' „ 70-6 'bad' „ 65-6 „ „ „ Pathological. question of lameness in horses must always occupy a prominent i in veterinary practice. It is intimately bound up with the tor apparatus, and it is not possible to attempt any useful ry of the troubles met with. The anatomy and physiology of imotor system should be thoroughly understood by the student to become a good practitioner. he Maximum Muscular Effort of the Horse,' Journal of 'ogy, vol. xix., 1896. Digitized by Microsoft® CHAPTEE XVII THE FOOT The foot is largely a modified form of skin, the vascular tissues represent the corium, while the horn represents the epidermis. It is no uncommon thing to have a horn-like tissue produced by the skin, as, for example, in the human nail, in the hand of the labourer, and in the chestnut and ergot found on the limbs of the horse. In spite of its origin from the skin, the foot is a specialised structure presenting not only a surface for wear and tear, but mechanisms for supporting the weight, and others devoted to warding off from both the foot and limb the concussion and jar to which such a structure is necessarily exposed. If it were not for the mechanisms just alluded to, and were the foot a structure simply devoted to offering a surface of sufficient density for the horse to stand upon, it would present little of special interest. The foot may primarily be divided into two parts, the insensitive or horn foot and the sensitive or vascular foot. The horn is produced from the vascular foot, but the latter does not exist solely for the production of horn; it is provided with a fibrous pad, elastic tissues, a peculiar arrangement of joint, and a remarkable corium, the collec- tive function of which is devoted to saving the parts from destruction during the battering process to which the foot is exposed, and further to support the weight. These two feet the sensitive and insensitive are closely united ; in their general configuration oiie is an exact counterpart of the r>fViai> onrl nnp. fits into the other ranch as a fineer fits into Digitized by Microsoft® A MANUAL OF VETEKINARY PHYSIOLOGY love. It would be out of place here to give, anything a detailed account of the anatomy of the foot, but re are certain structural features so intimately associated i the physiology of the organ that it is impossible to irate them. iones of the Foot. — The core of the foot consists of bone and which all the other structures are moulded. The e is not one solid piece, as we might imagine would be essary in such a position, but on the other hand consists hree pieces. One of these is the pedal bone which in pe resembles a miniature foot, and the substance of ch is porous to such an extent as to resemble pumice- le in appearance. A second bone, the navicular, is very ill, of peculiar shape, dense in structure, rests slightly the pedal bone, and is mainly held in position by mentous tissue. The third bone only belongs partly be foot and partly to the limb. One would suppose that pedal bone should occupy the whole of the interior he hoof, as high as the coronary edge and as far back the heels, but this is not so. It only occupies a com- atively small portion of the internal foot (Fig. 131), and t portion is mainly situated towards the anterior and iral parts ; the posterior part of the foot contains very e pedal bone, but the deficiency is made up by the ■oduction of two large plates of cartilage attached to bone, and over which the structures are reflected and ulded as on the bone itself. This singular deficiency of l6, in a part where one might be led to regard its itence in large amounts as a necessity, and the presence large cartilaginous plates to take its place, is due to the ious movements which the foot has to perform, and :ch could not be carried out if the bone of the foot were itively proportioned to the structure within which it fits, 'he Foot-joint. — Three bones form the foot-joint. The stion naturally arises why the joint is not composed of bones instead of three, and what advantage is gained by introduction of a small dense bone such as the navicular ) the articulation ? The articulation furnished by the Digitized by Microsoft® THE FOOT 539 pedis is much smaller than that furnished 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, therefore, of the navicular bone is to increase the articular surface of the pedis. But it is con- ceivable that this small articular surface of the pedis might have been increased in some other way than by the introduc- tion of a distinct bone and other complicated apparatus, and one is forced to recognise that the value of the navicular articulation does not depend entirely on the fact that it increases the size of the joint, but that it supplies what else- where we have spoken of as a yielding articulation. The value of this yielding articulation appears to be in the saving of direct concussion ; the weight through the corona comes in the first instance mainly on the navicular, which under its influence yields slightly in a downward direction ; from the navicular the weight is transferred to the pedis itself, which, as we shall later have to point out more particularly, also yields slightly under its influence, and in this way it is undoubted that direct concussion to the joint is prevented. The Navicular Bone and Bursa. — It is quite certain that 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. The chief support to the navicular bone is the broad expansion of the perforans tendon which passes beneath it ; between the tendon and the bone the most intimate fitting occurs, and a synovial apparatus exists here 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 articulations in the body, excepting those found in the knee and hock joints. Briefly, then, the small dense navicular bone is enabled to form a yielding articula- tion in the foot, owing to the manner in which it is supported in position by the powerful perforans tendon. It might be argued on purely theoretical grounds that a small bone thus placed in the foot would be very liable " to damage, and such we know practically to be the case. Digitized by Microsoft® 540 A MANUAL OF VETERINARY PHYSIOLOGY It is not our intention here to touch on the subject of navicular disease, excepting in so far as it helps to eluci- date the physiology of the part, but it is permissible to regard the lesions of navicular disease in the light of a physiological experiment, and we learn from them how intimately the freedom and elasticity of a horse's action depend upon the navicular bone, and the stilty, pottering, shuffling gait conferred on the animal when the navicular bone is no longer capable of properly performing its Fig. 131. — Longitudinal Section of the Foot. 1, 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 ; 11, junction of wall and sole, the ' white line.' functions. The very close support afforded to this bone by the perforans tendon may possibly be a cause of disease, for the conclusion has been forced on us 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. 131). This view is mentioned here not so much as a pathological as a physiological factor. We cannot recognise in the navicular bone any pulley function in connection with the perforans tendon, such as Digitized by Microsoft® THE FOOT 541 has been usually described, that is if by the use of the term pulley it is sought to convey the impression that some mechanical advantage is obtained. It is true that by passing beneath the navicular bone the direction of the pull of the tendon is slightly altered, but no mechanical advantage is thereby derived. The perforans tendon at its insertion spreads out fan-shaped, and is attached over a considerable semilunar 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 true that movement does occur 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 curious to observe the direction in which the largest amount of friction occurs between these two surfaces, Eeasoning from the position of the parts one would think the greatest amount of wear must occur 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 would point to the greatest friction occurring, not when the bone yields under the weight, but when it returns to its place as the body passes over the foot ; but it may be that the fact is capable of a different explanation. The frequency with which the central ridge of the navicular bone is affected with disease, would point to this part as being the position of the largest amount of pressure. Lateral Cartilages. — Attached to the heel of each 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 — viz., outside the foot in an upward direction as far forward as the coronet and as far back as the heel (Fig. 141, p. 568). There is no other structure in the body with which it can be compared : a bone possessed of two large cartilaginous wings is a something peculiar to the foot. The use of these cartilages is intimately connected with the main Digitized by Microsoft® 542 A MANUAL OF VETEEINAEY PHYSIOLOGY orinciples of the physiology of the foot, to be dealt with ater. Plantar Cushion. — Placed between the two plates of cartilage is a large somewhat pyramidal-shaped body known as the plantar cushion (Pig. 131, 7, 7). In appear- ance it resembles a fibro-fatty mass, composed of inter- acing fibres and fat, pale yellow in colour, almost destitute )f bloodvessels, firm to the touch, yet yielding in its nature, [t occupies the posterior part of the foot, rising above the tioof into the hollow of the heel, whilst its inferior face is V-shaped, and a complete counterpart of the horn cushion ar food-pad which covers it. The Corium of the foot completely covers the structures just described, viz., the whole of the pedal bone, a large surface of the lateral cartilages, and the plantar cushion. 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. The Vascular Wall or laminal tissue (Fig. 132-2) is com- posed 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, and they invest the entire surface 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 short and turned in under the foot, running for- wards beneath it to form the sensitive bars. If a single leaf, say at the toe, be removed and examined, it is found to commence immediately under the thick cornice-like structure known as the coronary substance, 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 edge of the leaf is not regular but denticulated, and when viewed from its face it is observed that it is narrower near the coronet than at its inferior part, at which latter place it Digitized by Microsoft® THE FOOT 543 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 in appearance. If the tissue be examined microscopically it is found that part of its substance is devoted to leaf formation, whilst the remainder is a sub-laminal tissue, the function of which is to secure the laminae firmly to the wall of the pedal bone. This sub-laminal tissue has been described by Fig. 132. — The Vascolar Wall, the Hoof being removed. 1, The wall-secreting substance with its papillae ; 2, the sensitive laminae ; 3, where the upper end of the sensitive laminae run into and fuse with the coronary substance ; 4, a line between 1 and the skin, which secretes the periople ; 5, the heels of the plantar cushion. 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. 133, e). Qutside this is a layer of fibrous connective tissue and elastic fibres, arranged in bundles, crossing and forming networks, and containing few cellular elements, though more than were found in the periosteal stratum ; this layer is extremely vascular and has been designated the stratum vasculosum. External to this layer are the laminas formed of elastic and connective Digitized by Microsoft® 544 A MANUAL OF VETEEINABY PHYSIOLOGY tissue fibres as in the previous layer, only the network is much finer. The laminae contain numerous bloodvessels and nerves. If a horizontal section ol the laminae be made and ,. | I 4m» Fig. 133. — Horizontal Section of the Horn and Vascular Wall of the Horse's Foot. Low Magnification. a, 6, c, The outer, middle and inner portions of the wall, showing the eanal system with the tubular and intertubular horn ; d, the horn laminae bearing on their side the lamellae, shown black; there are sometimes a few short laminse to be seen, one is shown in the figure ; e, the sub-laminal tissue, from which the sensitive laminse may be seen dovetailed between the horn laminae, and from the sides of which grow the sensitive lamellse. examined microscopically, it can easily be seen that each lamina has growing from its free edge a number of delicate processes which are miniature laminae, or as they have beemtermed secondary laminse or lamellae (Fig. 133, d) ; in Digitized by Microsoft® THE FOOT 545 number they are from 100 to 120 on each leaf, depending upon the size of the primary lamina. The appearance thus presented (Pig. 133, d) is very characteristic, and has been likened by Chauveau to a feather, the barb being the lamina and the barbules the secondary laminse. The Wall-secreting or Coronary Substance is a thick, half- round, cornice-shaped welt of material situated above the laminae (Fig. 132, 1) ; it has received several names, the most rational being that based on its function as the wall-secreting substance. Externally this body is covered by a membrane possessing long papillae which are highly vascular, and readily seen by immersing the foot in water, while the body itself on section is fibro-fatty in appearance. It extends all round the coronet from heel to heel, and here joins the plantar cushion. On its superior margin is a narrow groove (Fig. 132, 4), which is the dividing line between skin and hoof, and from which the periople is secreted. On the lower margin the substance fuses with fibres from the sensitive laminae. The entire coronary substance 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 to the lateral cartilages. The vascular papillated mem- brane covering the coronary substance is irregularly pig- mented, corresponding to the colour of the horn wall. ' The Vascular Sole is scarlet in colour, and covered by long papillae which are lodged 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 the foot-pad or horn frog. The Blood Supply to the foot is exceedingly rich. We have alluded to the scarlet appearance presented by the laminae, the vascular sole, and the tissue covering the plantar cushion ; but besides these the coronary cushion, pedal bone, etc., are richly supplied with blood. The pumicestone-like appearance presented by the latter is for 35 Digitized by Microsoft® 546 A MANUAL OF VETEEINAKY PHYSIOLOGY the purpose of affording passage to the innumerable vessels which are passing from the interior of the bone in an out- ward direction to reach the vascular tissues ; in fact, no description or drawing can adequately convey an idea of the appearance presented by this vascular body. The veins are large and numerous (Fig. 134) and are not pro- vided 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 points which will occupy our attention again when we deal with the use of the various parts of the foot. Fig. 134. — The Venous System of the Horse's Foot (Stoech). The insensitive foot or Hoof is moulded over the sensitive structures in such a way as to cover them completely, and form in horn a perfect counterpart of the sensitive foot. The hoof is composed of a wall with its inflections the bars, a sole, and a foot-pad or frog ; each of these parts must be considered separately. - The Wall is that part of the hoof which can be seen when the foot is on the ground ; its division into toe, quarters, and heels is for convenience of description, as no natural division exists. On the exterior of the wall is a layer of horn known as the periople, which is more apparent near the coronet where it is white, soft, and thick Digitized by Microsoft® THE FOOT 547 (Fig. 135), than lower down where it is extremely thin and more of the nature of a varnish, while at the toe it is practically absent. This layer is formed from the upper edge of the coronary substance. In a foot which has been poulticed, the periople at the coronet stands out as a white band running from heel to heel ; this appearance is due to the absorption of water by the layer of soft cells of which it is composed at the coronet. The use of the . pprinplp a.ppeyg in Via in wmnnt over the 1HP''*i Qn nf tTia» > skin and hoof, and by the covering it affords the wall to Fig. 135. — The External Foot or Hoof. The fibrous appearance of the wall may be seen, also the periople 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 photo- graph was swollen by immersion in water. assist in preventing evaporation from its surface. The colour of the wall is black, or black and so-called white, really buff; a black horn is produced by a pigmented coronary substance, a buff horn has no pigment. Non- pigmented horn is weak and brittle, and grows slowly. Such feet are always a source of trouble. The wall is thickest and longest at the toe, thinnest and shortest at the heel. A gradual decrease in thickness occurs from front to rear, but if a section of the wall be made in the direction of its fibres, it will be found that whatever 35—2 Digitized by Microsoft® 548 A MANUAL OF VETERINARY PHYSIOLOGY the thickness may be at that particular part, this thickness is maintained from the coronet to the ground surface. The greater thickness of the wall at the toe and quarters as compared with the heels, is connected with the wear and tear of the hoof, and the movements which the latter undergoes under the influence of the body weight. If the wall were as thick at the heels as at the toe it would have been a rigid box ; we shall have to show that it is a yielding box, and that the yielding which occurs corresponds to the thin wall of the heels. The reason why the wall is thick at the toe is that this is the region of the greatest friction. The wall at the heels is suddenly inflected, running under the foot in a forward direction for a short distance, and forming an acute angle with the wall. This inflected portion of the wall is called the Bars, and in the gap formed by the inflection 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 sharply bent on itself and caused to run in practically the opposite direction. When we consider this arrangement it is easy to see the advantages gained by it ; the foot is not a rigid body but a yielding one. It would be difficult to understand how any lateral move- ment could take place had the wall been a complete circle. The value of the inflected portion of the wall is rendered evident when we bear in mind the lateral movement of the foot. From their position the bars afford additional strength ; they knit the structures together at the heel in a remarkable way, and prevent any rupture between the wall and foot-pad during the lateral movements of the foot, such as would undoubtedly have occurred had the wall and foot-pad been directly united. The hind feet differ from the fore feet in shape, being more upright and narrower. On examining the inside of the hoof-wall a very complex arrangement presents itself. At the upper edge, corre- sponding to the coronet, is a deep semicircular groove, deepest at the toe and narrowest at the heels, in which is Digitized by Microsoft® THE FOOT 549 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, as may easily be seen, the papillae which project from the' substance ' are lodged. The thickness of the wall at any one place corresponds to this coronary substance, and from it the entire horn wall is secreted. The most perfect contact exists between the wall-secreting substance and the groove in which it is lodged, and this contact is further assisted by the vascular papillae which run for a short distance into the depths of the horn wall. Horn Laminae. — On the inside of the wall of the hoof a number of leaves are found arranged side by side, running all round the foot from heel to heel, and composed of delicate plates of horn. It is easy to see that they correspond in size, direction, and length, with the vascular or sensitive laminae previously described, and like them they possess secondary horn laminae or lamellae. These insensitive and sensitive laminae are arranged towards each other in a peculiar way, by which an enormous amount of strength is obtained, viz., by the process of dovetailing. Each insensitive . lamina fits in between two sensitive laminae, and so power- ful is this union, that in endeavouring to separate them the vascular laminae 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 horn wall is brought about, and in addition other advantages are obtained which will be dealt with shortly. The horn laminae as their name implies are composed of horn, but the secondary laminae which invest them are composed of cells which are a something between horn and epithelium, viz., the cells have not undergone a true horn conversion but remain protoplasmic in nature ; this is recognised by the fact that t hey rea d ily stain with carmine whereas horn does not. If our description has been clear, it will be observed that though the sensitive and insensitive laminae dovetail yet they are never in actual contact, for between them are the lamellae, both sensitive and insensitive, and it Digitized by Microsoft® 550 A MANUAL OF VETERINAEY PHYSIOLOGY is actually through these structures that the intimate union is maintained (Fig. 133, d). It will be remembered in speaking of the vascular laminae that we described some as being found beneath the foot ; in the same way horn laminas corresponding in position and number to these are also found under the foot, and are situated at that part which has been described as the bars. Clearly, therefore, the bars, though situated under the foot at its posterior part, are a part of the wall, inasmuch as they possess all the essential anatomical elements of the wall proper. The Sole of every normal foot is concave, that of the hind feet being more concave than 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. Soles vary in thickness, some being very rigid and firm, others very thin and yielding ; the sole cannot be too thick. The one shown in Fig. 131 is an excellent specimen of a good sole. The growth of the sole is peculiar ; in exactly the same way as noticed in the wall, the papillas 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 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 deter- mines for itself how thick the sole shall be, and without any assistance the fibres break off when the proper thick- ness has been attained, and allow the part to drop out. This shelling out of the sole is advantageous in the shod foot, inasmuch as the part not being exposed to friction cannot wear away. In parts of the foot such as the wall, which in the unshod foot are exposed to friction, no breaking off of horn fibres is required, as the wear and tear maintain the part at its proper length and thickness. The union between the vascular and horn sole is brought Digitized by Microsoft® THE FOOT 551 about by the papillae on the surface of the former. The extraordinary length and number of these can only be appreciated by examining the sensitive sole under water. The sole and wall are united, the place of union being marked by a white line which extends around the complete circumference of the hoof (Fig. 131). That part of the sole situated just within the white line is capable of bearing weight, inasmuch as it is not immediately under the vascular sole. The arrangement of horn at the junction of the wall and sole is peculiar. It will be remembered that the horn laminae have on them secondary laminae ; these secondary laminae exist wherever the sensitive horn laminae digitate. But in the horn of the line of junction of the wall and sole there are obviously no sensitive laminae, and though the horn laminae are there and can be distinctly seen with the naked eye, there are no traces of secondary laminae on them ; these have been left behind in the sensitive foot as the wall grows down. The horn formed between the junction of the wall and sole is softer than that of any other part of these two structures ; this softness allows of a slight yielding of the sole in an up and down direction, and this we shall find actually occurs. The Foot-pad* or 'frog,' as it is vulgarly known, is a pyramidal-shaped piece of horn, accurately moulded over ; the plantar cushion, and filling up the space left by the inflection of the wall at the posterior part of the foot. In the foot-pad we meet for the first time with a peculiar soft elastic horn, possessing something of the characters and appearance of indiarubber ; nothing in its microscopical appearance accounts for this physical difference in the horn of the pad as compared with that of the wall. Chemistry, however, comes to our assistance, and shows that the horn * The term ' Foot-pad ' is introduced not only to define the function of the part, but in order to eliminate the senseless language of the stable from scientific discussion. We should have preferred the term ' cushion ' to pad, but this would have created confusion with the plantar cushion. Foot ' buffer,' from some points of view would be a better term, but there are also objections to this. Digitized by Microsoft® 552 A MANUAL OP VETEEINAEY PHYSIOLOGY Df the pad contains much more moisture than that of any sther part of the foot, and it is the moisture which confers m it its peculiar soft pliable condition. The foot-pad grows :rom the vascular membrane covering the plantar cushion, n the same way as we have already seen it in the wall and iole. At the heels of the foot, where the wall is inflected, ihe soft horn of the pad not only fills up the gap between ihem, but plasters over the inflected edge of the wall for iome little distance, so that an inspection of the heel gives he impression that the horn found at the posterior part of he hoof is a continuation of the wall. The overgrowth of Fig. 136. — Horizontal Section of the Horn of the Wall, Highly Magnified. , Horn tube, a canal containing cellular elements ; 6, the tubular horn, that is, the horn secreted from the papillae, forming an oval or circular nest of cells around the canal ; c, the intertubular horn. tie foot-pad is provided against by a method which is a ombination of that found in the wall and sole, viz., it is ast off after growing to a certain thickness, while the part ext the ground is worn away by friction ; in consequence, wing to its rubber-like nature, rags of horn along the iges of the foot-pad are a common and natural condition. The Structure of Horn. — The horn of the foot consists of pithelial cells which have undergone compression and eratinisation, by which latter process they become hard ad tough. It is possible to have horn in the foot which not keratinised, and the two are very readily dis- Qguished by the process of staining. The double stain Digitized by Microsoft® THE FOOT 553 picro-carmine has a selective 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 all tissue which has undergone the process of keratinisation of a yellow colour. By means of this stain we possess a very easy means of determining the character of the horn under examination. The ultimate horn-cell is a very thin, spindle-shaped, oblong, or irregular body, containing granular matter, a nucleus, and frequently pigment (Fig. 136). In all cases the cells are united at their edges and sides by a cement substance. By acting upon horn with caustic alkalies the-, cells are in the first instance rendered clear, they then gradually dissolve, are converted into a gelatinous mass, and finally they disappear. There is no necessity to use a caustic alkali to destroy horn, any alkali has the effect of eroding it. Bearing in mind the highly alkaline nature of the horse's urine, 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. These tubes exist wherever the growing surface is invested with papillae or projections, so that where the papillae are numerous the tubes are numerous, where they are absent the tubes are absent. The only horny structure not secreted from a papillated surface is the horn laminae, and here conse- quently we find no horn tubes, but everywhere else the horn is found to possess a more or less tubular structure. The method of tube formation in horn is very simple ; the papillse growing from the various secreting surfaces are lodged in depressions in the horn, in this way a canal is formed for the reception of the papilla. As the horn grows down from the surface which secretes it, the canal lodging the papilla gradually slides off, but throughout the length of the horn a tubular appearance indicates where the papilla was at one time lodged, and the cells of these tubes Digitized by Microsoft® 54 A MANUAL OF VETE BINARY PHYSIOLOGY rom their reaction with carmine prove themselves to be ifferent to true horny structure. The horn which is secreted in the foot is therefore ormed (1) from papillae found on the secreting surface, ind (2) from the spaces between the papillae. The papillae orm tubular horn, the spaces between them form inter- ubular horn (Fig. 136, b and c). The tube or canal in horn s the outcome of the existence of papillae ; the horn is .rranged in an oval or circular manner around the canal Figs. 133 and 136), the cells composing it being so placed hat their edges are towards the papillae. There is, however, k layer of cells which actually forms the wall of the canal, tnd these are arranged with their sides next it, or, to put it inother way, they stand on their edges. The horn formed >y the papillae is consequently arranged concentrically, and his gives a laminated appearance around the canal, which s best seen in the external and middle layers of the wall Fig. 136, a, b). In the deep layer of the wall the papillae >roduce a much greater secretion, and here the circular or >val masses of cells investing the canal are more prominent, md further, unlike those in the anterior and middle parts )f the wall, they need no reagent to identify their cellular lature (Fig. 136, b). If a section of wall be stained with )icro-carmine only the canal-contents of the external and niddle wall stain with carmine ; all the remaining substance lakes up the picric acid. In the deep wall this is different ; lere the whole of the cellular material secreted by the mpillae is stained red, showing that these cells are proto- )lasmic rather than horny, and partly accounts for the act that this deep horn is always softer than the middle or sxternal horn of the wall. The horn formed between the papillae surrounds and knits together that formed by the papillae. If a section of horn be examined without under- going any special preparation, it is quite impossible to see he cells of which it is composed ; Fig. 133 gives a good idea )f this, and represents the fibrous appearance presented by i horizontal section of the wall. To see the cells the pre- paration has to be treated with a solution of potash or Digitized by Microsoft® THE FOOT 555 other reagent, when the appearance presented in Fig. 136 is obtained. At the junction of the wall and the sole the horn of the laminse is firmly interdigitated with the soft horn of the margin of the sole. This can be perfectly seen micro- scopically, and further it may be demonstrated that the portion of the sole thus thrust between the horn laminse is perforated in five or six places for the reception of the papillae which grow from the inferior extremity of the sensitive laminse. If a vertical section of horn be made, we can study the canals now divided in their length. It is readily seen that Fig. 137. — Microscopical Structure of Horn : Longitudinal Section of the Wall. Low Magnification (after Lungwitz). Note the different size of canals; those on the right are nearest the laminse ; 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. 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 appearance. 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. The use of the canal system in horn is for the purpose of irrigation ; the horn must be supplied with moisture, the bulk of this is obtained through these imperfect canals, the soft proto- plasmic canal wall readily admitting of transudation. It is not intended to represent that anything like a fluid is Digitized by Microsoft® 6 A MANUAL OP VETERINARY PHYSIOLOGY ■culating along the tubes, but moisture certainly does find way down, and is readily imbibed by the surrounding lis. Besides this arrangement for maintaining the aisture in horn, there is no doubt that in the intertubular rn moisture passes from the secreting surface from cell cell, and in this way is transmitted throughout the lgth of the foot. Constant evaporation is taking place >m the foot, and the loss is made good in the manner iicated. If the invisible moisture which is always escaping from e foot be hindered in its evaporation, the horn becomes dden, crumbles away, and closely resembles a grey cheese, lis experiment can readily be performed on the sole and )t-pad, by accurately moulding to their surface a sheet gutta-percha and leaving it there. The practical lesson obvious; no impervious material should be applied to e foot as a protection, or if used it should be venti- ;ed. Use of the Moisture in Horn. — The amount of moisture ntained in horn is something considerable, and the rate which it evaporates is quite extraordinary. If parings the foot-pad be enclosed in a bottle, in a short time the ;erior will become bedewed with moisture. The use of nsture in horn is to keep the foot elastic and so prevent from becoming brittle. The agency which is at work to event the too rapid evaporation of moisture from the ,11 is the periople, or thin varnish-like layer which covers a wall, and in addition there is the natural hardness of the ternal fibres of the wall ; the latter is sufficient to retain e fibres in their elastic condition by preventing evapora- m. In the case of the sole, the layers of exfoliated iterial which accumulate as the result of the breaking of the horn fibres prevent undue evaporation. Horn ataining but little moisture is in an abnormal condition, is rigid and brittle ; nails driven into the part cause it to ick, and that elasticity on which so largely depends the tural shape and usefulness of the foot becomes impaired, even destroyed. A museum specimen of a foot will ry clearly illustrate our meaning ; in its dried condition Digitized by Microsoft® THE FOOT 557 it is so brittle that if dropped it will occasionally fracture like a piece of glass ; but if this 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 instead of being probably twenty years old. All that the horn has done is to imbibe water, and the previously brittle substance now becomes yielding and elastic. The entire physiology of the horse's foot is centred around this question of the moisture contained in horn. One of our main objects in shoeing should be to protect the wall from unnecessary interference ; the removal of the varnish layer formed by the periople, and the cutting across of some thousands of horn-fibres by the unnecessary use of the rasp, lead to destruction owing to the evaporation of water. The necessity for elasticity in the foot is evident, when we consider the concussion to which the part is exposed during work, which would inevitably lead to its destruction by fracture or otherwise unless some such provision as this were present. Clinically we are perfectly acquainted with the fractures which occur in the wall of the hoof from violence. In one part of the foot undoubted eyidence of sweat glands exists ; these are found in a particular part of the plantar cushion near to and on the sides of the cleft. The glands are very large, coiled, and a spiral duct passes through the horn of the foot-pad and opens on the surface. Fig. 138 is after Franck, who carefully described these structures, though the original discovery was made by Ercolani.* Chemistry of Horn. — An analysis of the horn of the foot has given us the following results :t Water Organic matter Salts 100-000 100-000 100-00 * Veterinary Journal, vol. i., No. 1. t ' Chemistry of the Hoof of the Horse,' Veterinary Journal, vol. xxv., 1887. Digitized by Microsoft® Wall. Sole. Foot-pad. 24-735 37-065 42-54 74-740 62-600 57-27 •525 ■335 ■19 58 A MANUAL OP VETEKINARY PHYSIOLOGY The pad contains the largest amount of moisture, and le wall the least. The salts are small in amount, and msist principally of those of sodium, magnesium, iron and lica, in the form of chlorides, sulphates, and phosphates. Hoof consists of a horny material or keratin, a substance hich replaces the protoplasm originally existing in the alls. Keratin is a proteid-like body found in hair, nails, ?tg. 138. — The Sweat Glands of the Plantar Cushion (Peanck). 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 c is the deep-seated portions of the horn of the foot-pad, where it grows from the papillse of the corium of the plantar cushion ; g, g are horn tubes seen in longitudinal section. id even, in a modified form, in the nervous system; it insists of Carbon 51-41, Hydrogen 6 - 96, Nitrogen 17'46, xygen 19*49, and Sulphur 4*23 per cent. The sulphur loosely combined, and it is this in combination with pdrogen which causes horn undergoing decay or disease i have such an offensive odour, sulphuretted hydrogen jing formed. Keratin is a very insoluble substance, but is Digitized by Microsoft® THE FOOT 559 dissolved by strong and boiling acids and by alkalies. With sulphuric acid it yields leucine, tyrosine, and volatile sub- stances ; the latter conferring the peculiar odour on burnt horn, feather, nail, etc. 7 Provisions for Elasticity and Toughness. — From what we have previously said, it can be seen that it is the wall of the foot which supports the horse's weight. On examining the wall it is found to be thickest at the toe, thinner at the quarters and thinnest at the heels; it is thickest at the toe owing to the wear and tear of the foot at this part. 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 being given to the body by the toe, as we have seen in studying locomotion, we can readily understand how necessary it is for this part to be thick and strong. The object of the wall becoming thin towards the posterior part of the foot, is to allow of the elastic movement which we have yet to describe. Two physical conditions have, therefore, to be provided for in the wall — viz., elasticity of the posterior part, and toughness of the anterior portion. The first is provided 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 ensures its elasticity. The younger the horn, viz., the nearer to the coronet at which it is examined, the more moisture it contains ; the further away from the coronet the less moisture, the tougher and more resisting the horn. The wall grows evenly from the coronet all the way round ; if it grows half an inch in the month at the toe, * Fifty years ago Mr. J. Irvine Lupton communicated a paper on ' Physiological Beflections on the Position assumed by the Fore Foot of the Horse in the Varied Movements of the Limbs,' Veterinarian, vol. xxi., 1858. In this communication he states that the heel comes t@| the ground before the toe ; further, he clearly describes the use of the foot-pad, the expansion of the foot, and the final propulsion given by the toe. Mr. Lupton's advanced views did not meet with approval at the time ; to-day they are accepted facts. Digitized by Microsoft® 560 A MANUAL OP VBTEEINAEY PHYSIOLOGY it grows the same length at the quarters, and the same at ;he heels. The anterior part of the wall is longer than the posterior, therefore the anterior is tougher than the poB- ierior, for the reason that the horn is much older at the axtremity of the toe than it is at the heel, and being further iway from the coronet, it contains less moisture. The wall at the heel is some months younger than that at the Soe ; it is thinner, and contains more moisture, therefore it is more elastic but not so tough. The age of the wall is an important factor in the wear }f the foot. If it takes from nine to twelve months for the wall to grow from the coronet to the toe, the piece of wall Fig. 139. — Diagram Illustrating the Age of the Wall. i, b, c, d, e, f, are circles 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 the heel at a', the age of the wall at d corresponds with 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'. it /, Fig. 139, is, say, twelve months old, whilst that at a is ess than six months old. The horn of the quarter is older ;han the horn of the heel, and the horn of the toe older ;han that of the quarter. This excellent provision admits n the unshod foot of considerable friction occurring at the toe without producing undue wear, for the part is hard md tough, while the younger and moister horn at the posterior part of the foot allows of expansion. In this way varying degrees of toughness and elasticity are provided in ihe Wall. The toe of the wall appears to grow faster than either Digitized by Microsoft® THE FOOT 561 the quarters or the heels, but this is more imaginary than real ; it is the tendency of the foot to grow forward as well as downward which produces the illusion. That the foot does grow forward may readily be determined by experi- ment, for if we cut or saw a groove in the wall at the coronet, say an inch or so from the heelB, the groove will in course of time be carried some considerable distance towards the toe ; the exact amount 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 on the other, is perhaps the most remarkable feature in the physiology of the foot. We know how firm this union is, the extreme difficulty in a state of health in separating the two surfaces, even by mechanical means, while the structurally 'delicate nature of the parts yielding this firm connection we have previously considered. In one foot the weight is carried on 600 or more primary laminae, and 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 they are longer, and they have no plantar cushion and foot -pad to assist them as the shorter posteriorly-placed laminae have. Moreover, during progression, the final propulsion of the toe comes entirely on them. The short posteriorly-placed laminae have their strength increased by the direction in which the weight of the body comes upon them. Instead of bearing the weight on the length of the laminae, as at the toe, they carry it on the side, in such a manner that where we have, say, one lamina at the toe, there are twenty at the quarter. It is not possible to describe this condition clearly, but Fig. 140 will help to explain it. It will be remembered that the laminae are attached 36 Digitized by Microsoft® 562 A MANUAL OP VETEEINAEY PHYSIOLOGY Pig. 140. — Diagrams to Illustrate the Direction taken by the Laminje at Different Parts of the Foot, as seen in Trans- verse Section. In the upper figure the section is made through the toe of the foot : a, being part of the pedal bone; b, the horn wall ; c, the laminse, the latter are practically straight, the weight being imposed- from top to bottom in their length. The middle figure is a vertical section just behind the point of the foot-pad : the laminse,- c, give the appearance of being placed above one another. The lower figure is a vertical section through the posterior part of the foot : a, being the lateral cartilage, and c, the laminae. It will be observed that ' the laminae, as in the previous figure, are placed one above the other ; this arrangement. gives, strength, and is a compensation for shortness. Digitized by Microsoft® THE FOOT 563 at the anterior and part of the lateral face of the foot to bone, but for the remaining lateral face and posterior part of the foot they are attached to stout cartilage ; * if a line be drawn through the foot separating the osseous attach- ment of the laminaa from the cartilaginous attachment (see Fig. 141), it will be found that, roughly speaking, one- half is cartilaginous and one-half osseous ; the cartilaginous portion is situated just where elasticity is required, viz., the posterior face of the wall. One function of the lateral cartilages of the foot is to afford a movable wall-attachment to the sensitive laminae, and 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 laminae between the wall on the one hand, and the ossifying cartilage on the other. The folding up of the horn and sensitive leaves in the foot, in the manner above described, has another function besides that of merely supporting the weight and rendering the union firm. The first thing which strikes one in con- nection with the foot is its remarkably small size in proportion to the size of the body. Comparing the horse's foot, so far as size is concerned, with the human foot, the advantage 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 circumference, in reality it encloses a vast area due to the anatomical arrangement of the laminae. It is clear that by folding up this amount of material the surface of the foot is considerably increased. In other words, by this arrange- ment the foot has been kept within small proportions with- out 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 considerable. This is exactly * Some of the laminae are attached to the tendon of the extensor pedis and the lateral ligaments of the foot-joint. 36—2 Digitized by Microsoft® 564 A MANUAL OF VETERINARY PHYSIOLOGY 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 Clarke, who first had a calculation made as to the increased surface afforded by this arrangement, came to the conclu- sion that it was equal to 1|- square feet; but Moeller* has estimated that it is equivalent to 8 square feet, whilst Grader's estimate t is 10f square feet. For safety we adopt Moeller's number. The bearing surface afforded by each foot is equivalent to 8 square feet, giving a total area of 32 square feet ; but it is evident that as feet vary greatly in dze, this surface must be greater or less depending on the size of the foot. The physiological function of the leaves of the foot is iemonstrated by pathological observation. Inflammation sf the laminae occurs either through overwork, or through in animal standing too long in one position ; in either case ;he parts get strained, and resent it. The practical value Df exercising horses which from any reason have to stand for a length of time is well known ; the exercise overcomes ;he tendency of the laminae to congestion from continual strain, and the feet not only become cool, but the animal nay continue standing for a considerable time if exercised laily. The treatment of laminitis by exercise, first taught sy Broad, of Bath, possesses a sound physiological basis. [f any doubt exists as to the function of the laminae in supporting the weight of the horse's body, we have only ,o look at the processes which occur in them as the result )f disease. Laminitis is often attended by separation of ihe horn and sensitive laminae, when the horse's weight >eing no longer properly supported, the pedal bone under ts influence is actually forced through the sole of the foot. The Origin of the Horn Laminae. — No one doubts that the pall grows from the coronet, but great controversy has aken place over the origin of the horn laminae, some * Veterinary Journal, vol. v. t Quoted by Goubaux and Barriers, ' Exterior of the Horse ' translation). Digitized by Microsoft® THE FOOT 565 saying they grow like the wall from a part of the coronary cushion, and others affirming that they obtain their origin from the sensitive laminae. If we were to judge solely by the result of pathological processes, we should be inclined to say the sensitive secreted the horn laminae; but Moeller* points out that the sensitive and insensitive laminae are never in actual contact, and that between them are placed the secondary laminae both vascular and horny. Therefore he argues, and with great weight, that the vascular cannot secrete the horn laminae, but that the secondary vascular secrete the secondary horn laminae. If a portion of wall be removed experimentally and the vascular laminae be exposed, in the course of a short time the part becomes covered with a layer of horn, and this has been used as a strong argument in favour of the secretion of horn from sensitive laminae ; but the horn which is thus secreted is derived from the secondary vascular laminae, and no one contends that these secrete the primary horn laminae. The following explanation appears to us to be the correct one : The horn laminae are secreted from the lower edge of the coronary substance, here white protoplasmic cells are poured out between the papillae : these cells are carried down with the wall, being pressed into and moulded between the sensitive leaves, thus becoming horn laminae, the exact counterpart in shape of the mould in which they are made. All this occurs in the region marked 3, Pig. 132. As the wall grows down the horn-leaves are carried with it, so that there is a perpetual movement occurring between the slowly travelling insensitive and the fixed vascular laminae. The rate of this movement is probably about •0125 inch in twenty-four hours, on the assumption that the wall grows § of an inch in the month. During the time the horny are gliding through the sensitive leaves the vascular lamellae furnish them with horny lamellae ; and, as we have previously seen, when the wall reaches the sole the horny lamellae are left behind, while the laminae emerge with the wall destitute of these structures. * pp. eit. Digitized by Microsoft® 566 A MANUAL OF VBTBEINAEY PHYSIOLOGY The Use of the Bars. — The inflected portion of the wall, known as the ' Bars,' runs, as we have previously mentioned, forwards under the foot instead of completing the circle of ;he wall. The object of turning aside from completing the ring the wall originally looked like forming, is to make room for the elastic posterior foot, viz., the plantar cushion md foot-pad ; and the explanation why the wall is turned n instead of ending abruptly, is to afford a solid bearing to the posterior part of the foot, to give additional strength, md to secure a more intimate union with the sole. The bars being part of the wall are intended to bear weight ; in the foot of the wild horse and zebra, they present the most extraordinary development as the result of weight- bearing. The Use of the Sole is quite clear, it is to afford protection to the sensitive parts above. Its normally concave shape [Figs. 131 and 140) 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 ean bear weight. Under the influence of the body-weight the sole becomes slightly flatter, especially that portion of it situated posteriorly, the horns of the crescent. When we Borne to study the expansion of the foot the object of this flattening will be more apparent. The sole grows from the sensitive sole, as previously described. The Use of the Foot-pad. — This is one of the chief anti- soncussion mechanisms in the foot ; it is there to break the jar, and it does so by receiving, in conjunction with the posterior wall, the impact of the foot on coming to the ground ; this 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. In breaking the jar (not only to the foot but to the whole limb), it is assisted by its elastic rubber-like nature. The foot-pad needs for its perfectly healthy condition contact with the ground ; it is strange that in this respect two structures situated side by Digitized by Microsoft® THE FOOT . 567 side, viz., the sole and pad, should be so opposed in function. We know practically that if the latter be kept off the ground the part atrophies, the heels contract, the foot is rendered smaller, and the pad becomes diseased. This wasted condition of the pad and narrow foot may be restored 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 contracts : 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. The Lateral Cartilages. — We have dealt with certain functions of the lateral cartilages, but it will not be amiss to summarise our knowledge of their use. 1. They form an elastic wall to the sensitive foot, and afford attachment to the vascular laminae. 2. As the foot increases in width (expansion), the car- tilages carry outwards the sensitive laminae which are attached to them, and so prevent any disturbance of the union of the insensitive and sensitive laminae. 3. Large venous trunks pass through and close to the cartilages of the foot, and the movements of the cartilages assist the venous circulation. 4. The object of having lateral cartilages in the foot is to admit of expansion under the influence of the body- weight. This increase in the width of the foot is brought about by pressure on the pad, which widens and presses on the bars at H, Fig. 141, and at the same time tends to flatten the plantar cushion, both of which factors force the cartilages slightly outwards. When the posterior wall retracts the cartilages are carried back to their original position. Should this elastic cartilage under pathological conditions become converted into bone, its functions are destroyed, and lameness may occur. By a simple operation relief from this lameness in a large proportion of cases may be secured. It is possible to demonstrate that by Digitized by Microsoft® 68 A MANUAL OF VETEKINAEY PHYSIOLOGY urgical interference the hoof can be made permanently rider,, and thereby rendered capable of accommodating iteral cartilages which have undergone an increase in size s the result of ossification. This operation is based on hysiological principles.* Anti-Concussion Mechanism. — Practically the whole physi- logy of the foot is a consideration of the factors whereby G C D 'ig. 141. — Portion op the Wall removed, to show the Position of the blgid and elastic sensitive foot. , Wall of the foot ; b, the lateral cartilage ; g, a line which represents the coronet ; c, the pedal bone — the line of union between the pedal bone and lateral cartilage is well seen ; f, is a portion of the corona ; d, a portion of the sole exposed by the removal of the wall ; e, the heel of the wall left at its plantar surface to show the arrangement of the bar, h, which passes behind and within the lateral cartilage b. The figure, which is accurately drawn from a photograph, is intended to show what an extensive surface the lateral cartilage presents, and the variety of surfaces to which the sensitive laminae are attached ; they cover b, c, and f, the latter in the living animal being the position of the extensor pedis tendon and lateral ligament of the foot, to which the laminae are attached. Further, the figure shows the division of the internal foot into an elastic and a rigid portion. tie parts are saved from concussion, in spite of wear and sar, batter and jar. The weight carried on each fore foot 'hen the horse is standing is rather more than one quarter * ' A New Operation for the Cure of Lameness arising from Side- ones,' Veterinary Jon,mal, vol. xxv,, 1887, Digitized by Microsoft® THE FOOT 569 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 not only intended for the protection of the foot but also to save the limb, and they may be tabulated as follows : 1. 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 of the sole. Fig. 142. — 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. Expansion. — We have here retained a word warranted by custom though perhaps not free from objection. By its use is indicated the fact that the wall of the foot opposite to the heels becomes wider when the weight comes on this part (Fig. 142). The increase in the width of the foot is not due to a something being added to it, but to an alteration in the shape of certain of its structures ; if, therefore, the foot becomes wider it does so at the expense of other parts altering their shape. As a matter of fact an increase in Digitized by Microsoft® 570 A MANUAL OF VETERINAEY PHYSIOLOGY the width of the foot 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 back- wards and downwards, Fig. 143, A. In all fast paces, when the foot comes to the ground, the posterior wall and foot-pad first receive the weight. Fig. 143. — Diagrams to show the Area over which the Wall EXPANDS, AND TO ILLUSTRATE THE BeTREAT OF THE ANTERIOR Coronary Edge of the Hoop, and the sinking of the Heels (Ldngwitz). 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. B. In this figure the hoof is looked at from above ; the unbroken out- line 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. In A the shaded part of the wall is to illustrate the area which expands. Under the influence of the body-weight the foot-pad 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 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 apart the wall at the heels. When the weight is Digitized by Microsoft® THE FOOT 571 taken off the foot the heels return to their original position, and the foot becomes narrower. The increase in width which the foot undergoes is something very small ; this is probably the reason why for years it has never been accepted as a fact, and that in this country in particular few were found who gave the theory any support.* The employment of delicate apparatus such as that used by Lungwitzf and others, and experimenting upon feet which have not been mutilated "k^ shoeing, have placed the question beyond all doubt. The area over which the wall expands can be seen in Pig. 143, A ; the shaded portion of the heel represents the part which yields laterally. At times expansion is obtained at the coronet and little or none on the ground surface, but as a rule the amount obtained 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 but is influenced by the shape of the foot ; horses with low heels and full well-developed foot-pads register a larger amount than where the heels are high and rigid. The measurements obtained by us with very delicate apparatus were smaller than those obtained in Germany by Lungwitz. On an average we obtained, by simply lifting up one fore-foot, and so causing the horse to throw double weight on the other limb, an expansion of A- of an inch for half the foot, or A of an inch total increase in width. Doubtless during locomotion a greater expansion than this occurs. The question may be asked what advantage 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 between a yielding and an unyielding block of horn being brought to the ground ; it ' gives ' instead of offering resistance, 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. * See footnote, p. 559. f The Journal of Comparative Pathology and Therapeutics, vol. iv., 3. Digitized by Microsoft® !72 A MANUAL OF VETEEINAEY PHYSIOLOGY There is no point in the physiology of the foot which has ;iven rise to greater discussion than the question of expan- ion, but we submit that its existence is not only proved, but hat it is provided for by the anatomical construction of the >art and the elastic nature of horn. The retraction of the coronary edge of the foot in front, t,nd the sinking behind (see Fig. 143), are accompanied by i tense condition of the coronary substance which Lungwitz lescribes as an elastic ring. Macdonald* in this country •egards it as a hydraulic ligament which supports the pedal- oint under the strain to which it is exposed. The view we lold is that the tense state of the coronary substance is due io the alteration in the shape of the coronary edge of the oot, and that the value or existence of any hydraulic support in connection with the joint has yet to be demon- strated. In addition to the changes in the coronary edge of the 'oot during the period of expansion, another condition is jresent, viz., a comjiression of the wall under the influence )f the body-weight, which produces a diminution in its leight. This can be roughly demonstrated in the following nanner : If a portion of the wall, say between the heel md quarter, be cut away so as just to clear the shoe when he latter is fitted, it will be found on placing weight on he limb by lifting up the opposite fore-foot, that the wall las now descended sufficiently to touch the shoe. The >nly explanation which can be afforded is that given above, At,., the wall has undergone sufficient compression to tllow the part which was originally clear of the shoe to iome in contact with it, and to produce this it must have liminished in height. The Descent of the Pedal Bone is the last factor employed n saving concussion, and the existence of this has been as trenuously denied as the expansion of the wall ; there is, lowever, no difficulty in demonstrating it, and we can readily ee the value of this function. Concussion to the sensitive oot is prevented by a slight up-and-down play between the * Veterinary Becord, No. 145, 1892. Digitized by Microsoft® THE FOOT 573 sub-laminal 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 off it. As the pedal bone descends; the horn sole on which it is resting also slightly descends and comes nearer to the ground ; this is one reason why the sole is concave instead of flat. The descent of the internal foot saves concussion, in the same way that it is easier to catch a cricket-ball with a retreating move- ment of the hand than by rigid opposition ; further it 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. — Lying as the foot does furthest from the heart, we are led to inquire why it is that the blood is able to circulate through it so thoroughly, and whether other means are at hand for assisting the force of the heart in facilitating the circulation. Such means do exist. Though the contraction of the left ventricle is sufficient under ordinary circumstances to bring the blood back to the right side of the heart from any part of the body (as we have pointed out in dealing with the circulation), it is doubtful whether it would be wholly sufficient to empty the foot of blood or keep the considerable plexus of veins full. This plexus is shown in Fig. 134, p. 546, which is a reproduction from a photograph of a corrosion injection.* 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 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 figure appeared in an article by Dr. C. Storeh, of Vienna, on ' The Venous System of the Horse's Foot,' Oesterreichischen Monat- schrift fur Thierheilkwnde, 1893. Digitized by Microsoft® 574 A MANUAL OF VETERINARY PHYSIOLOGY 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 on the leg. With every compression of the foot the water rises in the manometer tube, and falls during the period of no pressure, a period corresponding in the living animal to the foot being off the ground. We must accept it, 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 the veins aspirate the blood into their interior. Indeed, so perfect are these mechanisms that, as previously pointed out, 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, 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. In conclusion we may with advantage summarise what has been said about the anti-concussion mechanisms : When the weight comes on to the foot, it is received by a yielding foot-articulation, an elastic wall, bars and pad, and through these by the plantar cushion. The elastic posterior wall is pressed outwards by the compressed indiarubber- like 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 connection with the sensitive laminae, and presses the sole down with it, while the wall of the foot diminishes in height under the compression to which it is exposed. Under these conditions the blood-pressure in the veins of the foot rises, and the vessels are emptied. When the weight is Digitized by Microsoft® THE FOOT 575 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. Such are the physiological features of the foot which facilitate the circulation and help to counteract concussion. Foot-lameness is only too frequent, but if it were not for the mechanisms we have described, it would not be possible for horses to work for a single day. Q Physiological Shoeing. — It is impossible to conclude this chapter on the foot without some mention of what might be termed physiological shoeing. We all recognise the evils of shoeing as strongly as we realise its necessity. By bearing in mind the functions of the various parts of the foot, we can certainly reduce these evils to comparatively narrow limits. The following rules form the basis of physiological shoeing : 1. 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_rendCT^thje_horn_ brittle, but is so 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 evapora- tion 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 pro- tection. 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. Digitized by Microsoft® 576 A MANUAL OF VETEEINAEY PHYSIOLOGY 7. The pattern of shoe is immaterial so long as it has a true and level bearing, and rests well and firmly on the wall and bars. We believe, the simpler the shoe the better, viz., one flat on both ground and foot surface. It should be secured with no more nails than are absolutely neces- sary, as the nails destroy the horn ; further, the nails should not be driven higher than needful, for high nailing is ruinous to feet. Such, briefly, are the conditions which fulfil physiological shoeing. Pathological. No useful summary of foot trouble can be given. Practically every structure in it is liable to disorder. It is the most common seat of in- curable lameness, and has always been so since the horse was subjected. ' 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. The wall may be so hard as even to resist a nail being driven in. Digitized by Microsoft® CHAPTER XVIII GENERATION AND DEVELOPMENT The Sexual Season of animals is a subject which in recent years has received exact expression at the hands of Heape,* whose communication, quoted below, we have followed in connection with this question. Heape divides female mammals into two classes, Monosstrous, or those which have one oestrous cycle, and Polycestrous, or those having a series of oestrous cycles. The first phase of generative activity at the beginning of a sexual season is known as Procestrum, or the Proosstrous period; it corre- sponds to the period ' coming on heat,' or ' coming in season.' The period lasts a variable time in different animals, and is succeeded by the period oj desire or CEstrus ; it is only during this period that sexual inter- course is permitted, or that fruitful coition is possible. If conception does not occur or is prevented, cestrus is followed by Metoestrum, or the metcestrous period, during which sexual activity passes away, and is succeeded by a period of complete rest or freedom from sexual excite- ment known as Ancestrum. The anoestrous period may last two, three, eleven, or more months, depending on the Species. The sexual cycle is not always as above described ; there are animals in which metoestrum is not followed by a period of complete rest, but by a short quiescence known as Dicestrum, which lasts a certain number of days, and is then followed by a new procestrum, oestrus, metoestrum, and dicestrum. * ' The Sexual Season of Mammals,' etc., by W. Heape, M.A., Quarterly Journal of Microscopical Science, vol. xliv., p. 1, 1901. 577 37 Digitized by Microsoft® 578 A MANUAL OP VETERINARY PHYSIOLOGY Among moncestrous mammals is the wolf, which in the wild state has only one sexual season in the year ; another is the dog, though in this ease the sexual season may recur during the year, but the periods in each case are quite distinct, and followed by complete rest, which is the essentially distinguishing feature. Among polycestrous mammals are the mare, cow, sheep, pig, and all of these during a portion of the year exhibit a series of dioestrous cycles (in the absence of pregnancy), followed by ancestrum until the next year. The number of annual sexual cycles which any given species passes through is vastly influenced by domestica- tion. Probably in all primitive species one sexual season yearly was the rule. Domestication alters this. The 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 ex- pended 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. The wild dog v and wolf breed once annually, in captivity twice annually. The lioness in a wild state has probably but a single breeding season, in captivity the cestrous period may be 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, polyosstrum being an acquired character. The frequency of oestrus under domestication is essentially influenced by food, temperature, and environment. The complete cestrous cycle in the dog** under domesti- * ' 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. 198. 1905. Digitized by Microsoft® GENEEATION AND DEVELOPMENT 579 cation is six months. Every six months, in spring and autumn, the majority of dogs come ' on heat,' though there are many exceptions to this rule, some of the smaller breed of dogs having a three and four heat period in the year. The period of procestrum lasts from seven to ten days and oestrus lasts another 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 rendered pregnant the dicestrous periods last twenty-one days, and are followed by procestrum, 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, with 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 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 year (Goodall). 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 * ' The (Estrous Cycle and Function of the Corpus Luteum in the Sheep,' by F. H. A. Marshall, B.A. Phil. Trans., B. vol. 196. 1903. 37—2 Digitized by Microsoft® 580 A MANUAL OF VETEEINAEY PHYSIOLOGY recurrent periods in any one cycle in the sheep have been observed to depend upon breed ; two, three, or four re- current periods have been noted. There are some breeds of sheep which may produce two sets of lambs in one year. The period of oestrus may be induced almost at any time in the late summer and autumn by the introduction of the ram to the ewes (Goodall). The soiv takes the boar about a week after she has weaned her litter, or about eight weeks after farrowing. The period of oestrus lasts about two days, the dicestrous period twenty-one days. The only known animal which in a wild state exhibits a continuous series of dicestrous cycles is the monkey, but even in this case there is a limited season when concep- tion is possible (Heape). The cestrous 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 any reflex act, that it may exist in the absence of any knowledge on the part of the animal, and that its production is under no central control. Furthermore, such an animal may become pregnant and be delivered in the ordinary way, though quite unconscious of the process. The external signs of prooestrum in all animals is a swelling of the vulva more or less pronounced, with a slight flow Of mucus which may be blood-stained. There is excitement, the mare may refuse to work, squeals and kicks when approached, elevates and protrudes the clitoris, micturates frequently, the material being very mucoid. The cow bellows, is excited, and mounts its fellows. Sheep become restless and follow the ram. The dog is playful, excited, and desires the attention and 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 extent. Changes in the Uterus during Sexual Excitement. — During prooestrum there is an increase in the uterine stroma, Digitized by Microsoft® GENEBATION AND DEVELOPMENT 581 injection of the mucous membrane in consequence of a dilatation of the capillaries, and usually a breaking down of the walls of the latter, leading to extravasation of blood into the stroma, or even into the cavity of the uterus. The glands of the uterus swell and pour out a slight secretion. In some animals such as the monkey the epithelial lining of the uterus is' destroyed during this period, but with ungulates desquamation of the uterus is probably very rare, while in carnivores it occurs more or less in every case. The pigmentation found in the mucous membrane of the uterus after cestrus is due to the extravasation of blood ; this blood is also the source of the blood-stained discharge, and on a more extensive scale is the cause of the menstrual flow in monkeys and women, in both of which there is in addition blood collected in lacunae in the wall of the uterus and destruction of the epithelium. Gradually in all animals the uterus recovers its normal appearance, prooestrum passes away, and is followed by oestrus. Bearing in mind the rapidity with which oestrus may follow prooestrum in such animals as the mare, cow, and sheep, it is evident that the whole of the above process cannot always be fully gone through ; but in the dog, whose cycle is far more regular, the uterus undergoes the changes described. 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 life be a source of danger from sexual excitement. The cause of oestrus is an internal Becretion produced by the stroma of the ovary. (See Corpus Luteum, p. 591.) When male animals suffer from a periodic sexual excite- ment it is known as rutting. This term should be confined entirely to a male sexual season, such as is experienced by the camel, stag, elephant, and oBtrich. In the rutting stag the neck becomes enormously swollen (Leeney),^the Digitized by Microsoft® 582 A MANUAL OF VETEEINAEY PHYSIOLOGY elephant experiences a discharge from the temporal gland, and the ostrich becomes red in the legs. All these are at this time dangerous to approach, and are frequently violent and aggressive. Effect of Removal of Testicles and Ovaries. — The influence of the removal of the ovaries and testicles on general meta- bolism is a subject which has been referred to in dealing with internal secretions (p. 265), and attention was there drawn to the fact that both in cats and dogs the complete removal of the ovaries, and, it may be added, of both horns of the uterus, may not in every case prevent an animal exhibiting oestrus. Such, of course, are exceptional cases, for ovariotomy usually suppresses the function. If an animal, for instance, be operated upon before puberty, viz., before an cestrual period has had time to appear, such an one will not subsequently experience any sexual excite- ment. If the operation be performed during the first preg- nancy, the ' heat ' period does not occur. If operated upon after one or more cestrual periods, and not being at the time pregnant, there may be a few returning heat periods and free sexual intercourse. If castration of the stag be practised the antlers fall off from the seventh to the ninth day after operation ; fourteen days is said to be the longest time they remain. This is evidence of an internal secretion of the testicle (p. 265) which influences the growth and shedding of the antlers, while the chain of evidence is completed by the fact that cas- tration on one side only affects the growth of the antler on that side. If the epididymis be left after complete castra- tion, its presence modifies the growth of the next pair of antlers.* Similarly the growth of parts in other male animals is affected by castration. Cats operated upon while very young have heads which are indistinguishable from the female ; the tissues of the jowl, which give the head of the male cat such a massive appearance, are lost after castration, * I am indebted to Mr. H. Leeney, M.R.O.V.S., Hove, for these facts and much other information on the subject. Digitized by Microsoft® GENERATION AND DEVELOPMENT 583 and this may occur even when the operation is performed late in life. Female cats operated upon while young acquire a head of the male type, and even if the operation be per- formed when approaching middle life, there is a disposition to broadening of the skull (Leeney). The alteration in the shape of the male and female skull observed in the cat when castration or ovariotomy is prac- tised in early life, supports the view advocated by Heape that no being is wholly male or wholly female, but a portion of each sex with one predominant. Cocks con- verted into capons when young do not develop such full male plumage, and the combs and wattles are more like those of the hen. Pullets from which the ' clutch ' has been taken grow fat, and sometimes put on male plumage. Hen pheasants injured by shot in the ovarium have fre- quently been found with male plumage, and disease of the ovary in hens or pheasants may lead to their crowing (Leeney). The Testicles are solid organs with an external covering of serous membrane, and possessing a tunica albuginea, and a stroma or framework of fibrous tissue. The spaces of the meshwork are occupied by the seminiferous tubules. These tubules are highly convoluted in the parts imme- diately concerned in the formation of spermatozoa, and commence usually by blind extremities. If the changes in the cells found on the basement membrane of the tubules be followed, it is found that the cells of the lining epithelium divide into two daughter cells, one remaining attached to the basement membrane, the sustentacular cell, the other becomes a spermatogen. The spermatogen-cells divide and subdivide to form other cells that are recognised as sperma- toblasts. These spermatoblasts elongate and pass into spermatozoa, collecting into sheaves as they do so, and becoming attached to the sustentacular cells that are placed on the basement membrane. These sustentacular cells minister to the needs of the developing sperms until they are fully matured. The latter are then set free, and pass into the lumen of the tubule. The spermatozoa are Digitized by Microsoft® 584 A MANUAL OF VETEEINAKY PHYSIOLOGY produced in enormous numbers ; it is estimated in man that each cubic centimetre of seminal fluid contains from sixty to seventy millions of cells. A mature spermatozoon under favourable conditions is active, moving about rapidly in the seminal fluid by means of its long vibratile tail. It is formed of a head, a middle piece or body, and a tail. The head corresponds to the nucleus, and is constantly present, the middle piece and tail are developed to a varying degree in different animals. In the horse the length of the head, which is bluntly pear- shaped, is about 5 ti* the tail is eight or nine times as long as the head. It is supposed that the sperm- cell extrudes polar bodies as does the ovum (p. 589), but they have not been recognised. The head of the sperm may be con- sidered as the male pronucleus. The Spermatic Fluid is alkaline or neutral in reaction, of viscid consistence, contains proteids, nuclein, lecithin, cholesterin, fat, leucine, tyrosine, kreatine, inosite, sulphur, alkaline earths, chloride of sodium, and phosphates. The essential element is the spermatozoa, without which the fluid is not fertile. Spermat6zoa exhibit spontaneous movement, the long tail moving from side to side, by which means the organism is propelled when placed in the Dody of the female. The vitality of spermatozoa under suitable conditions is considerable, and when placed in ;he body of the female they have been found very active nany days after copulation. Colin found them active in ihe vesiculae seminales eight days after castration. The jpermatozoa are readily killed by ordinary or acidulated rater, glycerin, etc. It is not known in what way the secretions of the pros- ate and seminal vesicles influence the main secretions with ivhich they are ejected, but it is supposed they maintain he motility of the spermatozoa. The prostatic fluid pre- :edes the spermatic in ejaculation, and in stallions and mils, when excessive daily demands are made, the fluid ijaculated is largely prostatic and infertile. The testicular iroducts of hybrids, such as the mule, are infertile. * H = a micron ; j^ millimetre = ^siwa mcn (nearly). Digitized by Microsoft® •GENEBATION AND DEVELOPMENT 585 The Period of Puberty, or that time in the animal's life when it is capable of procreation, has been put at 1J years for the horse, 8 to 12 months for bovines, and 6 to 8 months for the sheep, pig, and dog. There is, however, a great difference between capability and fitness for procreation. Breeding from immature animals is one explanation of a great deal of the worthless material in the shape of horses which may be seen in all countries. The advent of maturity is marked by certain changes in form, particularly in horses. They lose their awkwardness, the outline of the frame becomes more consolidated and in greater unison. 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 ex- tremely vicious. The age at which procreation ceases is not known. Fleming says that mares have been known to pro- duce foals at 28, 32, and 38 years of age, and it is certain that some good stallions have been advanced in years. 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 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, and they may also be produced 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 Digitized by Microsoft® 586 A MANUAL OF VETEEINAEY PHYSIOLOGY and glans are not fully erect in the stallion until the penis is introduced into the vagina ; at the moment of ejacula- tion in this animal the glans swells enormously, apparently to cover or grasp the os uteri. Though the organ in the horse assumes such considerable proportions, in the bull this is not marked. The peculiar penis in this animal comes to a narrow point without any of the swelling observ- able in the stallion. In the act of erection, the S-shaped curve of the penis is removed, and the organ elongates ; at the same time the retractor muscles of the sheath draw back the prepuce and the organ is exposed. In the ram, also, the penis is narrow and pointed, and the vermiform appendix at its extremity appears essential for successful impregnation, for if it be removed, it is said the animal proves sterile. In the dog the increase in the size of the penis is mainly at its posterior part, and the bulbous swellings there observable are the portions grasped by the spasm of the sphincter cunni of the female, rendering with- drawal impossible until complete relaxation occurs. The bone in the penis of the dog facilitates 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 nucleus of two different 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, pig, and camel. Colin places it at ten to twelve seconds for a vigorous stallion ; it is exceedingly rapid, almost instantaneous, in the bull and ram, probably from the peculiar shape of their intro- mittent organ. The spermatic fluid is forced into the vagina, or even directly into the uterus. The peculiar termination of the urethra of the horse, and the bulbous enlargement of the glans during the final act of coition, point to the organ grasping the os at the moment of Digitized by Microsoft® GENERATION AND DEVELOPMENT 587 ejaculation, while the projecting portion of the urethra is inserted into it, by which means some of the fluid is undoubtedly directly injected into the uterus ; the pointed penis of the bull and ram makes it certain that such is also the case in these animals. An examination of the uterus of the sheep and dog a few minutes after coition has revealed the presence within it of spermatozoa. There is ample evidence that the spermatozoa may remain alive for several days within the uterus. At the moment of intercourse the uterus becomes erect, and the introduction of the male element into it is further assisted by the aspiration following its subsidence. The actual mechanism of ejaculation is produced by a contraction of the vesiculse seminales, the prostate gland, and probably of the vasa deferentia, through the reflex action of the ejaculation centre in the lumbar and sacral portions of the cord. By this means the seminal fluid is forced out of the vesiculse into the urethra, and by means of the muscles of the perinseum is forcibly ejected from the urethra. In animals possessing no vesiculse, such as the dog, ejaculation takes place direct from the testicle and vas deferens. The Ovaries. — They are solid organs, and about half the size, or a little less than half the size, of the testicle of the male. An exception to this must be made in the case of the sheep ; here the ovary is very small compared to the testicle of the ram, this animal for its size having probably the largest of testicles, certainly among the domestic males. The ovaries of the mare, cow, and sheep, are somewhat ovoid with a slight depression termed the hilum ; the ovaries of the pig and dog are lobulated and resemble a bunch of grapes ; the ovary of the cat and rabbit is more or less lenticular. The substance of the ovary is divided into cortex and medulla ; the cortex being that portion containing the developing eggs or ova, the medulla being the solid, connective tissue, vascular core. Covering the ovary is a modified endothelium, the germinal epithelium. This is of the columnar type (a modification of the Digitized by Microsoft® 88 A MANUAL OP VETERINAEY PHYSIOLOGY ndothelium of the body cavity), and is found over ae whole ovary except where the ligament of the ovary asses to the uterine horn, and where the broad ligament f the uterus is attached to the ovary itself. The epithelium s called germinal because from it the eggs are developed uring intra-uterine life ; it is probable that no new ova re formed after birth. During development the germinal pithelium grows into the body of the ovary as a long plinder of cells. These cells eventually are cut off from ny connection with the epithelium covering the ovary, rid one cell, it may be two, takes on the appearance and haracters of an ovum. The other cells that have accom- anied and been constricted off with the ovum take on the uties of the membrana granulosa, which is merely a 3llular sphere containing the ovum. The earliest ova are >und in the cortex as large cells enclosed in the simple ae -layered membrana granulosa. The changes that occur from this primitive condition ntil the ovum is mature are chiefly indicated in the wall I the structure containing the egg, the so-called Graafian Hide. A connective-tissue capsule, the tunica fibrosa, :iginates around the follicle, and finally a cavity appears wing to a splitting of the membrana, a cavity containing fluid under pressure, the liquor folliculi. The ovum con- nues to grow slowly until it reaches about x^t" ("18 to mm.) in diameter, and is found in an upheaval of the ills of the membrana granulosa known as the discus or •mulus proligerus. The Graafian follicle of the adult animal consisting of Le above-mentioned parts, and containing the ovum, :tends throughout the thickness of the cortex of the ovary, id daily becoming larger, it appears eventually as a ssicle on the surface. The formation of the liquor lliculi under pressure, and its tendency to move in the rection of the least resistance, will influence the point of tpture, which is said generally to occur at the hilum or ereabouts. When rupture of the Graafian follicle occurs the ovum is Digitized by Microsoft® GENEBATION AND DEVELOPMENT 589 flushed out, and at the same moment, according to Henson, 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 abdominale. 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 fertilized abdominal foetation may occur, the peritoneum acting as a matrix in which the embryo may develop. The Ovum. — With the exception of those produced by the prototheria (duck-mole and spiny ant-eater), the mammalian ova are extremely small. They vary in size from T |- 7 to tIu of an inch, and although not to be compared to those of birds, reptiles, or amphibians, yet they are undoubtedly the largest cells found in the mammalian body. The greater size of the eggs of birds, reptiles and amphibians is due to the quantity of deutoplasm or yolk contained therein. In mammals this is small in amount, owing to the speedy union of the developing ovum to the uterine wall, which effects an intimate connection with an abundant food supply. The ovum is a typical cell, it is spherical and more or less translucent. It has a thick cuticle or zona radiata, within which lies the protoplasm and vitellus or yolk, confined in a special membrane the vitelline membrane. Within the vitellus or yolk is the germinal vesicle or the nucleus of the cell, and within this the germinal spot or nucleolus. This is the structure of the ovum prior to its extrusion from the Graafian follicle ; but either just before or immediately after escape from this, and prior to impreg- nation, changes occur. These changes, more especially involve the nucleus or germinal vesicle, and are known as the formation of the polar bodies. The extrusion of the Polar Bodies has been studied by Van Beneden in the ova of the ascaris megalocephala of the horse, and what is true of this is believed to be true for the Digitized by Microsoft® 90 A MANUAL OP VETERINAEY PHYSIOLOGY lammalian cell. The first stage is that of indirect division E the nucleus, and its movement towards the periphery E the cell. The nucleolus probably divides in a similar lanner, but its fate is not known. The nucleus having ivided, one half is extruded into a space beneath the zona idiata; thus the first polar body is got rid of. The half of le nucleus still remaining in the ovum divides, and for a scond time a nucleus is extruded, forming the second polar ody. There have been many explanations offered of the sig- ificance of the polar bodies. Two, however, are important nd worthy of mention. Minot and Balfour believed that bey were intended to prevent parthenogenesis, or the ossibility of a new creature developing from an ovum that ad never received a male element. Weismann believes bat by the loss of certain elements by means of the polar odies, the ovum is rendered receptive for characters of the aale ; in other words, it has a bearing upon the hereditary iroperties of ovum and sperm, the polar bodies carrying way superfluous histological and genetic properties. As a result of the rupture of the Graafian follicle, a rent 3 made in the ovary. This wound fills with blood from he opened vessels, and for some time afterwards appears ,s a pigmented spot. If pregnancy has not supervened, it indergoes a retrogressive metamorphosis and soon dis- .ppears. If, however, the ovum is fecundated, the corpus uteum, as this pigmented spot is termed, continues to grow, ,nd may be observed in the ovary even near term. The Corpus Luteum of the pregnant animal is very much arger than that of the non-pregnant, and it appears to be :onclusively settled that the existence of this yellow tissue n the ovary is not for the mere purpose of filling up a lavity in its structure, but that the yellow body is a luctless gland which becomes an active secreting agent, )roducing a secretion by which the ovum is anchored to he wall of the uterus, and its nourishment and develop- nent assisted. This ductless gland is functional until ibout the middle of pregnancy, when it is no longer a Digitized by Microsoft® GENEBATION AND DEVELOPMENT 591 necessary factor in the nourishment of the embryo and consequently degenerates. That the corpus luteum takes little or no share in the production of seasonal sexual excitement appears quite clear ; this is the function of the stroma of the ovary which pours an internal secretion into the blood, and so brings about menstruation and oestrus. Ovulation is the process of egg extrusion. In some animals, as the rabbit and ferret, it can only occur 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, donkey, cow, sheep, pig, and dog, ovulation oceurs during oestrus, but the act of copulation is not necessary to extrusion, and in such animals artificial insemination* is therefore possible. The period of oestrus is not necessarily identical with the period of ovulation, oestrus may occur without ovulation, and ovulation may occur without oestrus. Ewart says the mare may mature and discharge one or more eggs after she has become impregnated. Ovulation occurs at the moment the Graafian vesicle ruptures and the ovum is ejected. The number of ova which may be extruded during one sexual period is not known 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 laid is always greatly in excess of the number impregnated, and the mare which probably only produces 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 oestrous 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. Determination of Sex. — Heape t maintains that there is no * First practised on the dog by Spallanzani, 1784. f 'Notes on the Proportion of Sexes in Dogs,' W. Heape, M.A., F.R.S., Proceedings of the Cambridge Philosophical Society, vol. xiv., pt. ii., 1907. Digitized by Microsoft® 592 A MANUAL OF VETERINARY PHYSIOLOGY such thing as a pure male or female animal, but that all contain a dominant 'and a recessive sex, excepting hermaphrodites, in which both sexes are equally repre- sented. The assumption of male characteristics in old females, and of female characteristics in old males of the human species, is noted by Heape. We have referred on p. 582 to the remarkable effect of castration and ovariec- tomy on the skull of young cats, castration producing a female skull, ovariectomy a skull of the male type. Capon- ning also induces female plumage. Injuries of the ovarium in birds are associated with crowing and male plumage, all of which is evidence that the recessive sex asserts itself when the dominant sex becomes impaired, and supports the view held by Heape that there is no such thing as a pure male or female animal. If this be true, it naturally follows that a male ovum is fertilized by a female sperma- tozoan and a female ovum by a male spermatozoan (Heape). Everything, in fact, points to ova and spermatozoa being sexual — that is to say, there are male and female ova, male and female spermatazoa. Microscopic differences in the structure of spermatozoa have also been observed which have led to their classification into two groups, which are, in all probability, male and female. The bearing of Heape' s work on the determination of sex is of great importance. He maintains that the sex of the offspring is determined at the time of fertilization, and that no influence exerted subsequently can alter it. This is opposed to generally accepted doctrines, but results from an acceptance of his hypothesis that an ovum in which one sex is dominant must be fertilized by a spermatozoan in which the opposite sex is dominant ; whether the sex be determined by the ovum or spermatozoan depends upon which is the more powerful of the two. Heape's study of the ovary of the rabbit* shows that ova may degenerate, and that one of the chief causes is nutrition. It is possible that nutrition has a selective * ' Ovulation and Degeneration of Ova in the Babbit,' Proc. Boyal Society, vol. B., 76, 1905. Digitized by Microsoft® GENEBA.TION AND DEVELOPMENT 593 action on ovarian ova, and so 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 of ovarian ova produced is governed by the laws of heredity (Heape). Impregnation. — The male element meets with the female in the narrow passage of the Fallopian tube. Bearing in mind the enormous number of spermatozoa in even only a small amount of the secretion, it is easy to understand why the ovum cannot escape coming in contact with them. The ovum is transported towards the uterus by the cilia of the tube, the spermatozoa travel in the opposite direction by means of their own vibratile motion. One spermatozoan suffices for impregnation ; it passes through the zona radiata, reaches the vitelline membrane, and when within it the tail is lost, and only the head and part of the middle piece remain. The two pro-nuclei, male and female, come together, and a single nucleus results. The fusion of the nuclear element of these two different cells results in a new structure, the ovum is fertilized, and from this is developed a new being embodying the hereditary properties of both parents. In the sheep* the impregnated egg enters the uterus on the fourth or fifth day and travels slowly along it until the ninth day. On the ninth day the zona radiata ruptures and the blastocyst (that is the external cover of the cellular mass) lies in contact with the uterine epithelium. On the twelfth day the ovum has reached nearly to the lower limit of the horn in which it lies, the glands of the uterus enlarge, and the blastocyst rapidly elongates so that each end grows out to the tip of each horn of the uterus. If one embryo only be present it extends through both horns of the uterus ; if there are two they are each con- fined to one horn. On the seventeenth and eighteenth day the first attachment of the embryo to the uterus is effected, * ' The Morphology of the Ungulate Placenta,' by E. Assheton, M.A., whom in the above account of the sheep we have entirely followed. Phil. Trans., B. vol., 198. 19Q5. 38 Digitized by Microsoft® 594 A MANUAL OF VETEEINAEY PHYSIOLOGY a very important period in embryonic life. Up to this time the only nourishment available is that furnished by the juices poured into the uterine cavity by the glands, and until the twentieth day the ovum receives no other source of supply but this. On the twenty-eighth day villi on the external covering of the embryo are well developed, and on the maternal cotyledons are little depressions into which they fit. The allantois (see later) has grown rapidly, and the yolk sac (which Bee) has become reduced as the allantois increases. By the forty-fourth day the foetal cotyledons are scattered over the whole surface of the embryonic covering. On the seventy-eighth day the general character of the placenta is established. As the uterus swells owing to the increase in size of its contents, it does so generally excepting the upper part of the horns, which are but little longer than normal, and are engaged in active secretion. We have given this condensed account from Assheton of the development of the embryo of the sheep, as we have previously had but little exact knowledge how the embryo of a ruminant comports itself during the early days of development. The development of the embryo of the horse has been dealt with by Ewart,* not with the same degree of fulness as the above, as that is practically impossible, but sufficiently so to show not only the characteristic features of the process, but their profound practical bear- ing on the hygienic care of brood mares. We shall see presently that the "human decidua grows over the ovum on its arrival in the uterus,, and so prevents its escape. No such pouch is formed in the ungulates, and the escape of the ovum before it is securely fixed to the wall of the uterus is not unlikely, especially in the horse, where the connection between the embryonic sac and the uterus is easily broken down. To understand how this occurs Ewart points out that the remote ancestors of the horse were probably born on the forty-seventh or forty- eighth day of conception, and like the ancient and primi- * ' A Critical Period in the Development of the Horse,' by T. 0. Ewart, M.D., F.E.S. 1897. Digitized by Microsoft® GENERATION AND DEVELOPMENT 595 tive mammals the opossum and kangaroo, passed from the uterus to a pouch where they lay securely suspended by a teat until their perfect development was completed. The arrangement by which the equine embryo is anchored, as Ewart calls it, to the wall of the uterus, is in the first instance by some of the cells of the outer layer of the embryo, at a part which is in communication with the yolk sac. This connection (Fig. 144, a, b, c) is of a very slender kind and is the only one which exists up to the fifth week. cUi. Fig. 144. — Semi-diagrammatic Bepresentation of a Fode Weeks Hoese Embryo and its Fcetal Appendages, Natural Size (Ewart). cum., The amnion ; y.s., the yolk sac, which is vascular, v, as far as the circular bloodvessel, s.t., and crowded with granules which have entered by the absorbing area, a, b, c, of the yolk placenta ; all., the allantois. The embryo measures nearly three-eightbs of an inch in length, and is curved so that the tail lies under the head. At the fifth week additional means of securing the embryo to the wall are evident, by an increase in the size and strength of the original yolk sac adhesion. There is also a girdle about \ inch wide, not hitherto found in any mammal (Ewart), placed around the equator of the embryo (Fig. 144, t.g.). This girdle obtains adhesion to the uterine wall and so strengthens the original anchorage. About the end of the sixth week the attachment of embryo to uterus is again becoming precarious, for the yolk-sac 38—2 Digitized by Microsoft® 596 A MANUAL OF VETEEINAEY PHYSIOLOGY attachment area has become less (Pig. 145, a-c), while the girdle has shifted from the equator to near the pole (Fig. 145, t.g.). It is at this period Ewart considers the primitive ancestors of the horse were born. At the end of the seventh week the supply of nourish- ment through the medium of the yolk sac has nearly come to an end, the absorbing area next the uterus is considerably reduced, and it is at this period when an entirely new source of supply and attachment has to be found. The Fig. 145. — A Seven Weeks' Horse Embryo, Half Natural Size (Ewart). all., Allantois ; am., amnion ; c.v., non-vascular villi between the allantois and the yolk sac, not hitherto found in any mammal, and function unknown ; y.s., yolk sac ; a-c, absorbing area of the yolk placenta ; v 1 , vascular villi or allantois ; t.t., external vascular villi over the surface of the embryonic sac. supply is furnished by means of the allantois, while the additional attachment is furnished by the girdle becoming folded into ridges, which fit into grooves and depressions in the mucous membrane of the uterus. The outer cover of the embryo beyond the girdle is now dotted with numerous minute points, which subsequently become villi ; the villi are derived from a sprouting of the allantoic sac, and as they grow are accommodated in pits in the uterine wall. By the end of the eighth week this has been accomplished. The villi are not more than i inch long, even when full Digitized by Microsoft® GENEEATION AND DEVELOPMENT 597 grown, and at birth they are withdrawn from the uterine pits. Once the villi have become established the question of nourishment becomes no longer a difficulty, and the critical stage in the development of the horse is passed. The cause of mares ' breaking service ' from the sixth to the ninth week is answered by Ewart in the light of his inquiries. At the third, sixth, arid ninth week the physio- logical changes associated with oestrus are likely to super- vene and shake the reproductive system. At the third week the risk of casting off the embryo is not so great, as the area by which it is attached to the uterine wall is sufficiently large to render it moderately secure, but at the sixth week a change from yolk sac to placental nourishment is being effected, and the yolk sac area is less than it was at the third week. At such a time a contraction of the uterine horn will be followed by expulsion of the embryo. At the ninth week the question of not ' breaking service ' depends on whether the villi have appeared in time, and obtained a sufficiently intimate relation with the uterine vessels to supply the embryo with the additional nourish- ment its development requires, that through the yolk sac being insufficient. Ewart says that the embryo of the mare usually occupies the right horn of the uterus, and in the early days is suspended by the yolk sac from the upper wall of the organ, the head being towards the body of the womb. Later the foetus may lie in the body of the uterus, but the hind limbs remain to the last in the right horn. Development of the Ovum. — Ova are holohlastic or meroblastic according to the method of segmentation. This depends upon the amount of yolk contained in the egg ; if very little or none the seg- mentation is holoblastic and complete as in the eggs of mammals ; if abundant as in birds, the segmentation is meroblastic and partial. After fusion of the two pronuclei the resulting nucleus begins to divide, and there first results two cells which are not equal in size. These also divide, each into two, and the inequality of size of the first generation is impressed upon the second, and after the third division, when eight cells have resulted, we find four large cells and four some- what smaller. From this time the smaller cells divide more rapidly than Digitized by Microsoft® 598 A MANUAL OF VETEEINAEY PHYSIOLOGY the larger and become superficial, the larger cells remaining in the centre (Fig. 146, I.). Thus as the result of repeated division there results a mulberry mass of cells, the small cells being external, the large cells internal. It is at this stage that the segmenting ovum enters the uterine horn from the Fallopian tube; this has been observed in the rabbit. Fig. 146. — Section of a Babbit's Ovum at the Close or Segmenta- tion. II., III., IV., Stages in the Formation of the Blastodermic Vesicle (E. v. Beneden). Z.B., Zona radiata ; Ex.L., External layer of cells ; I.M., Inner mass of cells ; I.L.M., Inner lenticular mass of cells ; s.c, Segmenta- tion cavity. The next change observable is the appearance of the segmentation cavity (Fig. 146, II.). This first appears as a cleft between the inner large cells and the outer small cells ; this rapidly increases in size at the expense of the inner cells, which are pressed together forming a disc- like patch within the now hollow sphere of small external cells. This is the blastodermic vesicle, and is much larger than the original ovum. Digitized by Microsoft® GENEEATION AND DEVELOPMENT 599 The lens-like mass of inner cells flattens somewhat, still however remaining thicker in the centre ; this central thickening being the first sign of the embryonic or embryonal area (Fig. 146, III. and IV.). The spherical blastodermic vesical rapidly becomes ellipsoidal, and the membranes or coverings of the ovum become thin and attenuated ; the vitelline membrane indeed may have disappeared. The next stages in the blastodermic vesicle are not clearly under- ' stood, but it appears that the wall of the vesicle is one cell thick except at the embryonal area, where two layers are to be seen. This is the bilaminar blastoderm, the superficial layer of which is epiblast, the inner hypoblast. If the embryonal area be examined from a surface view it is seen to be pyriform in outline, and in its posterior part the primitive streak appears. This streak is due to a thickening — to the appearance of v the third permanent layer of cells — of the mesoblast, which is derived probably from both epiblast and hypoblast. These three layers constitute the trilaminar blastoderm, from which the various organs and tissues are developed. From the epiblast the following develop : The whole of the nervous system, including the brain, spinal cord, peripheral nerves, and sympa- thetic system. The epithelial structures of the organs of special sense. The epidermis and its appendages, including hairs, hoofs, and nails. The epithelium of all glands opening upon the surface of the skin, including mammary glands, sweat glands and sebaceous glands. The epithelium of the mouth (except that covering the tongue and posterior part of the mouth which is hypoblastic) and glands opening into it. Epithelial covering of anus. The enamel of the teeth. The epithelium of the nasal passages, upper part of pharynx, and cavities and glands opening into the narial passages, e.g., sinuses of head, etc. From the mesoblast: The urinary, and generative organs (except epithelium of urinary bladder and urethra). All the voluntary and involuntary muscles of the body. The whole of the vascular and lymphatic system, including serous membranes and spleen. The skeleton and all connective tissues. The amnion is partly epiblastic and partly mesoblastic. From the hypoblast : The epithelium of the alimentary tract from the back of the mouth to the anus, and all the glands opening into this part of the tract, such as the liver, pancreas, etc. The epithelium of the Eustachian tube and tympanum. The epithelium of the bronchial tubes and air-sacs of the lungs. The epithelium lining the vesicles of the thyroid body. Epithelial nests of the thymus. Epithelium of the urinary bladder and urethra. The allantois is partly hypoblastic and partly mesoblastic* At the stage mentioned in a previous paragraph, viz., the appearance * Schafer, Quain's 'Anatomy,' vol. i., Part I., p. 44. Digitized by Microsoft® 600 A MANUAL OP VETERINARY PHYSIOLOGY of the mesoblast, the hypoblast has grown along the inner surface of the epiblastic layer, and nearly lines the whole blastodermic vesicle, which now becomes ellipsoidal and filled with a coagulable fluid. In front of the primitive streak, the primitive groove appears as a linear depression bounded by two ridges, known as the medullary ridges, the groove is the medullary groove. The ridges continue to grow upwards, and then to curve inwards and approximate in the middle line from before backwards, forming a tube the foundation of the cerebro-spinal nervous system. If a section of the embryo be taken at this stage across the medullary groove and ridges, we find placed beneath the groove and derived from the hypoblast a mass of cells circular in section, the notochord or chorda dorsalis (Fig. 147). The notochord, which is rod-like, gives rise to nothing, but around it the vertebral column develops, and rudiments of it are found even in adult life in the pulpy centre of the intervertebral disc. The mesoblast has been rapidly growing as a sheet between the epiblast and hypoblast, and if the young embryo be examined from above, it is seen to be broken up into ' quadrangular masses ' the protovertebrae or somites. These somites- give rise to portions of the vertebra and to the muscles of the trunk. During the growth of the mesoblast the embryo, which is developing in front of the primitive streak, is. being gradually lifted from off the blastodermic vesicle. This is brought about by a process of tucking or folding off, and first appears at the tail-end of the embryo, and extends along either side to the head ; as a result there is a distinct depression or ' sulcus ' surrounding the embryo. The remainder of the blasto- dermic vesicle is filled with fluid, and forms the yolk sac (Pig. 147), and this may persist in some animals, as the dog, until birth. Many believe that this yolk sac, which in mammalia contains no yolk, but is abundant in birds and reptiles, points to the fact that the ancestors of mammals had large eggs even as the monotremes (prototheria) have to-day. The eggs of the Ornithorhynchus or duck-mole are as large as a hazel-nut. The medullary or neural groove, which has now been converted into a canal, becomes dilated and vesicular in the head region. These vesicles are at first three in number, then five, and give rise to various parts of the brain. The lumen of the canal and vesicles persists, and we see them in the adult as the minute central canal of the cord, and the ventricles of the brain. The nervous structures of the eyeball are derived as outgrowths of the brain ; the organ of smell is the nasal pit innervated from the fore part of the brain ; the ear is an involution 3f the epiblast that also speedily receives a nervous supply from the brain. The mesoblast about the time of the formation of the cerebral /esicles splits into two laminae, and the space between becomes the ',03lom or body cavity (Fig. 147). The upper lamina, consisting of ipiblast and mesoblast, is known as the somatopleure ; the lower Digitized by Microsoft® GENERATION AND DEVELOPMENT 601 lamina, consisting of mesoblast and hypoblast, becomes the splcmchno- pleure. Arising from the somatopleure, at first posteriorly and then at the sides of the embryo, are ridges that grow upwards over the embryo towards the head region, to fuse and form the amnion (Fig. 147). In front of the head the mesoblast has not as yet extended, and the epiblast and hypoblast are united forming the pro-amnion. This however soon disappears, and a ridge developed here grows over the F ctl se. Amnion or Chorion ~^^!£ 'Primitive. ""% :§^/f Aorta.. ~~~^ff Mid gut : - ' "iVoto chord. Fig. 147. — Diagram of a Teansveesb Section or a Mammalian Embryo, showing the Mode of Formation of the Amnion (Schafer). The amnion folds have nearly united in the middle line. head of the embryo to meet those advancing from behind. This fuses with those from the tail and sides, and as a cavity appears in the ridges the embryo has a dorsal covering (Figs. 147 and 148) of two layers, that next the embryo being the true amnion, and this is separated from the outer or false amnion (the chorion) by a cavity into which the allantois grows. Thus the amnion arises from the same portion of the embryo which gives rise to the body wall. The outer membrane of the embryo is an organ of paramount importance, known to morphologists in its early stages as the trophoblast, and to the anatomist in its complete Digitized by Microsoft® 602 A MANUAL OF VETERINARY PHYSIOLOGY form as the chorion. It brings about the connection between the fcetus and the mother through the medium of the villi. These villi are received into folds of the uterine mucous membrane, or into uterine crypts, and thus attachment to the mother is secured. The development of the organs of the body does not enter into a work of this kind ; the student, for fuller information, is referred to special works on Embryology. Reference however may be briefly Fig. 148. — Diagram of a Longitudinal Section of a Mammalian Ovum, after the Completion of the Amnion (Schafer). made to the so-called Chestnuts and Ergots of the horse, both of which are ancestral remains, the former being distinctly seen in the fcetus. Both 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 it may grow to a considerable size. The horn of which it is composed presents microscopically a tubular Digitized by Microsoft® GENEEATION AND DEVELOPMENT 603 structure, and is produced by the papillae of the skin. After growing a certain size it drops or is pulled off. Both ergots and chestnuts are found larger in horses wanting in quality than in those better bred. The Decidua. — 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 farther development in connection with the ovum. The latter on its arrival in the uterus becomes embedded in the folds of mucous membrane which grow up around and anchor it to the wall of the uterus. That portion of the mucous membrane which grows over and envelops the ovum is known as the decidua reflexa, that which lines the interior of the uterus is known as the decidua vera. Through the decidua vera the uterine glands grow, and later on in embryonic life when the final circulation is established between the fcetus and the mother, by means of the placenta, the latter on the maternal side is attached to a portion of the decidua vera, and to this part the term decidua serotina is given. After the birth of the child the membrane covering it, the placenta, and the uterine decidua, are all cast off, with the result that the interior of the uterus is converted into a large raw wound. Placenta. — No domesticated animal has a decidua ; the ovum is attached in quite another way to the uterine wall, and though a placenta exists it is differently arranged to that of the human female. This has led to the primary classification of placentas into deciduate and non-deciduate, but these terms in the light of recent enquiry are not appropriate, for it is no longer a matter of importance from a morphological point of view, whether a portion of the maternal tissue comes away with the afterbirth or not. The most recent work on the placenta of animals suggests another classification.* Assheton proposes to group * ' On the Morphology of the Ungulate Placenta,' by R. Assheton, M.A. Phil. Trans., B. vol., 198. 1905. Digitized by Microsoft® 604 A MANUAL OF VETEKINAEY PHYSIOLOGY placentas into two great types, placenta cumulata and placenta plicata, these terms being based on the arrange- ment of a certain group oi cells (the trophoblast) in the outer layer of the embryo, through which the embryo is secured to the wall of the uterus. Whatever form the placenta may be, or whatever the attachment between the fcetus and the mother, it is always originated by the tropho- blast cells. In the cumulate type of placenta the trophoblast cells heap themselves up and destroy the uterine epithelium, and form spaces into which the maternal blood escapes, while in the plicate there is no heaping up, but a process of folding and ingrowth takes place, the uterine epithelium in most cases being left intact. The pig is the extreme type of plicate placenta, then follows the mare, cow, sheep, while the placenta of man and carnivora is of the cumulate type. It must not be supposed that these types are sharply divided — for instance, the sheep has a plicate placenta which contains cumulate features, and the placenta of the dog though cumulate has features of a plicate type. Besides recognising placentas as deciduate and non- deciduate, or plicate and cumulate, they are further classified according to the disposition of the chorionic villi. If the villi are scattered over the whole surface of the chorion the placenta is diffuse, as seen in the sow, mare, and camel. ' The only parts of the chorion in these animals destitute of villi are the poles, and the smooth patch is very minute. If the villi are gathered into tufts upon the surface of the chorion, and these tufts correspond to elevations of the mucous membrane of the uterus, the placenta is cotyle- donary or polycotyledonary . The tufts and elevations are the foetal and maternal cotyledons respectively, and number sixty more or less. If the villi are disposed in a strap-like manner around the envelopes, leaving the poles for some distance free from villi, the placenta is zonary, and such a condition is found in the placenta of the dog and cat. In the rabbit and woman the placenta, from its shape, is discoidal or metadiscoidal. Digitized by Microsoft® GENERATION AND DEVELOPMENT 605 It is not known how the material passes from the maternal to the foetal tissues ; the blood of the two as previously mentioned does not come in contact, but active changes occur between them through the villi of the placenta. Proteid, fat, carbohydrate, and oxygen are received by the foetus, and carbon dioxide, nitrogenous waste products, etc., delivered to the mother. The nourish- ment of the mother directly influences that of the embryo, and pregnant animals imperfectly fed can only produce puny offspring. That material may pass from mother to foetus is proved by the bones of the embryo being stained if madder be administered to the parent. Yet we know that the placenta, under other circumstances, is an efficient filter for certain pathological substances, and that the tuberculous mother does not convey tuberculosis to the foetus. It has been suggested that the passage of water, salts, and sugar from mother to foetus may occur by diffusion, the passage of fat and proteid being perhaps connected with special enzymes. The presence of glycogen in all the embryonic tissues points to it as an important material in the nutrition of the foetus. Gradually, as development proceeds, the glycogenic function becomes largely centred in the liver and placenta. Fcetal Membranes. — If the egg of the hen be examined after incubating nine days, the appearance seen in Fig. 149 presents itself. A chick in an advanced stage of develop- ment 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 ; an identical arrange- ment exists in mammals. The supply of food required by the embryo chick during development is contained in a sac known as the yolk sac, to this food-supply the embryo is connected by a stalk through which the nourishment enters its body. The walls of the yolk sac are vascular and con- nected with the vessels of the embryo ; it is through the medium of the vascular wall that the altered yolk is taken up. A modified yolk sac is found in mammals (Figs. 145, 147, 148) ; it does not contain yolk, but it takes up the Digitized by Microsoft® 606 A MANUAL OP VETEKINAEY PHYSIOLOGY nourishment secreted by the uterine glands, and for a time this suffices for the needs of the embryo. 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 blood takes up oxygen from the air surrounding the allantoic sac ; an air space also exists at the end of the shell. An allantois exists in the mammal (Figs. 145, 148) ; unlike that of the bird it does not obtain aZUmtous -airspace. yolk- seta sfvelii Fig. 149. — Hen's Egg at the Ninth Day of Incubation (Ewaet after mllnes mabshall). oxygen from the air, though it is a breathing organ in the sense that it furnishes oxygen io the foetus. We have seen that immediately enveloping the mam- malian embryo is the amnion, Figs. 145 and 148 ; this sac contains a fluid in which the foetus lives. The fluid, or liquor amnii, is alkaline in reaction, and yellowish in colour during the early days of gestation, but reddish towards the end of it, probably due to discoloration with meconium. The amniotic fluid contains proteids, mucin, urea, sugar, lactic acid, keratin, and some salts ; besides these there are also portions of hoof, epithelium, etc., derived from the Digitized by Microsoft® GENEKA.TION AND DEVELOPMENT 607 foetus. The source of this fluid is probably by transudation from both the foetus and mother. Indigo blue injected into the vessels of the mother tinges the amniotic fluid, though it does not stain the foetal tissues. The function of this fluid is protective to mother and foetus ; the latter lies on it as on a water-bed, and during Pig. 150.— Diagram of the Fcbbal Envelopes of a Five Months House Embryo (Bonnet). parturition it assists in dilating the os and lubricating the maternal passage. The allantois grows out from the body of the embryo at the future umbilicus ; the part within the body forms the bladder, that outside it forms a sac which in the mare completely envelops the amnion (Pig. 150), but in ruminants only partly so (Fig. 151) ; the bladder and the cavity of the allantois are connected by a canal in the umbilical cord known as the urachus. The fluid found Digitized by Microsoft® 608 A MANUAL OP VETERINAEY PHYSIOLOGY 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 allied to it, allantoin, albumin, sugar, lactic acid, and certain salts. The allantois is the organ of respiration, and to a limited 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 effect an exchange of gases. Later on, as we have seen, p. 596, it furnishes the villi which penetrate into the walls of the uterus through the chorion. Floating in the allantoic fluid of the mare, or attached to the wall of the sac, are certain peculiar masses termed hippomanes ,■ their origin and use are quite unknown. It is usually considered that these bodies, which may be multiple, are found in the foal's mouth at birth, but we are assured by a close and reliable observer* that this is a fallacy. Hippomanes have also been observed in the cow.f The chorion envelops the two previous coverings. Through the umbilical cord it forms the vascular connection between the foetus and the mother, and the villi on its surface pro- ject themselves into the mucous membrane of the uterus, not through the medium of a decidua as in the woman, but directly into the uterine wall. The bloodvessels of the chorion and those of the uterus do not anastomose, but the foetal villi are bathed in the blood contained within sinuses in the uterus into which they run, and in this way, through the endothelial lining of the vessels of mother and embryo, the blood of the foetus receives oxygen and gets rid of carbonic acid. Umbilical Cord.— After the formation of the f cetal envelopes the body- walls rapidly close in, the splanchnopleure being received up into the body to form the primitive gut and its * Mr. T. B. Goodall, F.R.C.V.S., Christchurch. 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. Digitized by Microsoft® GENEKATION AND DEVELOPMENT 609 39 Digitized by Microsoft® 610 A MANUAL OF VETERINAEY PHYSIOLOGY derivatives, the somatopleure forming the body-wall and the limbs. The embryo or foetus retains its connections with the placenta by means of the umbilical cord, which is composed as follows : Structures in connection with the amnion and the body-wall at the umbilicus ; structures in connection with the allantois and the urachus, the latter being a funnel-shaped body connected with the urinary bladder, and the remains of which may be seen as a scar on the fundus of that organ, even in the adult ; the umbilical arteries and vein, or veins (ruminants). All these are cemented together by an embryonic connective tissue, the Whartonian jelly. Foetal Circulation. — With the formation of the foetal envelopes and the development of the heart, the circulation takes on a course altogether different from that in the vascular area in early embryonic life. The placenta acts as the fcetal respiratory and food-absorbing organ. Im- pure blood that has circulated through the tissues of the developing young is brought to the placenta by the umbilical arteries, these acting to the fcetus as the pulmonary arteries to the adult. After an interchange of gases and a renewal of food supply, the blood is carried away to the foetus by means of the umbilical vein or veins found in the cord. The vein enters the body at the navel or umbilicus, and passes forward along the floor of the abdomen, reaches the falciform ligament of the liver, travels along the free edge of that structure, and empties itself into the portal vein. After birth the remains of the umbilical vein are found as a thickening at the free edge of the falciform ligament, and is named 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, the ductus venosus (Fig. 152, d v), which passes upwards to join the posterior vena cava. After the blood*has circulated in the liver it Digitized by Microsoft® GENERATION AND DEVELOPMENT 611 leaves 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. In the horse the whole of the foetal blood passes through the liver before reaching the heart through the posterior cava ; in ruminants part of the blood passes through the liver, and part goes direct to the systemic circulation of the foetus through the ductus venosus. In the foetal 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 valve, that stretches from the mouth of the posterior vena cava to the annulus or thickened border of the foramen ovale ; it is absent from the heart of the foetal horse and pig. The function of this valve is to direct the blood-stream into the left auricle ; the blood in this way gets into the left auricle, 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 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 anterior vena cava. From the right auricle it passes to the right ventricle, and from this cavity it is pumped into the pulmonary artery. The lungs, however, are not functional, and are more or less solid organs, consequently they are not yet prepared to receive the blood as they will be after birth, when they become distended with air and have taken on their duties as breathing organs. The blood must therefore take another course than through the lungs. This course is provided by the ductus arteriosus (Fig. 152, 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, 39—2 Digitized by Microsoft® l.V. a .a . d.a. I. a. Jl.V a -re m \ a Htt^^^P p v wMmi v.A -/^^k\ '' miM) , !» Ib\. ,«/. V. Jl.Y.C. A- Fig. 152. — Diagram of the Fcetal Circulation (Ellenberger). u. v., Umbilical vein ; d. v., ductus venosus ; pt. v., portal vein'; I,, liver ; v. h., hepatic veins ; p. v. c, posterior Vena cava ; r. a., right auricle ; /. o., foramen ovale ; r. v., right ventricle ; p. a., pulmonary artery ; d. a., ductus arteriosus ; I. a., left auricle ; I. v., left ventricle ; a., the aorta ; a. a., arch of aorta ; amt. a., anterior aorta ; i.v., innominate veins ; a. v. c, anterior vena cava ; po. a., posterior aorta ; i.a., iliac artery; h. a., hypogastric artery; u. a., umbilical arteries; i. ve., iliac veins ; h. v., hypogastric veins ; u. c, umbilical cord. The diagram actually represents the foetal 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 indioate 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 foramen ovale (/.o.). Digitized by Microsoft® GENERATION AND DEVELOPMENT G13 and is conveyed to the hinder parts of the body and to the placenta. The allantoic or umbilical arteries 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, and 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 ligament of the bladder. The ductus arteriosus 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 ensuring that the purest blood reaches those organs which require it the 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. The fact is that the foetal blood at its best is far below the level of the arterial blood of the mother, and this is explained by saying that the demand of the foetus for oxygen is small owing to the low rate of its metabolism. Prom the blood of the umbilical artery and vein of the foetal sheep the following gases have been extracted, and may be compared with the arterial blood of the mother : Umbilical Umbilical Maternal Arterial Artery. Vein. Blood. Oxygen 2-3 6-3 20'0 Carbon dioxide ... 47'0 40-5 40-0 Digitized by Microsoft® 614 A MANUAL OF VETEEINAEY PHYSIOLOGY The liver is a very active organ in the foetus, and is abundantly supplied with a mixture of blood, the worst and the best in the body, the best predominating. Early in intra-uterine life the liver begins to secrete bile, which is discharged into the intestines as meconium (see p. 212). Uterine Milk. — If the villi of the chorion be separated from the tubular depressions of the mucous membrane of the uterus, a fluid may be expressed known as uterine milk. This is particularly observable in separating the fcetal and maternal cotyledons. Uterine milk is of a white or rosy- white colour, creamy consistence, and contains proteids, fat, and a small proportion of ash. Examined micro- scopically it is found to contain globules of fat, leucocytes, rod-like crystals, and structureless masses of proteid. The use of the fluid is for the nourishment of the embryo, and in the mare, cow, and sheep the uterine glands take a pro- minent part in providing nourishment throughout fcetal life, pouring their secretion into special depressions in the placenta (Assheton). The Duration of Pregnancy appears to be based on no fixed law. Judging from the length of time the elephant is in gestation 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 fifty-nine to sixty-three 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 the guinea-pig should require a period of gestation twice as long as the rabbit, which is also a rodent. The following are average periods of gestation : Elephant 2 years (nearly). Mare 11 months, and liable to vary within relatively wide limits. Ass 358 to 385 days. Zebra 13 months (and over). Cow... 40 weeks. Digitized by Microsoft® GENERATION AND DEVELOPMENT 615 Sheep ... 21 weeks (average) Camel .... 45 weeks. Pig ... 16 weeks. Dog 59 to 63 days. Cat ... 56 days. Babbit 32 days. Guinea-pig ... ... 63 days. Parturition. — 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 Fig. 153. — The Position occupied by the Equine Foetus during Intra- Uterine Life (France). bent at the knee, and the hind-legs in the right horn (Fig. 153). Preparatory to birth the foetus changes position and turns on its side, so as to assume first a lateral posi- tion (Fig. 154), and lastly an upright one (Fig. 155), 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. 155), 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 lies somewhat crooked, viz., the head inclining towards one side, and the hind extremities towards the Digitized by Microsoft® 610 A MANUAL OF VETERINARY PHYSIOLOGY other ; in all other respects it resembles the mare. The alteration in the position of the foetus does not occur through its own movements, but by the contraction of the uterus ; on the other hand, the stretching of the limbs is the result of fcetal movement.* There can be little doubt that the revolution of the fcetus prior to birth is the explanation of the complete torsion of the neck of the uterus and vagina which is sometimes found in both tbe cow and mare. The dilatation of the os is assisted by the amniotic and Fig. 154. — The First Stage in the Bevolution of the Fcetus ; Lateral Position. The Os is Dilated by the Membranes which have not yet Euptured (Franck.V allantoic fluids. Each contraction of the uterus is accom- panied by a pain ; the pains last from 15 to 90 seconds, and the interval between them is from 2 to 4 minutes. The contractions of the uterus 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). The mare is remarkable for the rapidity with which * This description of the change in the position of the fcetus pre- paratory to birth is taken from Ellenberger's ' Physiologie.' Digitized by Microsoft® GENEEATION AND DEVELOPMENT 617 delivery is effected; ruminants, on the other hand, are often very slow 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 why any difficulty in foaling invariably sacrifices the life of the foal. In ruminants, on the contrary, the circulation between the mother and foetus is kept up to the last by the gradual separation of the cotyledons, so that though the process may be delayed several hours, the animal is generally born alive. Fie. 155. — The Bevolution Completed, Membranes Buptured, and Foal in the Normal Position for Delivery (France). The cause of the first respiration of the foetus is dealt with at p. 112. The Secretion of Milk. 1 1 As the period of parturition approaches, the mammary glands become swollen owing to active changes occurring in them, and at or shortly after the birth of the animal milk is formed. Two processes contribute to the formation of milk; in one the cells lining the alveoli of the gland are bodily shed and form the fat of the milk, while in the other the Digitized by Microsoft® 618 A MANUAL OF VETBEINAEY PHYSIOLOGY water, proteids, salts, etc., are formed from the lymph in the gland by the ordinary process of secretion. We must examine the first of these at somewhat greater length. If the mammary gland of an animal which has never been pregnant be examined, the alveoli it contains are much smaller and less numerous than those of a secreting gland. The alveoli of the first-mentioned gland are found to be packed with small rounded cells of very slow growth ; when the animal becomes pregnant the gland enlarges, the alveoli increase in number, but remain packed with cells until parturition approaches or occurs. The solid masses of cells are now cast off, and leave behind them alveoli lined with a single layer of secretory epithelium, Loaded. Discharged. Fig. 156. — Mammary Gland of Dog during Lactation. After Hbidbnhain (Waller). the function of which is to furnish the milk. The shedding of the mass of cells which originally occupied the alveoli supplies the colostrum or first milk. The appearance presented by the single layer of cells lining the alveolus of the secretory gland, depends upon whether the gland is loaded or discharged. If the gland is loaded, viz., active secretion occurring, the cells are found to be large and columnar in shape, possessing two or more nuclei, one being at the base of the cell, and the other, giving indications of degeneration, placed near the apex (Fig. 156). In the apex or free portions of the cell fat globules can be seen, which may even have partly extruded themselves from the cell, and besides these there are other particles. Further, the cell gives the appearance Digitized by Microsoft® GENEKATION AND DEVELOPMENT 619 of the apex or free border being separated from the base by a process of constriction. If the gland be examined when discharged, viz., after the milk has been drawn off, the cells lining the alveolus are cubical or flattened, each containing a nucleus; the lumen of the alveolus is also increased in size, and within it may be seen some of the elements of the milk (Fig. 156). It is evident that the cells in the active gland are loaded with material, much of it being fat, and these cells break off leaving behind them the parent cell containing a nucleus from which another cell grows. In spite of this the formation of fat in milk is really a process of cell secretion, and this is supported by the fact that animals such as carnivora, whose food is deficient in fat, produce a fat- containing milk, and the fat is elaborated by the mammary cell from the proteid of the body. A fat diet does not increase the fat in milk, though a proteid diet has this effect. The proteids, sugar, and salts, found in milk are secreted in the ordinary way from the blood, or rather the lymph circulating in the gland, the cells lining the alveolus being the active factor in the matter, and that these substances are really elaborated by the cell, is supported by the fact that neither caseinogen nor milk sugar exists in any other tissue of the body. It has been supposed that the Becretion of milk is influenced by the nervous system, but there is no experimental evidence which places this beyond doubt. Composition. — The milk of herbivora has an alkaline reaction which may readily turn acid ; in carnivora the reaction is acid. Fresh cow's milk is amphoteric, viz., it gives both an acid and an alkaline reaction to test paper ; this is due to the presence of acid and alkaline salts. In the cow the specific gravity is 1028 to 1034. The secretion contains proteids (caseinogen and albumin), sugar (lactose), fats, and salts. An average secretion of milk from a cow may be taken at 6 quarts (6 '8 litres) per diem for forty weeks in the year. Digitized by Microsoft® 620 A MANUAL OF VETERINAEY PHYSIOLOGY In the following table is given an analysis of the milk of different animals : Water Solids .. Casein ■.. Albumin .. Fat Lactose .. Salts Cow. 84-28 15-72 3-57 •75 6-47 4-34 Mare. Sheep 92-5 .. . 82-84 7-5 .. . 15-17 1-8 i - ■3 5 4-7 •6 .. 4-8 4-7 .. . 3-4-6 ) •6i •3 .. Ass. 90-5 9-5 1-7 1-4 6-4 Dog. 76-0 24-0 10-0 10-0 3 5 ■5 It will be observed that the milk of the cow, dog, and sheep is remarkable for the high percentage of fat it con- tains ; the caseinogen of mare's milk is much less than that found in the cow, and more like that of the human. The milk of the dog is rich in caseinogen, fat, and calcium, but poor in lactose. Under the influence of rennin caseinogen becomes in- soluble, and the milk is coagulated, resulting in a clot and whey ; the clot or insoluble casein is now termed tyrein. Neither the albumin nor the caseinogen in milk is pre- cipitated by boiling; on the other hand, colostrum is precipitated by heating, and this is due to the fact that it contains globulin. The albumin of milk offers some peculiarities as compared with ordinary serum albumin, . and has been termed lactalbumin. The fats in milk are olein, stearin, and palmitin, and the proportion of these differs in various animals. The fat is contained within fat globules, and these form in milk 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 ; by the process of churning the emulsion is destroyed, and the fat is obtained as butter. Butter consists of 68 per cent, of palmitin and stearin, 30 per cent, of olein, and 2 per cent, of specific butter fats. Milk sugar or lactose is very liable to undergo fermentation, resulting in the production of lactic acid and the curdling of milk. It is not, however, capable of undergoing direct alcoholic fermentation, which would appear to be a pro- Digitized by Microsoft® GENEEATION AND DEVELOPMENT 621 vision against fermentative decomposition occurring either in the gland or in the alimentary canal (Lea). The milk of the mare in the presence of suitable ferments may- undergo alcoholic fermentation, as in the production of koumiss. The salts of milk are principally calcium phosphate, and salts of sodium and potassium. In the composition of the milk we obtain an insight into the nature and quantity of the salts required by growing animals. Bunge gives the following ash analysis of mare's and cow's milk : Mare's Mill. Cow's Milk. ... 1-04 . 1-76 Potassium . . . Sodium Calcium Magnesium Iron Phosphoric acid Chlorine Total ash per 1,000 ... 4-17 7-97 •14 1-11 1-23 1-59 •12 ... ... -21 •015 -003 1-31 1-97 •31 1-69 The phosphates are increased by those contained in the proteids; they are employed mainly in the construc- tion of the skeleton. The excess of potassium over sodium salts is a feature common to many of the secretions of the herbivora, but in milk, probably in all animals, the ash contains more potassium than sodium. Bunge states that this is due to the fact that as the animal grows it becomes richer in potassium and poorer in sodium salts, depending upon the relative increase in the muscular structure which is rich in potassium, and the relative decrease in the carti- laginous material which is rich in sodium. Bunge compared the ash of a puppy with the milk of the mother, and the milk with the blood. It was remarkable how closely the composition of the puppy's system agreed with the salts it was receiving in the milk, though when the ash of the milk was compared with the ash of the blood of the mother, the greatest diversity in composition was apparent. In comparing Bunge's analysis of the ash of cow's and mare's milk, one is struck by the fact that the calf requires much more salts for its nutrition than the foal. Digitized by Microsoft® 622 A MANUAL OP VETEEINAEY PHYSIOLOGY The first milk secreted is termed Colostrum. The source of colostrum, and some peculiarities in its composition, have already been dealt with. In appearance it is a yellowish- white fluid of an alkaline reaction, sweetish taste, and remarkable for the amount of proteid it contains, as much as 15 per cent., whilst ordinary milk only contains 4 per cent, or 5 per cent. Examined microscopically colostrum is found to contain bodies termed ' colostrum corpuscles.' These are large granular corpuscles containing fat. The use of colostrum is to act as a natural purge, by which means the intestinal canal of the newly born animal is cleared out. Digitized by Microsoft® GHAPTEE XIX GROWTH, DECAY, AND DEATH Growth. — 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 system in a high state of development ; in the course of a few hours 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 intelligence which nearly, if not quite, equals its parents ; it practically has nothing to learn but obedience to man. Not only is the nervous system in an advanced condition, but also the locomotor : the legs of the foal are remarkably long, some of the bones being nearly their full length, 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 development of the limb from what we have previously said, 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 length of 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 623 Digitized by Microsoft® 624 A MANUAL OF VETEKINAEY PHYSIOLOGY foal may be ascertained by measuring the fore limb from the fetlock to the elbow and multiplying it by two. Table showing the Length of the Bones op the Limbs op the Foal and Adult Horse. Adult Horse. Foal of Six Weeks. Difference. Scapula - Humerus - 15 in. 12 in. 8Jin. 8 in. 6} in. 4 in. Badius and ulna - 18 in. 12 in. 6 in. Knee-joint Metacarpal Suffraginis Femur 3^ x 3£ in. 9£in. 3im. 17 in. 3 x 3 in. 8f in. 3 in. lOiin. i in. i in. 1 in- 6§ in. Tibia Calcis to metatarsal bone 13£ in. 6 in. 9£in. 5 in. 4 in. lin. Metatarsal 11 in. 10 in. 1 in. Suffraginis 3±in. 3 in. £in. 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 contains much muscle. The oblique position of the scapula is due to the weight of the body on the limbs, the weakness of the muscles at this part allowing the angle formed by the Bcapula 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. The 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 112 lbs., that during the first three months the daily increase in weight was 2"2 lbs. ; from three up to six months the increase was 1*8 lbs., and * Quoted by Colin. Digitized by Microsoft® GROWTH, DECAY, AND DEATH 625 from six months up to three years of age the increase was at the rate of - 7 lb. per diem. The influence of feeding on development is most remark- able ; not only does the body increase in 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. Calves, according to Torcy,* have a mean weight at birth of 77 lbs., the daily increase during the first two years being 1*5 lbs. With sheep the daily increase in weight is more rapid ; a lamb will in ten days gain 50 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 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 - 44 lb. 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 t many years ago drew up a table, which he considered very imperfect, as to the rate at which some horses of his regiment grew, from which he showed that the increase in height between 2 years and 3 years was on an average one inch, between 3 years and 4 years one-third of an inch, and between 4 years and 5 years one-third of an 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 11 five-year-olds, 2 did not grow during the year. * Quoted by Colin. f ' Lectures on Form and Action.' 40 Digitized by Microsoft® 626 A MANUAL OF VETEEINAEY PHYSIOLOGY There can be no doubt that many horses grow much more than two-thirds of an inch between three and five years old. It is probable that many grow up to their sixth year. During the time the calf and foal are receiving their mother's milk the urine is acid, for the reason that the animal is practically carnivorous ; once a vegetable diet is taken the urine becomes alkaline, and it is probable de- creases in quantity. The activity of certain glands, such as the thymus, becomes considerably reduced as the animal grows, and finally they disappear at the adult period. One characteristic of the young animal is the necessity for sleep ; it is probably during slumber 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 ntero ; the following tables show the period at which changes take place in the teeth from birth to adult age : Horse. Eruption. Change. Incisors : \ Central At birth. 2-J years. Lateral 1 to 2 months. 3-j years. Corner 7 to 8 months. 4£ years. Molars : i First - ) f 2| years. Second V At birth. -J 3 years. Third - i s ( About 3^ years. Fourth About 1 year. Fifth - About 2i years. Sixth - About 3i to 4 years. Canines - About 4| years. Digitized by Microsoft® GEOWTH, DECAY, AND DEATH Ox* 627 Eruption. Change. Incisors : Central Middle Lateral Corner Molars : 1 At or soon after j birth. /irk y ears - - J 2^ to 2jSj years.f 1 2A to 3 years.f [2i§ to 3 & years.f First Second Third Fourth 1 At birth. 6 months. ("About 2j% years. \ About 2^ years. (.About 2 ^ years. Fifth About 12 months. Sixth 21 months. Sheep. * Eruption. Change. Incisors : Central \ /"About 1 year. Middle - At birth or soon J About 2 years. Lateral after. 1 Soon after 2 years. J Corner lAbout 3 years.J Molars : First Second ) At birth or soon | after. ("Soon after 18 J 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.' t There is considerable variation in the development of these teeth. % These teeth are liable to great variation in their development. 40—2 Digitized by Microsoft® 628 A MANUAL OF VETEEINAEY PHYSIOLOGY Pig. Eruption. Change. Incisors : Central 1 month. 12 months. Lateral 2 months. 18 months. Corner At birth. 8 months. Molars : First "I ) Second 1- 1 month. } About 15 months. Third J ) Fourth 5 months. Fifth 10 to 12 months. Sixth 18 months. Premolars - 5 months. Tusks At birth. 9 months. In all these tables the periods given are those of erup- tion 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. The completion of dentition 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 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 deposi- tion of fat in the body. 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 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 be stated that an old horse is liable to be killed by a hard day's Digitized by Microsoft® GEOWTH, DECAY, AND DEATH 629 work, and in this sense he is certainly old at seventeen. Arterial degeneration is not marked at this period of life, and few horses live long enough for their arteries to become rigid. Doubtless the work performed by horses is the chief cause of their rapid 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 preclude many of them from reaching a ripe old age, though 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-two years of age. Blaine* appears to have gone very carefully into the question of old age in equines, and he drew the following comparison, which is doubtless very close to the truth : ' The first five years of a horse may be considered 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.' Death. — Death from natural causes 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 and decay, or that he is allowed to live until the breath of life passes gradually from the body. 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. Natural death is described as commencing either at the heart, lungs, brain, or blood. Probably in the main most cases of natural death may be attributed to a failure of the heart's action ; but from what we know of the physiology of the heart, respiration, and blood, it is very difficult to separate these in discussing the causes of natural death, * ' Encyclopaedia of Rural Sports.' Digitized by Microsoft® 630 A MANUAL OF VETEEINAEY PHYSIOLOGY knowing as we do how completely one is dependent on the other. The cessation of the heart's action may be looked upon as the termination of life. We cannot enter upon the cause of death the result of disease, excepting to notice the interesting fact that horses seldom die quietly ; a large majority of them leave this world in powerful convulsions, fighting or struggling to the last, lying on their side, and galloping themselves to death. Especially is this marked in acute abdominal Fig. 157. — Convulsive Limb Movements at the Moment of Brain Destruction. Note the tail is affected as well as the limbs. The bandages are put on to assist the plate. trouble. The struggles at the end should not be mistaken for pain; the animal is quite unconscious. The violent convulsions which occur at the last moment are not present in death from acute chest diseases ; such cases stand per- sistently to the last, and either drop dead or die very shortly afterwards. In violent death by destruction of the brain in horses, remarkable 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 (Pig. 157). Digitized by Microsoft® GEOWTH, DECAY, AND DEATH 631 In spite of their rapidity, a marked interval between brain destruction and muscular contractions occurs ; in Pig. 158 the brain was destroyed by a charge of large shot, yet the horse is still standing, the impulses relaxing muscle tonus not yet having had time to pass out. At the moment of death the bladder and rectum are emptied, the horse sweats on the inside of the thighs, the pupil dilates widely, and occasionally the panniculus is called into play and the animal may shake the skin as if to dislodge a fly. Fig. 158. — Brain Destroyed by a Charge op Shot. 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 limb has left the ground. There is nothing, however, to indicate the fact that the horse is dead. Soon after death rigor mortis appears (see p. 379), and within a short time tympany of the abdomen is apparent in the herbivora, reaching such a degree in a few hours, especially during warm weather, that post-mortem rup- tures of the diaphragm and other viscera are exceedingly common. The explanation of the tympany is the con- siderable amount of gas generated by the fermentative decomposition of vegetable food. Digitized by Microsoft® CHAPTER XX THE CHEMICAL CONSTITUENTS OF THE BODY* A, large number of elements enter into the composition of the body. Oxygen, hydrogen, carbon, nitrogen, sulphur, phosphorus, chlorine, fluorine, silicon, potassium, sodium, calcium, magnesium, and iron are found, not, it is true, in a free state or only to a very slight extent, but brought together in such a way as to form compounds, and these may be divided into two classes, organic and inorganic. Carbon is present in the atmosphere in small amounts in the form of carbonic acid, viz., united to oxygen ; 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. Carbon enters the animal system with the carbon of the food, and leaves it either as carbonic acid or in compounds, such as urea ; as carbonic acid, therefore, it is again taken up by the plant. Hydrogen does not occur in a free state in nature, but principally as water, and a very small quantity as ammonia, * This brief outline of the chemistry of the body was originally based on a summary of the principal facts contained in Bunge's ' Physiological and Pathological Chemistry,' and Sheridan Lea's appendix to Foster's ' Physiology,' ' The Chemical Basis of the Animal Body.' This chapter is in no way intended to be a complete statement as to the chemical constituents of the animal body, but elucidatory and supplementary to the chemical statements scattered throughout the preceding chapters. Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OF THE BODY 633 and it is in these forms that hydrogen is taken up by plants. Animals give off hydrogen as water and ammonia, or as substances which may be readily made to yield these. Oxygen is the most widely distributed of the elements, forming one-quarter by weight of the atmosphere, and eight-ninths by weight of water ; it also forms, by means of its compounds, one-half the weight of the earth's crust. It is the only element which enters the animal or vegetable body in a free state. Nitrogen exists largely in a free state, since it forms no less than four-fifths of the atmosphere ; it has but little affinity for other elements. In the form of ammonia, nitrous and nitric acids, it enters the plant ; as proteid it enters the animal, leaving it as urea, etc., which by decomposition readily yields ammonia. The animal cannot utilize free nitrogen any more than the plant can, though leguminous plants appear to utilize the atmospheric nitrogen by symbiotic co-operation with certain bacteria. As a gas it is found dissolved to a slight extent in some of the fluids of the body. Sulphur exists largely in nature in combination as sulphates of alkalis and alkaline earths; in this form it is taken up by plants, and becoming a part of their proteid molecule is taken into the body of the animal, where by splitting up and oxidation it yields sulphuric acid, in which form it is excreted in the urine as sulphates or colligated with certain organic substances (see p. 300). Phosphorus enters plants as phosphoric acid united with alkalis; in soils it exists in only small quantities, hence the necessity of phosphates as manure. In the plant phosphoric acid forms a part of the complicated compounds known as lecithin and nuclein, in which condition it enters the animal body, forming a part of both the solid and fluid tissues. Chlorine does not exist in a free state in nature, but combined with potassium and sodium, in which form it enters plants, and from these passes in the same com- pounds into animals. Digitized by Microsoft® 634 A MANUAL OF VETERINARY PHYSIOLOGY Neither sodium, potassium, nor magnesium enter or leave the body or plant in any organic form, but simply as in- organic salts. On the other hand, Bunge considers that calcium does enter the body as an organic compound. Iron does not occur free in nature, but chiefly as com- pounds with oxygen in a ferrous and ferric state. In the animal it occurs chiefly in the highly complex body haemo- globin, which acts as an oxygen-carrier. Iron furnishes the plant with its colouring matter, for chlorophyll cannot be formed without its aid. It is not known in what form iron leaves the body. Silicon, in the form of silicic acid, is taken up by plants. From the plant it is taken into the body and passes into the tissues. It is largely of use in the growth of hair, and much of it passes out of the body of herbivora with the urine; in sheep, according to Bunge, it sometimes causes stone in the bladder. Bunge draws a contrast, in the following terms, between the methods employed by the vegetable and animal organism in the utilization of the various elements and compounds presented to them : 1. The plant forms organic substances ; the animal destroys organic substances. The vital process in the plant is preponderatingly synthetic, in the animal analytic. 2. The life of the plant is a process of reduction ; the life of the animal a process of oxidation. 3. The plant uses up kinetic energy and produces potential energy ; the animal uses up potential energy and produces kinetic energy. The organic compounds in the body may be broadly divided into nitrogenous and non-nitrogenous. NITKOGENOUS BODIES. Proteids. — This term is applied to a large number of substances more or less closely allied, which in one form or other go to make up by far the largest portion of the animal body. Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OF THE BODY 635 Proteids are highly complex substances, possessing as yet no definite chemical formula owing to the difficulty in obtaining them in a sufficiently pure state for analysis, and to the magnitude of their molecule. With some few excep- tions they have never been obtained in a crystalline con- dition, and their nature is colloidal, for they do not diffuse through an animal membrane, not even those which can be obtained in a crystalline form. All proteids contain carbon, hydrogen, oxygen, nitrogen, and very variable amounts of sulphur. Carbon Hydrogen Oxygen Nitrogen Sulphur 51'5 to 54 p 5 per cent. 6-9 to 7-8 „ 20-9 to 23-5 „ 15-2 to 17-0 „ 0-3 to 2-0 „ In spite of the fact that proteids exist in several forms in all animal and vegetable bodies, and that it is quite im- possible to maintain life without them, yet very little is known of them owing to their extreme complexity. Neither their chemical formula nor their molecular weight is known, though the latter must certainly be very large, and the chemist has never as yet been able to build them up synthetically, though the work now being carried on by Emil Fischer and his pupils is full of promise for a future synthesis. The decomposition products of proteids are very numerous and very varying in nature, according to the methods employed. In the body carbonic acid, water, urea, and uric acid are the final end products, but between these and proteids are glycine, leucine, and other substances. From the non-nitrogenous portions of the proteid glycogen and fat may be obtained, as we have previously seen. Proteids, when acted upon outside the body by means of heat, putrefaction, acids, alkalis, and oxidizing agents, yield a large and numerous class of substances. In the absence of adequate chemical knowledge, all classification of proteids must necessarily be artificial, and Digitized by Microsoft® 636 A MANUAL OF VETERINARY PHYSIOLOGY is at present based on their varying solubilities in water, saline solutions, acids, and alkalis. In the following table the proteids are thus roughly classified, and the distinguishing characteristics of each class given. Classification of the Proteids. A. Simple Proteids. Class I. Native Albumins. These are soluble in distilled water, and the solutions are coagulated by heat at 70° to 75° C, especially in the presence of dilute acetic acid. They are not precipitated by saturating their solutions with neutral salts other than sodio-magnesium sulphate and neutral ammonium sulphate. Examples of this class are egg and serum albumin, cell, muscle, and milk albumin, or lactalbumin (p. 620). Class II. Globulins. These are insoluble in distilled water, but soluble in dilute saline solutions ; from these they are precipitated by saturation with common salt or magnesium sulphate. In this class are found the globulin of the crystalline lens (crystallin), the globulin of the blood, para- or serum- globulin, the fibrinogen of the blood- and myosin of muscle. Class III. Derived Albumins {Albuminates). These are obtained by the action of acids or alkalis on albumins or globulins. They are insoluble in distilled water or in dilute neutral saline solutions, but soluble in acids and alkalies, and the solution is not coagulated by boiling, though it is precipitated by careful neutralization. Examples of this class are acid albumin, syntonin, and alkali albumin. Caseinogen (casein), which was at one time placed in this class, is now known to be a nucleo-proteid. Class IV. Fibrins. These are insoluble in water, but soluble by the pro- longed action of strong neutral saline solutions, whereby Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OF THE BODY 637 they are largely changed into globulins (Class II.), and with difficulty in strong acids and alkalis, being at the same time converted into acid or alkali albumin (Class III.). Examples are the fibrin formed during the clotting of the blood and lymph. Class V. Coagulated Proteids. Any of the above which have been coagulated either by heat or the prolonged action of alcohol. Class VI. Albumoses (Proteoses) and Peptones. Both of these are very soluble in water, albumose being precipitated by saturation with ammonium sulphate, while peptones are not. Peptones are not precipitated by any ordinary proteid precipitant excepting alcohol, and even the prolonged action of alchol does not coagulate them. Albumoses may be precipitated by the careful addition of nitric acid in the cold, and the precipitate characteristically disappears on heating and reappears on cooling. Albumoses (or proteoses) are formed as the primary product of the action of the gastric and pancreatic enzymes on proteids. Three well-marked forms of albumose are known, characterized by their varying solubilities and their precipitability by neutral salts or acetic acid and potasssium ferrocyanide. Peptones are the final product of the action of gastric and pancreatic enzymes on proteids. One of their most interesting characteristics is that they, alone among proteids, are diffusible through membranes. They differ from albumoses by the fact that they are not precipitated when their solution is saturated with neutral ammonium sulphate or any other neutral salt. B. Compound Proteids. Class I. Nucleo-proteids. The nucleo-proteids are, as the name implies, compounds of a proteid with nuclein, the characteristic constituent of nuclei. They form the bulk of the proteids present in Digitized by Microsoft® 638 A MANUAL OF VETERINARY PHYSIOLOGY most cell protoplasm and their solubilities are closely similar to those of the globulins. Their compound nature is shown by the fact that when digested with gaBtric juice they yield albumoses and peptones, together with an undis- solved residue of nuclein. Ordinarily the nuclein thus obtained is the true nuclein, which yields substances of the xanthine (purin) series when hydrolyzed by acids. In other cases the nuclein residue (pseudo-nuclein) does not yield xanthine bodies by hydrolysis, and typical examples of this form of nucleo-proteid are found in the caseinogen (casein) of milk and the vitellin of egg-yolk. They all contain phosphorus, since this element is characteristically a constituent of nuclein. Class II. Glyco-proteids. These forms of proteid are characterized by yielding, on hydrolysis, some kind of (carbohydrate) substance which reduces Fehling's fluid and gives osazones with phenyl- hydrazine. This reducing substance frequently contains nitrogen, and is probably in many cases glucosamine (C 6 H n 5 .NH 2 ), or amido-glucose. The characteristic members of the glyco-proteid group are the various kinds of mucin. Of these the mucin of saliva may be regarded as the truest and most typical form. Mucin confers on its solutions their well-known viscidity or 'ropiness-' From these it is readily precipi- table by the addition of acetic acid, and is resoluble in alkalis. i The Albuminoids. Under this name a number of substances are grouped together, which, while closely allied to the proteids, differ from them in some important particulars, and differ also in many respects the one from the other. The best known members of the group are collagen and gelatin, chondrin, elastin, and keratin. Collagen is the ground substance of which the fibres of connective tissue are formed and, under the name of ossein, Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OF THE BODY 639 forms a large part of the organic basis of bones. It is insoluble in water, salt solutions, and dilute alkalis and acids, though it swells up as a gelatinous mass by tbe action of the latter. Prolonged boiling with water, espe- cially in presence of dilute acids, converts it into gelatin, and the latter can be reconverted into collagen by a dry heat of 130° C. Gelatin. — A common and well-known substance of which isinglass is a typically pure form and glue an impure. Insoluble in cold water, it swells up by its action, and now dissolves readily when heated, the solution forming a jelly on cooling, even when it contains only 1 per cent, of gelatifln. When digested with pepsin or trypsin, gelatin yields diffusible substances known as gelatin-peptones. By hydrolysis it yields leucine and glycine, but no tyrosine or any member of the aromatic series, and hence gives no red reaction with Millon's reagent (see below). Chondrin. — This is obtained from hyaline cartilage by those processes which, when applied to connective tissue or bones, yield gelatin. Chondrin resembles gelatin in that its solutions gelatinize on cooling, but it differs chemically in many respects from gelatin. Thus it is precipitable by acetic acid, and when hydrolyzed yields a substance which reduces Fehling's fluid. Elastin. — This is the ground substance of the fibres of elastic tissue. It is extraordinarily insoluble and resistant to ordinary reagents, and is hence obtained by treating a tissue such as ligamentum nuchse with a succession of reagents which dissolve out everything except the elastin. By digestion with pepsin or trypsin or by hydrolysis, elastin yields products similar to many of those similarly obtainable from true proteids. Keratin. — The characteristic constituent of epidermal structures such as hair, nails, feathers, and horn. From these it is obtained as a residue by their extraction with a series of reagents, such as water, alcohol, ether, dilute acids, etc. Its elementary composition is closely allied to that of the true proteids, but it differs from them by the Digitized by Microsoft® 640 A MANUAL OP VETEBINAIlY PHYSIOLOGY large amount of loosely combined sulphur it contains, 5 per cent. It yields in hydrolysis large amounts of leucine and tyrosine and other substances similar to those thus obtainable from proteids. The various albumins we have spoken of belong to the animal body, but in the vegetable kingdom proteids are found which do not differ in any essential particular from animal proteids. The amount of proteid matter in plants is less than that found in animals, and globulins exist in Fig. 159. — Albumin Crystals from Horse-Sbrom (Gurber). larger amounts than albumins, in fact there are food substances used by animals, oats, maize, peas, etc., in which it is said that the whole of the proteid occurs as globulin and none as albumin. Some of the plant proteid matter crystallizes readily, vitellin for example. It is this substance which has furnished the so-called ' crystallized albumin,' the existence of which has been known for some time. Egg- albumin may be readily crystallized and the serum-albumin of horse's blood is remarkable for the ease with which it may be obtained in a crystalline form (Fig. 159). Both albuminates and proteoses occur in plants, but peptone does not appear to be found in them. Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OP THE BODY 641 The process by which plants form proteids is that of synthesis ; it is possible that such substances as asparagine, leucine, tyrosine, etc., which are met with in the plant are on their way to tissue construction, and are not, as in animals, the result of proteid destruction. A very remarkable fact about proteid substances is that, though they constitute the mainspring of organic life, yet they number amongst them, or amongst their decomposition products, some of the most powerful poisons known. Snake poison is a proteid, and even the albumose formed during the peptic digestion of albumin is highly poisonous if injected into the circulation. The principal tests employed to detect the presence of proteids are as follows : Proteid Reactions. 1. Xanthoproteic Reaction. — Solutions heated with strong nitric acid turn yellow, and on the addition of ammonia or caustic soda are changed to orange. 2. Millon's Reaction. — With Millon's reagent* they give a precipitate which turns red on heating. 3. Piotrowski's Reaction. — To the solution of proteid is added in excess a strong solution of caustic soda, and one or two drops of a weak (1 per cent.) solution of sulphate of copper; this gives a violet colour which deepens in tint on boiling. This test is also used to determine the presence of albumoses and peptones ; the colour reaction given by these is rose-red on the first careful and limited addition of the sulphate of copper, turning to violet at once on the addition of any excess of the copper salt, and is termed the biuret reaction. 4. Adamkiewicz's Reaction. — To a solution of the proteid is added strong sulphuric acid and glacial acetic acid ; a violet colour and slight fluorescence occur. * A mixture of mercurous and mercuric nitrates in presence of nitric acid, 41 Digitized by Microsoft® I 642 A MANUAL OF VETERINARY PHYSIOLOGY 5. Acetic acid and a solution of ferrocyanide of potassiun give a precipitate, except in the case of true peptones an< some forms of albumose. 6. Acetic acid and sulphate of soda give a precipitate oi boiling, except in the case of peptones. 7. Saturation of the solution with neutral ammoniun sulphate precipitates proteids other than peptones. 8. To a neutral or faintly acid solution of proteid abso lute alcohol is added in large excess, and a precipitati obtained. 9. Heating a solution of proteid (albumins and globulins causes a coagulum to form. The solution should be ren dered faintly acid with acetic acid, any excess of acid beinj avoided,' as otherwise no precipitate may be produced. The first three alone of the above reactions suffice t< detect the smallest traces of any proteid in solution. There are many other tests for proteids, mercuric chloride lead acetate, etc., but the above are those which are princi pally employed either to determine their presence, or ti free a solution entirely from proteid. Ferments. — The term ' fermentation ' was originally ap plied to the characteristic phenomena which occur durini the action of yeast in solutions of sugar, whereby the latte is actively and rapidly converted into alcohol, and th agent which gave rise to the phenomena was hence calle the ferment. Pasteur showed that in the case of th alcoholic fermentation of sugar the active agent is th yeast-cell, the process being dependent (as also in putre factions) on the activity of the cell as an organized livin structure. Previously to this soluble substances, such a diastase from malt and pepsin from gastric juice, had bee obtained, and since the conditions under which they worke best, and many of the phenomena attending their actio were closely similar to those holding good in the case ( yeast, they also came to be called ferments. As more an more of these ' substances ' were discovered and the supreme importance ascertained, as the causative agents i the chemical changes of digestion and numberless oth< Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OF THE BODY 643 physiological processes, and, since they were all soluble in water and therefore devoid of any organized structure, they were called the unorganized ferments or enzymes. This division of the ferments into two classes held good, and is even now convenient, until a few years ago, when Buchner showed that a soluble, unorganized substance (zymase) can be extracted from yeast-cells, and is able to produce alcohol by its action on sugar. He obtained similar results with other (bacterial) organisms, so that we cannot now speak of any essential differences in the activities of the living (organized) ferments and the non-living (unorganized) ferments or enzymes. . t/' Ferments are remarkable substances whose mode of action is still a mystery. The outcome of their action is usually hydrolytic — that is to say, they lead to the assump- tion of water by the substance on which they are working, and its decomposition into simpler, more Btable bodies of smaller potential energy. We cannot, however, enter here into the details of ferment action, but must be content to point out the chief characteristics of their activity and properties. 1. They are inactive at sufficiently low tempera- tures, and work best at some given medium temperature, such as 40° to 45° C. Above this temperature the animal enzymes show a gradually diminishing activity, which is finally and irretrievably destroyed at 70 °C, or at once when their solutions are boiled. 2. Their activity is closely dependent on the reaction of the solution in which they work, whether it be acid, alkaline or neutral, as in the case of pepsin, trypsin, and ptyalin. 3. Their action is tem- porarily lessened or even stopped by the presence of an excess of the products of their activity, to begin again when these products are removed. This is well seen in a diastatic conversion of starch into sugar. 4. In some cases their action appears to be reversible; as, for instance, the in- verting enzyme (maltase) of the intestinal juice can, if allowed to act on concentrated solutions of dextrose, recon- vert a part of this into maltose. 5. They are all soluble in water, and conveniently so in glycerin. From these 41—2 Digitized by Microsoft® 644 A MANUAL OF VETERINAEY PHYSIOLOGY solutions they may be precipitated by a sufficiency c absolute alcohol or by saturation with neutral ammoniur sulphate. When purified they resemble proteids in com position and reactions. 6. They are all non-diffusibl through membranes. 7. They are not apparently used u in the changes they produce, and they therefore influenc the velocity of any given conversion, not its total amounl Thus, a trace of enzyme will in time effect the conversioi of an unlimited amount of the substance on which it i working : more of the enzyme merely hastens the rate a which the final result is reached. The enzymes in tissues do not always exist in a free an active state, but as an inactive antecedent to which th term Zymogen has been applied ; a zymogen by appro priate means may be converted into an active enzym (see p. 234). Of the Pigments of the body comparatively little is known though they are widely distributed and perform importan functions. The best known animal pigment is haemoglobin the red colouring matter of the blood; it is of a proteii nature, yet crystallizable, and it also contains iron. I acts as an oxygen carrier, and is often spoken of as a re spiratory pigment ; it has several derivatives (see pp. 8-12) which supply the colouring matter of the bile, urine, am partly that of the fasces. The next pigment widely distributed is the black pigmen of the body or melanin ; it occurs in the skin, hair (p. 274] eye, horn, and is the chief constituent of the melanoti tumours so common in the horse. Both in the faeces and in the dandruff from the skin o the horse chlorophyll is found (p. 283) ; its function in th body is quite unknown. The bile pigments have been sufficiently dealt with o: p. 220. There are several other pigments, but none so importar as the above. Nitrogenous Fats. — Though true fatty substances eontai no nitrogen, yet there are certain complex nitrogenous fai Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OP THE BODY 645 and their derivatives which are found distributed in the body ; the most important of these is Lecithin, which is found in the white blood-corpuscles, the white matter of the brain, nerves, and spinal cord, semen, etc., and also in yolk of egg, where it is united with vitellin. Decomposition products of lecithin are glycero- phosphoric-acid and choline. The latter substance is poisonous, and by oxidation with nitric acid yields the extremely poisonous substance muscarine. Lecithin is largely introduced into the body by means of the food ; the poisonous action of the choline it contains is probably prevented by the substance being broken up by the bacteria of the intestines into carbonic acid, marsh gas, and ammonia. Neurine is a substance closely related to choline, but much more poisonous ; it is the active principle in the poisonous alkaloids produced by putrefactive decomposition of animal matter. Amides and Amido - Acids. — Many of the substances belonging to this series are of considerable importance, and very interesting from the point of view of their probable relationship to the formation of urea in the animal body. Glycine, C 2 H 5 N0 2 (also known as glycocoll and glyco- cine), is amido-acetic acid, CH 2 (NH 2 )C00H. It does not exist in the free state in the body, but in union with benzoic acid, to form hippuric acid (p. 298), and with cholalic acid to form the glycocholic acid of bile (p. 222). It is very soluble in water, the solutions having an acid reaction but sweet taste, and it crystallizes readily. Sarcosine, C 3 H 7 N0 2 , is methyl-glycine, CH 2 NH(CH 3 )C00H. Chemically it closely resembles glycine, and though not found in the body is an interesting substance, owing to its chemical relationship to creatine and its discussional relationship to the question of how urea is formed in the body (p. 295). Taurine, C 2 H 7 NS0 3 , or amido -isethionic acid, is one of the constituents of the bile acid of carnivora, viz., tauro- cholic acid. It is a substance with a neutral reaction and Digitized by Microsoft® 646 A MANUAL OF VETERINAEY PHYSIOLOGY is very stable, even when exposed to a high temperature and boiling dilute acid and alkalis. In the intestinal canal taurine in some animals, as man, is absorbed and reappears in the urine ; in dogs a large part is excreted unaltered ; in herbivora part is excreted and part oxidized, leading to an increase of sulphates in the urine. It is found in small amounts in horseflesh. Creatine, C 4 Ht,N 3 2 . — This is the chief and characteristic ' extractive ' of muscle-substance, in which it is present to the extent of 02 to 0*3 per cent. It is hence present in large amount in ' meat-extracts,' from which it may there- fore most conveniently be prepared, and is easily obtain- able, since it crystallizes readily. When boiled with baryta-water it takes up a molecule of water and splits into sarcosine and urea (p. 295). When heated with mineral acids it loses a molecule of water, and is thereby converted into Creatinine, C 4 H 7 N 3 0. — It is present in urine as a con- stant and characteristic constituent, varying greatly in amount, according to the amount of proteid in the food. Lysatine (or lysatinine), C s H 13 N 3 2 , or C 6 H n N 3 0, is a homologue of either creatine or creatinine. It is an interest- ing substance because it is obtained among the products of decomposition of proteids by means of boiling hydro- chloric acid and zinc chloride, and readily yields urea when it is itself heated with baryta-water. In this way the long-sought-for production of urea from proteids by purely chemical means became for the first time an accom- plished fact. Leucine, C 6 H 18 N0 2 , or amido-caproic acid, is a charac- teristic product of the pancreatic digestion of proteids, and is physiologically interesting, as a probable step, by the changes it undergoes in the liver, in the formation of urea in the body (pp. 231, 293). It may also be obtained in large quantities by boiling horn shavings with sulphuric acid. It crystallizes readily, and in forms so easily re- cognizable and so characteristic that they afford an in- fallible means of determining the presence of leucine in the minutest quanti^F^ ^ bfl@ THE CHEMICAL CONSTITUENTS OF THE BODY 647 Aspartic Acid, C 4 H 7 N0 4) or amido-succinic acid, may be obtained by the decomposition of proteids during pancreatic digestion, or their hydrolysis with acids. It is also found in plants, but forms no part of the animal body. Closely related to this acid is Asparagine, C 4 H 8 N 2 8 , which is principally of interest in the proteid metabolism of plants, though it does not occur in animals. When taken into the body of the carnivora, asparagine is wholly converted into urea; with herbivora it would appear that a part of the nitrogen of the asparagine can take the place of proteid and be stored up. Considering the frequency with which asparagine exists in plants, the conversion of asparagine into proteid is a valuable provision. Fig. 160. — Leucine Crystals (Krukenbbeg). The Urea and the Uric Acid Group. — Urea, or carbamide, is the end product of proteid decomposition, and the chief nitrogenous constituent of the urine. It has the formula (NH 2 ) 2 CO, and is found in minute quantities in some of the tissues of the body, though it is never found in muscle. In a pure state it crystallizes in long needles, but in the form of nitrate it separates out as six-Bided tables arranged in piles (Fig. 68, p. 294), and as oxalate in crystals re- sembling the nitrate, but of prismatic form. Urea is very soluble in water, soluble in alcohol, but insoluble in ether. The crystals have a bitter taste somewhat resembling salt- petre. It may be easily obtained in quantity by con- centrating urine to a syrupy state and extracting this with alcohol. The alcoholic extract then yields urea by slow crystallization. Two modes of the artificial preparation of urea outside Digitized by Microsoft® 648 A MANUAL OF VETERINABY PHYSIOLOGY the body are peculiarly interesting. When dry ammonia and carbon dioxide are brought together they form car- bamic acid, which at once unites with ammonia to form ammonium carbamate, 2NH 8 +C0 2 =NH 4 NH 2 C0 2 . Simple dehydration converts this at once into urea; hence the name carbamide, as being an amide of carbonic acid. Ammonium carbamate readily takes up one molecule of water to become ammonium carbonate. Urea may simi- larly be converted into ammonium carbonate by the assumption of two molecules of water, a change quickly completed by heating it in sealed tubes. The above purely chemical facts are important to the question of how urea is formed in the body (see p. 293). The second interesting synthesis of urea is by the action of ammonium sulphate on potassium cyanate ; this yields ammonium cyanate, NH 4 .CNO, which by mere evapora- tion to dryness is molecularly rearranged into urea, NH 2 .CO.NH 2 . The interest which attaches to this is that it was the first instance (in 1828) of the preparation by purely artificial means of a substance till then known only as a product of the living animal body. When urine is allowed to stand it rapidly becomes highly alkaline, due to the conversion of the urea it contains into ammonia and carbonic acid under the influence of organisms such as the Micrococcus urece. When urea is heated in a dry state for some time to 150° C. it gives off ammonia, and is largely converted into biuret. This substance yields a bright pink colour by the addition of sulphate of copper and caustic soda to its solu- tions. Since peptones (and some albumoses) yield a similar pink colour with the same reagents, it has come to be spoken of as the ' biuret reaction.' Uric Acid has the formula C 6 H 4 N 4 3 . It is the chief nitrogenous constituent of the urine of birds and reptiles, but only occurs in small quantities in the urine of the dog, and is absent from that of the herbivora. It is a crystal- line substance (Fig. 68, p. 297), odourless, tasteless, and extremely insoluble in water, very slightly soluble in ether Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OF THE BODY 649 and alpohol, but readily soluble in caustic potash. Uric acid does not occur free in the urine, but as urates com- bined with bases. Apart from its characteristic crystalline appearance, uric acid is readily recognizable by evaporating a fragment of the suspected substance carefully to dryness on a piece of white porcelain with a few drops of strong nitric acid. If the substance is uric acid, the residue thus obtained may be yellow, but is frequently pink, and is certain to turn to a bright reddish purple under the influence of the fumes of ammonia. This is the wellr known murexid test for uric acid, the colour being due to ammonium purpurate. There is a very close chemical relationship between urea and uric acid, but there is nothing to account for the fact that snakes and birds turn out the nitrogenous end-products of their metabolism as uric acid, while mammals get rid of it as urea. The chemical relationship of uric acid to urea is at once apparent on mere inspection of the consti- tutional formula of the acid — NH— CO I I . CO C— NEL I II >co NH— C— NH/ the groups on the right and left of the formula containing the obvious potentiality of becoming urea. In accordance with this we find that in nearly every possible decomposi- tion of uric acid, whatever other substances are obtained, urea is constantly present among them. We have previously dealt with the probable mode of origin of uric acid in the body, on p. 296, and have there also indicated its relations to the xanthine bodies as allied members of the purin group. Allantoin, C 4 H 6 N 4 3 , is a substance found in the allan- toic fluid, especially that of the calf, and in foetal urine and amniotic fluid. It can be obtained from urine after the administration of uric acid, and from uric acid by oxidation with potassium permanganate. Digitized by Microsoft® 650 A MANUAL OF VETERINAEY PHYSIOLOGY The Aromatic Series.— Many members of this series occur in the urine and some in the digestive canal. Benzoic Acid, C 7 H O 2 , is found principally in the urine of herbivora, and more commonly in stale than in the fresh secretion. In stale urine it is derived from the decom- position of hippuric acid. This acid does not exist free in the urine, but is combined with alkalis. It may be ob- tained as fine glistening needles which give microscopically the appearance presented in Pig. 71, p. 300. This acid is not very soluble in water, but readily dis- solves in alcohol and ether; on heating it sublimes, in which respect it differs considerably from hippuric acid. The source of benzoic acid in the body is discussed on p. 298. Hippuric Acid, C 9 H 9 N0 3 . — This acid exists largely in the urine of the herbivora ; it is formed within the body by the union of benzoic acid with glycine, and may readily be found in fresh urine, though when decomposition occurs it breaks up into its constituents. Hippuric acid is found in the urine united to an alkali, but may be obtained as a crystalline substance (Figs. 69, 70, p. 299). The acid is not very soluble in water, but is readily dissolved by alcohol ; it is insoluble in petroleum ether, a fluid in which benzoic acid is soluble. When heated dry in a small tube it yields a characteristic odour of new hay. The source of this acid in the body is dis- cussed on p. 298. Tyrosine, C 9 H n N0 8 . — This is found in many plants, and also in the intestinal canal as the result of the pancreatic digestion of proteids. It is, in fact, the close companion of leucine in nearly all the decompositions of proteids and other substances. In some ways it is less interesting physiologically than is leucine, since there is no evidence that it is in any way a forerunner of urea in the body, as is so often said to be the case. On the other hand, it is of great interest as indicating the presence or absence of aromatic groups in substances which do or do not yield tyrosine by hydrolysis. Thus gelatin yields no tyrosine by Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OF THE BODY 651 pancreatic digestion ; it lacks therefore the aromatic group in itB molecule, and consequently, unlike its allies the true proteidB, gives no reaction with Millon's reagent (see p. 641). Tyrosine crystallizes in fine needles which are sparingly soluble in water, insoluble in alcohol, but soluble in acids and alkalis. Tyrosine yields a very brilliant reddish pink colour when heated with Millon's reagent, if present even in minute traces, so that its identification is easy. Phenol and Cresol are formed in the animal body during the putrefactive decomposition of proteids, and are excreted by the bowels and urine, in the latter being found as an ethereal salt of sulphuric acid. This phenyl-sulphuric acid is also formed from the aromatic compounds in the food, especially that taken by the herbivora (p. 301). Pyrocatechin is found largely in the urine of the horse and other herbivora, and also after the administration of benzene or phenol. The dark colour of urine on standing, such as is well seen in the horse, is due to the oxidation of pyrocatechin. The source of this substance is from the phenol of the intestinal canal, and it may probably be introduced with certain constituents of the food (p. 301). Indigo Series. — This contains several substances found in the urine and digestive canal. Indol is the substance which gives the odour to faeces. It is present during the decomposition of proteids, and may be readily obtained from an artificial putrefactive pancreatic digestion, the odour of which is due to this substance. Part of the indol leaves the body by the urine as a potassium salt of indoxyl- sulphuric acid, and if this be oxidized it may be made to yield indigo blue; if indigo blue be acted upon by powerful reducing agents it yields indol. Indol administered to animals increases the output of indican, and whatever increases intestinal putrefaction increases the output of this substance; this is the reason why it is found more largely in herbivora than in carni- vora (p. 300). Digitized by Microsoft® 652 A MANUAL OP VETEEINAEY PHYSIOLOGY The presence of indican in the urine of the horse can readily be demonstrated by mixing the urine with an equal volume of hydrochloric acid, and adding a solution of hypo- chlorite of calcium until a blue colour appears. Skatol is a substance closely allied to indol ; it has much the same odour, and if excreted with the urine it passes off as a potassium salt of skatoxyl-sulphuric acid. The Bile Acids. — These have been sufficiently dealt with on p. 222. (e< THE NON-NITROGENOUS BODIES. Fats and Fatty Acids. — The fats met with in the animal body are compounds formed by substituting the radicles of certain acids of the acetic and acrylic series for the hydroxyls — OH — in the triatomic alcohol glycerin. The acids in question are the sixteenth and eighteenth in the acetic series — namely, palmitic and stearic — and the eighteenth of the acrylic series — oleic acid. The fats thus formed are therefore known as palmitin, stearin, and olein, and the mode of their formation is at once made clear by the following typical equation : Palmitic Acid. Glycerin. Palmitin. 3(C 15 H 31 .COOH)+C3H 6 (OH)3 = C 3 H 5 (C 16 H 31 .CO.O)3 + 3H 2 0. A certain proportion of the fats in milk, and hence in butter, are formed, as above, from acids lower down in the acetic series, such as caproie, caprylic, and capric acid. 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 alkalis or superheated steam may be decomposed into its respective fatty acid and glycerin, the process being simply a reversal of the equation given above for the formation of a fat. 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 which is what is known as soap. This decomposition and saponification take place to a greater or less extent in Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OF THE BODY 653 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 more 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 60° C, it is evident that the fluidity of living fat is due to the olein it contains, whose melting-point 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 is discussed in the chapter on the Pancreas (p. 236), whilst the origin of fat in the body and its function is dealt with under Nutrition (p. 324). Butyric Acid is found in the intestines, and in milk, it exists in union with glycerin as a neutral fat, and on the decomposition of this fat gives the odour to rancid butter. It may also be produced by the second stage of lactic fer- mentation in the stomach and alimentary canal, being derived from the carbohydrate matter ingested. Glycerin, which since it is an alcohol should really be known as glycerol, is a viscid, colourless, sweet fluid, soluble in all proportions in water and alcohol, but insoluble in ether. When heated strongly it yields acrolein, a substance which gives the pungent odour to burned fat. Lactic Acid exists in two forms in the body : ethylidene- lactic acid is the chief product of the lactic fermentation of sugars, and is found in the stomach and intestines especially after a diet containing carbohydrate ; sarco-lactic acid occurs in muscles, and is the cause of their acidity after activity. o -, Digitized by M/crasofi® 654 A MANUAL OP VETEEINARY PHYSIOLOGY Cholesterin is a peculiar substance extracted originally from gall-stones. It can be obtained in sparkling crystals whicb are soapy to the touch, and of characteristic micro- scopical shape. Cholesterin is the only alcohol which occurs free in the body ; it is not a fat, though, as a matter of convenience, it is generally dealt with in speaking of fats. Being an alcohol it should be called cholesterol. It is quite insoluble in water and cold alcohol, but readily soluble in solutions of bile salts, in ether and in chloroform. If an equal volume of strong sulphuric acid is added to a solution of cholesterin in chloroform the latter becomes at first blood-red, and then passes through blue and green to become finally yellow. This play of colours is very similar to that observed on the addition of nitrous acid to bile pigments (p. 220). If solid cholesterin be treated with strong sulphuric acid it turns red or violet, the colour changing additionally to blue or green on the addition of dilute solution of iodine. Cholesterin is thus a substance easily recognizable when present in even minute amounts. Cholesterin is found in the nervous system, and is especially common in the pia mater of the cerebellum and plexus choroidea of the horse, where it may give rise to tumours, the nature of the growth being readily recognized from its silvery fish-scale-like appearance. It is also found in lanoline or wool fat and in dandruff, where it replaces the glycerin in the fat. 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) may be found in the liver and other organs, minute amounts of sugar may be found in the blood, and a sugar exists in milk ; but very much less carbohydrate is recoverable from the body than enters it as food, for the Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OP THE BODY 655 reason that the bulk of it becomes converted into fat (p. 325) or is rapidly oxidized to carbonic acid and water as a source of heat and energy to the body (p. 323). The carbohydrates may be divided into the starch group or polysaccharides, the cane-sugar group or disaccharides, and the dextrose group or monosaccharides. Polysaccharides, (C 6 H 10 O 5 )n. Starch. — The formula for starch is unknown ; it is con- sidered to be (C 6 H 10 O 5 )ra, where n is not less than 5 or 6, and is probably very much larger. Starch exists in plants in the form of grains, the shape of which depends upon the group from which it is 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, and an interior called granulose. The granulose 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 trans- parent, and filters readily, while previously this was impos- sible. To this limpid fluid the term 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 Digitized by Microsoft® 656 A MANUAL OF' VETEEINAEY PHYSIOLOGY and finally of sugar. There are probably several dextrins, though two are generally more particularly described, viz., erythro-dextrin and achroo- dextrin. These are distin- guished 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 in a more perfect and complete form within the body. 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 in the economy have been previously discussed (see p. 225). 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 (p. 229). 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 masti- cation, 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 question which has given Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OF THE BODY 657 rise to great discussion, inasmuch as no animal is known to secrete a cellulose enzyme, although many, such as the herbivora, are known to digest cellulose. Bunge states that sheep are capable of digesting 30 to 40 per cent, of the cellulose of Bawdust and paper when mixed with hay. The two views most generally held at the present time with reference to the digestion of cellulose are that it is either due to putrefactive organism s or to a specific enzyme. Cellulose may be digested outside the body under the influence of putrefactive organisms, with the evolution of marsh gas and carbonic acid. Every condition necessary for this change exists within the body in most efficient form ; for example, in the rumen of the ox, and the large intestines of the horse ; but it would appear to be more than probable that a cellulose-dissolving enzyme exists. Young cellulose is more easily digested than old ; it is certain that the older parts of the plant are converted into lignin, and this to the majority of animals must be in- soluble. Cellulose when treated with strong sulphuric acid is converted into a dextrin-like product, and is finally converted into dextrose. Disaccharides, C 12 H 22 O n . Saccharose, or cane-sugar, is not found as part of the animal body, but exists largely in plants, and forms a well- known supply of carbohydrate 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 sonverted into equal parts of dextrose and levulose, and the same change may be effected by enzymes in the stomach md small intestines. This conversion of cane-sugar is recognised by the changed action of the solution on jolarized light, the rotation of the plane of polarization )eing now left-handed instead of right-handed as it was 42 Digitized by Microsoft® 658 A MANUAL OF VETERINAEY PHYSIOLOGY previously to the conversion, that is to say inverted ; hence the name invert sugar. If cane-sugar be injected into the circulation it passes out unaltered ; it is certain that before this sugar can be assimilated it must be converted into dextrose. 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 or glycogen. In its reactions it corresponds closely to dextrose, but it has a one-third less reducing action upon Fehling's solution, and it does not reduce Barfoed's reagent,* which dextrose is capable of doing. Its specific activity in rotating the plane of polarized light is considerably greater than that of dex- trose, being about + 140° as against + 52° for dextrose. If 5 c.c. of a \ per cent, solution of maltose is warmed for half an hour on a water-bath together with 1 decigramme of phenyl-hydrazine hydrochloride and 2 decigrammes of sodium acetate, a yellow compound is obtained in charac- teristically shaped crystals. These are phenyl-maltosazone. C 24 H 82 N 4 9 . When heated the crystals melt at 206° O, 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 is, like cane-sugar, non-assimilable, for ii injected into the circulation it is excreted unchanged, and it is probable that before absorption it has to be converted into dextrose. Lactose, or milk-sugar, is found solely in milk. Ii reduces Pehling'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 witl yeast. If boiled with dilute mineral acids it is converte< into equal parts of dextrose and galactose. Lactose readily undergoes lactic fermentation, as, fo: instance, in souring milk. The cause of this is a micro organism; but there are reasons for believing that ai enzyme may also bring it about. * A solution of cupric acetate to which aoetic acid is added. Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OP THE BODY 659 In spite of the fact noted above, 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 saccharose 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-lacto- sazone, which crystallizes in characteristic rounded clumps of yellow crystals. These crystals melt at 200° C, and are soluble in 80 to 90 parts of boiling water. Monosaccharides, C 6 H 12 6 . When the members of the preceding group of sugars, the disaccharides, are boiled with dilute acids or otherwise hydrolyzed, they take up a molecule of water and split into two molecules of a new sugar. Thus cane-sugar yields dextrose and levulose, maltose gives two molecules of dextrose, and lactose yields dextrose and galactose. Of these the most important is : 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 differ- ences in behaviour. Its specific rotatory power is only +52°. It reduces Barfoed's reagent (see Maltose). The osazone it forms, phenyl-glucosazone, crystallizes in fine yellow needles ; these melt at 205° C. and, unlike the corresponding com- pound of maltose, are almost insoluble in water. 42—2 Digitized by Microsoft® 660 A MANUAL OF VETEEINAEY PHYSIOLOGY Dextrose is capable of undergoing three fermentations, viz., alcoholic, lactic, and butyric ; the two latter are probably always present in the intestinal canals of animals, especially after a carbohydrate diet. Levulose. — 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 levulose. Inosite, C 6 H 12 6 . (CH.OH) 6 . This is a crystallizable substance, found among the ' extractives ' of many tissues, usually in very minute quantities, though it is markedly present in heart-muscle and in horse-flesh, which may contain as much as "003 per cent. It occurs also in semen. Inosite is found abundantly in vegetable tissues, especially in unripe beans, which thus provide a convenient source for its preparation. Possessed of a sweet taste, and as being originally found in muscles, inosite has at times been called ' muscle-sugar.' But although its empirical formula is the same as that of a monosaccharide, it is not a sugar at all : its solutions exert no rotatory power on polarized light, do not reduce metallic salts, and form no osazone with phenyl-hydrazine, nor are they capable of undergoing alcoholic fermentation. It is in fact a member of the benzene series, and consists of a closed ring of six CH.OH groups. The sugars of chief physiological importance are, as we have seen, the hexoses, that is to say a sugar such as dextrose which contains six atoms of carbon in the molecule, or the disaccharides which contain twelve. But the recent progress of organic chemistry has led to the synthesis not only of the sugars which are ordinarily met with, but of a series of artificial sugars containing three (trioses), four, five (pentoses), seven, eight, and nine atoms of carbon in their molecule. Of these the pentoses alone at present possess any physiological interest. This is due to the fact Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OF THE BODY 661 that a pentose may be obtained by the decomposition of the nucleo-proteids of the pancreas and of yeast-cells. These pentoses are not assimilable, as shown by their rapid appearance in the urine after their introduction into the body. Pentose yields an osazone which melts at 160° C. Tests for Sugar. 1. Trommer's. — An excess of caustic potash and a small amount of dilute solution of copper sulphate is 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 potassium. This is made to contain such an amount of the cupric oxide in each cubic centimetre as is exactly reduced, and the blue colour destroyed, by 0*005 gramme of dextrose. 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 becomes grey and then black, from the deposi- tion of metallic bismuth. 4. Picric Acid Test— Boil the solution of sugar with a little picric acid and caustic soda in small quantities ; a brown-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 respective sugars. They are very important for the dis- crimination of the various sugars, as well as for their identification. Digitized by Microsoft® 662 A MANUAL OP VETERINARY PHYSIOLOGY Inorganic Constituents. The inorganic substances found in the body are water, gases, and salts. Water forms about 60 per cent, of the whole body; it is taken in with the food and drink, and a small quantity may be formed within the system. The amount of water consumed by animals depends upon the nature of their food and the class of animal. Horses fed on dry food consume more water than cattle, the food of which contains as a rule a considerable amount of water. An excess of water leads to body waste by carrying off the solids through the kidneys, whilst reduction in the amount of water produces thirst and loss of nutrition. The Gases found in the body are oxygen, nitrogen, hydrogen, carbonic acid, sulphuretted hydrogen and marsh gas. The two former are taken in with the inspired air, carbonic acid is formed in the tissues, while hydrogen and its compounds are formed in the intestinal canal as the result of cellulose and other decompositions. The largest portion of the inorganic matter consists of the various Salts of sodium, potassium, calcium, magnesium, and iron, in the form of chlorides, sulphates, phosphates, and carbonates. The distribution of these salts throughout the tissues is very variable ; some, like bone, are excessively rich, whilst others are remarkably poor in them. Certain tissues and fluids have a preponderance of some salts to the exclusion of others. The amount of the salts existing in the body depends upon the age of the animal, and their nature is modified by the character of the food. The daily quantity ingested and stored up is largely affected by the rate of growth,. young growing animals storing up material which the adult rejects. The diet of the herbivora furnishes considerably more potassium than sodium salts to trje system, with the result Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OE THE BODY 663 that in the excretions salts of potassium are in excess over those of sodium. Sodium and Potassium.— Owing to the poorness of vegetable food in sodium salts, Bunge believes that the administration of common salt with the food of herbivora is a necessity. As this view is open to question his argu- ments in the matter should be known. Bunge says that in spite of the many inorganic salts found in the food, one only, viz., sodium chloride, is taken 'separately by the human subject in addition to that already existing in the food. But carnivora avoid salted food, as sufficient sodium chloride exists in the blood and tissues in the raw state in which these are consumed by them. Herbivora, on the other hand, have been known to travel considerable distances to obtain salt. According to Bunge the explanation of the desire shown by herbivora for common salt lies in the large amount of potassium consumed in their diet, the effect of potassium salts in the blood being to withdraw sodium salts from the system. Here are some tables given by him to show the pro- portion potassium bears to sodium in various articles of diet. In every 1,000 parts of dried material : Potassium. Sodi/um Bice 1 03 Bullock's blood 2 19 Oats "J Wheat 1 5 to 6 •1 to -4 Rye f Barley J Dog's milk ... 5 to 6 2 to 3 Peas 12 •2 MDk of herbivora . . . 9 to 17 1 to 10 Hay 6 to 18 •3 to 1-5 Beef 19 3 Beans 21 ■1 Clover Digit 23 zed by Microsoft® •1 664 A MANUAL OP VETERINAEY PHYSIOLOGY For one equivalent of sodium the equivalents of potassium are Equivalent K 2 0. Mangel-wurzel . Milk of herbivora Beef Wheat Barley Oats Eice Eye Hay Peas Clover Beans 2 ■8 to 6 4 12 to 23 14 to 21 15 to 21 24 9 to 57 3 to 57 44 to 50 90 110 The preponderance of potassium over sodium salts is here most marked, and Bunge considers that when a relation of from 4 to 6 equivalents of potassium to 1 equivalent of sodium is obtained in a diet no addition of sodium chloride is necessary ; but where the proportion of potassium is higher than this the animal instinctively seeks for sodium, for the reason previously given. We do not deny the stimulant to the palate which common salt may afford the herbivora, but so far as horses are concerned, and we think the same argument must apply to cattle, it is quite certain that no addition of common salt to the ordinary diet is necessary, and that the food furnishes ample sodium for the purposes of the body. Calcium forms the largest mineral deposit in the body ; it is taken in by means of the food. Bunge states that it is probable that the lime salts required for the growth of bone in young animals are contained in some organic compound, and that the administration of inorganic com- pounds of lime in rickets is irrational and useless. Lime exists largely in clover and hay, but only in small quantities in the cereal grains ; it is principally in the hay that the amount excreted by horses through the kidneys is supplied. In the urine it passes from the body in such quantities that it cannot be held in solution by the alkaline Digitized by Microsoft® THE CHEMICAL CONSTITUENTS OF THE BODY 665 fluid, and the urine of the horse is therefore always turbid. In the body calcium exists in the form of phosphate, sulphate, and carbonate, in the urine principally as carbonate and oxalate. Magnesium salts occur in the body principally as phos- phates, and in this form they enter largely into certain foods, such as oats. The amount of magnesium passing away from horses through the kidneys is small, but con- siderable quantities derived from the food pass out with the faeces, as they cannot be utilized in the body. By collecting in the bowels this salt produces the ammonio- magnesium phosphate calculi so common in horses. Phosphates are united with soda, potash, lime, and magnesia. They are principally taken in with the food» but may also be formed in the body from the metabolism of phosphorus-containing compounds. The foods richest in phosphoric acid are oilcake and bran, while hay and straw are poorest in this substance. Phosphoric acid is principally excreted by herbivora with the fasces, only small quantities passing away with the urine. Carbonates are found in several of the secretions of the body, notably in the urine, where they cause the most intense evolution of gas on the addition of an acid. The carbonates in the system of the herbivora result from the carbonates of the food, and the combustion of organic acids, malic, citric, tartaric, etc. ; these enter the body as salts of sodium and potassium, and the bases being set free unite with carbonic acid to form carbonates. The Sulphur in the body is derived from the albumin of the food ; in the system it is converted into sulphuric acid, and in this form 80 per cent, of the ingested sulphur appears in the urine. Sulphur exists in horn, hair, and epidermis. The importance of the sulphates in the urine is consider- able, as they afford a passage out of the body for the products of proteid decomposition. Phenol and allied compounds are in this way got rid of in the form of phenyl- sulphate of potassium. Digitized by Microsoft® 666 A MANUAL OF VETERINAEY PHYSIOLOGY Iron is an important constituent of the complicated substance haemoglobin. It is also found in the hair, skin, bile, lymph, most body fluids and tissues, and a small quantity in the fasces. Bunge considers that the iron which enters the system can only be absorbed when in the form of an organic compound. Inorganic iron, though largely used in the treatment of certain diseases, is not absorbed from the intestinal canal; food contains only organic and not inorganic iron, and the basmoglobin of the blood is formed from the complex organic compounds of iron which are produced by the vital process of the plant. Digitized by Microsoft® INDEX Abdominal muscles in respiration, 89 Abducens cranial nerve. 428 Aberration, chromatic, 483 spherical, 483 Abomasum, 175 Abscess of liver, 241 Absorption, 242 bands of haemoglobin, 9 of CO hsemoglobin, 10 by the csecum, 260 by the cellular tissue, 259 by the conjunctiva, 258 by the peritoneal cavity, 258 of strychnine, 258 of potassium iodide, 259 by the pleural cavity, 258 by' the respiratory passages, 257 by the skin, 258 by the stomach, 176, 259 by the vagina, 258 of acetic acid, 257 of alcohol, 257 of carbohydrates, 263 of chloroform, 257 of ether, 257 of fat, 261 of gases in liquids, 97 of hydrocyanic acid, 259 of liquid ammonia, 258 of morphia, 257 of pilocarpin, 257 ofproteid, 263 of potassium ferrocyanide, 257 of physostigmin, 257 of salts, 263 of turpentine, 258 of water, 257, 263 from the cellular tissue, 259 the stomach, 176 ansesthetics by rectum, 251 anthrax and conjunctiva, 258 atropin, from conjunctiva, 258 capillary, lymph, 243 chyle, 253, 256 chyme, 256 Absorption, chyme, Colin's observa- tion, 256 conjunctiva, 258 cow, lymph from, 246 curare and conjunctiva, 258 dialysis, 248 diffusion in lymph formation, 247 dog, lymph from, 247 stomach, 259 emulsification of fat, 261 ether, by air-passages, 257 per rectum, 261 fibrinogen in lymph, 245 filtration in lymph formation, 247 glands, lymphatic, 244 Heidenhain, lymph production theory, 250 herbivora, stomach in, 259 horse, 257 legs, oedema of, 251 lymph from, 247 in general, 257 intestinal, 259 lacteal vessel, 254 lymph, 242, 245 capillary, 243 Colin on, 246 formation of, 247 movement of, 251 Colin on, 252 Weiss on, 253 plasma, 246 production, Heidenhain on, 250 physical theory, 247 secretory theory, 250 Starling on, 249 sinus, 245 spaces, 242 quantity of, 246 lymphagogues, 250 lymphatic glands, 244 vessels, 243 667 Digitized by Microsoft® 668 A MANUAL OF VETEEINAEY PHYSIOLOGY Absorption of methylene blue from pleura, 259 nux vomica by air-passages, 257 in caecum, 260 oedema, production of, 251 osmosis in lymph formation, 247 osmotic pressure, 248 ox, lymph from, 247 paraglobulin in lymph, 245 peptonuria, 263 Peyer's patches, 255 potassium ferrocyanide, absorp- tion of, by air-pas- sages, 257 from bowel, 259 from cellular tissue, 258 from peritoneum, 259 from skin, 258 iodide, 259 serous cavities, 243 serum albumin in lymph, 245 sheep, lymph from, 247 solitary follicles, 255 Starling and Tubby on, 259 strychnine, by pleura, 259 by peritoneum, 259 thoracic duct, 243 villi, the, 253 Accelerator centre in medulla, 49 Accommodation, eye, 466 Helmholtz on, 467 Acid, acetic, absorption of, by air- passages, 258 in digestion, 207, 209 in stomach, 162 amido-acetic, 645 -caproic, 646 -isethionic, 645 -succinic, 647 aspartic, 647 pancreas, 236 urine, 293 benzoic, 650 Liebig on, 298 liver, 222 urine, 292, 298 butyric, 209, 653 in stomach, 162 capric, 652 caproic, 652 ' caprylic, 652 cholalic, 221 ethylidene-lactic, 653 formic, 209 glutaminic, 236 glycero-phosphoric, 645 glycocholic, 222 glycuronic, 301, 311 hippuric, 222, 292, 298, 302, 650 Acid, hydrochloric, 161 hydrocyanic, 259 hyoglycocholic, 221 hyotaurocholic, 221 indoxyl-sulphuric, 651 lactic, 161, 175, 209, 653 malic, 209 oleic, 652 oxalic, 301, 302 palmitic, 652 phenyl-proprionic, 199 phosphate of soda, 305 phosphoric, 210, 303 sarco-lactic, 119, 374, 378, 379, 653 skatoxyl-sulphuric, 652 stearic, 652 sulphuric, 231, 300, 302, 303 succinic, 209 tannic, 210 taurocholic, 645 uric, 292, 295, 379 Acids, biliary, 212 fatty, 212 of stomach, 161 Aehroo-dextrin, 656 Acrolein, 653 Acromegaly in man, 270 Adamkiewicz's reaction, 641 Addison's disease in man, 269 Adenine, 296 Adrenalin, 270 Adrenals, 269 Afferent nerves, 382 paths in the cord, 405 African ' horse-sickness, ' 27 Aids to the circulation, 74 Air, alveolar, 116 amount of, required, 113 atmospheric, 95 composition of, 95 moisture in, 95 complemental, 113 entrance of, into veins, 65 reserve, 113 residual, 113 tidal, 113 Albumin in milk, 620 Albuminates, 636 Albuminoids, 638 Albumins, derived, 636 native, 636 Albumose, pancreas, 236 Albumoses, 637 Alcohol, absorption of, by air-pas- sages, 257 Allantoin, 608, 649 Allantois, 596, 606 Alveolar air, 116 Amble of horse, 526 Digitized by Microsoft® INDEX 669 Amides, 615 Amido-acetic acid, 645 Amido-acids, digestion, 187, 199 -bodies in urine, 293 -oaproie acid, 646 -glucose, 638 -isethionio acid, 645 -succinic acid, 647 Ammonia, liquid, absorption by air- passages, 258 in urine, 304 salts in urine, 297 Ammonio - magnesium phosphate, digestion, 210 Ammonium carbamate in urine, 293 carbonate in urine, 304 sulphate, action on plasma, 3 Amoeboid movements of white cor- puscles, 13 Amount of air required, 113 respired (Boussingault), 116 of food required, 332 of heat produced, 350 Amnion, 601, 605 Amphoteric milk, 619 Amylolytic action, 143 Amylopsin (pancreas), 234, 236 Anacrotic limb of sphygmogram, 69 Anesthetics per rectum, 261 Analyses of blood, 4 Anelectrotonus (nerve), 387 Angle, visual, 485 Animal heat, 336 amount produced, 350 body temperature, 337 cattle, temperature of, 339 chloroform narcosis and heat production, 342 clipping, effect on temperature (Siedamgrotzky), 349 Colin on blood temperature, 338 on heat loss, 351 colour effect of, 345 conduction, 343 corpus striatum (heat puncture), 342 curare and heat production, 342 Despretz on heat loss, 351 Dieckerhoff on temperature, 339 dog, amount of heat produced, 351 loss of heat, 351 temperature of, 339 dormouse, hibernation of, 350 enzymes, intracellular, 336 evaporation, 343 fatigue fever, 341 grey horses and loss by heat, 346 Animal heat, loss of, 343 production, 340 puncture, 342 regulation, 343 hibernating animals, 338 hibernation, 349 homoithermal animals, 337 horse, amount of heat produced, 351 temperature of, 338 influence of heat and cold, 346 of nervous system on heat production, 341 intracellular enzymes, 336 marmot, hibernation, 350 negroes' akin, 346 nervous system, influence of, on heat production, 341 normal temperature of animals, 338 ox heat, loss by, 351 oxidases, 337 oxidations in animal heat, 336 peroxidases, 337 pig heat, lost by, 351 poikilothermal animal, 337 post-mortem rises of tempera- ture, 351 radiation, 343 Siedamgrotzky on effect of clip- ping, 349 on temperature, 338 sheep, amount of heat pro- duced, 351 temperature of, 339 swine, temperature of, 339 temperature, Colin on, 338 normal, of animals, 338 variations in body temperature, 339 varnishing the skin, 284, 346 wet, effect of, 347 Wolff on heat loss, 351 Wooldridge, on temperature, 339 Ancestrous period, 577 Anoestrum, 577 Ano-spinal centre in cord, 424 Anthrax and conjunctiva, 258 Antibody, 26 Anti concussion mechanisms in limbs, 518 in foot, 568 Antipepsin, 178 Antiperistalsis, 203 Aorta, pressure in, 63 Aortic valye, 29 Apex beat, non-existent, 39 Apncea, 107 Apomorphia, 180 Apoplexy of lungs, horse, 120 Digitized by Microsoft® 670 A MANUAL OF VETEEINAEY PHYSIOLOGY Aqueous humour, 455 Arginine, 318 pancreas, 236 Argon, 95 Arloing on sweating, 281 on sympathetic nerves, 448 Aromatic series, the, 650 Arrangement of food in the stomach, 157 of the spinal cord, 392 Arterial blood, 20 system, 55 'tone,' 75 Arteries, 55 contractility of, 56 elastic property of, 55 structure of, 56 pathological conditions, 83 Artery, anterior mesenteric, parasites in, 83 Artificial insemination, 591 Arytenoid cartilages, 94 Arytenoideus muscle, 122 Ascaris megalocephala, 589 Ascending tracts (spinal cord), 402 Ash, faeces, composition of, 210 milk, composition of, 621 muscle, composition of, 379 Asparagine, 647 Aspartic acid, 236, 647 Asphyxia, 106 Ass, amount of air required, 116 analysis of milk, 620 braying of, 129 period of gestation, 614 psj chical powers, 449 subepiglottic sinus, 129 Assheton on development of embryo of sheep, 594 on impregnation in sheep, 593 on uterine glands, 614 Astigmatism, 457 horse, 469 Astragalus, screw action of, 508 Atmospheric air, composition of, 95 Atropin, absorption from conjunc- tiva, 258 effects on eat, 468 on dog, 468 on horse, 468 on iris, 459 in secretion of saliva, 146 in sweating, 279 Auditory sensations, 501 Auriculo- ventricular valves, 33 Automatic action, 423 Axis-cylinder (nerve), 383 < Axone (nerve), 412 Azoturia, 10, 321 Bacteriolysis, 26 Balfour on polar bodies, 590 Barfoed's reagent, 658 Bars (hoof), 548 use of, 566 Basophile leucocytes, 13 Bayliss and Starling on pancreatic secretion, 233 on peristalsis, 203 Bear, generation, 578 Bell's experiment, horse, 427 Bellini, duct of (kidney), 289 Bellowing of ox, 129 Beneden, Van, on polar bodies, 589 Benzoic acid, 222, 650 Berlin, eye measurements, 480 Bernard, Claude, on division of facial, 429 on glycogen, 225 Bezoar stones, 210 Bicuspid valve, 33 Bile acids, origin of, 222 Pettenkofer's test, 222 amount of, secreted, 223 Ellenberger on, 219 Gmelin's test, 220 Hofmeistcr on, 225 horse, 218 percentage composition, 219 pigments, 220 ox, 218 ... salts, 221 sheep, 218 use of, 224 Voit's experiments, 225 Biliary calculi, 241 Bilirubin, 11, 12, 220 Hammarsten on, 221 Biliverdin, 220 Binocular vision, 473 Bischoff and Voit on urine of dog, 311 Biuret reaction, 641 Bladder, urinary, 311 Blaine on old age in horses, 629 Blastocyst, 593 Blastoderm, bilaminar, 599 Bleating of sheep, 129 Blind spot, retina, 463 Blood, 1 absorption bands of CO haemo- globin, 10 of haemoglobin, 9 African 'horse-sickness,' 27 ammonium sulphate, action on plasma, 3 amoeboid movements of white corpuscles, 13 amount of, in living animal, 22 analyses of, 4 antibody, 26 Digitized by Microsoft® INDEX 671 Blood, arterial, 20 azoturia of horse, 10 bacteriolysis, 26 basophile leucocytes, 13 bilirubin, 11, 12, 220 Bolir on C0 2 in, 25 on haemoglobin, 9 'buffy coat,' 2, 15 Bunge's table of salts in, 21 butyric acid, odour from, 2 calcium phosphate in, 21 salts, effect of, on milk- curdling, 20 on coagulation, 19 camel tribe, corpuscles of, 4 carbon dioxide in, 23, 21 carboxy-haemoglobin, 10 cause of coagulation, 17 cholesterin in, 5, 20 in leucocytes, 14 in red corpuscles, 5 clot, composition of, 16 coagulation of, 15 average time of, 16 cause of, 17 circumstances affecting, 18, 20 effect of salts of alkalis, 19 of acetic acid dilut., 18 of ammonium salts, 19 of calcium salts, 19 of citric acid, 19 of C0 2 , 19 of cold, 19 of leech extract, 20 of lime salts, 19 of potassium oxalate, 19 of peptone, 20 Hammarsten's theory, 17 living test-tube experiment, 19 colour of, 1 of venous, 20 composition of, 2, 25 corpuscles of, 4 colourless, 12 amceboid movements of, 13 origin of, 14 varieties of, 12, 13 red, 4, 5 crassamentum, 15 creatin in, 20 defensive mechanisms of the body, 25 defibrinated, 16 diapedesis of white corpuscles, 13 disease in, 26 distribution of, in the body, 23 dog, 2, 21 25 of Blood, dog, time of clotting, 16 Ellenberger's calculations, 6, 8 eosinophile cells, 13 extractives of, 20 fats in, 20 fibrin in, 4, 17 ferment, 3, 18 fibrinogen in liquor sanguinis, 3 precipitation of, 3 fibrino-globulin, 3 foot-and-mouth disease, 27 functions of, 1 gases of, 23 globin from, 11 globulicidal action of serum, glycogen in leucocytes, 14 grape-sugar in, 20 Grower's method for number corpuscles, 5 hsematin, 11 spectrum of, 11 hsematogen, 8 hasmatoidin, 11 haematoporphyrin, 11 haemin, 11 hsemochromogen, 11 haemoglobin, 1, 8 absorption bands of, 9 amount of, in horse's body, 8 carboxy-, 10 compounds of, 10 decomposition of, 11 Ellenberger's estimate, 8 in corpuscle, 5 meth-, 10 Muller's estimate, 8 nitric oxide, 11 oxy-, 7 reduced, 7 spectroscope test, 9 hemoglobinuria, 27 haemolysis, 25 Haldane and Lorraine Smith's process, 22 Hammarsten's theory of coagula- tion, 17 heated with sulphuric acid, 2 hippuric acid in, 20 horse, 2, 21 amount of, in, 23 amount of haemoglobin in, 8 specific gravity of, 2 azoturia of. 10 clotting of, in, 15 relation between, and body- weight, 22 time of clotting, 16 hyaline leucocytes, 12 hydrobilirubin from, 11, 12 in disease, 26 Digitized by Microsoft® 672 A MANUAL OF VETEEINAKY PHYSIOLOGY Blood, iron in, 21 ' laky,' 6 lecithin in, 20 in leucocytes, 14 in red corpuscles, 5 leech extract in coagulation, 20 letting, 27 leucocytes, 12 cholesterin in, 14 composition of, 13 diapedesis of, 13 glycogen in, 14 lecithin in, 14 origin of, 14 proportion of, 12 varieties of, 12 basophile, 13 eosinophile, 13 hyaline, 12 lymphocytes. 13 polynuclear, 12 liquor sanguinis, 2 fibrinogen in, 3 paraglobulin in, 3 serum albumin in, 3 serum globulin in, 3 ' living test-tube ' experiment, 19 lymph cell distinguished from white corpuscle, 12 lymphocytes, 13 magnesium phosphate in, 21 sulphate, actionon plasma, 3 malaria parasite, 27 Malassez's method for number of corpuscles, 5 mechanisms, defensive, of the body, 25 methaemoglobin, 10, 27 Metschnikoff, researches of 14 Miiller's estimate of hfemo- globin, 8 muscular work, effect upon blood, 2 Nasse, on time of coagulation, 16 nitric oxide haemoglobin, 11 nitrogen in, 23, 25 nucleo-proteid, 3 number of corpuscles in, Gower's method, 5 Malassez's method, 5 odour of, 2 olein in, 20 opsonins, 26 ox, 2, 21 relation between blood and body-weight, 22 specific gravity, 2 time of clotting, 16 oxygen in, 23, 24 oxy-hsemoglobin, 7, 10 Blood, palmitin in, 20 paraglobulin in liquor sangui- nis, 3 peptone, effect on coagulation, 20 phagocytosis, 14, 26 phosphate of calcium in, 21 physical characters of, 1 physiological salt solution, 6 Pig, 21 relation between, and body- weight, 22 specific gravity of, 2 time of clotting, 16 plasma, 2 separation of proteids from, 3 platelets, 7, 18 pleuritic effusion, 3 polynuclear colourless corpuscles, 12 potassium chloride in, 21 precipitin, 26 process for calculation of amount of, 22 proportion of, to body-weight, 22 proteids in plasma, 3 of serum, 3 pro-thrombin, 18 purpura, 27 quantity of, in the body, 22 rabies, 27 reaction of, 1 red cells, absorbing surface of, 6 production of, 7 seat of formation of, 7 red corpuscles of, 4 reduced haemoglobin, 7 rinderpest, 27 salts of, 21 serum, 3 albumin, in liquor san- guinis, 3 precipitation of, 3 globulin in liquor sangui- nis, 3 globulicidal action of, 25 proteids of, 3 relation between, and body-weight, 22 specific gravity of, 2 time of clotting, 16 soaps in , 20 sodium bicarbonate in, 1 carbonate in, 21 chloride in, 2, 21, 22 action on plasma, 3 phosphate in, 1, 21 specific gravity of dog, 2 of horse, 2 of ox, 2 Digitized by Microsoft® INDEX 673 Blood, specific gravity, of pig, 2 of sheep, 2 spectroscope test, haemoglobin, 9 spectrum of, hsemoglobin, 9 of oxy-haemoglobin, . 9 of CO hsemoglobin, 10 of hsematin, 11 splenic artery, leucocytes in, 12 vein, leucocytes in, 12 staining of leucocytes, 13 stearin in, 20 Stokes's fluid, 9 sugar in, 20, 226 supply, foot, 545 SussdorPs proportion of, to body- weight, 22 taste of, 2 temperature of, 22 raised experimentally, 71 Texas fever, organism in, 27 time occupied in clotting, 16 thrombin in, 18 transfusion, solution used for, 6 trypanosomes, 27 urea in, 20 uric acid in, 20 urobilin, 11 • venous, 20 whipped, 16 white corpuscles of, 12 Blood-pressure. See Bloodvessels Bloodvessels, 55 aids to the circulation, 74 air, entrance of, into veins, 65 anacrotic limb of sphygmogram, 69 aorta, pressure in, 63 arterial system, 55 ' tone,' 75 arteries, contractility of, 56 elastic properties of, 55 pathological conditions of, 83 structure of, 56 artery, anterior mesenteric, 83 blood-pressure, 43, 62 brachial vein, 64 capillaries, 64 crural vein, 64 dog, 63 effect of respiratory move- ments on, 63, 91 facial vein, 64 horse, 63 jugular vein, 64 rabbit, 63 sheep, 63 veins, 64 Volkmann's table, 63 blood, temperature of, raised experimentally, 71 Bloodvessels, blood, velocity of, in horse, 72 brain, exposed, pulsation in, 81 camel, pulse-rate of, 70 capillaries, 56, 64 carotid artery, horse, velocity in, 72 cat, temperature of, blood raised experimentally, 71 catacrotic limb of sphygmogram. 69 waves of sphygmogram, 69 Chauveau's observations on ve- locity in, 72 chorda tympani nerves, 76, 78 circle of Willis, 81 circulation, aids to, 74 arterial tone, 76 duration of the, 73 Hering's observations on time, 74 influence of the nervous system, 74 in the living tissues, 65 mean pressure, 60 mechanics of, 58 mesentery of mammal, 65 peculiarities in, 81 peripheral resistance, 58 pulse, 58 rete mirabile, 81 sphygmogram, 68 time of, in dog, horse, rabbit, 74 vaso-dilator nerves, 76 vaso-motor centre, 75 subcentres, 75 velocity of current, 68 of pulse-wave, 68 venous arrangements of brain, 82 "Volkmann's estimate of area, 72 corpuscles, white, migration of, 66 dicrotic wave of sphygmogram, 69 dilator nerves, 78 dog, division of spinal cord in, 74 pulse-rate of, 70 time of circuit of circula- tion, 74 vena cava, anterior, negative pressure in, 64 elasticity of arteries, 56 elephant, pulse-rate of, 70 frog's foot, circulation in, 65 haemorrhage, 65 thirst of, 65 horse, blood-pressure in, 63 43 Digitized by Microsoft® 674 A MANUAL OF VETEEINAEY PHYSIOLOGY Bloodvessels, horse, pulsation in jugulars, 57 pulse in, 57 pulse-rate of, 70 sphygmogram of, 69 time of circulation circuit, 74 inflammation, essential changes in, 66 jugular vein, velocity of blood in, 72 mesentery, circulation in, 65 metatarsal artery, velocity of blood in, 72 migration of white corpuscles, 66 negative pressure in veins, 64 nerves, brachial, 78 chorda tympani, 76, 78 dilator, 78 governing, 77 vaso-constrictor, 77 nervi erigentes, 78 nervous system, influence on circulation, 74 ox, pulse-rate of, 70 pathological conditions, 83 pig, pulse-rate of, 70 pulse, cause of, 66 character of, 83 explanation of, 58 hard, 83 large, 83 quick, 83 slow, 83 small, 83 solt, 83 pathological considerations, 83 pathologist's ' tension,' 70 physiologist's 'pressure,' 70 pulse-rate and blood-pressure, relation, 71 variations in, 71 tension of, 69 venous, 67 wave, 67 pulse- wave, graphic study of, 68 length of, 67 sphygmogram, 68 velocity of, 67 rabbit, division of cervical sym- pathetic, 75 time occupied by circuit of circulation, 74 rete mirabile, 81 sciatic nerves, 78 sheep, pulse-rate of, 70 Strongylus armatus, in horses, 83 use of, 55 Bloodvessels, vaso-constrictor centr 79 -dilator nerves, 76 -motor centre, 75 subcentres in cord, 7£ veins, 57 abdominal, 57 capacity of, 57 pregnant uterus, 57 pulse in, 66 structure of, 57 valves of, 57 vense cavae, 57 venous plexuses of penis, 8 without valves, 57 velocity of, 72 Volkmann's estimate, area c vascular system, 72 observations, velocity c blood, 72 Body temperature, 337 Bohr on carbon dioxide in blood, 21 103 on htemoglobin, 9 Bones of the foot, 538 Bouley on varnishing skin, 284 Boussingault on growth of foal, 624 tables, respiration, 116 Bovines, period of puberty, 585 Bowel, pendular movements of, 203 strangulation of, 215 Bowman, capsule of, 287 Brain, circulation in, 444 coverings of, 444 exposed, pulsation in, 81 movements of, 445 Braying of ass, 129 Broad's treatment of laminitis, 564 Broken wind, horse, 120, 335 Bronchitis, horse, 120 Brown, H. T., on cellulose-dissolvin enzyme, 171 Professor, dentition and old age 627 Brunner, glands of, 186 ' Brushing ' of horse, 512 Buchner on ferments, 643 Buek-jumping, horse, 532 ' Buffy coat ' in blood-clotting, 2, 1 Bulb, 404, 434. See Medulla obloi gata functions of, 437 Bunge on bile pigments, 220 on calcium, 634 on cellulose, 207 digestion, 657 on changes in liver, 231 on iron, 666 on lime salts, 664 on sodium salts, 663 Digitized by Microsoft® ■ INDEX 675 Bunge's table of salts in blood, 21 analysis, ash of milk, 621 Bunsen, partial pressure of, 98 Burdach, column of, 403 Bursa, navicular, 539 Butyric acid, 653 digestion, 209 odour of, from blood, 2 in stomach, 162 Csecum, the, 190 absorption from, 260 Calcareous degeneration, liver, 241 Calcium carbonate, 665 in food, 664 in urine, 301 oxalate, 283 phosphate, 212, 219, 233, 665 in blood, 21 salts, 212, 219 effect on coagulation, 10 on milk-curdling, 20 sulphate, 665 Calculi, biliary, 241 intestinal, 210 stomach, 210 Calories of heat, 324 Calves, Torcy on growth of, 625 Camel, .period of gestation, 615 pulse-rate of, 70 rutting of, 581 tribe, corpuscles of, 4 Canter of horse, 526 Cane-sugar, 657 digestion, 187 Capacity of heart, Colin, 42 Munk, 42 Capillaries, 56, 64 lymph, 243 'Capped elbow,' horse, 522 Capric acid, 652 Caproic acid, 652 Caprylic acid, 207, 652 Carbamide, 647 Carbon, 632 dioxide in blood, 23, 24 in nutrition, 316 monoxide, Haldane's experi- ments, 104 poisoning by, 105 Carbo-hydrates, 654 absorption of, 263 oxidation of, 323 Carbonates in the body, 665 Carbonic acid, Bohr's view, 103 fate of, 102 in cellulose digestion, 198 in large intestine, 208 in stomach, 178 respiration, 95 Carboxy-hsemoglobin, 10 Cardiac cycle, 35 sounds, 40 Cardiograph, 42 Carnine (muscle), 379 Carnivora, respiratory quotient in, 96 Carotid artery, horse, velocity in, 72 Cartilago nictitans, 477 Caseinogen, 619, 620 Castration, effects of, 265, 583 of, in fattening, 326 of, on voice, 128 Cat, blood of, 2 composition of body, 315 effects of atropin, 468 of pilocarpin, 281 emmetropia in, 470 experimental sweating in, 279 generation, 518 hair of, 276 hearing of, 496 heart, depressor nerve of, 50 iris of, 458 larynx of, 129 ovaries, removal of, 265 ovary of, 587 period of gestation, 615 section corpora quadrigemina, 439 spleen, 267 submaxillary ganglion, 427 sweating, 277 sympathetic nervous system, 447 temperature of blood, raised ex- perimentally, 71 vagus nerve, 45 voice centre in, 129 vomiting in, 180 'Cat-hairs,' horse, 273 Catacrotic limb of sphygmogram , 69 waves of sphygmogram, 69 Cattle, osteo-malacia in, 327 , repose of, 522 temperature of, 339 Causation of a muscular contraction, 372 Cause of coagulation of blood, 17 of first inspiration, 112 of heart beat, 51 Cells of Purkinje, 31 Cellular tissue, absorption from, 258 Cellulose, action of bacteria, 197 Bunge's observations on, 207 digestion of, 194, 656 dissolving enzyme, 171 fermentation, 171 Centre, diabetic, 230 of gravity, horse at rest, 514 Centres in the medulla, 435 Cerebellum, 438 43—2 Digitized by Microsoft® 676 A MANUAL OP VETEEINAKY PHYSIOLOGY Cerebellum, Luciani on, 439 Cerebral fluid, 445 Cerebrum, 440 Changes in active and resting muscle, 369 Chauveau and Marey, heart of horse, 38 on deglutition, 136 on heart valves, 39 on larynx, 129 on muscle work, 365 on tendon flexor metatarsi, 509 on velocity of blood, 72 of nerve impulses, 389 Check ligaments, function of, 514 Chemical changes during contraction of muscle, 370 composition of muscle, 377 constituents of thebody, 632 acid, amido-aeetic, 645 -caproic, 646 -isethionic, 645 -succinic, 647 aspartic, 647 benzoic, 650 butyric, 653 capric, 652 caproic, 652 caprylic, 652 ethylidene-lactic, 653 glycero-phosphoric, 645 hippuric, 650 indoxyl-sulphuric, 651 lactic, 653 oleic, 652 palmitic, 652 sarco-lactic, 653 skatoxyl-sulphuric, 652 stearic, 652 tauro-cholic, 645 acrolein, 653 achroo-dextrin, 656 Adamkiewicz's reaction, 641 albuminates, 636 albuminoids, 638 albumins, derived, 636 native, 636 album oses, 637 allantoin, 649 amides, 645 amido-acids, 645 -glucose, 638 aromatic series, the, 650 asparagine, 647 Barfoed's reagent, 658 biuret reaction, 641 Buchner on ferments, 643 Bunge on calcium, 634 cellulose digestion, 657 iron, 666 Chemical constituents of the body, Bunge on lime salts, 664 sodium salts, 663 butyric acid, 653 calcium carbonate, 665 in food, 664 phosphate, 665 sulphate, 665 cane-sugar, 657 capric acid, 652 caproic acid, 652 caprylic acid, 652 carbamide, 647 carbohydrates, 654 carbon, 632 carbonates in the body, 665 cellulose, digestion of, 656 choline, 645 chlorine, 633 chlorophyll, 644 cholesterin, 654 cholesterol, 654 chondrin, 639 coagulated proteids, 637 collagen, 638 creatine, 646 creatinine, 646 cresol, 651 dextrin, 655 dextrose, 656-7-9 disaccharides, 657 elastin, 639 erythro-dextrin, 656 ethylidene-lactic acid, 653 enzymes, 643 fats, nitrogenous, 644 ferments, 642 fibrins, 636 fatty acids, 652 fats, 652 Fischer, Emil, on proteids, 635 galactose, 658 gases found in the body, 662 gelatin, 639 globulins, 636 glycerin, 653 glycerol, 653 glycero-phosphoric acid, 645 glycine, 645 glycocoll, 645 glycocine, 645 glycogen, 656 glyco-proteids, 638 glucose, 659 glucosamine, 638 grape-sugar, 659 ruemoglobiD, 644 hexoses, 660 hippuric acid, 650 Digitized by Microsoft® INDEX 677 Ohemical constituents of the body, hydrogen, 632 indiean, 6,51 indigo series, 651 indol, 651 indoxyl-sulphuric acid, 651 inorganic, 662 inosite, 660 iron, 634 iron in the body, 665 invert sugar, 658 keratin, 639 lactose, 658 lactic acid, 653 lecithin, 645 leucine, 646 levulose, 657, 660 lysatine, 646 lysatinine, 646 maltose, 656-8 magnesium salts, 665 melanin, 644 niethyl-glycine, 645 Micrococcus urece, 648 Millon'a reagent, 641 reaction, 641 monosaccharides, 659 muscarine, 645 neurine, 645 nitrogen, 633 nitrogenous fats, 644 non-nitrogenous bodies, 652 nucleo-proteids, 637 oleic acid, 652 olein, 653 osazone tests, 658, 661 oxygen, 633 palmitic acid, 652 palmitin, 653 Pasteur on ferments, 642 pentoses, 661 pepsin, 643 peptones, 637 phenyl-glucosazone, 659 phenyl-maltosazone, 658 phenyl-sulphate of potas- sium, 665 phenol, 651, 665 phosphates, 665 phosphorus, 633 pigments of the body, 644 Piotrowski's reaction, 641 polysaccharides, 655 potassium salts in vegetable food, 663 proteid reactions, 641 proteids, 634 classification, 636 proteoses, 637 pseudo-nuclein, 638 Chemical constituents of the body, ptyalin, 643 purin series, 638 pyrocatechin, 651 reaction, biuret, 641 reactions, proteid, 641 Adamkiewicz's, 641 Millon'a, 641 Piotrowski's, 841 Xanthoproteic, 641 reagent, Barfoed's, 658 saccharose, 657 sarcolaotic acid, 653 sarcosine, 645 silicon, 634 skatol, 652 skatoxyl-sulphuric acid, 652 sodium salts in vegetable food, 663 starch, 655 stearic acid, 652 stearin, 653 sugar, invert, 658 tests for, 661 Bbttcher's, 661 fermentation, 661 Moore's, 661 picric acid, 661 Trommer's, 661 sulphur, 633, 665 taurine, 645 tauro-cholic acid, 645 trypsin, 643 tyrosine, 650 urea, 647 synthesis of, 648 uric acid, 649 vitellin, 638 water, 662 xanthine series, 638 xanthoproteic reaction, 641 zymase, 643 zymogen, 644 Chemical engine, muscle as, 366 excitant, specific, 264 Chemistry of respiration, 114 ' Chestnuts ' of the horse, 602 Chloral hydrate, effect on brain, 445 Chlorine, 633 . in urine, 303 Chloroform, absorption of, by air- passages, 257 cry of men and animals, 440 effect on brain, 445 narcosis and heat production, 342 Chlorophyll, 644 in dandruff, 283 Cholalic acid, 221-2 Cholcsterin, 212, 654 in bile, 219, 220 Digitized by Microsoft® 678 A MANUAL OF VETERINARY PHYSIOLOGY Choleaterin in blood, 20 in red corpuscle, 5 in spermal fluid, 584 leucocytes, 14 Cholesterol, 654 Choline, 645 Chondrin, 639 Chorda dorsalis, 600 tympani nerve, 76, 78, 133, 144, 428 Chordfe tendineas, 32, 33 Chorion, 602, 608 Choroid, 455, 461 Chromatic aberration, 483 Chyle, 253, 256 Chyme, 256 Colin's observations, 256 physical characters of, 188 Ciliary muscle, 461 effect of atropin, 468 processes, 458 zone, 461 Cilio-spinal centre in cord, 424 Circle of Willis, 81 Circulating proteid of Voit, 319 Circulation, aids to, 74 arterial tone, 76 aspiration in thorax, 35 in veins, 35 blood-pressure, 62 circle of Willis, 81 course of, 30 dilator nerves, 78 duration of the, 73 effect of respiration on, 91 Hering's observations on time of, 74 in mesentery of mammal, 65 in the living tissues, 65 influence of the nervous system, 74 mean pressure, 60 mechanics of, 58 peculiarities of, 81 peripheral resistance, 58 pulsation in exposed brain, 81 pulse, 58 rete mirabile, 81 sphygmogram, 68 time of, dog, 74 horse, 74 rabbit, 74 vaso-dilator nerves, 76 vaso-motor centre, 75 snbcentres, 75 velocity of blood- current, 68 of pulse-wave, 68 venous arrangement of brain, 82 Volkmann's estimate of area of, 72 Clarke, Bracy, on foot of horse, 564 Clarke's column, spinal cord, 395 Clipping horses, 2,75 effect of, on temperature, Siedam- grotsky, 349 Coagulated proteids, 637 Coagulation of blood, 14 average time of, 16 cause of, 17 circumstances affecting, 18, 20 effect of acetic acid (dilut.), 18 of ammonium salts, 19 of calcium salts, 19 of carbon dioxide, 19 of citric acid, 19 of cold, 19 of leech extract, 20 of peptone, 20 of potassium oxalate, 19 of salts of alkalis, 19 Hammarsten's theory, 17 living test-tube experiment, 19 Cocain, effect on iris, 459 in peristalsis, 203 Cochlea, 497 Cochlear canal, 500 Ccelom, 600 Colic, 213 Colin, figures, chest of horse, 85 observations, absorption, 257 antiperistalsis, 203 blood temperature, 338 capacity of heart, 42 of stomach, horse, 151 centre of gravity, horse at rest, 515 chyme, 256 digestion of hay, 156 experimental paralysis, horse, 444 experiments, seventh pair of nerves, 113 stomach, 177 guttural pouches, 128 heat loss, 351 ileum, function of, 189 insensible perspiration, 277 lymph, 246 movement, 252 mastication, 135 muscle temperature, 371 nervous mechanism, heart, 44 pancreas, 232 reticulum, 174 salivary glands, 139 succus entericus, 186 Collagen, 638 Colon, 194 absorption from, 199 Digitized by Microsoft® INDEX 679 Colon, digestive changes in, 197 Colostrum, 620, 622 Colour, effect of, 345 of blood, 1 Colouring matter of urine, 301 Colourless corpuscles, 12 Columnas carnese, 32 Comparison, body and engine, 365 Complemental air, 113 Composition of atmospheric air, in- spired, 95 expired, 95 of blood, 225 of the body, 314 of sweat of horse, 278 of urine, 292 Compression of propulsion, 517 Concussion of impact, 517 Condition in horses, 375 Conduction, 343 Conductivity of nerves, 389 Conjunctiva, absorption from, an- thrax, 258 atropin, 258 curare, 258 Consistence of urine, 306 Contractility of muscle, 356 Co-operative antagonism, muscles, 506 Co-ordinate movement, 409 Cornea, 455, 457 Corona, fracture of, 519 Coronary circulation, 53 substance (hoof), 545 Corpora nigra (horse), 460 quadrigemina, 439 striata, 437 Corpus arantii, 34 luteum, 590 striatum, heat puncture, 342 Corpuscles of blood, red, 4 colourless, 12 of camel tribe, 4 white, amoeboid movements of, 13 basophile leucocytes, 13 distinguished from lymph cell, 12 eosinophile cells, 13 hyaline leucocytes, 12 lymphocytes, 13 migration of, 66 origin of, 14 polynuclear, 12 varieties of, 12 Jorti, organ of, 500 toughing, 129 bourse of the circulation, 30 3ow, amount of air respired, 116 analysis of milk, 620 Cow, faeces of, 208 generation, 578 heart, foreign bodies in, 54 hippomanes in, 608 lymph from, 246 cestrous cycle, Goodall, 579 ovary of, 587 period of gestation, 614 position of fcetus in, 615 refraction, errors of, 470 sense of smell, 488 uterine glands in, 614 ' Cow-hocks,' horse, 512 ' Cow-kicking ' of horse, 532 Cranial nerves, 425 Crassamentum, 15 Creatine, 646 in blood, 20 in muscle, 379 in urine, 292 Creatinine, 292, 295, 646 Cresol, 651 ethereal sulphate of, 292 in urine, 300 Crico-arytenoideus lateralis, 122 posticus, 122 Curare and conjunctiva, 258 effects on end-plates, 354, 361 and heat production, 342 Curdling of milk, effect of calcium salts, 20 Current of action (nerve), 386 of rest (nerve), 386 Cumulus proligerus, 588 Cutaneous senses, cold, 490 pain, 490 pressure, 490 warmth, 490 Cuvier on organ of Jacobson, 487 Death, 629 convulsions in, 630 rigor mortis, 379, 631 Decay, 628 Decidua reflexa, 603 serotina, 603 vera, 603 Defensive mechanisms of the body, 25 Defibrinated blood, 16 De6eiency in oxygen, 105 Defsecation, act of, 211 Degeneration of nerves, 389 "Wallerian, 400 Deglutition, 136 Deiters, supporting cells of, 500 Dendrites, 412 Dentition, horse, 626 ox, 627 pig, 628 sheep, 627 Digitized by Microsoft® 680 A MANUAL OF VETEEINAEY PHYSIOLOGY Depressor nerve of heart, 50 Descending tracts (spinal cord), 402 Despretz on heat loss, 351 Determination of sex, 591 Deutoplasra, 589 Deutero-albumoses, 187 Development, 593 albumin in milk, 620 allantois, 596, 606 allantoin, 608 amnion, 601, 605 amphoteric milk, 619 ass, analysis of milk, 620 period of gestation, 614 Assheton, development of em- bryo of sheep, 594 on uterine glands, 614 blastocyst, 593 blastoderm, bilaminar, 599 Bunge's analysis of ash of milk . 621 camel, period of gestation, 615 caseinogen, 619, 620 cat, period of gestation, 615 ' chestnuts ' of the horse, 602 chorda dorsalis, 600 chorion, 602, 608 coelom, 600 colostrum, 620, 622 cow, analysis of milk, 620 hippomanes in, 608 period of gestation, 614 position of foetus in, 615 uterine glands in, 614 decidua reflexa, 603 serotina, 603 vera, 603 dog, analysis of milk, 620 period of gestation, 615 ductus arteriosus, 611 venosus, 610 duration of pregnancy, 614 elephant, period of gestation, 614 epiblast, structures derived from, 599 ' ergots ' of the horse, 602 Eustachian valve, 611 Ewart on development of embryo of horse, 594 fcetal circulation, 610 membranes, 605 foramen ovalis, 611 gestation, periods of, 614 glands, uterine, 614 guinea-pig, period of gestation, 615 hippomanes, Goodall on, 608 holoblastic ova, 597 horse, development of embryo of, 594 Development, hypoblast, structures derived from, 599 koumiss, 621 laetalbumin, 620 lactic acid in allantoic fluid, 608 lactose, 620 liquor amnii, 606 mare, analysis of milk, 620 period of gestation, 614 position of foetus in, 615 uterine glands in, 614 medullary groove, 600 meroblastic ova, 597 mesoblast, structures derived from, 599 milk, secretion of, 617 sugar, 620 neural groove, 600 notochord, 600 olein in milk, 620 ovum, development of the, 597 palmitin in milk, 620 parturition, 615 pig, period of gestation, 615 placenta, 603 cotyledonary, 604 cumulata, 604 discoidal, 604 metadiscoidal, 604 plicata, 604 polycotyledonary, 604 zonary, 604 pregnancy, duration of, 614 primitive groove, 600 streak, 599 pro-amnion, 601 rabbit, period of gestation, 615 rennin, 620 sheep, analysis of milk, 620 fcetal blood, gases in, 613 period of gestation, 615 uterine glands in, 614 somatopleure, 600, 610 splanchnopleure, 601, 608 stearin in milk, 620 trophoblast, 601, 604 tyrein, 620 umbilical cord, 608 uraohus, 607 urea in allantoic fluid, 608 uterine milk, 614 Whartonian jelly, 610 yolk sac, 600, 605 zebra, period of gestation, 614 Dextrin, 187, 655 Dextrose, 187, 656, 657, 659 Diabetes, 226 pancreatic, 239 Diabetic centre, 230 puncture, 230 Digitized by Microsoft® INDEX 681 Dialysis, 248 Diapedesis of white corpuscles, 13 Diaphragm, 86 in respiration, 89 movements of, 85 rupture of, 120 spasm of, 120 Diarrhoea, 217 Diastole of heart, 35 Dicrotic wave of sphygmogram, 69 Dieekerhoff on temperature, 339 Diffusion in lymph formation, 247 of gases, 98 Digastricus muscle, 136 Digestion, 131 abomasum, 175 absorption from the stomach, 176 acetic acid, 207, 209 in stomach, 162 acid, biliary, 212 butyric, 209 in stomach, 162 fatty, 212 formic, 209 hydrochloric, 161 lactic, 209 in stomach, 161 of pig, 175 malic, 209 phenyl-proprionic, 199 phosphoric, 210 . succinic, 209 tannic, 210 acids of stomach, 161 amido-acids, 187, 199 ammonio-magnesium phosphate, 210 amylolytio action, 143 antipepsin, 178 antiperistalsis, 203 apomorphia, 180 arrangement of food in the stomach, 157 atropin in secretion of saliva, 146 Bayliss and Starling on peri- stalsis, 203 Bezoar stones, 210 bowel, pendular movements of, 203 strangulation of the, 215 Brown, H. T. , on cellulose dis- solving enzyme, 171 Brunner, glands of, 186 Bunge on cellulose, 207 cascum, the, 190 calcium, salts of, 212, 219 calculi, intestinal, 210 stomach, 210 cane-sugar, 187 caprylic acid, 207 Digestion, carbonic acid in cellulose digestion, 198 in large intestine, 208 in stomach, 178 cat, vomiting in, 180 cellulose, action of bacteria, 197 Bunge's observations on, 207 digestion of, 194 dissolving enzyme, 171 fermentation, 171 Chauveau on deglutition, 136 cholesterin, 212 chorda tympani nerve, 133, 144 chyme, physical characters of, 188 cocaine in peristalsis, 203 colic, 213 Colin, experiments on stomach, 177 on antiperistalsis, 203 on capacity of stomach in horse, 151 on hay, 156 on ileum, functions of, 189 on mastication, 135 on reticulum, 174 on salivary glands, 139 on succus enterieus, 186 colon, 194 absorption from, 199 digestive changes in, 197 cow, feces of, 208 defecation, act of, 211 deglutition, 136 dextrin, 187 dextrose, 187 diarrhcea, 217 digastricus muscle, 136 deutero-albumoses, 187 drinking, horse, 134 ox, 134 dog, area of intestinal tract, 201 capacity of intestinal canal, 201 gastric juice, 165 intestinal, 201 lapping of, 134 mastication, 134, 135 oesophagus of, 138 stomach, 149, 176 submaxillary gland of, 144 tongue of, 133 vomiting of, 178 duodenum, 153 syphon-trap of, 153 Eber, observations of, 184 Ellenberger on, in ruminants, 173 on gastric acids, 161 on horse-feeding, 160 Digitized by Microsoft® 682 A MANUAL OF VETERINARY PHYSIOLOGY Digestion, Ellenberger on nervous mechanism, ruminants, 186 on intestinal, in horse, 187 on omasum, 174 on periods of stomach diges- tion, 171 on saliva, 143 on the csecum, 190 enteritis, 214 enterokinase, 187 enzymes in intestinal extract, 187 erepsin, 187 erythrodextrin, 142 ether, rectal administration of, 199 fats, in stomach, 171 fatty acids, 199 fifth nerve, gustatory division of, 144 lingual branch of, 133 Fletcher, experiments on salivary glands, 149 Flourens' experiments, 172 fteces, amount of, 211 ash of, 210 approximate composition of, 208 odour of, 211 of cow, 208 of horse, 208 of pig, 208 of sheep, 208 follicles of Lieberkiihn, 186 formic acid, 209 fundus glands, 164 galactose, 187 Gamgee on mastication, 134 gases of the intestines, 207 of the stomach, 178 gastric juice, secretion of, 162 gastritis, 215 glands, fundus, 164 gastric, 162 labial, 141 molar, 141 of Brunner, 186 palatine, 141 parotid, 139, 141 pyloric, 164 sublingual, 139 submaxillary, 139 glosso-pharyngeal nerve, 144 lingual branch of, 133 granulose, 142 hair-balls, 210 in rumen of cattle, 172 hay, 154 Heidenhain's view of secretory nerves, 146 Digestion, herbivora, saliva in, 140 teeth in, 132 hexone bases, 187 Hofmeister on gastric acids, 161 on periods of stomach, 171 horse, area of intestinal tract, 201 boiled food for, 171 capacity of intestinal canal, 201 of stomach, 151 drinking of, 134 feeding of, 158 faeces of, 208 gastric glands in, 162 intestinal affections in, 204 digestion in, 187 mastication, 134-5 medicines by mouth, 185 oesophagus of, 137 periods of stomach diges- tion, 171 salivary glands of, 139 saliva of, 143 stomach of, 149 absorption from, 176 digestion in, 150 sugar formation in, 170 tongue of, 133 vomiting of, 138, 178 watering of, 158 hydrochloric acid, 161 hydrogen in large intestine, 208 in stomach, 178 hypogastric plexus, 205 hypoglossal nerve, 133, 138 ileum, function of, 189 indol, 189, 199, 209 intestinal calculi, 210 canal, nervous mechanism of, 205 digestion, 186 in horse, 186 in other animals, 201 in ruminants, 199 intestines, gases of the, 207 movements of, 201 impaction, 217 inverting ferments, 187 invertase, 187 lactase, 187 maltase, 187 ipecacuanha, 180 Kiihne's table of peptones, 168 labial glands, 141 lactase, 187 lactic acid, 161, 209 Langley on gastric glands, 164 observation on secretion, 147 lapping of dog, 134 Digitized by Microsoft® INDEX 683 igestion, large intestine, 190 leucin, 209 levulose, 187 Lieberkiihn, follicles of, 186 lignin in diet of herbivora, 207 lion, intestinal canal of, 201 lips, horse, 131 ox, 131 pig, 131 sheep, 131 llama, stomach of, 182 magnesium salts, 212 phosphate, 219 sulphate, 219 masseter muscle, 136 Majendie's experiment, 180 malic acid, 209 maltase, 187 maltose, 187 marsh gas in cellulose digestion, 198 in large intestine, 208 in stomach, 178 mastication, Gamgee on, 135 nerves of, 136 dog, 134 horse, 134 ox, 134 sheep, 134 M'Kendrick on area of intestinal tract, 201 Meade Smith on saliva, 143 mechanism of rumination, 183 nervous, of the intestinal canal, 205 of secretion of saliva, 144 meconium, 212 milk-sugar, 187 molar glands, 141 movements of stomach, 184 of the intestines, 201 mucin, 140, 165 in stomach, 152 Munk, statistics of intestinal canal, 201 nerve, chorda tympani, 133 control, secretion of saliva, 144 fifth, in mastication, 136 lingual branch of, 133 ganglia, salivary glands, 147 glosso - pharyngeal, lingual branch of, 133 hypoglossal, 133, 138 recurrent laryngeal, 138 seventh in mastication, 136 superior laryngeal, 138 supply of stomach, 185 sympathetic, 144 nerves, deglutition, 138 Digestion, nerves, hypogastric plexus, 205 mastication, 136 pharyngeal plexus, 138 tongue, 133 nervous control of gastric juice, 169 of rumination, 183 mechanism of intestinal canal, 205 of secretion of saliva, 144 of stomach of rumi- nants, 186 nicotine in peristalsis, 203 in secretion, 147 nitrogen in stomach, 178 oats, 156 oesophageal groove, 180 oesophagus, dog, 138 horse, 137 ox, 138 sheep, 138 omasum, 174 ox, area of intestinal tract, 201 capacity of intestinal canal, 201 drinking, 134 intestinal, 201 mastication, 134-5 oesophagus of, 138 salivary glands of, 1 39 tongue of, 133 oxygen in stomach, 178 palate, grooves in, 141 palatine glands, 141 paracasein, 168 paralytic secretion, 146 parasitic diseases, 217 parotid gland, 139, 141 secretion in, 147 Pawlow, gastric juice, 165 pendular movements of bowel, 203 pepsin, 165-6 pig, 175 secretion of, 152 peptones, 168, 187, 199 periods of stomach digestion, 171 peristalsis, 203 effects of cocaine, 203 of nicotine, 203 Starling andBaylisson, 203 pharyngeal plexus, 138 phenol, 199, 209 phenyl-acetic acid, 199 -proprionic acid, 199 pig, area of intestinal tract, 201 capacity of intestinal canal, 201 Digitized by Microsoft® 684 A MANUAL OF VETEEINAEY PHYSIOLOGY Digestion, pig, faeces of, 208 intestinal digestion in, 201 saliva of, 143 stomach of, 150 digestion in, 175 vomiting of, 178 pilocarpin in secretion of saliva, 146 potassium in fasces, 210 sulphocyanide, 140 prehension of food, 131 proprionic acid, 207 proteid, 167 to peptone, conversion of, 168 proteoses, 199 primary, 168 secondary, 168 pterygoid muscles, 135-6 ptyalin, 140, 143 pyloric glands, 164 rabbit, saliva of, 143 reaction of contents of stomach, 161 rectum, absorption from, 199 recurrent laryngeal nerve, 138 rennin, 165 reticulum, 174 Roger on composition of faces, 210 rumen, 172 ruminants, intestinal, 199 stomach, 172 nervous mechanism of, 186 trouble in, 217 rumination, Colin's observa- tions, 180 Flourens' observations, 180 mechanism of, 182 rupture, 217 of stomach, 153 saliva, 139 amount of secretion, 139 chemical characters, 140 Meade Smith on, 143 non-amylolytic action of, 143 physical characters of, 140 salts of, 140 secretion of, 144 nerve control of, 144 use of, 142 salivary glands, changes in cells, 147 classification of, 139 sea-sickness, horse, 180 secretin, 187 secretion of gastric juice, 162 nerve control of, 169 Digestion, secretion of pepsin, 152 of saliva, 144 secretory nerves, Heidenhain's view of, 146 self-, of the stomach, 177 sheep, faces of, 208 intestinal digestion in, 201 mastication, 134 oesophagus of, 138 silica in faeces, 210 skatol, 189, 199, 209 Smith, Meade, on saliva, 143 sodium chloride, 212, 219 sphincters, 212 starch of plants, 142 Starling and Bayliss on peris- talsis, 203 sterno-maxillaris muscle, 136 stomach acids, 161 calculi, 210 contents, reaction of, 161 gases of, 178 in horse, 150 in dog, 176 in pig, 175 in ruminants, 172 movements of, 184 nerves of, 185 of dog, 150 of horse, 150 of llama, 182 of pig, 150 of ox, 150 periods of, 171 pouch of Pawlow, 166 rupture of, 153 self-digestion of, 177 strangulation of the bowels, 215 strychnine experiments, 177 per rectum, 199 stylo-maxillaris muscle, 136 sublingual gland, 139 submaxillary gland, 139 of dog, 144 succinic acid, 209 succus entericus, 186 sucking, 134 sulphuretted hydrogen in large intestine, 208 in stomach, 178 superior laiyngeal nerve, 138 swallowing centre, 138 syntonin, 168 syphon-trap of duodenum, 153 Tappeiner on cellulose, 171 tartar emetic, 180 teeth, 131 temporal muscle, 136 tiger, intestinal canal of, 201 tongue, dog, 133 Digitized by Microsoft® INDEX 685 •igestion, tongue, horse, 133 movements of, 133 nerves of, 133 ox, 133 trypsin, 187 trypsinogen, 187 tympany, 217 tyrosin, 209 vagus, action on small intestines, 205 and secretion of gastric juice, 169 motor nerve of stomach, 185 vomiting, 178 )igitalin, action on heart, 53 Dila'tor nerves, 78 )ioestrum, 577 Dioptrics, 480 ^saccharides, 657 Disassociation, process of, 103 )iscus proligerus, 588 Discharge of urine, 311 Disease, blood in, 26 Dishing ' of horse, 512 Distribution of blood in body, 23 of nerve fibres in the cord, 396 of the weight of the body, 515 Disynaptic arc (nervous system), 416 Division of the phrenic nerves, 112 of the seventh pair of nerves, 113 Dog, action of heart in, 54 amount of air required, 116 of heat produced, 351 analysis of milk, 620 area of intestinal tract, 201 bile, action of, 225 amount of, per hour, 224 blood of, 2, 21 of, time of clotting, 16 brain, localization, 441 capacity of intestinal canal, 201 division of spinal cord, 74 effect of atropin, 468 emmetropia, 470 experiments on kidney of, 285, 290 gastric juice, 165 generation, 578 hair of, 276 heart, pressure in, 37 valves, vegetations on, 54 intestinal digestion in, 201 intracardiac pressure, 42 iris of, 458 kidney, amount of blood through, 289 larynx of, 129 lapping of, 134 loss of heat, 351 Dog, lymph from, 247 mastication, 134, 135 number of respirations, 90 oesophagus of, 138 ovaries, removal of, 265 ovary of, 587 pancreatic fluid of, 232 period of gestation, 615 of puberty, 585 pilocarpin in, 281 pressure in pleural cavity, 90 psychical powers, 449 pulse-rate of, 70 reflex action in, 409 relation between blood and body- weight, 22 respiratory curves of, 92 sense of smell, 488 spleen, 267 stomach of, 149 absorption in, 259 digestion in, 176 submaxillary ganglion, 427 gland of, 144 sweating, 277 sympathetic nervous system, 447 temperature of, 339 tendon reflexes in, 423 time of circuit of circulation, 74 tongue of, 133 urine, Bischoff and Voit on, 310 vagus nerve, 45 vena cava anterior, negative pressure in, 64 voice centre in cerebral cortex, 128 vomiting of, 178 Donkey, sweating of, 277 Dormouse, hibernation of, 350 Draught, 534 Brunei on, 535 Drinking, horse, 134 ox, 134 Ductless glands, 264 acromegaly in man, 270 Addison's disease in man, 269 adrenals, 269 adrenalin, 270 castration, effects of, 265 cat, ovaries, removal of, 265 spleen, 267 chemical excitant, specific, 264 dog spleen, 267 ovaries, removal of, 265 Edkins on gastric juice, 264 hormone, 264 internal secretion, 264 iodothyrin, 269 Digitized by Microsoft® 686 A MANUAL OF VBTEEINAEY PHYSIOLOGY Ductless glands, Leeney's observa- tions, ovarian tissue, 265 ovary, influence of, 265 parathyroid, 268 phagocytosis in spleen, 267 pineal body, 270 pituitary body, 270 secretin of Starling and Bayliss, 233, 264 spleen, 266 enzyme in, 267 use of, 267 thymus, 269 influence of castration on, 266 thyroid gland, 268 Ductus arteriosus, 611 venos s, 610 Duodenum, 153 syphon-trap of, 153 Duration of the circulation, 73 of pregnancy, 614 Durham, researches on hair pigment, 274 Dyspncea, 106 Ear, auditory sensations, 501 cat, hearing of, 496 cochlea, 497 cochlear canal, 500 Corti, organ of, 500 Deiters, supporting cells of, 500 endolymph, 499 Eustachian tube, 497 external, 496 movements of, 496 fenestra ovalis, 498 rotunda, 498 Galton on sounds, 496 guttural pouches, 497 harmonics, 495 hearing, 495 helicotrema, 500 Hensen's cells, 501 incus, 497 internal, 497 labyrinth, 497, 502 lamina spiralis, 499 macula acustica, 502 malleus, 497 membrana basilaris, 499 membrane of Eeissner, 500 middle, 497 musical sounds, 495 noise, 496 perilymph, 498 pillars of Corti, 499 otoliths, 499 calcium carbonate, 504 saccule, 499 Ear, scala tympani, 499 vestibuli, 499 semicircular canals, 497, 502 Sherrington on the labyrinth, 503 sound, nature of, 495 stapes, 497 tympanum, 497 utricle, 499 vestibule, 497 Eber, observations of, on rumen, 184 Edkins on gastric juice, 264 Effect of respiration on circulation, 91 Effector organ, 412 Efferent nerves, 382 paths in the cord, 405 Eighth pair cranial nerves, 429 Elastic tension, muscle, 364 Elasticity of arteries, 56 of muscle, 363 provisions for, in the foot, 559 Elastin, 639 Electric phenomena of muscle, 369 of nerves, 385 Electrotonus (nerve), 387 Elephant, period of gestation, 614 pulse-rate of, 70 psychical powers of, 449 rutting of, 581 Eleventh pair cranial nerves, 434 Ellenberger on bile, 219 on caecum, the, 190 calculation of number of red cells, 6 on digestion in ruminants, 173 estimate of haemoglobin per- centage, 8 on gastric acids, 161 on horse-feeding, 160 on intestinal digestion in horse, 187 on nervous mechanism, stomach of ruminants, 186 on omasum, 174 on periods of stomach digestion, 171 on saliva, 143 on varnishing the skin, 284 Emmetropic eyes, 468 Emulsification of fat, 261 Endolymph, 499 End-plate in muscle, 354 Enlargements of liver, 241 Enteritis, 214 Enterokinase, 187, 234 Enzymes, 643 diastatip, 234 in intestinal extract, 187 intracellular, 336 Digitized by Microsoft® INDEX 687 Inzymes, lipolytic, 234 proteolytic, 234 losinophile cells, 13 Ipiblast, structures derived from ,599 !piglottis, 125 h'colani on sweat-gland in foot of horse, 557 Srection, act of, 585 centre in cord, 424 irepsin, 187, 236 Ergots ' of the horse, 602 irythro-dextrin, 142, 656 Eserin, effect on iris, 459 Ether, absorption of, by air-passages, 257 effect on brain, 445 per rectum, 199, 261 Ethylidene-lactic acid, 653 Evaporation, 343 Sversbusch on iris of horse, 459 eustachian tube, 497 valve, 611 Ewart on development of embryo of horse, 594 on ovulation in mare, 591 Excitability of nerves, 385 Excretion, definition of, 285 Expansion of foot, 569 Lungwitz on, 571 Expenditure and income of body, 315 Experimental paralysis, horse, 444 Expiration, 88 muscles of, 89 Expired air, composition of, 95 Extensibility of muscle, 363 External ear, 496 intercostal muscles in respira- tion, 89 respiration, 99 Extractives of blood, 20 of muscle, 379 Eye, 454. See Sight schematic, 478 structure of the, 454 Eyeball, movements of the, 470 muscles of, 471 Eyelashes of horse, 477 Tacial nerve, 428 sinuses, 94 Talse nostril, 92 fatigue fever, 341 muscle, 372 Wedenski on, 375 Fat, absorption of, 261 emulsification of, 261 fatty acids, 199, 652 liver, 241 r ats, description of, 652 in blood, 20 Fats, in stomach, 171 nitrogenous, 644 Fenestra ovalia, 498 rotunda, 498 Ferment, liver, 229 Ferments, 642 Fetlock joint, 513 Fever, effect on nutrition, 335 Fibrin, 4, 17 ferment, 3, 18 Fibrinogen in liquor sanguinis, 3 in lymph, 245 precipitation of, 3 Fibrino-globulin, 3 Fibrins, 636 Fick on muscle-work, 365 Fifth nerve, gustatory division of, 144 lingual branch of, 133 pair, cranial nerves, 425 inferior maxillary, 426 ophthalmic, 426 superior maxillary, 426 Filtration in lymph formation, 247 Fischer, Emil, on proteids, 635 on purin, 296 Fishes, sight of, 468 Fleming on age-limit of procreation, 585 Fletcher, experiments on salivary glands, 149 Flourens, experiments on digestion in ruminants, 172 Foal, bones of, 624 growth of, Boussingault, 624 Focus, lenses, 482 Ffeces, amount of, 211 ash of, 210 approximate composition bf, 208 cow, 208 horse, 208 odour of, 211 pig, 208 sheep, 208 Fcetal circulation, 610 lung, 88 membranes, 605 inspiration, first, 112 Follicles of Lieberkiihn, 186 Food, amount of, required, 333 inorganic, 327 Foot-and-mouth disease, 27 Foot, the, 537 anti-concussion, mechanism of, 568 bars (hoof), 548 use of, 566 blood-supply, 545 bones of the, 538 Broad, laminitis, treatment of, 564 Digitized by Microsoft® 688 A MANUAL OF VETEEINAEY PHYSIOLOGY Foot, bursa, navicular, 539 Clarke, Bracy, on foot of horse, 564 coronary substance, 545 elasticity, provisions for, in foot, 559 Ercolani on sweat-gland in foot of horse, 557 expansion, 569 Lungwitz on, 571 Franck on sweat-glands in foot of horse, 557 'frog,' 551 hoof, 546 horn, chemistry of, 557 growth of, 559 laminee, 549 origin of, 564 salts of, 558 structure of, 552 use of the moisture in, 556 horse, bones of, 538 laminal tissue, 538 how it carries the weight, 561 joint, 538 keratin, 558 laminse, area of, 564 Clarke, Bracy, 564 Gader, 564 Moeller, 564 laminal tissue, 542 laminitis, 564 Broad's treatment, 564 lateral cartilages, 541, 567 Lupton, J. Irvine, on foot of horse, 559 Lungwitz on expansion of, 571 Macdonald on horse's, 572 Mechanism of foot, anti-concus- sion, 568 Moeller on sublaminal tissue, 543 laminal tissue, 565 navicular bone, 538 pad (frog), 551 use of, 566 pathological, 576 pedal bone, 538 descent of, 572 periople, 546 physiological shoeing, 575 plantar cushion, 542 ' side-bone, ' cause of lameness in, 563 sole, 550 use of, 566 Storch, venous system, horse's, 573 stratum, periostale of sublaminal tissue, 543 Foot, stratum vasculosum, 543 toughness, provision for, in foot, 559 vascular mechanism of, 573 sole, 545 wall, 542 wall - secreting substance of, 545 weight, how carried by, 561 Foramen ovale, 611, 613 Foreign bodies in heart, 54 Formic acid, 209 Foster on levers, 506 Fourth pair cranial nerves, 425 Franck on sweat-glands in foot of horse, 557 ' Frog,' 551 Frog's foot, circulation in, 65 Frog, reflex action in, 409 Functions of blood, 1 of spinal cord, 425 nerves, 399 Fundus glands, 164 Furfurol, 222 Galactose, 187, 658 Gall-bladder, 223, 224 animals without, 224 Gallop of horse, 528 Galton on sounds, 496 Gamgee on mastication, 134 Ganglia, collateral, 447 terminal, 447 vertebral, 446 on nerves, 384 Gases, absorption of, in liquids, 97 of blood, 23 Dal ton and Henry's law, 97 diffusion of, 98 found in the body, 662 of the intestines, 207 stomach, 178 partial pressure of Bunsen, 98 Gaskell on the heart, 49 sympathetic nerves, 446 Gasserian ganglion, 426 Gastric juice, secretion of, 162 Gastritis, 215 Gastrocnemius of frog, 357 Gelatin, 639 Generation, 577 ancestrous period, 577 anoestrum, 577 artificial insemination, 591 Ascaris megalotephala, 589 Assheton on impregnation in sheep, 593 Balfour on polar bodies, 590 bear, 578 Digitized by Microsoft® INDEX 689 eneration, Benedeu, Van, on polar bodies, 589 bovines, puberty, period of, 585 camel, rutting of, 581 castration, effects of, 583 cat, 518 ovary of, 587 cholesterin in spermal fluid, 584 corpus luteum, 590 cow, 578 cestrous oycle, Goodall, 579 ovary of, 587 cumulus proligerus, 588 deutoplasm, 589 determination of sex, 591 dicestrum, 577 discus proligerus, 588 dog, 578 ovary, 587 period of puberty of, 585 elephant, rutting of, 581 erection, act of, 585 Ewart on ovulation in mare, 591 Fleming on age limit of procrea- tion, 685 germinal epithelium, 587 spot, 589 vesicle, 589 Graafian follicle, 588 Heape on ovary of rabbit, 592 sex determination, 592 sexual season of mammals, 577 Henson on Graafian follicle, 589 horse, period of puberty, 585 spermatozoa, 584 impregnation, 593 inosit in spermatic fluid, 584 kreatin in spermatic fluid, 584 lecithin in spermatic fluid, 584 Leeney on stag (rutting), 581 leucin in spermatic fluid, 584 lioness, 578 liquor folliculi, 588 mare, 578 ovary, 587 Marshall and Jolly, oestrous cycle in dog, 578 mechanism of ejaculation, 587 membrana granulosa, 588 metcestrous period, 577 metcestrum, 577 Minot on polar bodies, 590 monoestrous mammals, 577 monkey, dicestrous cycles (Heape), 580 nuclein in spermatic fluid, 584 oestrus, 577 ostium abdominale, 589 ostrich, rutting of, 581 Digitized by Generation, otter, 578 ovaries, 587 effect of removal of, 582 ovariotomy, effects of, 583 ovulation, ass, 591 cat, 591 cow, 591 deer, 591 dog, 591 elephant, 591 ferret, 591 mare, 591 monkey, 591 pig, 591 rabbit, 591 sheep, 591 wolf, 591 ovum, 589 parthenogenesis, 586 pig, 578 oestrous cycle, 580 ovary of, 587 puberty, period of, 585 polyoestrous mammals, 577 prooestrous period, 577 prooestrum, 577 external signs of, 580 prostate, secretion of, 584 prototheria, 589 puberty, period of, 585 rabbit, ovary of, 587 seminal vesicles, secretion of, 584 seminiferous tubules, 583 sexual ova, 592 intercourse, 586 season of animals, 577 spermatozoa, 592 sheep, 578 impregnation in, 593 oestrous cycle (Goodall), 479 ovary of, 587 puberty, period of, 585 spermatic fluid, 584 spermatoblasts, 583 spermatogen, 583 spermatozoa, 583 stag, rutting of, 581 sustentacular cells, 583 testicles, 583 effect of removal, 582 tunica albuginia, 583 fibrosa, 588 tyrosine in spermatic fluid, 584 uterus, changes in, during pro- oestrum, 581 Van Beneden on polar bodies, 589 vitelline membrane, 589 Weissmann on polar bodies, 590 wolf, 578 44 Microsoft® 690 A MANUAL OF VETEEINAEY PHYSIOLOGY Generation, zona radiata, 589 Genito-spinal centre in cord , 424 Germinal epithelium, 587 spot, 589 vesicle, 589 Gestation, periods of, 614 Glands, fundus, 164 gastric, 162 Harderian, 477 lachrymal, 477 labial, 141 lymphatic, 244 Meibomian, 477 molar, 141 of Brunner, 186 palatine, 141 parotid, 139, 141 pyloric, 164 sublingual, 139 submaxillary, 139 uterine, 614 Glia cells, cord, 395 Globin, 11 Globulicidal action of serum, 25 Globulins, 636 Glosso-pharyngeal nerve, 144, 430 lingual branch of, 133 Glottis, 94, 124 Glucosamine, 638 Glucose, 659 Glutaminic acid, 236 Glycerin, 653 Glycerol, 653 Glycero-phosphoric acid, 645 Glycine, 221, 222, 292, 645 Glyoocholate of soda, 221 Glycocholic acid, 222 Glycocine, 645 Glycocoll, 222, 293, 645 Glycogen, description, 656 in leucocytes, 14 in liver, 225 in muscle, 227 sources of, 228 use of, 227 Glyco-proteids, 638 Glycosuria, 228, 239 Goat, larynx of, 129 number of respirations, 90 Golgi, organ of, 391 tendon, organs of, 353 Goll, column of, 403 Goodall on hippomanes, 608 Gowers' method for number of cor- puscles in blood, 5 Graafian follicle, 568 Grandeau on amount of sweat, 277 and Leclerc on diet, 333 Granulose, 142 Grape-sugar, 659 Grape-sugar in blood, 20 Grey horses and loss of heat, 346 Growth, 623 Guinea-pig, period of gestation, 615 Guttural pouches, 128, 497 Hsematin, 11 spectrum of, 11 Haematogen, 8 Hsemin, 11 Hiematoporphyrin, 11 Hsematoidin, 11 Haemoehromogen, 11 Haemoglobin, 1, 8, 644 absorption bands of, 9 amount of, in horse's body, 8 carboxy-, 10 compounds of, 10 decomposition of, 11 Ellenberger's estimate, 8 in blood-corpuscles, 5 met-, 10 Miiller's estimate, 8 nitric oxide, 11 oxy-, 7 reduced, 7 spectroscope test, 9 Hemoglobinuria, 27 in horse, 335 Hemolysis, 25 Haemorrhage, 65 thirst in , 65 Hair, 272 cat, 276 dog, 276 horse, 272 clipping of, 275 permanent, 273 pigment in, 274 ' Hair-balls ' in rumen of cattle, 172, 210 Haldane and Smith's process for amount of blood, 22 results, respiration, 119 and Priestley's experiments, res- piration, 117 Hammarsten on bilirubin, 221 theory of coagulation, 17 Harderian gland, 477 Harmonies, 495 Hay, digestion of, 154 Heape on ovary of rabbit, 592 on sex determination, 592 on sexual season of mammals, 577 Hearing, 495. See Ear Heat, calorie of, 324 kilocalorie of, 324 loss, 343 production, 340 puncture, 342 Digitized by Microsoft® INDEX 691 it regulation, 343 ait, 28 accelerator centre in medulla, 49 action of aconitin, 53 of adrenalin, 53 of atropin, 53 of calcium salts, 52 of digitalin, 53 of drugs, 53 of muscarin, 48, 53 ofnicotin, 53 of physostigmin, 48, 53 of pilocarpin, 53 of potassium salts, 52 of sodium salts, 52 of valves of heart, 39 aortic valve, 29 apex-beat, non-existent, 39 auriculo-ventricular valves, 33 beat, cause of, 51 of the, 31 ratio of, to respiration, 91 Bicuspid valve, 33 blood-pressure, 43 capacity of, 42 Colin, 42 Munk, 42 cardiac cycle, 35 sounds, 40 cardiograph, 42 cat, depressor nerve, 50 vagus nerve, 45 cause of heart-beat, 51 cells of Purkinje, 31 Chauveau and Marey's experi- ments, 38 on valves, 39 chords tendineee, 32, 33 circulation, aspiration in thorax, 35 in veins, 35 Colin on capacity of, 42 on nervous mechanism of, 44 columnse carnese, 32 coronary circulation, 53 corpus Arantii, 34 course of circulation, 30 cow, foreign bodies in, 54 depressor nerve of, 50 diastole of, 35 digitalin, action on, 53 dog, action of, 54 intracardiac pressure, 42 pressure in, 37 vagus nerve, 45 valves, vegetations on, 54 fibrous rings in, 32 foreign bodies in, 54 Gaskell's observations, 49 horse, aortic ring, 32 Digitized Heart, horse, depressor nerve, 50 endocardium of, 32 pleurisy in, 54 time of cycle, 38 valvular disease in, 54 horse-power of, 43 impulse of the, 31, 38 internal pressure, 40 dog, 42 . intracardiac pressure, 40 Marey's law, 48 moderator bands, 34 movements of, 35 Munk on horse-power of, 43 muscle, 31, 356 musculi papillares, 32, 33 negative pressure in, 37 nerve-centre for, 48 nervous mechanism of, 44 mitral valve, 29, 33 ox, aortic ring of, 32 pathological conditions, 53 pericarditis in horse, 54 pericardium, use of, 39 pleurisy in horse, 54 pig, vegetations on valves, 54 pneumogastric nerves, 45 position of the, 31 pulmonary valve, 29 pulmonic circulation, 29 rabbit, depressor nerve, 50 refractory period, 52 relations of, 31 rotation of, 36 rupture of, 54 semilunar valves, 29, 34 sigmoid valves, 34 sounds of, 36, 40 sympathetic, 45, 49 systemic circulation, 29 systole of, 35 tricuspid valve, 29 vagus in neck, cat, 45 dog, 45 valves of, 29 action of, 39 aortic, 29 auriculo-ventricular, 33 right, 29 bicuspid, 33 Chauveau on, 39 mitral, 29, 33 pulmonary, 29 semilunar, 29, 34 sigmoid, 34 tricuspid, 29 use of the, 30 valvular disease, horse, 54 work of, 43 Helicotrema, 500 44—2 by Microsoft® 692 A MANUAL OP VETEEINARY PHYSIOLOGY Heidenhain, lymph production theory, 250 on secretory nerves, 146 Helmholtz on accommodation, 467 Henle, ascending limb of, 289 loop of (kidney), 289 Hensen on Graafian follicle, 589 Hensen's cells, 501 Herbivora, respiratory quotient in, 96 saliva in, 140 stomach absorption in, 259 teeth in, 132 Hexone bases, 187 Hexoses, 660 Hibernating animals, 338 Hibernation, 349 Hiccough, 130 Hip-joint, 512 Hippomanes, 608 Goodall on, 608 Hippuric acid, 650 in blood, 20 in urine, 222 Histidine, 236 Hock-joint, 508 Hofmeister on bile, 225 on gastric acids, 161 on periods of stomach digestion, 171 Holoblastic ova, 597 Homoithermal animals, 337 Hoof, 546 Hoppe Seyler, analysis of pancreatic juice, 232 Hormone, 264 Horn, 272 chemistry of, 557 growth of, 559 laminae, 549 origin of, 564 salts of, 558 structure of, 552 use of the moisture in, 556 Horse, absorption in, 257 act of standing, 521 amble of, 526 amount of air required, 113, 116 of blood in, 23 of haemoglobin in, 8 of heat produced, 351 aortic ring, 32 apoplexy of lungs, 120 area of acute vision, 464 of intestinal tract, 201 astigmatism of, 469 azoturia in, 10 Bell's experiment, 427 bile, action of, 225 amount of, per hour, 224 specific gravity of, 218 Horse, blood of, 2, 21 time of clotting, 16 pressure in, 63 boiled food for, 171 broken wind, 120 bronchitis, 120 ' brushing,' 512 buck-jumping, 532 canter of, 526 capacity of intestinal canal, 201 of stomach, 151 ' cat-hairs,' 273 chest, Colin's figures, 85 ciliary muscle, 461 and atropin, 468 clotting of blood in, 15 corpora nigra, 460 'cow-kicking,' 512, 532 dandruff, 283 development of embryo of, 594 diaphragm, 86 'dishing,' 512 division of phrenic nerves in, 112 draught, 534 drinking of, 134 endocardium, 32 experimental paralysis, 444 eyelashes, 477 feeding of, 158 feces of, 208 foot, bones of, 538. See Foot fundus oculi, 465 gallop of, 528 gastric glands in, 162 growth of, Percival on, 625 hair of, 273 heart, depressor nerve of, 50 experiments by Chauveau and Marey, 38 intestinal affections in, 204 digestion in, 187 iris of, 458 Eversbusch on, 459 'jibbing,' 450 jump of, 530 kicking of, 532 laryngitis, 121 larynx, 121, 122 lying down, 521 lymph from, 247 lymphangitis in, 335 mastication, 134 maximum muscular effort, 536 medicines by the mouth, 185 muscles of eyeball ,471 myopia in, 468, 481 neighing of, 129 normal daily work of, 532 nostril of, 93 Digitized by Microsoft® INDEX 693 Horse, number of respirations, 90 oedema of legs, 251 - oesophagus of, 137 old age in, 629 osteoporosis in, 335 pancreatic fluid of, 232 paralysis, larynx, 432 pericarditis, 54 period of puberty, 585 periods of stomach digestion, 171 pilocarpin in, 281 plantar neurectomy in, 317 pleural cavities of, 84 pleurisy in, 54, 120 pneumonia in, 120 pressure, negative, in pleural cavity, 90 pulsation in jugulars, 57 pulse-rate of, 70 psychical powers, affection, 449 intelligence, 449 memory, 449 Rankine on normal daily work, 532 rearing of, 532 Redtenbacher on daily work, 533 refraction, errors of, 470 relation between blood- and body-weight, 22 retina of, 465 rising, 522 roaring, 120, 126, 431 rupture of the diaphragm, 120 saliva of, 143 salivary glands of, 139 sense of smell, 488 spasm of the diaphragm, 120 spermatozoa, 584 sphygmogram of, 69 'speedy-cutting,' 512 ' staleness, ' 375 Stilhnan on motion, 506 stomach of, 149 absorption from, 176 digestion in, 150 sugar formation in, 170 sweat, composition of, 278 sweating of, 276 temperature of, 338 tendon reflex in, 423 thirst in, 494 thrombosis of iliac arteries in, _ 281 time of circulation circuit, 74 tongue of, 133 urine of, colour, 306 odour, 306 quantity, 305 salts in, 301 solids in, 306 Horse, urine of, specific gravity, 306 valvular disease, 54 velocity of gallop, 533 of trot, 533 voice production of, 127 vomiting of, 138, 178 walk of, 523 ' wall-eyed,' 458 watering of, 158 weight he can carry, 534 Zuntz and Lehmann's experi- ments, 105 Horse-power, 365 Watt oh, 535 Hunger, 494 Hyaline leucocytes, 12 Hydrobilirubin, 11, 12 Hydrochloric acid, 161 Hydrogen, 632 in expired air, 97 in large intestine, 208 in nutrition, 315 in stomach, 178 Hyoglycocholic acid, 221 Hyotaurocholic acid, 221 Hypermetropia, 469 Hyperpnoea, 105 Hypoblast, structures derived from, 599 Hypogastric plexus, 205 Hypoglossal nerve, 133, 138 Hypoxanthine, 296, 379 Iliac arteries, thrombosis in horse, 281 Ileum, function of, 189 Impaction, 217 Impregnation, 593 Impulse of the heart, 31 how given, 38 Inoo-ordinate movements, 409 Incus, 497 Indican, 300, 651 Indigo series, 651 Indol, 236, 651 in digestion, 189, 199, 209 in urine, 300 Indoxyl-sulphuric acid, 651 Inferior laryngeal nerve, 431 Inflammation, essential changes in, 66 Influence of heat and cold, 346 of nervous system on heat pro- duction, 341 of vagus on respiration, 110 of work on respiration, 117 Inogen, 372 Inorganic constituents of the body, 662 substances in urine, 301 Inosit, 660 Digitized by Microsoft® 694 A MANUAL OF VETERINARY PHYSIOLOGY Inosit in spermatic fluid, 584 Insensible perspiration, 277 Inspiration, 84 cause of the first, 112 muscles of, 89 Inspiratory centre, 108 tetanus, 111 Inspired air, composition of, 95 Instinct in animals, 452 Intelligence in animals, 452 Internal intercostals in respiration, 89 respiration, 99 secretions, 264 Intestinal absorption, 259 calculi, 210 canal, nervous mechanism of, 205 digestion, 186 in horse, 187 in other animals, 201 in ruminants, 199 Intestines, gases of the, 207 movements of, 201 Intracardiac pressure, 40 Intracellular enzymes, 336 Invert sugar, 658 Inverting ferments, invertase, 187 lactase, 187 maltase, 187 Involuntary muscle, 355 Iodothyrin, 269 Ipecacuanha, 180 Iris, 457, 458 Langley and Anderson on, 458 Iron, 634 in blood, 21 in the body, 665 in red cells, 327 phosphate of, in bile, 219 Irradiation in reflex action, 408 Irritability of muscle, 356 Islands of Langerhans, 240 Jacobson, organ of, 487 Jaundice, 240 Joints, 507 elbow, 512 fetlock, 513 foot, 538 hip, 512 hock, 508 knee, 512 shoulder, 512 stifle, 511 ' Jugular vein, horse, velocity of blood in, 72 pulse in, 57 Jump of horse, 530 Katabolism, 316 Kathelectrotonus, 387 Katoptric tost, 467 Keratin, 558, 639 Kicking of horse, 532 Kidney. See Urine amount of blood through, 289 Malpighian tufts, 286 movements of, 286 oncometer of Roy, 286 pathological, 313 structure of, 286 uriniferous tubules, 286 Kinase, 234 Knee-jerk, 423 -joints, 512 Koumiss, 621 Krause, end-bulbs of, 391 Kreatin, in spermatic fluid, 584 Krypton, 95 Kiihne, pancreas of rabbit, 238 table of peptones, 168 Labial glands, 141 Labyrinth, 497, 502 and muscle tonus, 379 Lachrymal gland, 477 Lactalbumin, 620 Lactase, 187 Lacteal vessel, 254 Lactic acid, 653 in allantoic fluid, 608 (digestion), 161, 209 Lactose, 620, 658 ' Laky ' blood, 6 Lameness, production of, 516 Lamina spiralis, 499 Laminae, area of, Bracy Clarke, 564 Moeller, 564 Gader, 564 Laminal tissue, 542 Laminitis, Broad's treatment, 564 Lang and Barrett on ciliary muscle, 468 on errors of refraction, 470 Langerhans, islands of, 240 Langley and Anderson on iris, 458 on gastric glands, 164 < on nerve fibres, 446 on secretion, 147 Lanolin in dandruff, 283 Lapping of dog, 134 Large intestine, 190 Laryngitis, horse, 121 Larynx, the, 122 cat, 129 dog, 129 goat, 129 horse, muscles of, 121 nervous mechanism of, 125 ox, 129 sheep, 129 Digitized by Microsoft® INDEX 695 Larynx, the, ventricles of, 129 Lateral cartilages, 541, 567 Laihyrus sativus, poisoning by, 432 Latissinius dorsi in respiration, 89 Law of Dalton and Henry, 97 Lawes and Gilbert, on composition of body, 314 , on storage, 328 Lea, Sheridan, on pancreas, 238 Lecithin, 220, 645 in blood, 20 in leucocytes, 14 in bile, 219, 220 in red corpuscle, 5 in spermatic fluid, 584 Leech extract, in coagulation, 20 Leeney on rutting of stag, 581 on ovarian tissue, 265 Lens, 457, 458 passage of light through, 481 Letting blood, effects of, 27 Leucin,-209, 236, 293, 318, 584, 646 Leucocytes, 12 cholesterin in, 14 composition of, 13 diapedesis of, 13 glycogen in, 1 4 lecithin in, 14 origin of, 14 proportion of, 12 varieties of, 12 basophile, 13 eosinophile, 13 hyaline, 12 lymphocytes, 13 polynuclear, 12 Leucocythasmia, 296 Levatores costarum in respiration, 89 Levers, 505 Foster on, 506 Levulose, 187, 657, 660 Lieberkiihn, follicles of, 186 Ligamentum inhibitorium, iris, 460 pectinatum, 456, 460 Lignin in diet of herbivora, 207 Limbs, function of, in relation to lameness, 516 structure of, in relation to lame- ness, 516 Lingual nerve, 434 Lion, intestinal canal, 201 Lioness, generation, 578 Lipase, 234, 236, 325 Lips, horse, 131 ox, 131 pig, 131 sheep, 131 jiquids, absorption of gases in, 97 jiquor amnii, 606 folliculi, 588 Liquor sanguinis, 2 fibrinogen in, 3 paraglobulin in, 3 serum albumin in, 3 globulin in, 3 Liver, 218 abscess of, 241 acid, benzoic, 222 eholalic, 221 glyeocholie, 222 hippuric, 222 hyoglycocholic, 221 hyotaurocholic, 221 sulphuric, 231 Bernard, Claude, on glycogen, 225 bile acids, origin of, 222 amount of, secreted, 223 Ellenberger on, 219 Gmelin's test, 220 Hofmeister on, 225 of horse, 218 of ox, 218 of sheep, 218 percentage composition, 219 Pettenkofer's test, 222 pigments, 220 salts, 221 use of, 224 Voit's experiments, 225 biliary calculi, 241 bilirubin, 220 Hammarsten on, 221 biliverdin, 220 blood, sugar in, 226 blood-supply of, 218 Bunge on bile pigments, 220 on changes in, 231 calcareous degeneration of, 241 calculi, biliary, 241 centre, diabetic, 230 eholalic acid, 221, 222 cholesterin, 219, 220 diabetes, 226 centre, 230 puncture, 230 disorders of, in tropics, 321 dog, bile, action of, 225 amount of, per hour, 224 Ellenberger on bile, 219 enlargements of, 241 fatty, 241 ferment, 229 furfurol, 222 gall-bladder, 223, 224 animals without, 224 glycine, 221, 222 glycocoll, 222 glycogen, 225 Digitized by Microsoft® 696 A MANUAL OF VETEEINAEY PHYSIOLOGY Liver, glycogen in muscle, 227 sources of, 228 use of, 227 glycocholate of soda, 221 glycocholic acid, 222 glycosuria, 228 Gmelin's test for bile, 220 Hammarsten on bilirubin, 221 hippuric acid, 222 histidinej 236 Hofmeister on bile, 225 horse, bile, action of, 225 amount of, per hour. 224 specific gravity of, 218 hyoglycocnolic acid, 221 hyotaurocholic acid, 221 iron, phosphate of, in bile, 219 jaundice, 240 lecithin, 219, 220 nucleo-albumin in bile, 219 ox, bile, action of, 225 amount of, per hour, 224 specific gravity, 218 sulphur in, 219 parasitic disease of, 241 pathological conditions, 240 rupture of, 241 Pettenkofer's test for bile acids, 222 phloridzin, 228 pig, bile, action of, 225 amount of, per hour, 224 sulphur in, 219 puncture, diabetic, 230 secretin, 223 sheep, bile, action of, 225 amount of, per hour, 224 specific gravity of, 218 sulphur in, 219 soda, glycocholate, 221 taurocholate, 221 stercobilin, 210, 221 sugars, conversion of, 228 in the blood, 226 supply, how regulated, 229 sulphuric acid, 231 sulphur in bile, 219 taurine, 222 taurocholate of soda, 221 urea in, 231 Voit's experiments, bile, 225 Living test-tube experiment, 19 Llama, stomach of, 182 Locomotion, 522 Locomotor apparatus, 505 amble of horse, 526 Locomotor apparatus, anti-concussion mechanisms, 518 astragalus, screw action of, 508 ' brushing ' of horse, 512 buck-jumping of horse, 532 canter of horse, 526 ' capped elbow,' horse, 522 cattle, repose of, 522 centre of gravity, horse at rest, 514 Chauveau on tendon flexor metatarsi, 509 check ligaments, function of, 514 Colin on centre of gravity, horse, 515 compression of propulsion, 517 concussion of impact, 517 co - operative antagonism, muscles, 506 corona, fracture of, 519 'cow-hocks,' horse, 512 ' cow-kicking ' of horse, 532 ' dishing ' of horse, 512 distribution of the weight of the body, 515 draught, 534 Brunei on, 535 fetlock-joint, 513 Foster on levers, 506 gallop of horse, 528 hip-joint, 512 hock-joint, 508 ' horse-power,' Watt on, 535 jump of horse, 530 kicking of horse, 532 knee-joint, 512 lameness, production of, 516 lying down, horse, 521 levers, 505 limbs, function and struc- ture of, in relation to lameness, 516 locomotion, 522 Lupton on paces of horse, 530 Marey on locomotion in horse, 522 maximum muscular effort of horse, 535 mechanisms, anti - concus- sion, 518 Muybridge on locomotion in horse, 522 normal daily work of horse, 532 pastern, fracture of, 519 pathological, 536 Digitized by Microsoft® INDEX 697 Locomotor apparatus, Rankine on daily work, horse, 533 rearing of horse, 532 Redtenbacher on daily work, horse, 533 rising of horse, 522 spavin, position of, 510 'speedy-cutting,' horse, 512 shoulder -joint, 512 standing, act of, 521 Stanford on locomotion in horse, 522 stifle-joint, discussion of, 508 description of, 511 Stillman on function of suspensory ligament, 513 on locomotion in horse, 522 on muscles of propul- sion, 506 suffraginis, fracture of, 519 suspensory ligament, func- tion of, 513 synovia, 507 trot of horse, 524 velocity of gallop, 533 of trot, 533 walk of horse, 523 Waller's ' co-operative an- tagonism,' 506 weight of the body, distri- bution of, 515 which a horse can carry, 534 [iuciani on cerebellum, 438 Jungs, 84. See Respiration apoplexy of, in horse, 120 Jungwitz on expansion of foot, 571 jupton, J. I., on foot of horse, 530, 559 Luxus consumption ' of nitrogen, 321 jying down, horse, 521 jymph, 242, 245 capillary, 243 cell distinguished from white corpuscle, 12 Colin on, 246 formation of, 247 movement of, 251 Colin on, 252 Weiss on, 253 plasma, 246 production, Heidenhain on, 250 physical theory, 247 secretory theory, 250 Starling on, 249 quantity of, 246 Lymph spaces, 242 sinus, 245 Lymphagogues, 250 Lymphangitis, 321 Lymphatic glands, 244 vessels, 243 Lymphocytes, 13 Lysatinino, 646 Lysine, 236 Maedonald on foot of horse, 572 Macula aeustica, 502 Magnesium in urine, 301, 303 phosphate, 21, 212, 219, 233 salts, 212, 219, 665 sulphate, action on plasma, 3 Majendie's experiment (vomiting), 180 Malaria parasite, 27 Malassez's method for number of corpuscles, 5 Malic acid, 209 Malleus, 497 Maltase, 187 Maltose, 187, 656, 658 Mare, analysis of milk, 620 generation, 578 ovary of, 587 period of gestation, 614 position of foetus in, 615 uterine glands in, 614 Marey's law, circulation, 48 observations, locomotion in horse, 522 Marmot, hibernation in, 350 Marsh gas in cellulose digestion, 198 in expired air, 97 in large intestine, 208 in stomach, 178 Marshall and Jolly, cestrous cycle in dog, 578 Masseter muscle, 136 Mastication, dog, 134 Gamgee on, 135 horse, 134 nerves of, 136 ox, 134 sheep, 134 Maximum muscular effort, horse, 535 Medulla oblongata, 434 centres, cardiac, 48, 49 diabetic, 230 mastication, 436 respiratory, 108 saliva, 436 swallowing, 436 sweat, 280 vaso-motor, 75 vomiting, 436 Medullary groove, 600 Digitized by Microsoft® 698 A MANUAL OP VETERINARY PHYSIOLOGY Medullary sheath, nerves, 383 Mechanism, anti-concussion, 518 of foot, 568 defensive, of the body, 25 of ejaculation, 587 nervous, of the intestinal canal, 205 of the larynx, 125 of sweating, 279 pancreatic secretion, 233 rumiuation, 183 secretion of saliva, 144 Meconium, 212 Meibomian glands, 477 Melanin, 274, 644 Membrana basilaris, 499 granulosa, 588 nictitans, 454 Membrane of Reissner, 500 Mendel's theories of heredity, 274 Meroblastic ova, 597 Mesentery, circulation in, 65 Mesoblast, structures derived from, 599 Metabolism, 316 views of Pfliiger, 318 ofVoit, 318 Metatarsal artery, horse, velocity of blood in, 72 Methaemoglobin, 10, 27 Methylene blue experiment, 102 absorption of, from pleura, 259 Methyl-glycine, 645 Metoestrous period, 577 Metcestrum, 577 Metschnikoff, researches of, 14 M'Kendrick on area of intestinal tract, 201 on taste goblets, 489 Micrococcus urece, 648 Micturition, act of, 313 Mid-brain, 439 Middle ear, 497 Migration of white corpuscles, 66 Milk, secretion of, 617 sugar, 187, 620 Millon's reagent, 641 Minot on polar bodies, 590 Mitral valve, 29, 33 Moderator bands (heart), 34 Moeller on laminae, 565 on sublaminal tissue, 543 on urine of calves, 309 Moisture in air, 95 Molar glands, 141 Monoestrous mammals, 577 Monkey, dicestrous cycle of, 580 Monocular vision, 473 Monosaccharides, 659 Morgan, on intelligence, instinct, and reason, 452 Morphia, absorption of, by air-pas- sages, 257 effect on iris, 459 Motor areas of brain, 441 oculi nerves, 425 Movements of diaphragm, 85 of eyeball, 470 of heart, 35 of intestines, 201 of stomach, 184 Mucin, 140, 152, 165 Mule, psychical powers of, 449 subepiglottic sinus of, 129 sweating, 277 Midler's estimate of haemoglobin, 8 Munk on capacity of heart, 42 on horse-power of heart, 43 on ox urine, 308 on phosphates in urine, 304 statistics of intestinal canal, 201 Murmur, respiratory, 119 vesicular, 119 Muscarin, 645 action on heart, 48, 53 Muscle antagonism, 354 currents, 366 curve, 360 nerve preparation, 357 plasma, 377 sense, 353, 492 wave, 361 Muscles of eyeball, 471 of respiration, 89 Muscular system, 352 acid, sarco-lactie, 374, 378, 379 uric (muscle), 379 active muscles, changes in, 370 ash, composition of (muscle), 379 carnine, 379 causation of a muscular con- traction, 372 changes in active and resting muscles, 369 Chauveau on muscle work, 365 chemical changes during con- traction of muscle, 370 composition of muscle, 377 engine (muscle as), 366 Colin on muscle temperature, 371 comparisons, body and engine, 365 condition in horses, 375 contractility of muscle, 356 creatine, 379 curare, effect on end-plates, 354, 361 elasticity, muscle, 363 Digitized by Microsoft® INDEX 699 Muscular system, elastic tension muscles, 364 electric phenomena of muscles, 366-369 effect of muscular work on blood, 2 end-plate in muscle, 354 extensibility of muscle, 363 extractives (muscle), 379 fatigue (muscle), 372 Wedenski on, 375 Fiek on muscle work, 365 gastrocnemius of frog, 357 Golgi, tendon organs of, 351 heart muscle, 356 horso-power, 365 ' staleness,' 375 hypoxanthine, 379 inogen, 372 involuntary muscle, 355 irritability of muscle, 356 labyrinth and muscle tonus, 379 muscle antagonism, 354 currents, 366 curve, 360 -nerve preparation, 357 plasma, 377 sense, 353 wave, 361 myosin, 378 myosinogen, 378 neuro-muscular spindles, 353 pale muscle, 355 phenomena of contraction of, 380 phosphoric acid (muscle), 379 potassium salts, 379 resting muscle, changes in, 379 rigor mortis, 379 sarcolemma, 352 sarcomere, 353 sarcoplasm, 353 sarcostyles, 353 sarco-lactic acid, 374, 378, 379 Schafer's views on muscle, 353 smooth muscle, phenomena of contraction, 380 structure of muscle, 352 summation of contractions, 361, 380 taurine, 379 tetanus, 362 ' tone,' 364 ' training,' 377 urea (muscle), 379 voluntary muscle, 352 xanthine, 379 Zuntz on muscle work, 365 Musculi papillares, 32, 33 Musical sounds, 495 Muybridge on locomotion in horse, 522 Myopia, 468 Myosin, 378 Myosinogen, 378 Nasal chamber, olfactory portion, 93 respiratory portion, 93 Nasse on time of coagulation, 16 Nature of nervous impulses, 389 Navicular bone, 538 Negative pressure in heart, 37 in pleural cavity, dog, 90 horse, 90 sheep, 90 in respiration, 90 Negative pressure in veins, 64 variation (nerve), 386 Negro's skin, 346 Neighing of horse, 129 Neural groove, 600 Neurilemma, 383 Neurine, 645 Neuroglia, 395 Neuro-muscular spindles, 353, 492 Neurone, 412 Neutral point (nerve), 387 Nerve centre for heart, 48 brachial, 78 chorda tympani, 76, 78, 133 facial, 108 fifth, in mastication, 136 lingual branch of, 133 ganglia. See Ganglia salivary glands, 147 hypoglossal, 133, 138 supply of stomach, 185 Nerves, afferent, 382 deglutition of, 138 dilator, 78 dorso-lumbar, 108 glosso-pharyngeal, 110 lingual branch of, 133 hypogastric plexus, 205 mastication, 136 motor, of respiration, 108 nasal of fifth, 110 ocular muscles, 472 pharyngeal plexus, 138 phrenic, 108 division of, 112 recurrent laryngeal, 125, 138 sciatic, 78 seventh pair, division of, 113 in mastication, 136 superior laryngeal, 110, 125, 138 vaso-constrictor, 77 sympathetic, 144 tongue, 133 Nervi erigentes, 78 Digitized by Microsoft® 700 A MANUAL OP VETEEINAEY PHYSIOLOGY Nervi nervorum, 385 Nervous control of bloodvessels, 77 of gastric j uice, 169 of pancreatic secretion, 233 of rumination, 183 of secretion of saliva, 144 of heat production, 341 Nervous mechanism of heart, 44 of the intestinal canal, 205 of the larynx, 125 of stomach of ruminants, 186 of respiration, 108 of sweating, 279 Nervous system, 382 abducens cranial nerve, 428 afferent nerves, 382 paths in the cord, 405 anelectrotonus, 387 ano-spinal centre in cord, 424 arrangement of the cord (spinal), 392 Arloing on sympathetic nerves, 448 ass, psychical powers of, 449 ascending tracts (spinal cord), 402 automatic action, 423 axis-cylinder, nerves, 383 axone, 412 Bell's experiment, horse, 427 Bernard on division of facial, 429 brain, circulation in, 444 coverings of, 444 brain, movements of, 445 bulb, functions of, 434, 437 used for medulla oblongata, 404 Burdach, column of, 403 cat, section of corpora quadri- gemina, 439 submaxillary ganglion, 427 sympathetic, 447 centres in the medulla, 435 cerebellum, 438 Luciani on, 439 cerebral fluid, 445 cerebrum, 440 Chauveau on velocity of nerve impulses, 389 chloral hydrate, effect on brain, 445 chloroform, effect on brain, 445 cry of men and animals, 440 chorda tympani of facial, 428 cilio-spinal centre in cord, 424 Clarke's column (spinal cord), 395 Colin on experimental paralysis, horse, 444 conductivity of nerves, 389 Nervous system, co-ordinate move- ment, 409 corpora quadrigemina, 439 corpora striata, 437 cranial nerves, 425 eighth, 429 eleventh, 434 fifth, 425 fourth, 425 ninth, 430 seventh, 428 sixth, 428 tenth, 430 third, 425 twelfth, 434 current of action, 386 of rest, 386 degeneration of nerves, 389 Wallerian, 400 dendrites, 412 descending tracts (spinal cord), 402 distribution of nerve fibres in the cord, 396 disynaptic arc, 416 dog, brain, localization, 441 psychical powers, 449 reflex action in, 409 submaxillary ganglion, 427 sympathetic, 447 tendon reflexes in, 423 efferent nerves, 382 paths in the cord, 405 effector organ, 412 eighth pair cranial nerves, 429 electric phenomena of nerves, 385 electrotonus, 387 elephant, psychical powers, 449 ether, effect on brain, 445 eleventh pair cranial nerves, 434 erection centre in cord, 424 excitability of nerves, 385 experimental paralysis, horse, 444 facial nerve, 428 fifth pair cranial nerves, 425 inferior maxillary, 426 ophthalmic, 426 superior maxillary, 426 fourth pair cranial nerves, 425 frog, reflex action in, 409 functions of spinal cord, 425 nerves, 399 ganglia, collateral, 447 terminal, 447 ganglia on nerves, 384 Gaskell on sympathetic nerves, 446 Gasserian ganglion, 426 Digitized by Microsoft® INDEX 701 Nervous system, genito-spinal centre in cord, 424 glia cells, cord, 395 glossopharyngeal nerve, 430 Goll, column of, 403 Golgi, organ of, 391 horse, Bell's experiment, 427 experimental paralysis, 444 'jibbing,' 450 paralysis of larynx, 432 psychical powers, 444 memory, 444 affection, 444 intelligence, 444 roaring in, 4, 31, 126 tendon reflex in, 423 inco-ordinate movements, 409 inferior laryngeal nerve, 431 influence on circulation, 74 instinct in animals, 452 v. reason, 452 intelligence in animals, 452 irradiation in reflex action, 408 katheleetrotonus, 387 knee-jerk, 423 Krause's end-bulbs, 391 Langley on nerve fibres, 446 larynx, paralysis in horses, 432 Lathyrus sativus, poisoning, 432 lingual nerve, 434 Luciani on cerebellum, 438 medulla oblongata centres, 434 cardiac, 48, 49 diabetic, 230 masticatory, 436 respiratory, 108 saliva, 436 swallowing, 436 vaso-motor, 75 vomiting, 436 medullary sheath, 383 mid-brain, 439 Morgan on intelligence, instinct, and reason, 452 motor areas, brain, 441 oculi nerves, 425 mule, psychical powers, 449 nature of nervous impulses, 389 negative variation (nerve), 386 nerves, afferent, 382 nervi nervorum, 385 nervus intermedius of facial, 428 neurilemma, 383 neuroglia, 395 neurone, 412 neutral point (nerve), 387 nicotin, action on nerve-cells, 446 ninth pair cranial nerves, 430 nociceptive arc, 417 ox, sympathetic, 448 Nervous system, parturition centre in cord, 424 pathetic nerve, 425 pars trigemini nerve, 425 perikaryon, 412 pharyngeal nerve, 431 pneumogastric nerve, 430 pons Varolii, 437 portio dura, 428 mollis, 429 principle of the common path, 411" psychical powers of animals, 448-9 rabbit, sympathetic, 447 reason in animals, 452 v. instinct, 452 receptor organ, 412 recurrent laryngeal nerve, 431 sensibility, 399 reflex act, time occupied by, 422 action, 409 arc, 411 roaring in horses, 126, 431 Romanes on instinct and reason, 452 scratch reflex, 414 sensory areas, brain, 441 seventh pair cranial nerves, 428 sixth pair cranial nerves, 428 Smith, Sydney, on instinct and reason, 452 special centres in the spinal cord, 424 spinal accessory nerves, 434 cord, 391 nerves, 392 ■function of, 399 stepping reflex, 410 structure of nerves, 383 strychnine, effect on brain, 445 submaxillary ganglion, cat and dog, 427 superior laryngeal nerve, 431 sympathetic, 446 tactile cells, 391 sweat centres in cord, 424 synapses, 412 tendon reflexes, 423 tenth pair cranial nerves, 430 termination of nerves, 391 thalami optici, 437 third pair cranial nerves, 425 tracts in spinal cord, 401, 403 trophic centres in cord, 424 Tiirck, column of, 403 twelfth pair cranial nerves, 434 vaso-motor centres in cord, 424 velocity of nerve impulses, 389 Digitized by Microsoft® 702 A MANUAL OF VETERINARY PHYSIOLOGY Nervous system, vesico-spinal centre in cord, 424 Wallerian degeneration (spinal nerve), 400 Nervus intermedins of facial, 428 Newsom, calculation, hair of horse, 273 Nieotin, action on heart, 53 on nerve cells, 446 in peristalsis, 203 in secretion, 147 Ninth pair cranial nerves, 430 Nitre in veterinary practice, 282 Nitric oxide haemoglobin, 11 Nitrogen, 95, 633 in blood, 23, 25 in nutrition, 315 in stomach, 178 Nitrogenous equilibrium, 319, 320 food, 318 fats, 644 substances in urine, 292 Nociceptive arc, 417 Noise, 496 Non-nitrogenous bodies, 652 food, 322 Normal temperature of animals, 338 Nostrils, 92 false, 92 Notochord, 600 Nuclein in spermatic fluid, 584 Nucleo-albumin in bile, 219 -proteid, 3, 637 Number of respirations, 90 Nutrition, 314 amount of food required, 332 anabolism, 316 arginine, 318 azoturia, 321 broken wind, 120, 335 calories of heat, 324 carbohydrates, oxidation of, 323 carbon in, 316 castration, effect of, on fattening, 326 cat, composition of body, 315 cattle, osteomalacia in, 327 circulating proteid of Voit, 319 composition of the body, 314 expenditure and income of the body, 315 fever, effect on nutrition, 335 food, amount of, required, 333 inorganic, 327 Grandeau and Leelerc on diet, 333 haemoglobinuria in horse, 335 heat, calorie of, 324 kilocalorie of, 324 horse, lymphangitis in, 335 osteoporosis in, 335 Nutrition, horse, plan tar neurectomy, ' 317 hydrogen in, 315 iron in red cells, 327 katabolism, 316: Lawes and Gilbert on composi- tion of body, 314 on storage, 328 leucine, 318 lipase, 325 liver disorders of tropical climates, 321 ' luxus consumption ' of nitrogen , 321 lymphangitis, 321- metabolism, 316 Pfluger on, 318 Voit on, 318 nitrogen in, 315 nitrogenous equilibrium, 319, 320 food, 318 non-nitrogenous food, 322 pig, composition of body, 314 obesity in show cattle, 326 ox, composition of body, 314 pathological disorders of nutri- tion, 335 potassium in red cells, 327 in sweat, 327 Rubner's experiments, 332 salts in nutrition, 316 sheep, composition of body, 314 sodium in blood plasma, 327 starch, proteid-sparing action of, 320 starvation, 329 storage of tissue, 328 subsistence diet, 333 sulphur in hair, 327 in nutrition, 316 tissue proteid of Voit, 319 storage of, 328 tyrosine, 318 Voit's theory, metabolism, 318 water in tissues, 328 Wolff on diet, 334 Nux vomica, absorption of, by air- passages, 257 in Cfeonm, 260 Oats, digestion of, 156 Obesity in show cattle, 326 Odour of blood, 2 OMema, production of, 251 Oesophageal groove, 180 OJsophagus of horse, 137 of dog, 138 of ox, 138 of sheep, 138 Digitized by Microsoft® INDEX 703 (Estrus, 577 Oleic acid, 652 Olein, 653 in blood, 20 in milk, 620 Omasum, 174 Omnivora, respiratory quotient in, 96 Oncometer of Roy, 286 Ophthalmia, sympathetic, 455 Ophthalmoscope, 464 Opsonins, 26 Optic disc, 464 nerve, 454 - decussation of, 455 Optics, physiological, 478 Osazone tests, 658, 661 Osmosis in lymph formation, 247 Osmotic pressure, 248 Ostrich, rutting of, 581 Ostium abdominale, 589 Otoliths, 499, 504 Otter, generation of, 578 Ovaries, 587 effect of removal of, 582, 583 influence of, 265 Overtones, 495 Ovulation : in ass, cat, cow, deer, dog, elephant, ferret, mare, mon- key, pig, rabbit, sheep, wolf, 591 Ovum, 589 development of the, 597 Ox, aortic ring of, 32 area of intestinal tract, 201 bellowing of, 129 bile, action of, 225 amount of, per hour, 224 specific gravity of, 218 sulphur in, 219 blood of, 2, 21 time of clotting, 16 capacity of intestinal canal, 201 composition of body, 314 drinking of, 134 heat lost by, 351 intestinal digestion in, 201 iris of, 458 larynx of, 129 lymph from, 247 mastication, 134 number of respirations, 90 oesophagus of, 138 pulse-rate of, 70 relation between blood- and body-weight, 22 salivary glands of, 139 sweating, 277 sympathetic nervous system, 448 tongue of, 133 Oxidases, 337 Oxidations in animal heat, 336 Oxygen, 95, 633 ■ in blood, 23, 24 deficiency in, 105 excess of, 106 fate of, in the tissues, 100 inhalation of, in disease, 107 intramolecular, 101 in stomach, 178 Oxyhemoglobin, 7 Palate, grooves in, 133 Palatine glands, 141 Pale muscle, 355 phenomena of contraction of, 380 Palmitic acid, 652 Palmitin, 653 in blood, 20 in milk, 620 Pancreas, 232 acid, aspartie, 236 glutaminic, 236 albumose, 236 amido-acids, 236 amylopsin, 234, 236 arginine, 236 calcium phosphate, 233 Colin on, 232 enterokinase, 234 enzyme, diastatic, 234 lipolytic, 234 proteolytic, 234 erepsin, 236 kinase, 234 Kiihne on rabbit, 238 Langerhans, islands of, 240 Lea, Sheridan on, 238 leucin, 236 lipase, 234, 236 lysin, 236 magnesium phosphate, 233 pathological disturbances, 241 Pancreatic cells, changes in, 237 diabetes, 239 fluid, dog, 232 Hoppe-Seyler on, 232 horse, 232 Pawlow on, 237 secretion, amount of, 238 Bay liss and Starling on, 233 mechanism of, 233 nerve control of, 233 peptones, 236 phenol, 236 potassium chloride, 233 Schmidt's analysis, 232 secretin, 233 skatol, 236 sodium carbonate, 233 chloride, 233 Digitized by Microsoft® 704 A MANUAL OF VETERINARY PHYSIOLOGY Pancreatic secretion, sodium phos- phate, 233 Starling on, 235 steapsin, 234, 236 trypophan, 236 trypsin, 234 trypsinogen, 234 tyrosine, 236 uses of, 233 Papilla (retina), 464 Papillae of tongue, eircumvallate, 489 filiform, 489 fungiform, 489 Paracasein, 168 Paraglobulin, 3, 245 Paralytic secretion, 146 Parasitic diseases, digestive canal, 217 of liver, 241 Parathyroid, 268 Parotid gland, 139, 141 secretion, 147 Pars trigemini, nerve, 425 Parthenogenesis, 586 Partial pressure of Bivnsen, 98 Parturition, 615 centre in cord, 424 Pastern, fracture of, 519 Pasteur on ferments, 642 Pathetic nerve, 425 Pathological conditions, arteries, 83 foot, 576 heart, 53 liver, 240 locomotor apparatus, 536 nutrition, 335 pulse, 83 respiration, 120 ' roaring,' 126 Pawlow on gastric juice, 165 on pancreatic juice, 237 Pedal bone, 538 descent of, 572 Pendular movements of bowel, 203 Pentoses, 661 Pepsin, 152, 165-6, 175, 643 Peptones, 168, 187, 199, 236, 637 effect on coagulation, 20 Peptonuria, 263 1'ercival on growth of horse, 625 Pericarditis in horse, 54 Pericardium, use of, 39 Perikaryon, 412 Perilymph, 498 Periods of stomach digestion, 171 Periople, 546 Peristalsis, 203 effects of cocain, 203 of nicotin, 203 Starling and Bayliss on, 203 Peritoneal cavity, absorption from, 258 Peroxidases, 337 Perspiration, insensible, Colin, 277 Pettenkofer's test for bile acids, 222 Peyer's patches, 255 Phagocytosis, 14, 26 in spleen, 267 Pharyngeal plexus, 138 nerve, 431 Phenol, 651, 665 digestion, 199, 209 pancreas, 236 urine, 300 Phenol, ethereal sulphate of, 292 Phenyl-acetic acids, 199 -glucosazone, 659 -maltosazone, 658 -proprionic acid, 199 -sulphate of potassium, 665 Phloridzin, 228 Phonation, 127 Phosphates, 21, 665 Phosphoric acid, 379 Phosphorus, 633 Phrenic nerves, division of, 112 Physical characters of blood, 1 of chyme, 188 Physiological optics, 478 salt solution, 6 shoeing, 575 Physostigmin, absorption by air- passages, 257 action on heart, 53 Pig, area of intestinal tract, 201 amount of air respired, 116 bile, action of, 225 auiount of, per hour, 224 sulphur in, 219 blood of, 21 time of clotting, 16 capacity of intestinal canal, 201 composition of body, 314 generation, 578 feces of, 208 growth of, 625 heart valves, vegetations on, 54 heat lost by, 351 intestinal digestion in, 201 cestrous cycle, 580 ovary of, 587 period of gestation, 615 puberty, 585 pulse-rate of, 70 relation between blood- and body-weight, 22 number of respirations, 90 saliva of, 143, 150 stomach, digestion in, 175 sweating, 277 Digitized by Microsoft® INDEX 705 urine of, 305, 309 vomiting of, 178 lent in hair, 274 lents of the body, 644 rs of Gorti, 499 arpin, absorption by air-pas- sages, 257 lotion on heart, 53 in cat, 281 in dog, 281 in horse, 281 in man, 281 in secretion of saliva, 146 in sweating, 280 il body, 270 owski's reaction, 641 tary body, 270 mta, 603 jotyledonary, 604 ;umulata, 604 liscoidal, 604 netadiscoidal, 604 Dlicata, 604 Jolycotyledonous, 604 :onary, 604 ax cushion, 542 la, blood, 2 ieparation of proteids from, 3 lets, blood, 7, 18 al cavity, absorption from, 258 ^ methylene blue, 259 isy, effusion in, 3 n horse, 54, 120 nonia, horse, 120 nogastric nerve, 45, 430, lothermal animal, 337 luclear colourless corpuscles, 12 2strous mammals, 577 accharides, 655 Varolii, 437 i dura, seventh nerve, 428 mollis, eighth nerve, 429 on of the heart, 31 nortem rises of temperature, sium chloride in blood, 21 in pancreas, 233 srrocyanide, absorption from bowel, 259 from air-passages, 257 from cellular tissue, 258 from skin, 258 i faeces, 210 i red cells, 327 i sweat, 301, 303, 327 dide, absorption from peri- toneum, 259 Its, action on heart, 52 muscle, 379 in vegetable food, 663 Potassium sulphocyanide, 140 in wool, 284 Pr.-ecipitin, 26 Praecrucial gyrus, dog, 128 Pregnancy, duration of, 614 Prehension of food, 131 Principle of the common path (nerves), 411 Primitive groove, 600 streak, 599 Prooestrous period, 577 Procestrum, 577 external signs of, 580 Process for calculation of amount of blood, 22 Proamnion, 601 Proprionic acid, 207 Prostate, secretion of, 584 Proteid, 167, 634 absorption of, 263 classification, 636 in plasma, 3 of serum, 3 reactions, 641 separation from plasma, 3 to peptone, conversion of, 168 Proteoses, 199, 637 primary, 168 secondary, 168 Prothrombin, 18 Prototheria, 589 Pseudo-nuclsin, 638 Psychical powers (ass, dog, elephant, horse, mule), 448 Pterygoid muscles, 135, 136 Ptyalin, 140, 143, 643 Puberty, period of, 585 Pulmonary valve, 29 circulation, 29 Pulse, cause of, 66 character of, 83 explanation of, 58 pathological, 83 rate and blood-pressure, relation, 71 rate, variations in, 71 tension, 70 venous, 67 wave, 67 graphic study of, 68 length of, 67 sphygmogram, 68 velocity of, 67 Puncture, diabetic, 230 Purin bases, 296 series, 638 Purkinje, cells of, 31 Purpura, 27 Pyloric glands, 164 Pyrocatechin, 301, 651 45 Digitized by Microsoft® 706 A MANUAL OF VETERINARY PHYSIOLOGY Quantity of blood in the body, 22 Rabbit, division of cervical sympa- thetic, 75 heart, depressor nerve, 50 ovary of, 587 pancreas of, 238 period of gestation, 615 saliva of, 143 sympathetic nervous system, 447 time occupied by circuit of circu- lation, 74 Rabies, 27 Radiation, 343 Rankine on daily work of horse, 533 Ratio of heart-beats to respiration, 91 Reagent, Barfoed's, 658 Rearing of horse, 532 Reason in animals, 452 v. instinct, 452 Reaction, biuret, 641 of bile, 21S of blood, 1 of contents of stomach, 161 of urine, 304 Reactions, proteid, of Adamkiewicz, 641 ofMillon, 641 of Piotrowski, 641 xanthoproteic, 641 various, 642 Receptor organ, 412 Rectum, absorption from, 199 ether by, 261 Recurrent laryngeal nerve, 138, 431 division of, 126 sensibility, 399 Red corpuscles of blood, 4 Redtenbacher on horse's daily work, 533 Reduced hemoglobin, 7 Reflex acts, time occupied by, 422 action, 407 arc, 411 Refraction, errors of, in horses, 470 Refractory period of heart-beat, 52 Reiset on gases in expired air, 97 Reissner's membrane, 499 Relations of the heart, 31 Rennin, 165, 620 Reserve air, 113 Residual air, 113 Respiration, 84 abdominal muscles in, 89 absorption of gases in liquids, 97 acid, sarco-lactic, 119 air, amount of, required, 113 atmospheric, 95 moisture in, 95 alveolar, 116 Respiration, eompleinental, 113 reserve, 113 residual, 113 tidal, 113 amount of air respired, Boussin- gault, 116 apncea, 107 apoplexy of lungs, horse, 120 argon, 95 arytenoid cartilages, 94 asphyxia, 106 ass, amount of air respired, 116 braying of, 120 subepiglottic sinus of, 129 atmospheric air, composition of, 95 arytenoideus, 122 bellowing of ox, 129 bleating of sheep, 129 blood-pressure, effect on, 91 Bohr on CO, in blood, 103 Boussingault's tables, 116 ' broken wind' in horse, 120 bronchitis in the horse, 120 Bunsen on ' partial pressure, ' 98 carbon monoxide, poisoning by, 105 Haldane's experiments, 104 carbonic acid, 95 Bohr's view, 103 fate of, 102 carnivora, respiratory quotient in, 96 castration, effect on voice, 128 cat, larynx of, 129 voice centre in, 129 cause of first respiration, 112 Ohauveau on larynx, 129 chemistry of, 114 circulation, effect on, 91 Golin's figures, chest of horse, 85 experiments, seventh pair of nerves, 113 guttural pouches, 128 complemental air, 113 composition of atmospheric air, 95 inspired, 95 expired, 95 coughing, 129 cow, amount of air respired, 116 crico-arytenoideus lateralis, 122 posticus, 122 Dalton and Henry's law, 97 deficiency in oxygen, 105 diaphragm, description of, 86 horse, rupture of, 120 spasm of, 120 in, 89 Digitized by Microsoft® INDEX 707 aspiration, diaphragm, movements of, 85 diffusion of gases, 98 disassociation, process of, 103 division of seventh pair of nerves, 113 of phrenic nerves, 112 dog, amount of air required, 116 larynx of, 129 number of respirations, 90 pressure in pleural cavity, 96 respiratory curves of, 92 voice centre in cerebral cortex, 128 dyspnoea, 106 effect of, on circulation, 91 epiglottis, 125 expiration, 88 muscles of, 89 expired air, composition of, 95 external intercostal muscles in, 89, 99 facial sinuses, 94 false nostril, 92 foetal lung, 88 fcetus, first inspiration in, 112 gases, absorption of, in liquids, 97 Dalton and Henry's law, 97 diffusion of, 98 ' partial pressure ' of Bunsen, 98 glottis, 94, 124 goat, larynx of, 129 number of respirations, 90 guttural pouches, 128 Haldane and Lorraine Smith's results, 119 and Priestley's experiments, 117 Haldane's experiments with CO, 104 heart-beats, ratio of, to, 91 herbivora, respiratory quotient in, 96 hiccough, 130 horse, amount of air required, 113, 116 apoplexy of lungs, 120 'broken wind,' 120 bronchitis, 120 chest of, Colin 's figures, 85 diaphragm of, 86 division of phrenic nerves in, 112 laryngitis in, 121 larynx, 121, 122 neighing of, 129 nostril of, 93 Respiration, horse, number of respira- tions in, 90 pleural cavities of, 84 pleurisy in, 120 pneumonia, 120 pressure in pleural cavity, 90 roaring, 120 rupture of diaphragm in, 120 spasm of diaphragm in, 120 voice production in, 127 Zuntz and Lehmann's ex- periments, 105 hydrogen in expired air, 97 hyperpncea, 105 influence of the vagus on, 110 of work on, 117 inspiration, 84 cause of first, 112 muscles of, 89 inspiratory centre, 108 tetanus, 111 inspired air, 95 internal intercostals in, 89, 99 krypton, 95 laryngitis, horse, 121 larynx, 122 muscles of, 121 nervous mechanism of, 125 of cat, 129 of dog, 129 of goat, 129 of horse, 121, 122 of ox, 129 of sheep, 129 ventricles of, 129 latissimus dorsi in, 89 law of Dalton and Henry, 97 levatores costarum in, 89 liquids, absorption of gases in, 97 lungs, 84 apoplexy of, in horse, 120 marsh gas in expired air, 97 mechanism, nervous, of the larynx, 125 methylene blue experiment, 102 moisture in air, 95 movements of diaphragm, 85 mule, subepiglottic sinus of, 129 murmur, respiratory, 119 vesicular, 119 muscles of, 89 nasal chamber, olfactory portion, respiratory portion, 93 pressure in pleural cavity, dog, 90 horse, 90 45—2 Digitized by Microsoft® 708 A MANUAL OF VETERINARY PHYSIOLOGY Respiration, negative pressure in pleural cavity of sheep, 90 neighing of horse, 129 nerves, dorso-lumbar, 108 facial, 108 glossopharyngeal, 110 motor of, 108 nasal of fifth, 110 phrenic, 108 division of, 112 recurrent laryngeal, of vagus, 108, 125 sciatic, 78 seventh pair, division of, 113 superior laryngeal, 110, 125 vaso-constrictor, 77 nervous mechanism of the larynx, 125 governing respiration, 108 nitrogen, 95 nostrils, 92 false, 92 omnivora, respiratory quotient in, 96 ox, bellowing of, 129 larynx of, 129 number of respirations, 90 oxygen, 95 deficiency in, 105 excess of, 106 fate of, in the tissues, 100 inhalation of, in disease, 107 intramolecular, 101 pathological, 120 - pathology of ' roaring,' 126 ' partial pressure ' of Bunsen, 98 phonation, 127 phrenic nerves, division of, 112 pig, amount of air respired, 116 number of respirations, 90 pleurisy, horse, 120 pneumonia, horse, 120 prsecrucial gyrus, dog, 128 ratio of heart-beats to, 91 recurrent laryngeal nerves, divi- sion of, 126 Eeiset on gases in expired air, 97 reserve air, 113 residual air, 113 respiratory centre, 108 changes in air and blood, 94 exchange, 104 influenced by age, 104 by food, 105 by muscular work, 104 by temperature, 105 Respiration, respiratory exchange, in lungs and tissues, 99 murmur, 119 passages, absorption from, 257 quotient, carnivora, 96 herbivora, 96 omnivora, 96 ribs in, 85 roaring, horse, 120 pathology of, 126 rumination, effect of, on, 90 rupture of the diaphragm, horse, 120 sarcolactic acid, 118 soaleni muscles in, 89 serratus anticus in, 89 magnus in, 89 posticus in, 89 sheep, amount of air required, 116 bleating of, 129 larynx of, 129 number of respirations, 90 pressure in pleural cavity, 90 sneezing, 129 spasm, diaphragm, horse, 120 subepiglottic sinus, ass, 129 mule, 129 Sussdorf on division of phrenics, 112 tetanus, inspiratory, 111 thyro-arytenoideus, 122 tidal air, 113 transversalis costarum in, 89 triangularis sterni in, 89 vagus, influence on, 110 vesicular murmur, 119 voice, cerebral centre for, cat, 129 dog, 128 effect of castration on, 128 production, 127 work, influence of, on, 117 yawning, 129 Zuntz and Lehmann's experi- ments, 105, 114 Respirations, number of, 30 Resting muscle, changes in, 369 Rete mirabile, 81 Reticulum, 174 Retina, 455, 462 Retinal image, formation of, 484 Rhodopsin, 463 Ribs, in respiration, 85 Rigor mortis, 379, 631 Rinderpest, 27 ' Roaring ' in horses, 120, 126, 431 Roger on composition of feces, 210 Digitized by Microsoft® INDEX 709 Romanes on instinct and reason, 452 Kubner's experiments on proteids, 332 Rumen, the, 172 Rumination, Oolin's observations, 180 effect of, on respiration, 90 Flourens on, 180 mechanism of, 182 Ruminants, intestinal digestion in, 199 stomach digestion in, 172 nervous mechanism of, 186 trouble in, 217 Rupture, 217 of the diaphragm, horse, 120 of the heart, 54 of the stomach, 153 Saccharose, 657 Saccule, 499 Saliva, 139 amount of secretion, 139 chemical characters, 140 Meade Smith on, 143 non-amylolytic action of, in herbivora, 143 physical characters of, 140 salts of, 140 secretion of, 144 nerve control of, 144 use of, 142 Salivary glands, changes in cells, 147 Salkowski on urine, 303 composition of urine, 307 Salt solution, physiological 6 Salts, absorption of, 263 in nutrition, 316 of blood, 21 Sarco-lactic acid, 653 in muscle, 374 in respiration, 118 Sarcolemma, 352 Sarcomere, 353 Sarooplasm, 353 Sarcosine, 295, 645 Sarcostyles, 353 Scala tympani, 499 vestibuli, 499 Scaleni in respiration, 89 Schafer's views on muscle, 353 Schematic eye, 478 Schmidt, analysis of pancreatic fluid, 232 Sciatic nerves, 78 Sclerotic, 455 Scratch reflex, 414 Sea-sickness, horse, 180 Sebaceous secretion, 282 Sebum, 276, 282 Secretin, 187, 223, 233 of Starling and Bayliss, 233, 264 Secretion of gastric juice, 162 nerve control of, 169 of pepsin, 152 of saliva, 144 Secretory nerves, Heidenhain's view, 146 Self-digestion of the stomach, 177 Semicircular canals, 497, 502 Semilunar valves, 29, 34 Seminal vesicles, secretion of, 584 Seminiferous tubules, 583 Sensory areas of brain, 441 Serous cavities, 243 Serratus anticus in respiration, 89 magnus in respiration, 89 posticus in respiration, 89 Serum, 3 albumin in liquor sanguinis, 3 in lymph, 245 precipitation of, 3 globulin in liquor sanguinis, 3 precipitation of, 3 proteids of, 3 globulicidal action of, 25 Seventh pair cranial nerves, 428 Sexual intercourse, 586 ova, 592 season of animals, 577 spermatozoa, 592 Sheep, amount of air required, 116 of heat produced, 351 bile, action of, 225 amount of, per hour, 224 specific gravity of, 218 sulphur in, 219 bleating of, 129 blood, time of clotting, 16 composition of body, 314 faeces of, 208 fcetal, gases in blood of, 613 generation, 578 growth of, 625 impregnation in, 593 intestinal digestion in, 201 iris of, 458 larynx of, 129 mastication, 134 milk, analysis of, 620 number of respirations, 90 oesophagus of, 138 cestrous cycle, Goodall, 579 ovary of, 587 period of gestation, 615 pressure, negative, in pleural cavity, 90 • puberty, period of, 585 pulse-rate of, 70 relation between blood- and body-weight, 22 sweating, 277 Digitized by Microsoft® 710 A MANUAL OF VETERINARY PHYSIOLOGY Sheep, temperature of, 339 urine of, 309 uterine glands in, 614 Sherrington on the labyrinth, 503 Shoeing, physiological, 575 Shoulder -joint, 512 ' Side-bone,' cause of lameness in, 563 Siedamgrotzky on effect of clipping, 349 on temperatures, 338 Sigmoid valves, 34 Sight, 454 aberration, chromatic, 483 spherical, 483 accommodation (eye), 466 Helmholtz on, 467 angle, visual, 485 aqueous humour, 455 astigmatism, 457 horse, 469 atropin, cat, 468 dog, 468 effect on iris, 459 horse, 468 Berlin, eye measurements, 480 blind spot (retina), 463 binocular vision, 473 cartilago nictitans, 477 cat, atropin, effect of, 468 emmetropia, 470 iris of, 458 choroid, 455, 461 ciliary muscle, horse, 461 effect of atropin, horse, 468 processes (eye), 458 zone, 461 chromatic aberration, 483 cocaine, effect on iris, 459 cornea, 455, 457 corpora nigra (horse), 460 cow, refraction, errors of, 470 dioptrics, 480 dog, iris of, 458 emmetropia of, 470 emmetropic eyes, 468 eserin, effect on iris, 459 Eversbusch on iris of horse, 459 eyeball, movements of, 470 muscles of, horse, 471 eyelashes of horse, 477 eye, schematic, 478 structure of the, 454 fishes, sight of, 468 foeus, lenses, 482 gland, Harderian, 477 lachrymal, 477 gland, Meibomian, 477 horse, area of acute vision, 464 fundus oculi, 465 Sight, horse, iris of, 458 myopic, 468, 481 refraction, errors of, 470 retina of, 465 'wall-eyed,' 458 hypermetropia, 469 iris, Langley and Anderson on, 457, 458 katoptric test (eye), 467 Lang and Barrett on ciliary muscle and atropin, 468 on errors of refraction, 470 lens, 457, 458 lenses, passage of light through, 481 ligamentura pectinatum, 456, 460 inhibitorium (iris), 460 membrana nictitans, 454 monocular vision, 473 morphia, effect on iris, 459 myopia, 468 nerves of ocular muscles, 472 ophthalmia, sympathetic, 455 ophthalmoscope, 464 optic disc, 464 nerve, 454 decussation of, 455 ox, iris of, 458 papilla (retina), 464 physiological optics, 478 refraction, errors of, in horses, 470 retina, 455, 462 retinal image, formation of, 484 rhodopsin, 463 sclerotic, 455 sheep, iris of, 458 spherical aberration, 483 tapetum lucidum, 457, 461 tears, 477 theory of vision, 484 visual angle, 485 purple, 463 vitreous humour, 456, 462 wild animals, hypermetropic, yellow spot (retina), 464 zonule of Zinn, 462, 467 Silica in dandruff, 283 in feces, 210 Silicon, 634 Sixth pair cranial nerves, 428 Skatol, 652 in digestion, 189, 199, 209 pancreas, 236 urine, 300 Skatoxyl-sulphuric acid, 652 Skin, the, 271 absorption by the, 258 Digitized by Microsoft® INDEX 711 Skin, Arloing on sweating, 281 atropin in sweating, 279 Bouley on varnishing, 284 calcium oxalate in, 283 cat, hair of, 276 pilocarpin in, 281 sweating, 277 experimental, 279 'cat-hairs,' horse, 273 chlorophyll in dandruff, 283 clipping horses, 275 Colin on insensible perspiration, 277 composition of sweat of horse, 278 dandruff, composition of, 283 dog, hair of, 2 c 6 pilocarpin in, 281 sweating, 277 donkey, sweating, 277 Durham, researches on hair pig- ment, 274 effect of varnishing, 284 EUenberger on varnishing, 284 Grandeau on amount of sweat, 277 hair, 272 cat, 276 dog, 276 horse, 272 clipping of, 275 permanent, 273 pigment in, 274 horn, 272 horse, 'cat-hairs,' 273 dandruff, 283 hair of, 272 pilocarpin in, 281 sweat of, composition , 278 sweating, 276 thrombosis of iliac arteries, 281 insensible perspiration, 277 lanolin in dandruff, 283 mechanism, nervous, of sweat- ing, 279 melanin, 274 Mendel's theories of heredity, 274 mule, sweating of, 277 nitre in veterinary practice, 282 nerve ' sweat centre,' 280 Newsom's calculation, hair of horse, 273 ox, sweating of, 277 parasitic disease, 284 pathological, 284 perspiration, insensible, Colin on, 277 pig, sweating, 277 Skin, pigment in hair, 274 pilocarpin, action of, in cat, 281 in dog, 281 in horse, 281 in man, 281 in sweating, 280 potash in wool, 284 respiratory function of, 284 sebaceous secretion, 282 sebum, 276, 282 sheep, sweating, 277 silica in dandruff, 283 ' suint ' in wool, 284 sweat, 276 amount of, daily, 277 Grandeau on amount of, 277 horse, composition of, 278 urea in, 279 sweating, Arloing on, 281 atropin in, 279 nervous mechanism of, 279 pilocarpin in, 280 thrombosis of iliac arteries in horse, 281 tyrosin, pigment from, 275 tyrosinase, 275 urea in dandruff, 283 in sweat, 279 varnishing, effect of, 284 Warrington on potash in wool, 284 Smell, 485 Smith, Meade, on saliva, 143 Sydney, on instinct and reason, 452 Smooth muscle, phenomena of con- traction, 380 Sneezing, 129 Soaps in blood, 20 Sodium carbonate in blood, 1, 21 in pancreas, 233 chloride, action on plasma, 3 in blood, 2, 21, 22 in digestion, 212, 219 in pancreas, 233 phosphate in pancreas, 233 in blood, 1, 21 in blood plasma, 327 in urine, 301 salts, action on heart, 52 in vegetable food, 663 Soda, glycocholate, 221 taurocholate, 221 Sole, 550 use of, 566 Solitary follicles, 255 Somatopleure, 600, 610 Sound, the nature of, 495 Sounds, cardiac, 40 Spasm of the diaphragm, horse, 120 Digitized by Microsoft® 712 A MANUAL OF VETEEINARY PHYSIOLOGY Spavin, position of, 510 Special centres in the spinal eord, 424 Specific gravity, blood (dog, horse, ox, sheep, pig), 2 bile, 218 Spectroscope test, liEemoglobin, 9 Spectrum of CO haemoglobin, 10 of haemoglobin, 9 of oxy-haemoglobin, 9 of hsematin, 11 ' Speedy-cutting,' horse, 512 Spermatic fluid, 584 Spermatoblasts, 583 Spermatogen, 583 Spermatozoa, 583 Spherical aberration, 483 Sphincters, 212 Spinal accessory nerves, 434 cord, 391 nerves, 392 function of, 399 Splanchnopleure, 601, 608 Spleen, 266 enzyme in, 267 use of, 267 Splenic artery, leucocytes in, 12 vein, leucocytes in, 12 Stag, rutting of, 581 Staining of leucocytes, 13 Standing, act of, horse, 521 Stanford on locomotion in horse, 522 Stapes, 497 Starch, 655 of plants, 142 proteid-sparing action of, 320 Starling on lymph production, 249 on pancreatic extract, 235 and Bayliss on peristalsis, 203 and Tubby on absorption, 259 Starvation, 329 Steapsin, 234, 236 Stearic acid, 652 Stearin, 653 in blood, 20 in milk, 620 Stepping reflex, 410 Stercobilin, 210, 221 Sterno-maxillaris muscle, 136 Stifle-joint, description of, 511 discussion of, 508 Stillman on function of suspensory ligament, 513 on locomotion in horse, 522 on muscles of propulsion , 506 Stokes's fluid, 9 Stomach absorption, 176, 259 acids, 161 calculi, 210 contents, reaction of, 161 digestion in dog, 176 Stomach, digestion in horse, 150 in, periods of, 171 in pig, 175 in ruminants, 172 gases of, 178 movements of, 184 nerves of, 185 of dog, horse, pig, ox, 150 of llama, 182 pouch of Pawlow, 166 rupture of, 153 self-digestion of, 177 Storage of tissue, 328 Storch, venous system of horse's foot, 573 Strangulation of the bowels, 215 Stratum periostale, 543 vasculosum, 543 Strongylus artnatus, 83 Structure of muscle, 352 of nerves, 383 Strychnine absorbed by peritoneum, 259 by pleura, 259 effect on brain, 445 experiments, 177 per rectum, 199 Stylo-maxillaris muscle, 136 Subepiglottic sinus of ass, 129 of mule, 129 Sublingual gland, 139 Submaxillary ganglion, cat and dog, 427 gland, 139 of dog, 144 Subsistence diet, 333 Succinic acid, 209 Succus entericus, 186 Sucking, 134 Suffraginis, fracture of, 519 Sugar in blood, 20, 226 invert, 658 supply, how regulated, 229 tests for, Bdttcher's, 661 fermentation, 661 Moore's, 661 picric acid, 661 Trommer's, 661 Sugars, conversion of, 228 ' Suint' in wool, 284 Sulphindigotate of soda, 290 Sulphur, 633 in bile, 219 in body, 665 in hair, 327 in nutrition, 316 Sulphuretted hydrogen in large in- testines, 208 in stomach, 178 Sulphuric acid, 231 Digitized by Microsoft® INDEX 713 Summation of contractions, 361, 380 Superior laryngeal nerve, 138, 431" Suspensory ligament, function of, 513 Sussdorf on division of phrenics, 112 on proportion of blood to body- weight, 22 Sustentacular cell, 583 Swallowing centre, 138 Sweat, 276 amount of daily, 277 centres in cord, 424 Grandeau on amount of, 277 horse, composition of, 278 urea in, 279 Sweating, Arloing on, 281 atropin in, 279 pilocarpin in, 280 nervous mechanism of, 279 Swine, temperature of, 339 Sympathetic system, nerves, 446 Synapses, 412 Synovia, 507 Syntonin, 168 Syphon- trap of duodenum, 153 Systemic circulation, 29 Systole of heart, 35 Tactile cells, 391 sensations, 491 Tapetum lucidum, 457, 461 Tappeiner on cellulose, 171 Tartar emetic, 180 Taste, 489 of blood, 2 goblets, M'Kendrick, 489 Taurine, 222, 379, 645 Taurocholate of soda, 221 Taurocholic acid, 645 Tears, 477 Teeth, horse, 132 ox, 131 sheep, 131 Temperature of the blood, 22 Colin on, 338 normal, of animals, 338 Temporal muscle, 136 Tendon reflexes, 423 Tension of pulse, 69 Tenth pair of cranial nerves, 430 Tereg on urines, 308 Termination of nerves, 391 Testicles, 583 effect of removal of, 582 Test, Gmelin's, for bile, 220 for sugar, 661 Tetanus, 362 inspiratory, 111 Texas fever, organism of, 27 Thalami optici, 437 Theories of urinary secretion, 290 of vision, 484 Third pair cranial nerves, 425 Thirst, 494 Thoracic duct, 243 Thrombin, 18 Thrombosis of iliac arteries in horse, 281 Thymus, 269 gland, influence of castration on, 266 Thyro-arytenoideus muscle, 122 Thyroid gland, 268 Tidal air, 113 Tiger, intestinal canal of, 201 Tissue proteid of Voit, 319 storage of, 328 Tongue, dog, horse, ox, 133 movements of, 133 nerves of, 133 Torcy on growth of calves, 625 Toughness, provision for, in foot, 559 Tracts in the spinal cord, 401, 403 Training, 377 Transfusion, solution used for, 6 Transversalis costarum in respiration, 89 Triangularis sterni in respiration, 89 Tricuspid valve, 29 Trophic centres in cord, 424 Trophoblast, 601, 604 Trot of horse, 524 Trypanosomes, 27 Trypophan, 236 Trypsin, 187, 234, 643 Trypsinogen, 187, 234 Tunica albuginea, 583 fibrosa, 588 Tiirck, column of, 403 Turpentine, absorption by air-pas- sages, 257 Twelfth pair cranial nerves, 434 Tympanum, 497 Tympany, 217 Tyrein, 620 Tyrosin, 650 from proteid, 318 in feces, 209 in spermatic fluid, 584 in urine, 293 pancreas, 236 pigment from, 275 Tyrosinase, 275 Umbilical cord, 608 Urachus, 607 Urea, as measure of work, 294 description of, 647 in allantoic fluid, 608 in blood, 20 Digitized by Microsoft® 714 A MANUAL OF VETERINARY PHYSIOLOGY Urea in dandruff, 283 in liver, 231 in muscle, 379 in sweat, 279 in urine, 292 synthesis of, 648 variations in amount of, 295 Urethra, 312 Uric acid, 20, 295, 649 Urine, 285 acid, aspartic, 293 benzoic, 292 Liebig on, 298 glycuronic, 301, 311 hippurie, 292, 302 production of, 298 oxalic in, 301, 302 phosphate of soda, 305 phosphoric in, 303 sulphuric in, 300-2-3 uric, origin of, 292, 295 adenine, 296 amido-bodies, 293 ammonia in, 304 salts in, 297 ammonium carbamate, 293 carbonate in, 304 Bellini, duct of, 289 Bischoff and Voit on urine of dog, 311 bladder, urinary, 311, 312 blood, amount of, through kid- ney, 289 Bowman, capsule of, 287 calcium in, 301 chlorine in, 303 colouring matter of, 301 composition of, 292 consistence of, 306 creatine, 292 creatinine, 292, 295 cresol, 300 ethereal sulphate of, 292 discharge of, 311 dog, experiments on kidney of, 285, 290 kidney, blood through, 289 urine of, Bischoff and Voit on, 310 excretion, definition of, 285 Fischer, Emil, on purin, 296 glycine, 292 glycocoll, 293 Henle, ascending limb of, 289 loop of, 289 horse, salts in, 301 colour, 306 odour, 306 quantity, 305 solids, 306 Urine, horse, specific gravity, 305 hypoxanthine, 296 indican, 300 indol, 300 inorganic substances in, 301 kidney glomeruli, 286 Malpighian tufts, 286 movements of, 286 pathological, 313 structure of, 286 uriniferous tubules, 286 leucine, 293 leucocythsemia, 296 magnesium in, 301, 303 micturition, act of, 313 Moeller on, of calves, 309 Munk on phosphates in, 304 on ox, 308 nitrogenous substances, 292 oncometer of Koy, 286 phenol, 300 ethereal sulphate of, 292 pig, urine of, 305, 309 potassium in, 301, 303 purin bases, 296 pyrocatechin, 301 reaction of, 304 Salkowski on (chlorides), 303 composition of, 307 sarcosine, 295 sheep, urine of, 309 skatol, 300 sodium in, 301 sulphindigotate of soda, 290 Tereg on urines, 308 theories of urinary secretion, 290 tyrosine, 293 urea as measure of work, 292 variations in amount of, 295 urethra, 312 uric acid, 295 formation, 296 urobilin, 301 urochrome in, 301 vascular mechanism of kidney, 289 Wolff, E., on, 306 xanthine, 296 Urobilin, 11, 301 Urochrome, 301 Uterine milk, 614 Uterus, changes in, during pro- cestrum, 581 Utricle. 499 Vagina, absorption from, 258 Vagus, action on small intestines, 205 and secretion of gastric juice, 169 Digitized by Microsoft® INDEX 715 Vagus in neck, stimulation of, 45 influence of, on respiration, 110 motor nerve of stomach, 185 "Valves, aortic, 29 auriculo-ventricular, 29, 33 bicuspid, 33 Chauveau on, 39 mitral, 29, 33 of the heart, 29 action of, 39 of veins, 57 pulmonary, 29 semilunar, 29, 34 sigmoid, 34 tricuspid, 29 use of the, 30 Valvular disease, horse, 54 Variations in body temperature, 339 Varnishing the skin, effect of, 284, 346 Vascular mechanism of foot, 573 of kidney, 289 sole, 545 wall (foot), 542 Vaso-constrictor centre, 79 Vaso-dilator nerves, 76 Vaso-motor centre, 50, 75 centres in cord, 424 subcentres in cord, 75 Veins, 57 abdominal, 57 capacity of, 57 construction of, 57 of pregnant uterus, 57 pulse in, 66 without valves, 57 Velocity of blood, 72 of gallop, 533 of nerve impulses, 589 of trot, 533 Vense eavse, 57 Venous blood, 20 plexuses of corpus cavernosum, 82 Vesico-spinal centre in cord, 424 Vesicular murmur, 119 Vestibule, 497 Villi, the, 253 Vision, theory of, 484 Visual angle, 485 purple, 463 Vitellin, 638 Vitelline membrane, 589 Vitreous humour, 456, 462 Voice, cerebral centre for, cat, 129 dog, 128 effect of castration on, 128 production, 127 Voit, experiments, bile, 225 theory of metabolism, 318 Volkmann's estimate of area of vascular system, 72 observations, velocity of blood, 72 Voluntary muscles, 352 Vomiting, 178 Walk of horse, 523 Wall-secreting substance of foot, 545 Waller's co-operative antagonism,506 degeneration, spinal nerve, 400 Warrington on potash in wool, 284 Water, 662 absorption of, 263 by air-passages, 257 in tissues, 328 Weight, how carried by foot, 561 of the body, distribution of, 515 which a horse can carry, 534 Weissmann on polar bodies, 590 Wet, effect of, 347 Whartonian jelly, 610 White corpuscles of blood, 12 Wild animals, hypermetropia in, 470 Wolf, generation, 578 Wolff on diet, 334 on heat loss, 351 on urine, 306 Wooldridge on temperature, 339 Work of the heart, 43 influence on respiration, 117 Xanthine (muscle), 379 series, 638 (urine), 296 Xanthoproteic reaction, 641 Vawning, 129 Yellow spot, retina, 464 Yolk sac, 600, 605 Zebra, period of gestation, 614 Zona radiata, 589 Zonule of Zinn, 462, 467 Zuntz on muscle work, 365 and Lehmann, experiments, res- piration, 105, 114 Zymase, 643 Zymogen, 644 Baillie're, Tindall and Cox, 8, Henrietta Street, Covenl Garden. Digitized by Microsoft® Digitized by Microsoft® Frog (on the JUit) (side view) Bird Mammal Camel Frog's Corpuscle after addition of water Mammalian after addition of syrup Blood of mammal ® '%??&,& ° '■■'■I "' ( H ' Mammalian a/ter addition of salt !PhiU 1. Red blood-corpuscles. ■s 8 I . * ■8 ^ (*$> # I 8 N 6" Si ■8 J is 411 2. The colourless corpuscles of human blood, * 1000. a, eosinophile cells ; 6, finely granular oxyphile cells ; c, hyaline cells ; d, lymphocyte ; e, polymorphonuclear neutrophile cells (Kanthack and Hardy). The magnification is much greater than in 1. 3. Cover-glass preparation of spinal cord of ox, x 250. (Stained with methylene blue). DtndrUic processes Ophthalmoscopic view of fundus. of the lxsfse Digitized by Microsoft® Digitized by Microsoft® Digitized by Microsoft® Digitized by Microsoft® Digitized by Microsoft® Digitized by Microsoft® Digitized by Microsoft® Digitized by Microsoft® i "liiiSSiisiJSiHB |l|ji|